the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jul 2024
The role of siliceous sponges in pre-Eocene marine Si cycle from the perspective of rock mineralogy
Abstract. The process of siliceous sponge dissolution during diagenesis was interpreted not only as an important part of marine Si cycle (comprising Si burial) but also as a significant mechanism of chert formation (so-called “chertification”; Maliva and Siever, 1989a). Both ideas were widely accepted by researches and are commonly used in geological studies. New research contradicts these seminal assumptions and indicates that in pre-Eocene marine Si cycle, although siliceous sponges were an important part of the ecosystems, did not play a controlling role in regulating dSi (= dissolved silicon) concentration in the porewater as well as in chert formation. The presