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the Creative Commons Attribution 4.0 License.
| 11 Apr 2024
Were early Archean carbonate factories major carbon sinks on the juvenile Earth?
Abstract. Paleoarchean carbonates in the Pilbara Craton (Western Australia) are important archives for early life and environment on early Earth. Amongst others, carbonates occur in interstitial spaces of ca. 3.5–3.4 Ga pillow basalts (North Star-, Mount Ada-, Apex-, and Euro Basalt, Dresser Formation) and associated with bedded deposits (Dresser- and Strelley Pool Formation, Euro Basalt). This study aims to understand the formation and geobiological significance of those early Archean carbonates by investigating their temporal-spatial distribution, petrography, mineralogy, and geochemistry (e.g., trace elemental compositions, δ13C, δ18O). Three carbonate factories are recognized: (i) an oceanic crust factory, (ii) an organo-carbonate factory, and (iii) a microbial carbonate factory. The oceanic crust factory is characterized by carbonates formed in interspaces between pillowed basalts (“interstitial carbonates”). These carbonates precipitated inorganically on and within the basaltic oceanic crust from CO2-enriched seawater and seawater-derived alkaline hydrothermal fluids. The organo-carbonate factory is characterized by carbonate precipitates that are spatially associated with organic matter. The close association with organic matter suggests that the carbonates formed taphonomically via organo-mineralization, that is, linked to organic macromolecules (either biotic or abiotic) which provided nucleation sites for carbonate crystal growth. Organo-carbonate associations occur in a wide variety of hydrothermally influenced settings, ranging from shallow marine environments to terrestrial hydrothermal ponds. The microbial carbonate factory includes carbonate precipitates formed through mineralization of extracellular polymeric substances (EPS) associated with microbial mats and biofilms. It is commonly linked to shallow subaquatic environments, where (anoxygenic) photoautotrophs might have been involved in carbonate formation. In case of all three carbonates factories, hydrothermal fluids seem to play a key-role in the formation and preservation of mineral precipitates. For instance, alkaline earth metals and organic materials delivered by fluids may promote carbonate precipitation, whilst soluble silica in the fluids drives early chert formation, delicately preserving authigenic carbonate precipitates and associated features. Regardless of the formation pathway, Paleoarchean carbonates might have been major carbon sinks on the early Earth, modulating the carbon cycle and, hence, climate variability.
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RC1: 'Comment on egusphere-2024-1007', Graham Shields, 21 May 2024
This fascinating contribution presents new carbon and oxygen stable isotope data, mainly from early Archean (Paleoarchean) carbonates in western Australia, and comes to the conclusion that Paleoarchean carbonates, which the authors divide into three main types, could have been major carbon sinks at the time, thus moderating the global carbon cycle and contributing to early climate regulation. The geochemical data are new, sedimentological context well described, and conclusions of broad interest. I have a few comments for the authors to consider when framing the final version of this thoughtful manuscript.
- Major carbon sink. The major source of carbon into the Archean exogenic system would have been volcanic outgassing (in the absence of oxidative weathering), while the major sink would have to have been carbonate, just as today. The premise of the paper is that we do not yet know what the carbon sinks would have been at the time, but I would ask the authors what other sinks might there have been, because they do not specify the alternatives anywhere and do not provide a conceptual box model of the Archean carbon cycle, which would have been useful. If they draw the same conclusion as me, that carbonate minerals must have been the dominant sink, then I suggest rewording the title, conclusions and other relevant sections throughout the manuscript. Even accepted that it might not have been the dominant sink, which I very much doubt, is it likely that carbonate was ever not a “major” sink as implied by the title? In this regard, the final sentence of the Abstract is also rather weak.
- I would appreciate more quantification and quantified comparisons with the modern. For example, on line 178 “highly carbonatized” could be quantified, while what I really want to know as reader is how this compares with ocean basalts today? Do your observations match the constraints mentioned later from the literature on lines 514-516, for instance?
- Carbonate mineralogy. Wherever possible I would encourage the authors to specify the mineralogy, e.g. in Table 1 or line 233. This is particularly important where the stable isotope data are outlined / discussed, as different minerals fractionate differently. On line 225, and elsewhere, we learn that the primary mineralogy of interstitial carbonates was calcite, and yet the crystals are described as being “acicular, a habit more commonly associated with aragonite. Can the authors rule out aragonite as the precursor carbonate mineral?
- Other geochemical data. On lines 368-374, we learn about other pertinent geochemical data, such as strontium isotopes, which come from a doctoral thesis. This section comes rather out-of-the-blue, and it is not apparent why these data, which are evidently from the same project, have not been presented more fully. As they are key to the interpretation, I’d recommend that the reader be told more relevant methodological and contextual details about these analyses.
- Carbon (isotope) mass balance. Carbon sinks and the global carbon cycle do not get enough attention until the end of the paper, but could have come already in the Introduction considering their importance to the "take-home" message. In this regard, the authors need to use their new data and compiled literature data to constrain the isotopic value of the carbonate sink at that time, and carry out a simple isotope mass balance calculation. The isotopic composition of the other sink, organic matter, can be estimated from the literature, but it is not mentioned in the paper. These values allows a very simple isotope mass balance to be proposed if we can assume a likely input value or range of input values (this is mentioned in the paper). As far as I can tell, the conclusion from such a simple mass balance would confirm that the sub-seafloor calcite carbon sink was likely the dominant carbon sink of the Paleoarchean (lines 511-512). A pertinent paper in this regard would be Mills et al (2016) Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering, in PNAS, 11, 9073-9078, but there are other relevant papers not mentioned here that pertain to the carbon budget on the early Earth, e.g. Canfield (2021) Carbon cycle evolution before and after the great oxidation of the atmosphere. American journal of Science, 321, 297-331.
English formulation and other minor issues:
Line 20 (Abstract): “Interspaces between pillowed basalts” – this is not the same as interstitial carbonate, and needs rewording for clarity.
Line 23 (Abstract): “formed taphonomically” can be omitted here and elsewhere, as it is not clear what extra meaning this adds, as this term usually refers to fossil preservation.
Line 74: I suggest omitting “comprehensively” here. Likewise, on line 149 “high-precision” seems unnecessary.
Line 158: “into”, not “in”.
Lines 194-195: Here, and elsewhere I would have appreciated a chemical equation to illustrate the process being described.
Lines 244-245 – the evidence for dolomitization is given as the presence of ankerite. I find this confusing. Isn’t this “ankeritization”?
Lines 250-251 – sentence needs rewording for clarity. Likewise, lines 438-439, 463-465, 486-487.
Line 417: “consummated” probably needs a different word?
Line 461: “complexation” – for me, this terms means something else, as in complexed CaCO30.
Line 468: “higher”, not “heavier”.
Table 2 – How are evaporite minerals identified in this study? And which minerals are these?
Citation: https://doi.org/10.5194/egusphere-2024-1007-RC1 -
AC1: 'Reply on RC1', Wanli Xiang, 26 Jun 2024
RC1: 'Comment on egusphere-2024-1007', Graham Shields, 21 May 2024
This fascinating contribution presents new carbon and oxygen stable isotope data, mainly from early Archean (Paleoarchean) carbonates in western Australia, and comes to the conclusion that Paleoarchean carbonates, which the authors divide into three main types, could have been major carbon sinks at the time, thus moderating the global carbon cycle and contributing to early climate regulation. The geochemical data are new, sedimentological context well described, and conclusions of broad interest. I have a few comments for the authors to consider when framing the final version of this thoughtful manuscript.
1. Major carbon sink. The major source of carbon into the Archean exogenic system would have been volcanic outgassing (in the absence of oxidative weathering), while the major sink would have to have been carbonate, just as today. The premise of the paper is that we do not yet know what the carbon sinks would have been at the time, but I would ask the authors what other sinks might there have been, because they do not specify the alternatives anywhere and do not provide a conceptual box model of the Archean carbon cycle, which would have been useful. If they draw the same conclusion as me, that carbonate minerals must have been the dominant sink, then I suggest rewording the title, conclusions and other relevant sections throughout the manuscript. Even accepted that it might not have been the dominant sink, which I very much doubt, is it likely that carbonate was ever not a “major” sink as implied by the title? In this regard, the final sentence of the Abstract is also rather weak.
- R: Thank you very much for offering us these valuable and helpful suggestions. Firstly, we agree that carbonate minerals must have been the dominant carbon sink at that time. Therefore, we need to make a correction to illustrate it, e.g. rewording the title and relevant sentences with “dominant” replacing of “major”. Secondly, we will revise the Introduction to specify the present-day and early Archean carbon sinks. Based on the previous work, the early Archean carbon sinks were organic matter in chert units and carbonate minerals preserved in carbonatized basaltic rocks, interstitial carbonates, and stromatolites. Thirdly, considering the suggestions about carbon sink and carbon (isotope) mass balance, we will add a new section after “5.2 Early Archean carbonate factories – implications”, where to summarize the conceptual box model of the Archean carbon cycle and carry out a simple isotope mass balance calculation. At last, the Abstract and Conclusion may be revise little to fit our work.
2. I would appreciate more quantification and quantified comparisons with the modern. For example, on line 178 “highly carbonatized” could be quantified, while what I really want to know as reader is how this compares with ocean basalts today? Do your observations match the constraints mentioned later from the literature on lines 514-516, for instance?
- R: Thank you for pointing those problems. We describe “Spherulitic and variolitic zones in the basalts are highly carbonatized”, compared to other basaltic parts. This is mainly based on observations elemental mappings via micro-XRF about sample thin sections, which were presented in Fig.3b and Fig.S1a-c, respectively. In this paper, we just focus on interstitial carbonates and only briefly introduce the host basalt. In fact, we are preparing a paper about pillow basalts of Apex Basalt, which preserves the primary interstitial carbonates, with more details such as the zonation and elemental compositions of zones (see Chapter 4 in Xiang, 2023). Considering the RC2, we will reword it into “carbonate minerals are particularly prominent in voids, veins and variolites within spherulitic and variolitic zones” to avoid confusion. However, for the reader’s interest, we will try to add a quantification of the carbonatized degree and a comparison with modern oceanic crust, somewhere suitable in the text or in the supplementary materials. To do this, we can use Si and Ca concentrations of minerals within basalt via micro-XRF point spectrum and line analysis (in our unpublished work), CO2 concentrations of basalt samples from Apex Basalt in Nakamura and Kato (2004), carbonate proportion (vol.%) in the modern ocean crust in Rausch (2012).
- For the latter question, we think the carbon flux into the oceanic crust factory is much higher than the present-day flux. As Rausch (2012) illustrated, carbonate minerals are in veins, vesicles and breccias of modern oceanic crusts, and more abundant in older crusts (Gillis et al., 2001; Heft et al., 2008; Coogan and Gillis, 2013). However, they could precipitate as interstitial carbonates in the Archean oceanic curst (Nakamura and Kato, 2004; Shibuya et al., 2012; Marien et al., 2023), which were considerable portions (vol%) of strata preserving the pillow basalts. Nakamura and Kato (2004) reported carbon flux into the early Archean oceanic crust by the seafloor carbonatization to be 3.8 × 1013 mol/yr, while Rausch (2012), Alt and Teagle (1999) suggested a global carbon uptake flux of 1.03 × 1012 mol/yr and 1.5–2.4 × 1012 mol/yr into modern oceanic crust, respectively. Notably, the estimates did not include any interstitial carbonate. However, this work aims to emphasize the significance of carbonates formed in interpillow void space of the basalts within the oceanic crust which we call interstitial carbonates. Although hard to measure, interstitial carbonates look far more than carbonate minerals within basalts (Fig. 2, 3, 5). Therefore, we think the carbon flux into the oceanic crust factory should be beyond 3.8 × 1013 mol/yr, and far more than that into modern oceanic crust.
3. Carbonate mineralogy. Wherever possible I would encourage the authors to specify the mineralogy, e.g. in Table 1 or line 233. This is particularly important where the stable isotope data are outlined / discussed, as different minerals fractionate differently. On line 225, and elsewhere, we learn that the primary mineralogy of interstitial carbonates was calcite, and yet the crystals are described as being “acicular, a habit more commonly associated with aragonite. Can the authors rule out aragonite as the precursor carbonate mineral?
- R: Thank you for your suggestion and pointing the problem. Following your advice, we will add a column in Table 1 to specify the mineralogy. For the question, we do not think of the presence of aragonite as precursor. Even in modern oceanic crust, aragonite is not so pervasive, for that their precipitation may be determined by several factors such as Mg/Ca ratio of parenting fluid and nucleation template. In our work, we have done XRD and Raman analyses, and had the geochemical compositions, which indicate they are now low-Mg calcite. The habit of acicular crystal fan can also be found in calcite, and there should still be some evidence preserved if aragonite was the precursor due to the rather low diagenetic overprint. However, we did not find any convincing evidence to prove "the primary mineralogy of interstitial carbonates was aragonite”. Therefore, we chose to describe and believe what we see today.
4. Other geochemical data. On lines 368-374, we learn about other pertinent geochemical data, such as strontium isotopes, which come from a doctoral thesis. This section comes rather out-of-the-blue, and it is not apparent why these data, which are evidently from the same project, have not been presented more fully. As they are key to the interpretation, I’d recommend that the reader be told more relevant methodological and contextual details about these analyses.
- R: Thank you for pointing out this issue. As you said, this work, the Sr isotope data and the aforementioned data of Apex pillow basalts are from the same project that will be in three publications. The Sr isotope data are all unpublished, and crucial to the works in preparation. Therefore, we just used some results of them in this work. However, considering your kind suggestion, we will provide an information in the supplementary materials. This will include relevant methodological and contextual details about these analyses, and a table to present some 87Sr/86Sr ratios of the basalt-carbonate system, including the average value of the primary and secondary interstitial carbonates, fracture-filling calcite (D-2-W), and the whole rock value of pillow basalts. This is sufficient to support this work. Nonetheless, we encourage readers to find more details in Xiang, 2023, Carbonate factories in the early Archean and their geobiological impacts (Ph.D. thesis, http://dx.doi.org/10.53846/goediss-10047). The three works are shown with more details in Chapter 2 to 4, respectively. And they can also find some interesting applications of our works.
5. Carbon (isotope) mass balance. Carbon sinks and the global carbon cycle do not get enough attention until the end of the paper, but could have come already in the Introduction considering their importance to the "take-home" message. In this regard, the authors need to use their new data and compiled literature data to constrain the isotopic value of the carbonate sink at that time, and carry out a simple isotope mass balance calculation. The isotopic composition of the other sink, organic matter, can be estimated from the literature, but it is not mentioned in the paper. These values allow a very simple isotope mass balance to be proposed if we can assume a likely input value or range of input values (this is mentioned in the paper). As far as I can tell, the conclusion from such a simple mass balance would confirm that the sub-seafloor calcite carbon sink was likely the dominant carbon sink of the Paleoarchean (lines 511-512). A pertinent paper in this regard would be Mills et al (2016) Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering, in PNAS, 11, 9073-9078, but there are other relevant papers not mentioned here that pertain to the carbon budget on the early Earth, e.g. Canfield (2021) Carbon cycle evolution before and after the great oxidation of the atmosphere. American journal of Science, 321, 297-331.
- R: Thank you for your kind suggestions and recommending nice references. We will revise the Introduction to introduce carbon sinks and carbon cycle earlier, and highlight the significance of them to moderate climate. And we will add a new section after “5.2 Early Archean carbonate factories – implications”. In this section, we will carry out a simple isotope mass balance calculation following the method described in Canfield (2021), using our δ13C data of carbonates and data of organic matter in Flannery et al. (2018) (-29 to - 45 ‰; more data will be considered if they were from the relevant EPT formations). And we will also quantify carbon uptake flux into the oceanic crust factory and compare it with the modern one. We believe all will support that carbonate precipitated in the oceanic crust factory associated with seafloor hydrothermal alteration was the dominant carbon sink on early Earth.
6. English formulation and other minor issues:
（1）Line 20 (Abstract): “Interspaces between pillowed basalts” – this is not the same as interstitial carbonate, and needs rewording for clarity.
- R: Thanks for pointing this problem. Following your advance, the sentence (Line 20) “The oceanic crust factory is characterized by carbonates formed in interspaces between pillowed basalts (‘interstitial carbonates’)” will be reworded into “The oceanic crust factory is characterized by carbonates formed in interpillow void spaces of pillowed basalts (referred to as ‘interstitial carbonates’ in this work)”. Besides, considering the rewording of the Introduction, “The geobiological significance of other carbonates, such as precipitates in the interspaces of (ultra)mafic rocks that possibly formed through seafloor hydrothermal alteration (referred to as ‘interstitial carbonates’ in this work)” (Line 70-71) may be revised to “However, their calculation on carbon flux did not included carbon in carbonates precipitated in the interpillow void spaces of the pillow basaltic rocks, referred to as ‘interstitial carbonates’ in Marien et al. (2023) and this work”.
（2）Substitution or modification of words. We organized some comments to answer here:
a. Line 23 (Abstract): “formed taphonomically” can be omitted here and elsewhere, as it is not clear what extra meaning this adds, as this term usually refers to fossil preservation.
b. Line 74: I suggest omitting “comprehensively” here. Likewise, on line 149 “high-precision” seems unnecessary.
c. Line 158: “into”, not “in”.
d. Line 417: “consummated” probably needs a different word?
e. Line 461: “complexation” – for me, this term means something else, as in complexed CaCO30.
f. Line 468: “higher”, not “heavier”.
- R: Thank you for pointing out these problems. Following your advice, we will omit “taphonomically” on Line 23, “comprehensively” on Line 74, “high-precision” on Line 149, and correct “in” on Line 158 with “into”, “consummated” on Line 417 with “confirmed”, “complexation” on Line 461 with “combination”, “heavier” on Line 468 with “higher”. After the corrections, the sentences will be presented as below:
- a. Line 23: “The close association with organic matter suggests that the carbonates formed via organo-mineralization, that is, linked to organic macromolecules (either biotic or abiotic) which provided nucleation sites for carbonate crystal growth.”
- b. Line 74: “This study investigates early Archean carbonates in the EPT, including interstitial carbonates associated with basalts, carbonate stromatolites and other sedimentary carbonates.”
- Line 149: “Additionally, some carbonate facies, including carbonate veinlets and carbonate inclusions, were extracted using a drill from individual mineral phases from polished rock slabs.”
- c. Line 158: “The host basalts are pillow-shaped, internally subdivided into more crystalline interiors and quenched glassy rims, and commonly locally cut by tectonic fractures (Fig. 2).”
- d. Line 417: “To distinguish minerals formed through mineralization linked to organic matrices and compounds from those whose formation is induced by living organisms, the terms “organomineral” and “organo-mineralization” were introduced at the 7th International Symposium on Biomineralization in 1995 and further developed in the following decade (…), before being finally confirmed in following studies (…).”
- e. Line 461: “Certain functional groups of organic substances in the EPS (e.g. Asp- and Glu-rich macromolecules) efficiently bind and sequester divalent cations such as Ca2+ and Mg2+, thereby inhibiting their combination with carbonate anions and subsequent precipitation (Reitner et al. 1995a, b, c).”
- f. Line 468: “More specifically, δ13C values of carbonates from SPF stromatolites (3.08 ± 0.30 ‰ on average) are higher than those of the interstitial carbonates (0.22 ± 0.98 ‰ on average) and the sedimentary carbonates (1.85 ± 0.48‰ on average).”
7. Lines 194-195: Here, and elsewhere I would have appreciated a chemical equation to illustrate the process being described.
- R: Thank you for your kind suggestion. Following your advice, we will make a correction here to illustrate it. Firstly, we will supplement an explanation in the end of the 2nd paragraph in Section 4.1.1 and add a simple relationship after the relevant text. It will look like:
- “Except for the devitrified volcanic glass, Si is rich in the interior of the pillow basalt but rare in the spherulitic and variolitic zones, implying a Si loss during basalt carbonatization (Fig. 3b). Si yielded during this process was likely enriched in fluids, resulting in chert cementation of interstitial carbonates (Fig. 4). The process can be summarized as follows (see Eq.1 where refers to calcium silicate minerals):
- "CaSiO3" + CO2 +H2O → "CaSiO3" +H2CO3 → CaCO3 + SiO2 + H2O (Eq.1) ”
- Secondly, we will revise the caption of Fig. 3 on Line 194-195 into “In addition, the quenched margin of the basalt seems to relatively depleted in Si as compared to the core, implying a loss of Si during carbonatization processes (see Eq.1).”
8. Lines 244-245 – the evidence for dolomitization is given as the presence of ankerite. I find this confusing. Isn’t this “ankeritization”?
- R: Thank you for pointing this issue. As you said, this is ankeritization. It was me who insisted to use “dolomitization”, while some of the coauthors thought of “ankeritization”. I preferred to use the usual term, because dolomitization and recrystallization are two common processes during diagenetic alteration, and ankerite [Ca(Fe,Mg,Mn)(CO3)2] is an Fe- and Mn-bearing dolomite. However, considering your advice and the truth that ankerite is a characteristic mineral which is only found in interstitial carbonates, we will correct “dolomitization” with “ankeritization” in the relevant sentences.
9. Lines 250-251 – sentence needs rewording for clarity. Likewise, lines 438-439, 463-465, 486-487.
- R: Thank you for your kind suggestions. Following your advice, we will make corrections as follows:
- Line 250-251: we will revise the paragraph into “The secondary carbonates are either Mn- or Sr-enriched (see Fig. 5), indicating at least two diagenetic fluids during later alteration. For instance, the Mn-enriched carbonates include the recrystallized interstitial calcites, fibrous calcite cement within basalt fractures, and the interstitial ankerite. To be noted, some interstitial ankerites, formed through recrystallization and neomorphism or closer to the basaltic parts, are more Mn-enriched than other interstitial ankerites (Fig. 5a). On the other hand, the calcite overgrowth of the interstitial ankerites and the fracture-filling calcite are Sr-enriched, especially the latter, which is most Sr-enriched among all analyzed carbonate phases (Fig. 5).”
- Line 438-439: “Some EPT bedded sedimentary carbonates (except the Dresser bedded carbonates) show an average δ13C value of 1.85 ± 0.48‰, within the range of the values of modern seawater (Kroopnick, 1980; Tan, 1988) and the Strelley Pool stromatolites (Lindsay et al., 2005; Flannery et al., 2018; this work), reflecting their formation in marine environments.”
- Line 463-465: “Distinct from biological induced biomineralization linked to contemporaneous biological activity, organo-mineralization involves mineralizing organic matrices and compounds remote in space and time from the organisms which they derive from, or it can be in association with prebiotic or abiotic organic matter (Trichet and Défarge, 1995; Défarge, 2009, 2011).”
- Line 486-487: “Carbonates in this carbonate factory precipitated from CO2-rich seawater-derived hydrothermal fluids, which were in a high alkalinity and high cation loads.”
10. Table 2 – How are evaporite minerals identified in this study? And which minerals are these?
- R: Thanks for pointing this problem out. We assume there should be some evaporite minerals in organo-carbonate factory and microbial carbonate factory due to their deposition environments (supported by references and this work) and mineral morphologies. For example, the organo-carbonate factory could occur on land, as showed in the case of the DB bedded carbonates, in a hydrothermal pond. “The clusters of radiating calcite crystals at the base of each carbonate-chert layer (Figs. 6f, 8a), which were initially proposed to be gypsum or aragonite (Runnegar et al., 2001; Van Kranendonk et al., 2008; Otálora et al., 2018), are likely indicative of evaporitic conditions” (Line 442-445). In the microbial carbonate factory, we ascribed the formation of carbonate fans beneath the SPF stromatolites to be evaporation, considering their morphology and the depositional environment of the SPF stromatolites. Therefore, the evaporate minerals in our study are carbonate minerals (calcite and dolomite).
- However, it seems that we have lost to tell our readers some important messages. To solve this problem, we will make some corrections as follows:
- (1) We will supplement the information of carbonate fans beneath the SPF stromatolites in the end of the 2nd paragraph in Section 4.3:” Beneath the stromatolites, large carbonate fans (~ 40 cm) positioned on a chert layer and were cemented by chert (Fig. 9). The carbonate fans encompass fusiform dolomite aggregations (Fig. 9f).”
- (2) We will add the introduction of depositional environments in the first paragraph in Section 5.2 from Line 485 (italic and bold)；“The oceanic crust factory includes abiotically formed carbonates such as Mn- or Sr-enriched calcite and ankerite that are associated with pillow basalts within the upper oceanic crust. Carbonates in this carbonate factory precipitated from CO2-rich seawater-derived hydrothermal fluids, which were in a high alkalinity and high cation loads. The organo-carbonate factory is dominated by authigenic carbonates formed through taphonomy-controlled organo-mineralization (i.e. organomicrites). Importantly, and in contrast to the microbial carbonate factory, the involved organic matter can be of either biological or abiotic origin. This pathway of carbonate precipitation makes it possible to form in various environments, ranging from shallow marine to terrestrial settings. The microbial carbonate factory is somewhat similar to the organo-carbonate factory, but specifically refers to EPS-controlled carbonate precipitation, that is, mineralization of biologically derived organic substances. However, as in case of the organo-carbonate factory, organomicrite is formed as a typical product. Associated with biological activities, this carbonate factory occurred in the photic, relatively restricted, shallow marine environments like lagoons. Given that most of these carbonates formed in shallow-water environments under anoxic conditions, anoxygenic phototrophs appear a plausible source of biological organic matter, but this remains to be tested in future studies.”
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Nakamura, K. and Kato, Y.: Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO2 sink in the Early Archean, Geochimica et Cosmochimica Acta, 68, 4595–4618, https://doi.org/10.1016/j.gca.2004.05.023, 2004.
Otálora, F., Mazurier, A., Garcia-Ruiz, J. M., Van Kranendonk, M., Kotopoulou, E., El Albani, A., and Garrido, C.: A crystallographic study of crystalline casts and pseudomorphs from the 3.5 Ga Dresser Formation, Pilbara Craton (Australia), Journal of Applied Crystallography, 51, 1050– 1058, https://doi.org/10.1107/S1600576718007343, 2018.
Rausch, S.: Carbonate veins as recorders of seawater evolution, CO2 uptake by the ocean crust, and seawater-crust interaction during low- temperature alteration, Ph.D. thesis, Universität Bremen, 2012.
Runnegar, B., Dollase, W.A., Ketcham, R.A., Colbert, M., Carlson, W.D.: Early Archean sulfates from Western Australia first formed as hydrothermal barites not gypsum evaporites. In: Geological Society of America Abstracts with Programs, vol. 33, 2001.
Shibuya, T., Tahata, M., Kitajima, K., Ueno, Y., Komiya, T., Yamamoto, S., Igisu, M., Terabayashi, M., Sawaki, Y., Takai, K., et al.: Depth variation of carbon and oxygen isotopes of calcites in Archean altered upper oceanic crust: Implications for the CO2 flux from ocean to oceanic crust in the Archean, Earth and Planetary Science Letters, 321, 64–73, https://doi.org/10.1016/j.epsl.2011.12.034, 2012.
Tan, F. C.: Stable carbon isotopes in dissolved inorganic carbon in marine and estuarine environments, in: Handbook of environmental isotope geochemistry, edited by Fritz, P. and Fontes, J., vol. 3, pp. 171–190, Elsevier, Amsterdam, 1988.
Trichet, J. and Défarge, C.: Non-biologically supported organomineralization, Bulletin de l’Institut océ anographique, Monaco. Numé ro special 14(2), 203–236, 1995.
Van Kranendonk, M. J., Philippot, P., Lepot, K., Bodorkos, S., and Pirajno, F.: Geological setting of Earth’s oldest fossils in the ca. 3.5 Ga Dresser formation, Pilbara Craton, Western Australia, Precambrian Research, 167, 93–124, https://doi.org/10.1016/j.precamres.2008.07.003, 2008.
Xiang, W.: Carbonate factories in the early Archean and their geobiological impacts, Ph.D. thesis, University of Göttingen, 1055 Germany, http://dx.doi.org/10.53846/goediss-10047, 2023.
Citation: https://doi.org/10.5194/egusphere-2024-1007-AC1
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RC2: 'Comment on egusphere-2024-1007', Anonymous Referee #2, 28 May 2024
In this manuscript, the authors characterize three different sources of carbonates commonly found in the East Pilbara Terrain (Australia). They describe and differentiate the potential origin of said carbonates in three different marine carbonate factories, as reflected in their distinct d13C signal, these being: 1. The oceanic crust (samples obtained from interstitial carbonate within basalts); 2. Organo-carbonates (sedimentary carbonates form through mediation of an organic component); and 3. Microbial carbonate (stromatolites).
Although I enjoyed reading this manuscript, and it made me think about the carbon cycle from a different perspective that I normally would, I recommend some changes below to be carried out before its publication.
First, and foremost, either the title needs to be changed, or the discussion needs to be rewritten. As it stands, it is unclear to me that this manuscript answer their own title “Were early Archean carbonate factories major carbon sinks on the juvenile Earth?”. The last sentence in the manuscript says “[…] Paleoarchean carbonates might have been major carbon sinks at the time of formation […]”. So, were they major carbon sinks? There is no mention of any quantification of the discussed carbon sink, nor mention of any other potential carbon sinks during this geological period. After reading this manuscript, I cannot answer the question that the authors are postulating. Considering this journal, it is likely that non-experts in Archean carbonates will read this paper, and a further expansion on the carbon cycle at the time, literature looking at carbon sinks in this time and how this work fits with the literature are needed.
Also, the introduction needs to be organized in a different way in order to bring to the front the research question the authors are trying to answer, which is not clear until the very end of the introduction. As before, given the varied potential readership of the journal, the point of why to investigate Archean carbonates, and the ones in the EPT in particular, needs to be specified early in the introduction. For example: 1) The carbon cycle and carbon sinks and their importance for the climate system during the Precambrian. There is no mention of this until the very end of the Introduction (Line 79), and then again nothing until the very end of the discussion (Line 505), this needs to come up earlier, and expanded. 2) Then, as per lines 51 – 54, carbonate formations in this period are poorly constrained, especially in the early Archean. 3) This sets the framework to drive the reader into why more effort needs to be paid, and why you do the work you do. This is only an example of a potential structure that could highlight the importance of this work. Essentially, the introduction needs to be “punchier”.
Finally, in the results (sections 4.1.2 and 4.1.3) as data is presented on primary and secondary carbonate facies and different minerals found. Is it possible to include any sort of quantification/percentage on the presence of minerals on the analyzed samples? This would give a more complete characterization of the interstitial carbonates, the diagenetic processes and their stable isotopic composition.
Minor comments:
Line 92: Repeated “consisting” word twice within a sentence. Rewrite for clarity.
Lines 125 – 128: “For comparison, we additionally analyzed […]”. What is the purpose of this comparison? Include this info in this paragraph, it will make it easier for the reader to understand why you included these samples at all.
Line 146: Change for “were cleaned three times in ethanol using an ultrasonic bath”, or similar. “Using ultrasound” might be confusing.
Line 147: No need to “gently” dry samples. You can delete this adjective.
Lines 177 – 181: These sentences are repetitive and need to be reworded/rewritten.
“Spherulitic and variolitic zones in the basalts are highly carbonatized, with carbonate minerals being particularly prominent in variolites and concentric syngenetic veins.” – What does highly carbonatized mean? First and second part of the sentence is saying the same thing?
“Notably, elemental distributions in basalts do not seem to relate to the degree of weathering (e.g. in sample A22 of the Apex Basalt; Fig. 3b) and hence might be pristine.” – I do not know what do you mean by this. What would be different in the data if elemental distribution related to weathering? Also, likely need some references to justify why does not relate to degree of weathering. And then, this would be more fitting included in the discussion.
“Except for the devitrified volcanic glass, Si is rich in the interior of the pillow basalt but rare in the zone of spherulites and variolites, which are dominated by calcite” – You are mentioning again that calcite is predominant in spherulites and variolites. Which you mentioned two sentences ago.
Line 301: “Notably, it occurs between a unit consisting of sulfidic stromatolites […]” Remove consisting.
Line 368: On the sentence referring to Sr isotopes. I had to dig where this information was coming from. I was confused as to where the calcite Sr signal (0.700596) was coming from. Is it the same samples as this manuscript? Different samples but same location? Data from a completely different location? And why is typical of Archean seawater? I think this needs to be expanded, as it is in strong support of your stable isotopic data. There is no other mention of Sr isotopes in the rest of the manuscript, or other supporting literature. Also repeated in Line 393.
Line 438: “δ13C signatures of some EPT bedded sedimentary carbonates except the Dresser bedded carbonates (1.85 ± 0.48‰ on average) are generally in line with a formation in marine environments”. As it reads, seems like the 1.85% value is that of the Dresser carbonates. Include “(1.85 ± 0.48‰ on average)” after “EPT bedded sedimentary carbonates”.
Line 440: “[…], which in good accordance with d13C signatures […]”. Remove word “which”.
Lines 451 – 452: “Biogenicity” repeated twice in the same sentence. Rewrite for clarity.
Line 458 – 459: “In any case, extracellular polymeric substances (EPS) secreted by microorganisms, amongst others to cope with environmental stressors, play a key-role in mineralization”. The part of “among others to cope with environmental stressors” is confusing and distracting. Although I understand what the authors are inferring to, I would rewrite it, and there is no need to dig into the secretion of EPSs to cope with environmental stressors, which made me think about whether the discussion was heading that way (environmental stressors of the stromatolites in this location and period time). For example, you can entirely delete “among others to cope with environmental stressors” and the sentences would still perfectly fit into the discussion.
Line 469: “(~0.22 -1.85 ‰ on average; Fig. 11)”. These values are confusing, I understand they come from interstitial carbonates (0.22) and sedimentary carbonates (1.85) but all results through the manuscript are given as “X ±Y ‰”, so these are not consistent. I had to go back to results to check that 1.85 was from the sedimentary carbonates, and not a ±‰.
Line 470 – 471: “This difference is well in line with a sequestration of 12C by photoautotrophic microorganisms in the microbial mats, resulting in an enrichment of 13C in the environment and, consequently, in the carbonate”. This sentence needs a reference.
Figure 3: In 3b, Close-up area imaged under white light seems narrower than the uXRM maps. Can this be amended so they reflect the exact same area?
Figure 7: Is the scale bar in 7d and 7e both 200um? If 7e is a close-up of 7d, shouldn’t the scale bar be wider in the close-up? All scale bars in the images are 200um except that in 7f? Clarify.
Caption in Figure 7/Line 291: “(c) Raman spectra for spots in (a) and (c)”. It should be “in (a) and (b)".
Figure 11: This is only a suggestion, but it took me a while to digest the legend and what each symbols is. I would organize the legend grouping by the three carbonate factories discussed in the manuscript. So first group, Oceanic Crust, and include below the interstitial carbonate, veinlet carbonate and fracture carbonate. Then Sedimentary Carb, and include below these samples, and finally microbial. Also another extra group for the extra samples added for comparison. I think this would make it easier to identify the distribution of each carbonate factory in the plot rapidly.
Citation: https://doi.org/10.5194/egusphere-2024-1007-RC2 -
AC2: 'Reply on RC2', Wanli Xiang, 26 Jun 2024
RC2: 'Comment on egusphere-2024-1007', Anonymous Referee #2, 28 May 2024
In this manuscript, the authors characterize three different sources of carbonates commonly found in the East Pilbara Terrain (Australia). They describe and differentiate the potential origin of said carbonates in three different marine carbonate factories, as reflected in their distinct d13C signal, these being: 1. The oceanic crust (samples obtained from interstitial carbonate within basalts); 2. Organo-carbonates (sedimentary carbonates form through mediation of an organic component); and 3. Microbial carbonate (stromatolites).
Although I enjoyed reading this manuscript, and it made me think about the carbon cycle from a different perspective that I normally would, I recommend some changes below to be carried out before its publication.
1. First, and foremost, either the title needs to be changed, or the discussion needs to be rewritten. As it stands, it is unclear to me that this manuscript answers their own title “Were early Archean carbonate factories major carbon sinks on the juvenile Earth?”. The last sentence in the manuscript says “[…] Paleoarchean carbonates might have been major carbon sinks at the time of formation […]”. So, were they major carbon sinks? There is no mention of any quantification of the discussed carbon sink, nor mention of any other potential carbon sinks during this geological period. After reading this manuscript, I cannot answer the question that the authors are postulating. Considering this journal, it is likely that non-experts in Archean carbonates will read this paper, and a further expansion on the carbon cycle at the time, literature looking at carbon sinks in this time and how this work fits with the literature are needed.
- R: Thank you for pointing the problems and providing kind suggestions. On our side, we think the early Archean carbonates were the dominant carbon sinks at that time. We wanted to use a question as the title to appeal the readers. However, it makes the readers confused, which is not our intention. Therefore, following your suggestion, we will reword the title into “Early Archean carbonate factories acting as the dominant carbon sinks on the juvenile Earth” (or “Early Archean carbonate factories – the dominant carbon sinks on the juvenile Earth”?). Besides, we will revise the Introduction, making an expansion on the carbon sinks, carbon cycle and how they moderate the climate at that time. At last, a conceptual box model of the Archean carbon cycle and a simple isotope mass balance calculation of carbon flux will be given in a new section in the Discussion. In this case, the Conclusion may be revised too.
2. Also, the introduction needs to be organized in a different way in order to bring to the front the research question the authors are trying to answer, which is not clear until the very end of the introduction. As before, given the varied potential readership of the journal, the point of why to investigate Archean carbonates, and the ones in the EPT in particular, needs to be specified early in the introduction. For example: 1) The carbon cycle and carbon sinks and their importance for the climate system during the Precambrian. There is no mention of this until the very end of the Introduction (Line 79), and then again nothing until the very end of the discussion (Line 505), this needs to come up earlier, and expanded. 2) Then, as per lines 51 – 54, carbonate formations in this period are poorly constrained, especially in the early Archean. 3) This sets the framework to drive the reader into why more effort needs to be paid, and why you do the work you do. This is only an example of a potential structure that could highlight the importance of this work. Essentially, the introduction needs to be “punchier”.
- R: Thank you for pointing out this issue. We will revise the Introduction following the RC1 and your advice. A potential structure can be (the orders also work for the paragraphs): (1) a brief introduction of the present-day carbon cycle and carbon sinks and their importance for the climate system, which lead to the study of carbonate factory; (2) an introduction of carbonate factory, and in the end “However, carbonate factories are still poorly understood, which is particularly true for the Archean eon, when life still was in its infancy”; (3) an explanation why to study the Archean carbonate factories (because “The Archean eon is an important period for understanding biological and geological evolution on our planet and Earth-like exoplanets (Catling and Zahnle, 2020)”, and the conditions at that time were different from that of the present day); (4) a summery about the previous relevant works (there were two carbon reservoirs including carbonaceous organic matter mainly in chert, and carbonate rocks like stromatolites and interstitial carbonates), and specify why to choose the EPT carbonate rocks; (5) Shortcomings of the studies of the EPT carbonate rocks; (6) the aims and methods of this work.
3. Finally, in the results (sections 4.1.2 and 4.1.3) as data is presented on primary and secondary carbonate facies and different minerals found. Is it possible to include any sort of quantification/percentage on the presence of minerals on the analyzed samples? This would give a more complete characterization of the interstitial carbonates, the diagenetic processes and their stable isotopic composition.
- R: Thank you for pointing this issue. We have measured the percentage of minerals on some samples via XRD. However, it was not more helpful than observation on sample thin sections for identifying the diagenetic processes of carbonates. For example, it cannot tell the difference between the primary calcite and various recrystallized calcites. Therefore, we preferred to study the samples via in-situ analyses. Even when we needed to measure stable carbon and oxygen isotopes, we used a tiny drill to collect sample powers of an individual mineral facies on sections as possible, based on the observation of the related thin sections (the collected powers were not enough for XRD measurement). In this regard, we think data of the most analyzed samples in Table 1 were based on a single carbonate facies, except the interstitial ankerite samples from the Mount Ada Basalt which were influenced by calcite overgrowth (see Fig. 4i). Therefore, we cannot find a good method at the present to include any sort of quantification/percentage on the presence of minerals on the analyzed samples. On the other hand, it is hard to quantify the presence of interstitial carbonates on the analyzed samples, which is the key point to calculate the carbon flux into the oceanic crust factory. In a word, a nice method to quantify mineral percentage in-situ is interesting to explore in the future.
Minor comments:
4. Substitution or modification of words. We organized some minor comments to answer here:
a. Line 92: Repeated “consisting” word twice within a sentence. Rewrite for clarity.
b. Line 146: Change for “were cleaned three times in ethanol using an ultrasonic bath”, or similar. “Using ultrasound” might be confusing.
c. Line 147: No need to “gently” dry samples. You can delete this adjective.
d. Caption in Figure 7/Line 291: “(c) Raman spectra for spots in (a) and (c)”. It should be “in (a) and (b)".
e. Line 301: “Notably, it occurs between a unit consisting of sulfidic stromatolites […]” Remove consisting.
f. Line 440: “[…], which in good accordance with d13C signatures […]”. Remove word “which”.
g. Lines 451 – 452: “Biogenicity” repeated twice in the same sentence. Rewrite for clarity.
- R: Thank you for kindly pointing out these mistakes. Following your advice, we will omit the second “consisting” on Line 92, “consisting” on Line 301, “which” on Line 440, and reword the sentence on Line 146-147, “(a) and (c)” with “(a) and (b)” on Line 291, the “biogenicity” with “that” on Line 452. For your convenience to review, the sentences after the correction will be presented as below:
- a. Line 92: “A characteristic feature of the EPT is the so-called dome-and-keel structure, consisting of a central nucleus of the 3459 ± 18 Ma North Pole Monzogranite (“North Pole Dome”) surrounded by little-deformed, predominantly mafic volcanic rocks of the Warrawoona Group and Kelly Group (Hickman and Van Kranendonk, 2012a) (Fig. 1).”
- b-c. Line 146-147: “The sample chips were cleaned three times in ethanol using an ultrasonic bath and dried at room temperature before being crushed into small pieces.”
- d. Line 291: “(c) Raman spectra for spots in (a) and (b), supporting the presence of ankerite and organic matter.”
- e. Line 301: “Notably, it occurs between a unit of sulfidic stromatolites and bladed barite below, and wave rippled volcanoclastic sediments above (Fig. 6e).”
- f. Line 440: “At the same time, δ13C values of the Dresser bedded carbonates are relatively depleted (-5.72 ±1.36 ‰ on average), in good accordance with δ13C signatures of carbonatites (-4.99 ± 1.22 ‰ on average) (Fig.11), indicating hydrothermal admixture of mantle-derived carbon.”
- g. Line 451: “Although the biogenicity of early Archean stromatolites is commonly controversial, that of stromatolites from the Dresser Formation and the SPF has been widely accepted (…)”
5. Lines 125 – 128: “For comparison, we additionally analyzed […]”. What is the purpose of this comparison? Include this info in this paragraph, it will make it easier for the reader to understand why you included these samples at all.
- R: Thank you for pointing this out. Following your advice, we will reword this paragraph: “In order to better understand depositional environments of the studied EPT carbonates, we additionally analyzed carbon and oxygen isotopic compositions of diverse carbonates for comparison. They include carbonate inclusions in black barites…”
6. Lines 177 – 181: These sentences are repetitive and need to be reworded/rewritten.
(1) “Spherulitic and variolitic zones in the basalts are highly carbonatized, with carbonate minerals being particularly prominent in variolites and concentric syngenetic veins.” – What does highly carbonatized mean? First and second part of the sentence is saying the same thing?
- R: Thank you for pointing those problems. “Highly carbonatized” was used to describe spherulitic and variolitic zones, compared to other basaltic parts. The second part of the sentence is an explanation for the first part. In order to avoid confusion, we will reword it into “Carbonate minerals are particularly prominent in voids, veins and variolites within spherulitic and variolitic zones”. In addition, considering the advice of RC1, we will try to add a quantification of the carbonatized degree and a comparison with modern oceanic crust, somewhere suitable in the text or in the supplementary materials.
(2) “Notably, elemental distributions in basalts do not seem to relate to the degree of weathering (e.g. in sample A22 of the Apex Basalt; Fig. 3b) and hence might be pristine.” – I do not know what do you mean by this. What would be different in the data if elemental distribution related to weathering? Also, likely need some references to justify why does not relate to degree of weathering. And then, this would be more fitting included in the discussion.
- R: Thank you for pointing this issue. We wrote it because we noticed the lower, smaller basalt part in Fig. 3a was weathered. In the previous work, we learned that some elements (Ca, Mg, Al and so on) are easy to lose during rock weathering. Therefore, we mentioned this in advance. The reason why the elemental distribution in the weathered part shows no obvious difference with the unweathered part may be that the preserved weathered part mainly consists of silicates and quartz, which are relatively resistant to weathering. However, considering it is meaningless and confusing in this work, we will omit the sentence.
(3) “Except for the devitrified volcanic glass, Si is rich in the interior of the pillow basalt but rare in the zone of spherulites and variolites, which are dominated by calcite” – You are mentioning again that calcite is predominant in spherulites and variolites. Which you mentioned two sentences ago.
- R: Thank you for pointing this issue. To reduce the repetition, we will reword it into “Except for the devitrified volcanic glass, Si is rich in the interior of the pillow basalt but rare in the spherulitic and variolitic zones, implying a Si loss during basalt carbonatization (Fig. 3b)”. And taking other corrections into account, we will revise the paragraph. For your convenience, we attach the new one here:
- “Although the host basalts show secondary mineral assemblages indicative of greenschist metamorphism (calcite + chlorite + anatase + quartz ± pyrite), phenocrysts (i.e., plagioclase and pyroxene) can still be recognized in the basalt interior of the well-preserved samples, e.g. A22 from the Apex Basalt (Figs. 3a, 4a). Notably, the well-preserved basalts exhibit concentric green ophitic-holohyaline interiors and yellow-green quenched margins. In the margins, the size and density of ovoid spherulites and variolites (amygdules) decrease outwards, merging into the glassy zone (Fig. S1a-c). Carbonate minerals are particularly prominent in voids, veins and variolites within spherulitic and variolitic zones, as illustrated by an overlapping image of Si, Ca and Mg mappings via micro-XRF (see Fig. 3b). Except for the devitrified volcanic glass, Si is rich in the interior of the pillow basalt but rare in the spherulitic and variolitic zones, implying a Si loss during basalt carbonatization (Fig. 3b). Si yielded during this process was likely enriched in fluids, resulting in chert cementation of interstitial carbonates (Fig. 4). The process can be summarized as follows (see Eq.1 where refers to calcium silicate minerals):
- "CaSiO3" + CO2+ H2O → "CaSiO3"+ H2CO3 →CaCO3+ SiO2+ H2O (Eq.1)”
7. Line 368: On the sentence referring to Sr isotopes. I had to dig where this information was coming from. I was confused as to where the calcite Sr signal (0.700596) was coming from. Is it the same samples as this manuscript? Different samples but same location? Data from a completely different location? And why is typical of Archean seawater? I think this needs to be expanded, as it is in strong support of your stable isotopic data. There is no other mention of Sr isotopes in the rest of the manuscript, or other supporting literature. Also repeated in Line 393.
- R: Thank you for pointing out this issue. We have studied the EPT carbonate rocks about their sedimentology, mineralogy and geochemical compositions, which have been written into two articles. Based on the identification and classification of various carbonate facies including primary and secondary facies (this work), we selected typical samples and measured their geochemical compositions (including REEs and Sr isotopes) via acid digestion (the work in preparation). Therefore, some samples have a data system of their mineralogy, elemental compositions, and isotopic composition (i.e. C, O, Sr) based on the same sample powders, while the others have a data system of their mineralogy, C and O isotopic compositions (in Table 1). The calcite Sr signal (0.700596) belongs to the fracture-filling calcite (D-2-W). We think it to be typical of the Archean seawater with the support from the REE+Y pattern (low REE concentrations, no Eu anomaly, positive Y anomaly and so on) and its 87Sr/86Sr ratio (0.700596) close to the lowest 87Sr/86Sr ratios of barites from the same, Dresser Formation (0.700502 in McCulloch, 1994; 0.700447 in Chen et al., 2022), which will be discussed in detail in the other article.
- On the other hand, we also used the same procedure to study pillow basalts of Apex Basalt, which preserve the primary interstitial carbonates. This work is also in preparation. We got the whole rock value of 0.706337 ± 0.000078 given by the Rb-Sr errorchron of the basalts and linked to the study of interstitial carbonates. Therefore, you could see on Line 393-395 “The mixture of different fluids is also supported by 87Sr/86Sr ratios of primary interstitial calcite associated with the Apex basalt (0.703094 ± 0.000979), laying between those of early Archean seawater and Apex pillow basalt (0.700596 and 0.706337 ± 0.000954, respectively; Xiang, 2023)”.
- They are all unpublished, and crucial to our works in preparation. However, considering your advice and readers’ interest, we will provide an information in the supplementary materials. This will include relevant methodological and contextual details about these analyses, and a table to present some 87Sr/86Sr ratios of the basalt-carbonate system, including the average value of the primary and secondary interstitial carbonates, fracture-filling calcite (D-2-W), and the whole rock value of pillow basalts. Nonetheless, we encourage readers to find more details in Xiang, 2023, Carbonate factories in the early Archean and their geobiological impacts (Ph.D. thesis, http://dx.doi.org/10.53846/goediss-10047), where they can also find some interesting applications of the relevant works.
8. Line 438: “δ13C signatures of some EPT bedded sedimentary carbonates except the Dresser bedded carbonates (1.85 ± 0.48‰ on average) are generally in line with a formation in marine environments”. As it reads, seems like the 1.85% value is that of the Dresser carbonates.
- R: Thank you for pointing out this problem. To solve it, we will reword this sentence for clarity: “Some EPT bedded sedimentary carbonates (except the Dresser bedded carbonates) show an average δ13C value of 1.85 ± 0.48‰, within the range of the values of modern seawater (Kroopnick, 1980; Tan, 1988) and the Strelley Pool stromatolites (Lindsay et al., 2005; Flannery et al., 2018; this work), reflecting their formation in marine environments.”
9. Line 458 – 459: “In any case, extracellular polymeric substances (EPS) secreted by microorganisms, amongst others to cope with environmental stressors, play a key-role in mineralization”. The part of “among others to cope with environmental stressors” is confusing and distracting. Although I understand what the authors are inferring to, I would rewrite it, and there is no need to dig into the secretion of EPSs to cope with environmental stressors, which made me think about whether the discussion was heading that way (environmental stressors of the stromatolites in this location and period time). For example, you can entirely delete “among others to cope with environmental stressors” and the sentences would still perfectly fit into the discussion.
- R: Thank you for your kind suggestion. We will make a correction following your advice: “In any case, extracellular polymeric substances (EPS) secreted by microorganisms play a key-role in mineralization (Decho, 2011; see in Fig. 10)” (Line 458).
10. Line 469: “(~0.22 -1.85 ‰ on average; Fig. 11)”. These values are confusing, I understand they come from interstitial carbonates (0.22) and sedimentary carbonates (1.85) but all results through the manuscript are given as “X ±Y ‰”, so these are not consistent. I had to go back to results to check that 1.85 was from the sedimentary carbonates, and not a ±‰.
- R: Thank you for pointing this problem. Following your advice, we will reword the sentence into “More specifically, δ13C values of carbonates from SPF stromatolites (3.08 ± 0.30 ‰ on average) are higher than those of the interstitial carbonates (0.22 ± 0.98 ‰ on average) and the sedimentary carbonates (1.85 ± 0.48‰ on average)”.
11. Line 470 – 471: “This difference is well in line with a sequestration of 12C by photoautotrophic microorganisms in the microbial mats, resulting in an enrichment of 13C in the environment and, consequently, in the carbonate”. This sentence needs a reference.
- R: Thank you for your suggestion. Following your advice, we will add a reference “(Arp et al., 2011)” in the end of this sentence. For your convenience, we attach the reference here: Arp, G., Helms, G., Karlinska, K., Schumann, G., Reimer, A., Reitner, J., & Trichet, J. (2011). Photosynthesis versus Exopolymer Degradation in the Formation of Microbialites on the Atoll of Kiritimati, Republic of Kiribati, Central Pacific. Geomicrobiology Journal, 29(1), 29–65. https://doi.org/10.1080/01490451.2010.521436
12. Figure 3: In 3b, Close-up area imaged under white light seems narrower than the uXRM maps. Can this be amended so they reflect the exact same area?
- R: Thank you for pointing this problem out. It is a display mistake happened when we converted the MS word into pdf. We are sorry for not observing it. We will make a correction and check for the next submission.
13. Figure 7: Is the scale bar in 7d and 7e both 200um? If 7e is a close-up of 7d, shouldn’t the scale bar be wider in the close-up? All scale bars in the images are 200um except that in 7f? Clarify.
- R: Thank you for pointing these problems. The scale bar in 7d and 7e are both 200um, however, 7e is not a close-up of 7d (they are two areas). Besides, 7f and 7g share the same scale bar of 5 um. To avoid the potential confusion, we will make a correction by lining the crystals in 7e, adding a scale bar in 7g, and rewording the caption “The scale bars in (f) and (g) are 5 um, while others 200 um” on Line 295.
14. Figure 11: This is only a suggestion, but it took me a while to digest the legend and what each symbols is. I would organize the legend grouping by the three carbonate factories discussed in the manuscript. So first group, Oceanic Crust, and include below the interstitial carbonate, veinlet carbonate and fracture carbonate. Then Sedimentary Carb, and include below these samples, and finally microbial. Also another extra group for the extra samples added for comparison. I think this would make it easier to identify the distribution of each carbonate factory in the plot rapidly.
- R: Thank you for your kind suggestion. It looks better than the original one. We would like to make this correction following your advice.
References
Arp, G., Helms, G., Karlinska, K., Schumann, G., Reimer, A., Reitner, J., and Trichet, J.: Photosynthesis versus Exopolymer Degradation in the Formation of Microbialites on the Atoll of Kiritimati, Republic of Kiribati, Central Pacific. Geomicrobiology Journal, 29(1), 29–65. https://doi.org/10.1080/01490451.2010.521436, 2011.
Catling, D. C. and Zahnle, K. J.: The Archean atmosphere, Science advances, 6, eaax1420, http://dx.doi.org/10.1126/sciadv.aax1420, 2020.
Chen, X., Zhou, Y., and Shields, G. A.: Progress towards an improved Precambrian seawater 87Sr/86Sr curve, Earth-Science Reviews, 224, 103 869, 2022.
Decho, A. W.: Extracellular polymeric substances (EPS), in: Encyclopedia of Geobiology, edited by Reitner, J. and Thiel, V., pp. 359–361, Springer, Berlin, 2011.
Flannery, D. T., Allwood, A. C., Summons, R. E., Williford, K. H., Abbey, W., Matys, E. D., and Ferralis, N.: Spatially- resolved isotopic study of carbon trapped in 3.43 Ga Strelley Pool Formation stromatolites, Geochimica et Cosmochimica Acta, 223, 21–35, 2018.
Hickman, A. H. and Van Kranendonk, M.: A Billion Years of Earth History: A Geological Transect Through the Pilbara Craton and the Mount Bruce Supergroup-a Field Guide to Accompany 34th IGC Excursion WA-2, Geological Survey of Western Australia, Record 2012/10, 2012a.
Kroopnick, P.: The distribution of 13C in the Atlantic Ocean, Earth and Planetary Science Letters, 49, 469–484, https://doi.org/10.1016/0012- 821X(80)90088-6, 1980.
Lindsay, J., Brasier, M., McLoughlin, N., Green, O., Fogel, M., Steele, A., and Mertzman, S.: The problem of deep carbon— an Archean paradox, Precambrian Research, 143, 1–22, https://doi.org/10.1016/j.precamres.2005.09.003, 2005.
McCulloch, M. T.: Primitive 87Sr/86Sr from an Archean barite and conjecture on the Earth’s age and origin, Earth and planetary science letters, 126, 1–13, https://doi.org/10.1016/0012- 821X(94)90238-0, 1994.
Tan, F. C.: Stable carbon isotopes in dissolved inorganic carbon in marine and estuarine environments, in: Handbook of environmental isotope geochemistry, edited by Fritz, P. and Fontes, J., vol. 3, pp. 171–190, Elsevier, Amsterdam, 1988.
Xiang, W.: Carbonate factories in the early Archean and their geobiological impacts, Ph.D. thesis, University of Göttingen, 1055 Germany, http://dx.doi.org/10.53846/goediss-10047, 2023.
Citation: https://doi.org/10.5194/egusphere-2024-1007-AC2
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AC2: 'Reply on RC2', Wanli Xiang, 26 Jun 2024
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-1007', Graham Shields, 21 May 2024
This fascinating contribution presents new carbon and oxygen stable isotope data, mainly from early Archean (Paleoarchean) carbonates in western Australia, and comes to the conclusion that Paleoarchean carbonates, which the authors divide into three main types, could have been major carbon sinks at the time, thus moderating the global carbon cycle and contributing to early climate regulation. The geochemical data are new, sedimentological context well described, and conclusions of broad interest. I have a few comments for the authors to consider when framing the final version of this thoughtful manuscript.
- Major carbon sink. The major source of carbon into the Archean exogenic system would have been volcanic outgassing (in the absence of oxidative weathering), while the major sink would have to have been carbonate, just as today. The premise of the paper is that we do not yet know what the carbon sinks would have been at the time, but I would ask the authors what other sinks might there have been, because they do not specify the alternatives anywhere and do not provide a conceptual box model of the Archean carbon cycle, which would have been useful. If they draw the same conclusion as me, that carbonate minerals must have been the dominant sink, then I suggest rewording the title, conclusions and other relevant sections throughout the manuscript. Even accepted that it might not have been the dominant sink, which I very much doubt, is it likely that carbonate was ever not a “major” sink as implied by the title? In this regard, the final sentence of the Abstract is also rather weak.
- I would appreciate more quantification and quantified comparisons with the modern. For example, on line 178 “highly carbonatized” could be quantified, while what I really want to know as reader is how this compares with ocean basalts today? Do your observations match the constraints mentioned later from the literature on lines 514-516, for instance?
- Carbonate mineralogy. Wherever possible I would encourage the authors to specify the mineralogy, e.g. in Table 1 or line 233. This is particularly important where the stable isotope data are outlined / discussed, as different minerals fractionate differently. On line 225, and elsewhere, we learn that the primary mineralogy of interstitial carbonates was calcite, and yet the crystals are described as being “acicular, a habit more commonly associated with aragonite. Can the authors rule out aragonite as the precursor carbonate mineral?
- Other geochemical data. On lines 368-374, we learn about other pertinent geochemical data, such as strontium isotopes, which come from a doctoral thesis. This section comes rather out-of-the-blue, and it is not apparent why these data, which are evidently from the same project, have not been presented more fully. As they are key to the interpretation, I’d recommend that the reader be told more relevant methodological and contextual details about these analyses.
- Carbon (isotope) mass balance. Carbon sinks and the global carbon cycle do not get enough attention until the end of the paper, but could have come already in the Introduction considering their importance to the "take-home" message. In this regard, the authors need to use their new data and compiled literature data to constrain the isotopic value of the carbonate sink at that time, and carry out a simple isotope mass balance calculation. The isotopic composition of the other sink, organic matter, can be estimated from the literature, but it is not mentioned in the paper. These values allows a very simple isotope mass balance to be proposed if we can assume a likely input value or range of input values (this is mentioned in the paper). As far as I can tell, the conclusion from such a simple mass balance would confirm that the sub-seafloor calcite carbon sink was likely the dominant carbon sink of the Paleoarchean (lines 511-512). A pertinent paper in this regard would be Mills et al (2016) Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering, in PNAS, 11, 9073-9078, but there are other relevant papers not mentioned here that pertain to the carbon budget on the early Earth, e.g. Canfield (2021) Carbon cycle evolution before and after the great oxidation of the atmosphere. American journal of Science, 321, 297-331.
English formulation and other minor issues:
Line 20 (Abstract): “Interspaces between pillowed basalts” – this is not the same as interstitial carbonate, and needs rewording for clarity.
Line 23 (Abstract): “formed taphonomically” can be omitted here and elsewhere, as it is not clear what extra meaning this adds, as this term usually refers to fossil preservation.
Line 74: I suggest omitting “comprehensively” here. Likewise, on line 149 “high-precision” seems unnecessary.
Line 158: “into”, not “in”.
Lines 194-195: Here, and elsewhere I would have appreciated a chemical equation to illustrate the process being described.
Lines 244-245 – the evidence for dolomitization is given as the presence of ankerite. I find this confusing. Isn’t this “ankeritization”?
Lines 250-251 – sentence needs rewording for clarity. Likewise, lines 438-439, 463-465, 486-487.
Line 417: “consummated” probably needs a different word?
Line 461: “complexation” – for me, this terms means something else, as in complexed CaCO30.
Line 468: “higher”, not “heavier”.
Table 2 – How are evaporite minerals identified in this study? And which minerals are these?
Citation: https://doi.org/10.5194/egusphere-2024-1007-RC1 -
AC1: 'Reply on RC1', Wanli Xiang, 26 Jun 2024
RC1: 'Comment on egusphere-2024-1007', Graham Shields, 21 May 2024
This fascinating contribution presents new carbon and oxygen stable isotope data, mainly from early Archean (Paleoarchean) carbonates in western Australia, and comes to the conclusion that Paleoarchean carbonates, which the authors divide into three main types, could have been major carbon sinks at the time, thus moderating the global carbon cycle and contributing to early climate regulation. The geochemical data are new, sedimentological context well described, and conclusions of broad interest. I have a few comments for the authors to consider when framing the final version of this thoughtful manuscript.
1. Major carbon sink. The major source of carbon into the Archean exogenic system would have been volcanic outgassing (in the absence of oxidative weathering), while the major sink would have to have been carbonate, just as today. The premise of the paper is that we do not yet know what the carbon sinks would have been at the time, but I would ask the authors what other sinks might there have been, because they do not specify the alternatives anywhere and do not provide a conceptual box model of the Archean carbon cycle, which would have been useful. If they draw the same conclusion as me, that carbonate minerals must have been the dominant sink, then I suggest rewording the title, conclusions and other relevant sections throughout the manuscript. Even accepted that it might not have been the dominant sink, which I very much doubt, is it likely that carbonate was ever not a “major” sink as implied by the title? In this regard, the final sentence of the Abstract is also rather weak.
- R: Thank you very much for offering us these valuable and helpful suggestions. Firstly, we agree that carbonate minerals must have been the dominant carbon sink at that time. Therefore, we need to make a correction to illustrate it, e.g. rewording the title and relevant sentences with “dominant” replacing of “major”. Secondly, we will revise the Introduction to specify the present-day and early Archean carbon sinks. Based on the previous work, the early Archean carbon sinks were organic matter in chert units and carbonate minerals preserved in carbonatized basaltic rocks, interstitial carbonates, and stromatolites. Thirdly, considering the suggestions about carbon sink and carbon (isotope) mass balance, we will add a new section after “5.2 Early Archean carbonate factories – implications”, where to summarize the conceptual box model of the Archean carbon cycle and carry out a simple isotope mass balance calculation. At last, the Abstract and Conclusion may be revise little to fit our work.
2. I would appreciate more quantification and quantified comparisons with the modern. For example, on line 178 “highly carbonatized” could be quantified, while what I really want to know as reader is how this compares with ocean basalts today? Do your observations match the constraints mentioned later from the literature on lines 514-516, for instance?
- R: Thank you for pointing those problems. We describe “Spherulitic and variolitic zones in the basalts are highly carbonatized”, compared to other basaltic parts. This is mainly based on observations elemental mappings via micro-XRF about sample thin sections, which were presented in Fig.3b and Fig.S1a-c, respectively. In this paper, we just focus on interstitial carbonates and only briefly introduce the host basalt. In fact, we are preparing a paper about pillow basalts of Apex Basalt, which preserves the primary interstitial carbonates, with more details such as the zonation and elemental compositions of zones (see Chapter 4 in Xiang, 2023). Considering the RC2, we will reword it into “carbonate minerals are particularly prominent in voids, veins and variolites within spherulitic and variolitic zones” to avoid confusion. However, for the reader’s interest, we will try to add a quantification of the carbonatized degree and a comparison with modern oceanic crust, somewhere suitable in the text or in the supplementary materials. To do this, we can use Si and Ca concentrations of minerals within basalt via micro-XRF point spectrum and line analysis (in our unpublished work), CO2 concentrations of basalt samples from Apex Basalt in Nakamura and Kato (2004), carbonate proportion (vol.%) in the modern ocean crust in Rausch (2012).
- For the latter question, we think the carbon flux into the oceanic crust factory is much higher than the present-day flux. As Rausch (2012) illustrated, carbonate minerals are in veins, vesicles and breccias of modern oceanic crusts, and more abundant in older crusts (Gillis et al., 2001; Heft et al., 2008; Coogan and Gillis, 2013). However, they could precipitate as interstitial carbonates in the Archean oceanic curst (Nakamura and Kato, 2004; Shibuya et al., 2012; Marien et al., 2023), which were considerable portions (vol%) of strata preserving the pillow basalts. Nakamura and Kato (2004) reported carbon flux into the early Archean oceanic crust by the seafloor carbonatization to be 3.8 × 1013 mol/yr, while Rausch (2012), Alt and Teagle (1999) suggested a global carbon uptake flux of 1.03 × 1012 mol/yr and 1.5–2.4 × 1012 mol/yr into modern oceanic crust, respectively. Notably, the estimates did not include any interstitial carbonate. However, this work aims to emphasize the significance of carbonates formed in interpillow void space of the basalts within the oceanic crust which we call interstitial carbonates. Although hard to measure, interstitial carbonates look far more than carbonate minerals within basalts (Fig. 2, 3, 5). Therefore, we think the carbon flux into the oceanic crust factory should be beyond 3.8 × 1013 mol/yr, and far more than that into modern oceanic crust.
3. Carbonate mineralogy. Wherever possible I would encourage the authors to specify the mineralogy, e.g. in Table 1 or line 233. This is particularly important where the stable isotope data are outlined / discussed, as different minerals fractionate differently. On line 225, and elsewhere, we learn that the primary mineralogy of interstitial carbonates was calcite, and yet the crystals are described as being “acicular, a habit more commonly associated with aragonite. Can the authors rule out aragonite as the precursor carbonate mineral?
- R: Thank you for your suggestion and pointing the problem. Following your advice, we will add a column in Table 1 to specify the mineralogy. For the question, we do not think of the presence of aragonite as precursor. Even in modern oceanic crust, aragonite is not so pervasive, for that their precipitation may be determined by several factors such as Mg/Ca ratio of parenting fluid and nucleation template. In our work, we have done XRD and Raman analyses, and had the geochemical compositions, which indicate they are now low-Mg calcite. The habit of acicular crystal fan can also be found in calcite, and there should still be some evidence preserved if aragonite was the precursor due to the rather low diagenetic overprint. However, we did not find any convincing evidence to prove "the primary mineralogy of interstitial carbonates was aragonite”. Therefore, we chose to describe and believe what we see today.
4. Other geochemical data. On lines 368-374, we learn about other pertinent geochemical data, such as strontium isotopes, which come from a doctoral thesis. This section comes rather out-of-the-blue, and it is not apparent why these data, which are evidently from the same project, have not been presented more fully. As they are key to the interpretation, I’d recommend that the reader be told more relevant methodological and contextual details about these analyses.
- R: Thank you for pointing out this issue. As you said, this work, the Sr isotope data and the aforementioned data of Apex pillow basalts are from the same project that will be in three publications. The Sr isotope data are all unpublished, and crucial to the works in preparation. Therefore, we just used some results of them in this work. However, considering your kind suggestion, we will provide an information in the supplementary materials. This will include relevant methodological and contextual details about these analyses, and a table to present some 87Sr/86Sr ratios of the basalt-carbonate system, including the average value of the primary and secondary interstitial carbonates, fracture-filling calcite (D-2-W), and the whole rock value of pillow basalts. This is sufficient to support this work. Nonetheless, we encourage readers to find more details in Xiang, 2023, Carbonate factories in the early Archean and their geobiological impacts (Ph.D. thesis, http://dx.doi.org/10.53846/goediss-10047). The three works are shown with more details in Chapter 2 to 4, respectively. And they can also find some interesting applications of our works.
5. Carbon (isotope) mass balance. Carbon sinks and the global carbon cycle do not get enough attention until the end of the paper, but could have come already in the Introduction considering their importance to the "take-home" message. In this regard, the authors need to use their new data and compiled literature data to constrain the isotopic value of the carbonate sink at that time, and carry out a simple isotope mass balance calculation. The isotopic composition of the other sink, organic matter, can be estimated from the literature, but it is not mentioned in the paper. These values allow a very simple isotope mass balance to be proposed if we can assume a likely input value or range of input values (this is mentioned in the paper). As far as I can tell, the conclusion from such a simple mass balance would confirm that the sub-seafloor calcite carbon sink was likely the dominant carbon sink of the Paleoarchean (lines 511-512). A pertinent paper in this regard would be Mills et al (2016) Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering, in PNAS, 11, 9073-9078, but there are other relevant papers not mentioned here that pertain to the carbon budget on the early Earth, e.g. Canfield (2021) Carbon cycle evolution before and after the great oxidation of the atmosphere. American journal of Science, 321, 297-331.
- R: Thank you for your kind suggestions and recommending nice references. We will revise the Introduction to introduce carbon sinks and carbon cycle earlier, and highlight the significance of them to moderate climate. And we will add a new section after “5.2 Early Archean carbonate factories – implications”. In this section, we will carry out a simple isotope mass balance calculation following the method described in Canfield (2021), using our δ13C data of carbonates and data of organic matter in Flannery et al. (2018) (-29 to - 45 ‰; more data will be considered if they were from the relevant EPT formations). And we will also quantify carbon uptake flux into the oceanic crust factory and compare it with the modern one. We believe all will support that carbonate precipitated in the oceanic crust factory associated with seafloor hydrothermal alteration was the dominant carbon sink on early Earth.
6. English formulation and other minor issues:
（1）Line 20 (Abstract): “Interspaces between pillowed basalts” – this is not the same as interstitial carbonate, and needs rewording for clarity.
- R: Thanks for pointing this problem. Following your advance, the sentence (Line 20) “The oceanic crust factory is characterized by carbonates formed in interspaces between pillowed basalts (‘interstitial carbonates’)” will be reworded into “The oceanic crust factory is characterized by carbonates formed in interpillow void spaces of pillowed basalts (referred to as ‘interstitial carbonates’ in this work)”. Besides, considering the rewording of the Introduction, “The geobiological significance of other carbonates, such as precipitates in the interspaces of (ultra)mafic rocks that possibly formed through seafloor hydrothermal alteration (referred to as ‘interstitial carbonates’ in this work)” (Line 70-71) may be revised to “However, their calculation on carbon flux did not included carbon in carbonates precipitated in the interpillow void spaces of the pillow basaltic rocks, referred to as ‘interstitial carbonates’ in Marien et al. (2023) and this work”.
（2）Substitution or modification of words. We organized some comments to answer here:
a. Line 23 (Abstract): “formed taphonomically” can be omitted here and elsewhere, as it is not clear what extra meaning this adds, as this term usually refers to fossil preservation.
b. Line 74: I suggest omitting “comprehensively” here. Likewise, on line 149 “high-precision” seems unnecessary.
c. Line 158: “into”, not “in”.
d. Line 417: “consummated” probably needs a different word?
e. Line 461: “complexation” – for me, this term means something else, as in complexed CaCO30.
f. Line 468: “higher”, not “heavier”.
- R: Thank you for pointing out these problems. Following your advice, we will omit “taphonomically” on Line 23, “comprehensively” on Line 74, “high-precision” on Line 149, and correct “in” on Line 158 with “into”, “consummated” on Line 417 with “confirmed”, “complexation” on Line 461 with “combination”, “heavier” on Line 468 with “higher”. After the corrections, the sentences will be presented as below:
- a. Line 23: “The close association with organic matter suggests that the carbonates formed via organo-mineralization, that is, linked to organic macromolecules (either biotic or abiotic) which provided nucleation sites for carbonate crystal growth.”
- b. Line 74: “This study investigates early Archean carbonates in the EPT, including interstitial carbonates associated with basalts, carbonate stromatolites and other sedimentary carbonates.”
- Line 149: “Additionally, some carbonate facies, including carbonate veinlets and carbonate inclusions, were extracted using a drill from individual mineral phases from polished rock slabs.”
- c. Line 158: “The host basalts are pillow-shaped, internally subdivided into more crystalline interiors and quenched glassy rims, and commonly locally cut by tectonic fractures (Fig. 2).”
- d. Line 417: “To distinguish minerals formed through mineralization linked to organic matrices and compounds from those whose formation is induced by living organisms, the terms “organomineral” and “organo-mineralization” were introduced at the 7th International Symposium on Biomineralization in 1995 and further developed in the following decade (…), before being finally confirmed in following studies (…).”
- e. Line 461: “Certain functional groups of organic substances in the EPS (e.g. Asp- and Glu-rich macromolecules) efficiently bind and sequester divalent cations such as Ca2+ and Mg2+, thereby inhibiting their combination with carbonate anions and subsequent precipitation (Reitner et al. 1995a, b, c).”
- f. Line 468: “More specifically, δ13C values of carbonates from SPF stromatolites (3.08 ± 0.30 ‰ on average) are higher than those of the interstitial carbonates (0.22 ± 0.98 ‰ on average) and the sedimentary carbonates (1.85 ± 0.48‰ on average).”
7. Lines 194-195: Here, and elsewhere I would have appreciated a chemical equation to illustrate the process being described.
- R: Thank you for your kind suggestion. Following your advice, we will make a correction here to illustrate it. Firstly, we will supplement an explanation in the end of the 2nd paragraph in Section 4.1.1 and add a simple relationship after the relevant text. It will look like:
- “Except for the devitrified volcanic glass, Si is rich in the interior of the pillow basalt but rare in the spherulitic and variolitic zones, implying a Si loss during basalt carbonatization (Fig. 3b). Si yielded during this process was likely enriched in fluids, resulting in chert cementation of interstitial carbonates (Fig. 4). The process can be summarized as follows (see Eq.1 where refers to calcium silicate minerals):
- "CaSiO3" + CO2 +H2O → "CaSiO3" +H2CO3 → CaCO3 + SiO2 + H2O (Eq.1) ”
- Secondly, we will revise the caption of Fig. 3 on Line 194-195 into “In addition, the quenched margin of the basalt seems to relatively depleted in Si as compared to the core, implying a loss of Si during carbonatization processes (see Eq.1).”
8. Lines 244-245 – the evidence for dolomitization is given as the presence of ankerite. I find this confusing. Isn’t this “ankeritization”?
- R: Thank you for pointing this issue. As you said, this is ankeritization. It was me who insisted to use “dolomitization”, while some of the coauthors thought of “ankeritization”. I preferred to use the usual term, because dolomitization and recrystallization are two common processes during diagenetic alteration, and ankerite [Ca(Fe,Mg,Mn)(CO3)2] is an Fe- and Mn-bearing dolomite. However, considering your advice and the truth that ankerite is a characteristic mineral which is only found in interstitial carbonates, we will correct “dolomitization” with “ankeritization” in the relevant sentences.
9. Lines 250-251 – sentence needs rewording for clarity. Likewise, lines 438-439, 463-465, 486-487.
- R: Thank you for your kind suggestions. Following your advice, we will make corrections as follows:
- Line 250-251: we will revise the paragraph into “The secondary carbonates are either Mn- or Sr-enriched (see Fig. 5), indicating at least two diagenetic fluids during later alteration. For instance, the Mn-enriched carbonates include the recrystallized interstitial calcites, fibrous calcite cement within basalt fractures, and the interstitial ankerite. To be noted, some interstitial ankerites, formed through recrystallization and neomorphism or closer to the basaltic parts, are more Mn-enriched than other interstitial ankerites (Fig. 5a). On the other hand, the calcite overgrowth of the interstitial ankerites and the fracture-filling calcite are Sr-enriched, especially the latter, which is most Sr-enriched among all analyzed carbonate phases (Fig. 5).”
- Line 438-439: “Some EPT bedded sedimentary carbonates (except the Dresser bedded carbonates) show an average δ13C value of 1.85 ± 0.48‰, within the range of the values of modern seawater (Kroopnick, 1980; Tan, 1988) and the Strelley Pool stromatolites (Lindsay et al., 2005; Flannery et al., 2018; this work), reflecting their formation in marine environments.”
- Line 463-465: “Distinct from biological induced biomineralization linked to contemporaneous biological activity, organo-mineralization involves mineralizing organic matrices and compounds remote in space and time from the organisms which they derive from, or it can be in association with prebiotic or abiotic organic matter (Trichet and Défarge, 1995; Défarge, 2009, 2011).”
- Line 486-487: “Carbonates in this carbonate factory precipitated from CO2-rich seawater-derived hydrothermal fluids, which were in a high alkalinity and high cation loads.”
10. Table 2 – How are evaporite minerals identified in this study? And which minerals are these?
- R: Thanks for pointing this problem out. We assume there should be some evaporite minerals in organo-carbonate factory and microbial carbonate factory due to their deposition environments (supported by references and this work) and mineral morphologies. For example, the organo-carbonate factory could occur on land, as showed in the case of the DB bedded carbonates, in a hydrothermal pond. “The clusters of radiating calcite crystals at the base of each carbonate-chert layer (Figs. 6f, 8a), which were initially proposed to be gypsum or aragonite (Runnegar et al., 2001; Van Kranendonk et al., 2008; Otálora et al., 2018), are likely indicative of evaporitic conditions” (Line 442-445). In the microbial carbonate factory, we ascribed the formation of carbonate fans beneath the SPF stromatolites to be evaporation, considering their morphology and the depositional environment of the SPF stromatolites. Therefore, the evaporate minerals in our study are carbonate minerals (calcite and dolomite).
- However, it seems that we have lost to tell our readers some important messages. To solve this problem, we will make some corrections as follows:
- (1) We will supplement the information of carbonate fans beneath the SPF stromatolites in the end of the 2nd paragraph in Section 4.3:” Beneath the stromatolites, large carbonate fans (~ 40 cm) positioned on a chert layer and were cemented by chert (Fig. 9). The carbonate fans encompass fusiform dolomite aggregations (Fig. 9f).”
- (2) We will add the introduction of depositional environments in the first paragraph in Section 5.2 from Line 485 (italic and bold)；“The oceanic crust factory includes abiotically formed carbonates such as Mn- or Sr-enriched calcite and ankerite that are associated with pillow basalts within the upper oceanic crust. Carbonates in this carbonate factory precipitated from CO2-rich seawater-derived hydrothermal fluids, which were in a high alkalinity and high cation loads. The organo-carbonate factory is dominated by authigenic carbonates formed through taphonomy-controlled organo-mineralization (i.e. organomicrites). Importantly, and in contrast to the microbial carbonate factory, the involved organic matter can be of either biological or abiotic origin. This pathway of carbonate precipitation makes it possible to form in various environments, ranging from shallow marine to terrestrial settings. The microbial carbonate factory is somewhat similar to the organo-carbonate factory, but specifically refers to EPS-controlled carbonate precipitation, that is, mineralization of biologically derived organic substances. However, as in case of the organo-carbonate factory, organomicrite is formed as a typical product. Associated with biological activities, this carbonate factory occurred in the photic, relatively restricted, shallow marine environments like lagoons. Given that most of these carbonates formed in shallow-water environments under anoxic conditions, anoxygenic phototrophs appear a plausible source of biological organic matter, but this remains to be tested in future studies.”
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Coogan, L. A. and Gillis, K. M.: Evidence that low-temperature oceanic hydrothermal systems play an important role in the silicate-carbonate weathering cycle and long-term climate regulation, Geochemistry, Geophysics, Geosystems, 14, 1771– 1786, 2013.
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Défarge, C.: Organomineralization, in: Encyclopedia of geobiology, edited by Reitner, J. and Thiel, V., pp. 697-701, Springer, Berlin, 2011.
Flannery, D. T., Allwood, A. C., Summons, R. E., Williford, K. H., Abbey, W., Matys, E. D., and Ferralis, N.: Spatially- resolved isotopic study of carbon trapped in 3.43 Ga Strelley Pool Formation stromatolites, Geochimica et Cosmochimica Acta, 223, 21–35, 2018.
Gillis, K. M., Muehlenbachs, K., Stewart, M., Gleeson, T., and Karson, J.: Fluid flow patterns in fast spreading East Pacific Rise crust exposed at Hess Deep, Journal of Geophysical Research: Solid Earth, 106, 26 311–26 329, https://doi.org/10.1029/2000JB000038, 2001.
Heft, K. L., Gillis, K. M., Pollock, M. A., Karson, J. A., and Klein, E. M.: Role of upwelling hydrothermal fluids in the development of alteration patterns at fast spreading ridges: Evidence from the sheeted dike complex at Pito Deep, Geochemistry, Geophysics, Geosystems, 9, https://doi.org/10.1029/2007GC001926, 2008.
Kroopnick, P.: The distribution of 13C in the Atlantic Ocean, Earth and Planetary Science Letters, 49, 469–484, https://doi.org/10.1016/0012- 821X(80)90088-6, 1980.
Lindsay, J., Brasier, M., McLoughlin, N., Green, O., Fogel, M., Steele, A., and Mertzman, S.: The problem of deep carbon— an Archean paradox, Precambrian Research, 143, 1–22, https://doi.org/10.1016/j.precamres.2005.09.003, 2005.
Marien, C. S., Jäger, O., Tusch, J., Viehmann, S., Surma, J., Van Kranendonk, M. J., and Münker, C.: Interstitial carbonates in pillowed metabasaltic rocks from the Pilbara Craton, Western Australia: A vestige of Archean seawater chemistry and seawater-rock interactions, Precambrian Research, 394, 107 109, https://doi.org/10.1016/j.precamres.2023.107109, 2023.
Nakamura, K. and Kato, Y.: Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO2 sink in the Early Archean, Geochimica et Cosmochimica Acta, 68, 4595–4618, https://doi.org/10.1016/j.gca.2004.05.023, 2004.
Otálora, F., Mazurier, A., Garcia-Ruiz, J. M., Van Kranendonk, M., Kotopoulou, E., El Albani, A., and Garrido, C.: A crystallographic study of crystalline casts and pseudomorphs from the 3.5 Ga Dresser Formation, Pilbara Craton (Australia), Journal of Applied Crystallography, 51, 1050– 1058, https://doi.org/10.1107/S1600576718007343, 2018.
Rausch, S.: Carbonate veins as recorders of seawater evolution, CO2 uptake by the ocean crust, and seawater-crust interaction during low- temperature alteration, Ph.D. thesis, Universität Bremen, 2012.
Runnegar, B., Dollase, W.A., Ketcham, R.A., Colbert, M., Carlson, W.D.: Early Archean sulfates from Western Australia first formed as hydrothermal barites not gypsum evaporites. In: Geological Society of America Abstracts with Programs, vol. 33, 2001.
Shibuya, T., Tahata, M., Kitajima, K., Ueno, Y., Komiya, T., Yamamoto, S., Igisu, M., Terabayashi, M., Sawaki, Y., Takai, K., et al.: Depth variation of carbon and oxygen isotopes of calcites in Archean altered upper oceanic crust: Implications for the CO2 flux from ocean to oceanic crust in the Archean, Earth and Planetary Science Letters, 321, 64–73, https://doi.org/10.1016/j.epsl.2011.12.034, 2012.
Tan, F. C.: Stable carbon isotopes in dissolved inorganic carbon in marine and estuarine environments, in: Handbook of environmental isotope geochemistry, edited by Fritz, P. and Fontes, J., vol. 3, pp. 171–190, Elsevier, Amsterdam, 1988.
Trichet, J. and Défarge, C.: Non-biologically supported organomineralization, Bulletin de l’Institut océ anographique, Monaco. Numé ro special 14(2), 203–236, 1995.
Van Kranendonk, M. J., Philippot, P., Lepot, K., Bodorkos, S., and Pirajno, F.: Geological setting of Earth’s oldest fossils in the ca. 3.5 Ga Dresser formation, Pilbara Craton, Western Australia, Precambrian Research, 167, 93–124, https://doi.org/10.1016/j.precamres.2008.07.003, 2008.
Xiang, W.: Carbonate factories in the early Archean and their geobiological impacts, Ph.D. thesis, University of Göttingen, 1055 Germany, http://dx.doi.org/10.53846/goediss-10047, 2023.
Citation: https://doi.org/10.5194/egusphere-2024-1007-AC1
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RC2: 'Comment on egusphere-2024-1007', Anonymous Referee #2, 28 May 2024
In this manuscript, the authors characterize three different sources of carbonates commonly found in the East Pilbara Terrain (Australia). They describe and differentiate the potential origin of said carbonates in three different marine carbonate factories, as reflected in their distinct d13C signal, these being: 1. The oceanic crust (samples obtained from interstitial carbonate within basalts); 2. Organo-carbonates (sedimentary carbonates form through mediation of an organic component); and 3. Microbial carbonate (stromatolites).
Although I enjoyed reading this manuscript, and it made me think about the carbon cycle from a different perspective that I normally would, I recommend some changes below to be carried out before its publication.
First, and foremost, either the title needs to be changed, or the discussion needs to be rewritten. As it stands, it is unclear to me that this manuscript answer their own title “Were early Archean carbonate factories major carbon sinks on the juvenile Earth?”. The last sentence in the manuscript says “[…] Paleoarchean carbonates might have been major carbon sinks at the time of formation […]”. So, were they major carbon sinks? There is no mention of any quantification of the discussed carbon sink, nor mention of any other potential carbon sinks during this geological period. After reading this manuscript, I cannot answer the question that the authors are postulating. Considering this journal, it is likely that non-experts in Archean carbonates will read this paper, and a further expansion on the carbon cycle at the time, literature looking at carbon sinks in this time and how this work fits with the literature are needed.
Also, the introduction needs to be organized in a different way in order to bring to the front the research question the authors are trying to answer, which is not clear until the very end of the introduction. As before, given the varied potential readership of the journal, the point of why to investigate Archean carbonates, and the ones in the EPT in particular, needs to be specified early in the introduction. For example: 1) The carbon cycle and carbon sinks and their importance for the climate system during the Precambrian. There is no mention of this until the very end of the Introduction (Line 79), and then again nothing until the very end of the discussion (Line 505), this needs to come up earlier, and expanded. 2) Then, as per lines 51 – 54, carbonate formations in this period are poorly constrained, especially in the early Archean. 3) This sets the framework to drive the reader into why more effort needs to be paid, and why you do the work you do. This is only an example of a potential structure that could highlight the importance of this work. Essentially, the introduction needs to be “punchier”.
Finally, in the results (sections 4.1.2 and 4.1.3) as data is presented on primary and secondary carbonate facies and different minerals found. Is it possible to include any sort of quantification/percentage on the presence of minerals on the analyzed samples? This would give a more complete characterization of the interstitial carbonates, the diagenetic processes and their stable isotopic composition.
Minor comments:
Line 92: Repeated “consisting” word twice within a sentence. Rewrite for clarity.
Lines 125 – 128: “For comparison, we additionally analyzed […]”. What is the purpose of this comparison? Include this info in this paragraph, it will make it easier for the reader to understand why you included these samples at all.
Line 146: Change for “were cleaned three times in ethanol using an ultrasonic bath”, or similar. “Using ultrasound” might be confusing.
Line 147: No need to “gently” dry samples. You can delete this adjective.
Lines 177 – 181: These sentences are repetitive and need to be reworded/rewritten.
“Spherulitic and variolitic zones in the basalts are highly carbonatized, with carbonate minerals being particularly prominent in variolites and concentric syngenetic veins.” – What does highly carbonatized mean? First and second part of the sentence is saying the same thing?
“Notably, elemental distributions in basalts do not seem to relate to the degree of weathering (e.g. in sample A22 of the Apex Basalt; Fig. 3b) and hence might be pristine.” – I do not know what do you mean by this. What would be different in the data if elemental distribution related to weathering? Also, likely need some references to justify why does not relate to degree of weathering. And then, this would be more fitting included in the discussion.
“Except for the devitrified volcanic glass, Si is rich in the interior of the pillow basalt but rare in the zone of spherulites and variolites, which are dominated by calcite” – You are mentioning again that calcite is predominant in spherulites and variolites. Which you mentioned two sentences ago.
Line 301: “Notably, it occurs between a unit consisting of sulfidic stromatolites […]” Remove consisting.
Line 368: On the sentence referring to Sr isotopes. I had to dig where this information was coming from. I was confused as to where the calcite Sr signal (0.700596) was coming from. Is it the same samples as this manuscript? Different samples but same location? Data from a completely different location? And why is typical of Archean seawater? I think this needs to be expanded, as it is in strong support of your stable isotopic data. There is no other mention of Sr isotopes in the rest of the manuscript, or other supporting literature. Also repeated in Line 393.
Line 438: “δ13C signatures of some EPT bedded sedimentary carbonates except the Dresser bedded carbonates (1.85 ± 0.48‰ on average) are generally in line with a formation in marine environments”. As it reads, seems like the 1.85% value is that of the Dresser carbonates. Include “(1.85 ± 0.48‰ on average)” after “EPT bedded sedimentary carbonates”.
Line 440: “[…], which in good accordance with d13C signatures […]”. Remove word “which”.
Lines 451 – 452: “Biogenicity” repeated twice in the same sentence. Rewrite for clarity.
Line 458 – 459: “In any case, extracellular polymeric substances (EPS) secreted by microorganisms, amongst others to cope with environmental stressors, play a key-role in mineralization”. The part of “among others to cope with environmental stressors” is confusing and distracting. Although I understand what the authors are inferring to, I would rewrite it, and there is no need to dig into the secretion of EPSs to cope with environmental stressors, which made me think about whether the discussion was heading that way (environmental stressors of the stromatolites in this location and period time). For example, you can entirely delete “among others to cope with environmental stressors” and the sentences would still perfectly fit into the discussion.
Line 469: “(~0.22 -1.85 ‰ on average; Fig. 11)”. These values are confusing, I understand they come from interstitial carbonates (0.22) and sedimentary carbonates (1.85) but all results through the manuscript are given as “X ±Y ‰”, so these are not consistent. I had to go back to results to check that 1.85 was from the sedimentary carbonates, and not a ±‰.
Line 470 – 471: “This difference is well in line with a sequestration of 12C by photoautotrophic microorganisms in the microbial mats, resulting in an enrichment of 13C in the environment and, consequently, in the carbonate”. This sentence needs a reference.
Figure 3: In 3b, Close-up area imaged under white light seems narrower than the uXRM maps. Can this be amended so they reflect the exact same area?
Figure 7: Is the scale bar in 7d and 7e both 200um? If 7e is a close-up of 7d, shouldn’t the scale bar be wider in the close-up? All scale bars in the images are 200um except that in 7f? Clarify.
Caption in Figure 7/Line 291: “(c) Raman spectra for spots in (a) and (c)”. It should be “in (a) and (b)".
Figure 11: This is only a suggestion, but it took me a while to digest the legend and what each symbols is. I would organize the legend grouping by the three carbonate factories discussed in the manuscript. So first group, Oceanic Crust, and include below the interstitial carbonate, veinlet carbonate and fracture carbonate. Then Sedimentary Carb, and include below these samples, and finally microbial. Also another extra group for the extra samples added for comparison. I think this would make it easier to identify the distribution of each carbonate factory in the plot rapidly.
Citation: https://doi.org/10.5194/egusphere-2024-1007-RC2 -
AC2: 'Reply on RC2', Wanli Xiang, 26 Jun 2024
RC2: 'Comment on egusphere-2024-1007', Anonymous Referee #2, 28 May 2024
In this manuscript, the authors characterize three different sources of carbonates commonly found in the East Pilbara Terrain (Australia). They describe and differentiate the potential origin of said carbonates in three different marine carbonate factories, as reflected in their distinct d13C signal, these being: 1. The oceanic crust (samples obtained from interstitial carbonate within basalts); 2. Organo-carbonates (sedimentary carbonates form through mediation of an organic component); and 3. Microbial carbonate (stromatolites).
Although I enjoyed reading this manuscript, and it made me think about the carbon cycle from a different perspective that I normally would, I recommend some changes below to be carried out before its publication.
1. First, and foremost, either the title needs to be changed, or the discussion needs to be rewritten. As it stands, it is unclear to me that this manuscript answers their own title “Were early Archean carbonate factories major carbon sinks on the juvenile Earth?”. The last sentence in the manuscript says “[…] Paleoarchean carbonates might have been major carbon sinks at the time of formation […]”. So, were they major carbon sinks? There is no mention of any quantification of the discussed carbon sink, nor mention of any other potential carbon sinks during this geological period. After reading this manuscript, I cannot answer the question that the authors are postulating. Considering this journal, it is likely that non-experts in Archean carbonates will read this paper, and a further expansion on the carbon cycle at the time, literature looking at carbon sinks in this time and how this work fits with the literature are needed.
- R: Thank you for pointing the problems and providing kind suggestions. On our side, we think the early Archean carbonates were the dominant carbon sinks at that time. We wanted to use a question as the title to appeal the readers. However, it makes the readers confused, which is not our intention. Therefore, following your suggestion, we will reword the title into “Early Archean carbonate factories acting as the dominant carbon sinks on the juvenile Earth” (or “Early Archean carbonate factories – the dominant carbon sinks on the juvenile Earth”?). Besides, we will revise the Introduction, making an expansion on the carbon sinks, carbon cycle and how they moderate the climate at that time. At last, a conceptual box model of the Archean carbon cycle and a simple isotope mass balance calculation of carbon flux will be given in a new section in the Discussion. In this case, the Conclusion may be revised too.
2. Also, the introduction needs to be organized in a different way in order to bring to the front the research question the authors are trying to answer, which is not clear until the very end of the introduction. As before, given the varied potential readership of the journal, the point of why to investigate Archean carbonates, and the ones in the EPT in particular, needs to be specified early in the introduction. For example: 1) The carbon cycle and carbon sinks and their importance for the climate system during the Precambrian. There is no mention of this until the very end of the Introduction (Line 79), and then again nothing until the very end of the discussion (Line 505), this needs to come up earlier, and expanded. 2) Then, as per lines 51 – 54, carbonate formations in this period are poorly constrained, especially in the early Archean. 3) This sets the framework to drive the reader into why more effort needs to be paid, and why you do the work you do. This is only an example of a potential structure that could highlight the importance of this work. Essentially, the introduction needs to be “punchier”.
- R: Thank you for pointing out this issue. We will revise the Introduction following the RC1 and your advice. A potential structure can be (the orders also work for the paragraphs): (1) a brief introduction of the present-day carbon cycle and carbon sinks and their importance for the climate system, which lead to the study of carbonate factory; (2) an introduction of carbonate factory, and in the end “However, carbonate factories are still poorly understood, which is particularly true for the Archean eon, when life still was in its infancy”; (3) an explanation why to study the Archean carbonate factories (because “The Archean eon is an important period for understanding biological and geological evolution on our planet and Earth-like exoplanets (Catling and Zahnle, 2020)”, and the conditions at that time were different from that of the present day); (4) a summery about the previous relevant works (there were two carbon reservoirs including carbonaceous organic matter mainly in chert, and carbonate rocks like stromatolites and interstitial carbonates), and specify why to choose the EPT carbonate rocks; (5) Shortcomings of the studies of the EPT carbonate rocks; (6) the aims and methods of this work.
3. Finally, in the results (sections 4.1.2 and 4.1.3) as data is presented on primary and secondary carbonate facies and different minerals found. Is it possible to include any sort of quantification/percentage on the presence of minerals on the analyzed samples? This would give a more complete characterization of the interstitial carbonates, the diagenetic processes and their stable isotopic composition.
- R: Thank you for pointing this issue. We have measured the percentage of minerals on some samples via XRD. However, it was not more helpful than observation on sample thin sections for identifying the diagenetic processes of carbonates. For example, it cannot tell the difference between the primary calcite and various recrystallized calcites. Therefore, we preferred to study the samples via in-situ analyses. Even when we needed to measure stable carbon and oxygen isotopes, we used a tiny drill to collect sample powers of an individual mineral facies on sections as possible, based on the observation of the related thin sections (the collected powers were not enough for XRD measurement). In this regard, we think data of the most analyzed samples in Table 1 were based on a single carbonate facies, except the interstitial ankerite samples from the Mount Ada Basalt which were influenced by calcite overgrowth (see Fig. 4i). Therefore, we cannot find a good method at the present to include any sort of quantification/percentage on the presence of minerals on the analyzed samples. On the other hand, it is hard to quantify the presence of interstitial carbonates on the analyzed samples, which is the key point to calculate the carbon flux into the oceanic crust factory. In a word, a nice method to quantify mineral percentage in-situ is interesting to explore in the future.
Minor comments:
4. Substitution or modification of words. We organized some minor comments to answer here:
a. Line 92: Repeated “consisting” word twice within a sentence. Rewrite for clarity.
b. Line 146: Change for “were cleaned three times in ethanol using an ultrasonic bath”, or similar. “Using ultrasound” might be confusing.
c. Line 147: No need to “gently” dry samples. You can delete this adjective.
d. Caption in Figure 7/Line 291: “(c) Raman spectra for spots in (a) and (c)”. It should be “in (a) and (b)".
e. Line 301: “Notably, it occurs between a unit consisting of sulfidic stromatolites […]” Remove consisting.
f. Line 440: “[…], which in good accordance with d13C signatures […]”. Remove word “which”.
g. Lines 451 – 452: “Biogenicity” repeated twice in the same sentence. Rewrite for clarity.
- R: Thank you for kindly pointing out these mistakes. Following your advice, we will omit the second “consisting” on Line 92, “consisting” on Line 301, “which” on Line 440, and reword the sentence on Line 146-147, “(a) and (c)” with “(a) and (b)” on Line 291, the “biogenicity” with “that” on Line 452. For your convenience to review, the sentences after the correction will be presented as below:
- a. Line 92: “A characteristic feature of the EPT is the so-called dome-and-keel structure, consisting of a central nucleus of the 3459 ± 18 Ma North Pole Monzogranite (“North Pole Dome”) surrounded by little-deformed, predominantly mafic volcanic rocks of the Warrawoona Group and Kelly Group (Hickman and Van Kranendonk, 2012a) (Fig. 1).”
- b-c. Line 146-147: “The sample chips were cleaned three times in ethanol using an ultrasonic bath and dried at room temperature before being crushed into small pieces.”
- d. Line 291: “(c) Raman spectra for spots in (a) and (b), supporting the presence of ankerite and organic matter.”
- e. Line 301: “Notably, it occurs between a unit of sulfidic stromatolites and bladed barite below, and wave rippled volcanoclastic sediments above (Fig. 6e).”
- f. Line 440: “At the same time, δ13C values of the Dresser bedded carbonates are relatively depleted (-5.72 ±1.36 ‰ on average), in good accordance with δ13C signatures of carbonatites (-4.99 ± 1.22 ‰ on average) (Fig.11), indicating hydrothermal admixture of mantle-derived carbon.”
- g. Line 451: “Although the biogenicity of early Archean stromatolites is commonly controversial, that of stromatolites from the Dresser Formation and the SPF has been widely accepted (…)”
5. Lines 125 – 128: “For comparison, we additionally analyzed […]”. What is the purpose of this comparison? Include this info in this paragraph, it will make it easier for the reader to understand why you included these samples at all.
- R: Thank you for pointing this out. Following your advice, we will reword this paragraph: “In order to better understand depositional environments of the studied EPT carbonates, we additionally analyzed carbon and oxygen isotopic compositions of diverse carbonates for comparison. They include carbonate inclusions in black barites…”
6. Lines 177 – 181: These sentences are repetitive and need to be reworded/rewritten.
(1) “Spherulitic and variolitic zones in the basalts are highly carbonatized, with carbonate minerals being particularly prominent in variolites and concentric syngenetic veins.” – What does highly carbonatized mean? First and second part of the sentence is saying the same thing?
- R: Thank you for pointing those problems. “Highly carbonatized” was used to describe spherulitic and variolitic zones, compared to other basaltic parts. The second part of the sentence is an explanation for the first part. In order to avoid confusion, we will reword it into “Carbonate minerals are particularly prominent in voids, veins and variolites within spherulitic and variolitic zones”. In addition, considering the advice of RC1, we will try to add a quantification of the carbonatized degree and a comparison with modern oceanic crust, somewhere suitable in the text or in the supplementary materials.
(2) “Notably, elemental distributions in basalts do not seem to relate to the degree of weathering (e.g. in sample A22 of the Apex Basalt; Fig. 3b) and hence might be pristine.” – I do not know what do you mean by this. What would be different in the data if elemental distribution related to weathering? Also, likely need some references to justify why does not relate to degree of weathering. And then, this would be more fitting included in the discussion.
- R: Thank you for pointing this issue. We wrote it because we noticed the lower, smaller basalt part in Fig. 3a was weathered. In the previous work, we learned that some elements (Ca, Mg, Al and so on) are easy to lose during rock weathering. Therefore, we mentioned this in advance. The reason why the elemental distribution in the weathered part shows no obvious difference with the unweathered part may be that the preserved weathered part mainly consists of silicates and quartz, which are relatively resistant to weathering. However, considering it is meaningless and confusing in this work, we will omit the sentence.
(3) “Except for the devitrified volcanic glass, Si is rich in the interior of the pillow basalt but rare in the zone of spherulites and variolites, which are dominated by calcite” – You are mentioning again that calcite is predominant in spherulites and variolites. Which you mentioned two sentences ago.
- R: Thank you for pointing this issue. To reduce the repetition, we will reword it into “Except for the devitrified volcanic glass, Si is rich in the interior of the pillow basalt but rare in the spherulitic and variolitic zones, implying a Si loss during basalt carbonatization (Fig. 3b)”. And taking other corrections into account, we will revise the paragraph. For your convenience, we attach the new one here:
- “Although the host basalts show secondary mineral assemblages indicative of greenschist metamorphism (calcite + chlorite + anatase + quartz ± pyrite), phenocrysts (i.e., plagioclase and pyroxene) can still be recognized in the basalt interior of the well-preserved samples, e.g. A22 from the Apex Basalt (Figs. 3a, 4a). Notably, the well-preserved basalts exhibit concentric green ophitic-holohyaline interiors and yellow-green quenched margins. In the margins, the size and density of ovoid spherulites and variolites (amygdules) decrease outwards, merging into the glassy zone (Fig. S1a-c). Carbonate minerals are particularly prominent in voids, veins and variolites within spherulitic and variolitic zones, as illustrated by an overlapping image of Si, Ca and Mg mappings via micro-XRF (see Fig. 3b). Except for the devitrified volcanic glass, Si is rich in the interior of the pillow basalt but rare in the spherulitic and variolitic zones, implying a Si loss during basalt carbonatization (Fig. 3b). Si yielded during this process was likely enriched in fluids, resulting in chert cementation of interstitial carbonates (Fig. 4). The process can be summarized as follows (see Eq.1 where refers to calcium silicate minerals):
- "CaSiO3" + CO2+ H2O → "CaSiO3"+ H2CO3 →CaCO3+ SiO2+ H2O (Eq.1)”
7. Line 368: On the sentence referring to Sr isotopes. I had to dig where this information was coming from. I was confused as to where the calcite Sr signal (0.700596) was coming from. Is it the same samples as this manuscript? Different samples but same location? Data from a completely different location? And why is typical of Archean seawater? I think this needs to be expanded, as it is in strong support of your stable isotopic data. There is no other mention of Sr isotopes in the rest of the manuscript, or other supporting literature. Also repeated in Line 393.
- R: Thank you for pointing out this issue. We have studied the EPT carbonate rocks about their sedimentology, mineralogy and geochemical compositions, which have been written into two articles. Based on the identification and classification of various carbonate facies including primary and secondary facies (this work), we selected typical samples and measured their geochemical compositions (including REEs and Sr isotopes) via acid digestion (the work in preparation). Therefore, some samples have a data system of their mineralogy, elemental compositions, and isotopic composition (i.e. C, O, Sr) based on the same sample powders, while the others have a data system of their mineralogy, C and O isotopic compositions (in Table 1). The calcite Sr signal (0.700596) belongs to the fracture-filling calcite (D-2-W). We think it to be typical of the Archean seawater with the support from the REE+Y pattern (low REE concentrations, no Eu anomaly, positive Y anomaly and so on) and its 87Sr/86Sr ratio (0.700596) close to the lowest 87Sr/86Sr ratios of barites from the same, Dresser Formation (0.700502 in McCulloch, 1994; 0.700447 in Chen et al., 2022), which will be discussed in detail in the other article.
- On the other hand, we also used the same procedure to study pillow basalts of Apex Basalt, which preserve the primary interstitial carbonates. This work is also in preparation. We got the whole rock value of 0.706337 ± 0.000078 given by the Rb-Sr errorchron of the basalts and linked to the study of interstitial carbonates. Therefore, you could see on Line 393-395 “The mixture of different fluids is also supported by 87Sr/86Sr ratios of primary interstitial calcite associated with the Apex basalt (0.703094 ± 0.000979), laying between those of early Archean seawater and Apex pillow basalt (0.700596 and 0.706337 ± 0.000954, respectively; Xiang, 2023)”.
- They are all unpublished, and crucial to our works in preparation. However, considering your advice and readers’ interest, we will provide an information in the supplementary materials. This will include relevant methodological and contextual details about these analyses, and a table to present some 87Sr/86Sr ratios of the basalt-carbonate system, including the average value of the primary and secondary interstitial carbonates, fracture-filling calcite (D-2-W), and the whole rock value of pillow basalts. Nonetheless, we encourage readers to find more details in Xiang, 2023, Carbonate factories in the early Archean and their geobiological impacts (Ph.D. thesis, http://dx.doi.org/10.53846/goediss-10047), where they can also find some interesting applications of the relevant works.
8. Line 438: “δ13C signatures of some EPT bedded sedimentary carbonates except the Dresser bedded carbonates (1.85 ± 0.48‰ on average) are generally in line with a formation in marine environments”. As it reads, seems like the 1.85% value is that of the Dresser carbonates.
- R: Thank you for pointing out this problem. To solve it, we will reword this sentence for clarity: “Some EPT bedded sedimentary carbonates (except the Dresser bedded carbonates) show an average δ13C value of 1.85 ± 0.48‰, within the range of the values of modern seawater (Kroopnick, 1980; Tan, 1988) and the Strelley Pool stromatolites (Lindsay et al., 2005; Flannery et al., 2018; this work), reflecting their formation in marine environments.”
9. Line 458 – 459: “In any case, extracellular polymeric substances (EPS) secreted by microorganisms, amongst others to cope with environmental stressors, play a key-role in mineralization”. The part of “among others to cope with environmental stressors” is confusing and distracting. Although I understand what the authors are inferring to, I would rewrite it, and there is no need to dig into the secretion of EPSs to cope with environmental stressors, which made me think about whether the discussion was heading that way (environmental stressors of the stromatolites in this location and period time). For example, you can entirely delete “among others to cope with environmental stressors” and the sentences would still perfectly fit into the discussion.
- R: Thank you for your kind suggestion. We will make a correction following your advice: “In any case, extracellular polymeric substances (EPS) secreted by microorganisms play a key-role in mineralization (Decho, 2011; see in Fig. 10)” (Line 458).
10. Line 469: “(~0.22 -1.85 ‰ on average; Fig. 11)”. These values are confusing, I understand they come from interstitial carbonates (0.22) and sedimentary carbonates (1.85) but all results through the manuscript are given as “X ±Y ‰”, so these are not consistent. I had to go back to results to check that 1.85 was from the sedimentary carbonates, and not a ±‰.
- R: Thank you for pointing this problem. Following your advice, we will reword the sentence into “More specifically, δ13C values of carbonates from SPF stromatolites (3.08 ± 0.30 ‰ on average) are higher than those of the interstitial carbonates (0.22 ± 0.98 ‰ on average) and the sedimentary carbonates (1.85 ± 0.48‰ on average)”.
11. Line 470 – 471: “This difference is well in line with a sequestration of 12C by photoautotrophic microorganisms in the microbial mats, resulting in an enrichment of 13C in the environment and, consequently, in the carbonate”. This sentence needs a reference.
- R: Thank you for your suggestion. Following your advice, we will add a reference “(Arp et al., 2011)” in the end of this sentence. For your convenience, we attach the reference here: Arp, G., Helms, G., Karlinska, K., Schumann, G., Reimer, A., Reitner, J., & Trichet, J. (2011). Photosynthesis versus Exopolymer Degradation in the Formation of Microbialites on the Atoll of Kiritimati, Republic of Kiribati, Central Pacific. Geomicrobiology Journal, 29(1), 29–65. https://doi.org/10.1080/01490451.2010.521436
12. Figure 3: In 3b, Close-up area imaged under white light seems narrower than the uXRM maps. Can this be amended so they reflect the exact same area?
- R: Thank you for pointing this problem out. It is a display mistake happened when we converted the MS word into pdf. We are sorry for not observing it. We will make a correction and check for the next submission.
13. Figure 7: Is the scale bar in 7d and 7e both 200um? If 7e is a close-up of 7d, shouldn’t the scale bar be wider in the close-up? All scale bars in the images are 200um except that in 7f? Clarify.
- R: Thank you for pointing these problems. The scale bar in 7d and 7e are both 200um, however, 7e is not a close-up of 7d (they are two areas). Besides, 7f and 7g share the same scale bar of 5 um. To avoid the potential confusion, we will make a correction by lining the crystals in 7e, adding a scale bar in 7g, and rewording the caption “The scale bars in (f) and (g) are 5 um, while others 200 um” on Line 295.
14. Figure 11: This is only a suggestion, but it took me a while to digest the legend and what each symbols is. I would organize the legend grouping by the three carbonate factories discussed in the manuscript. So first group, Oceanic Crust, and include below the interstitial carbonate, veinlet carbonate and fracture carbonate. Then Sedimentary Carb, and include below these samples, and finally microbial. Also another extra group for the extra samples added for comparison. I think this would make it easier to identify the distribution of each carbonate factory in the plot rapidly.
- R: Thank you for your kind suggestion. It looks better than the original one. We would like to make this correction following your advice.
References
Arp, G., Helms, G., Karlinska, K., Schumann, G., Reimer, A., Reitner, J., and Trichet, J.: Photosynthesis versus Exopolymer Degradation in the Formation of Microbialites on the Atoll of Kiritimati, Republic of Kiribati, Central Pacific. Geomicrobiology Journal, 29(1), 29–65. https://doi.org/10.1080/01490451.2010.521436, 2011.
Catling, D. C. and Zahnle, K. J.: The Archean atmosphere, Science advances, 6, eaax1420, http://dx.doi.org/10.1126/sciadv.aax1420, 2020.
Chen, X., Zhou, Y., and Shields, G. A.: Progress towards an improved Precambrian seawater 87Sr/86Sr curve, Earth-Science Reviews, 224, 103 869, 2022.
Decho, A. W.: Extracellular polymeric substances (EPS), in: Encyclopedia of Geobiology, edited by Reitner, J. and Thiel, V., pp. 359–361, Springer, Berlin, 2011.
Flannery, D. T., Allwood, A. C., Summons, R. E., Williford, K. H., Abbey, W., Matys, E. D., and Ferralis, N.: Spatially- resolved isotopic study of carbon trapped in 3.43 Ga Strelley Pool Formation stromatolites, Geochimica et Cosmochimica Acta, 223, 21–35, 2018.
Hickman, A. H. and Van Kranendonk, M.: A Billion Years of Earth History: A Geological Transect Through the Pilbara Craton and the Mount Bruce Supergroup-a Field Guide to Accompany 34th IGC Excursion WA-2, Geological Survey of Western Australia, Record 2012/10, 2012a.
Kroopnick, P.: The distribution of 13C in the Atlantic Ocean, Earth and Planetary Science Letters, 49, 469–484, https://doi.org/10.1016/0012- 821X(80)90088-6, 1980.
Lindsay, J., Brasier, M., McLoughlin, N., Green, O., Fogel, M., Steele, A., and Mertzman, S.: The problem of deep carbon— an Archean paradox, Precambrian Research, 143, 1–22, https://doi.org/10.1016/j.precamres.2005.09.003, 2005.
McCulloch, M. T.: Primitive 87Sr/86Sr from an Archean barite and conjecture on the Earth’s age and origin, Earth and planetary science letters, 126, 1–13, https://doi.org/10.1016/0012- 821X(94)90238-0, 1994.
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Citation: https://doi.org/10.5194/egusphere-2024-1007-AC2
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AC2: 'Reply on RC2', Wanli Xiang, 26 Jun 2024
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Wanli Xiang
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Andreas Pack
Mark van Zuilen
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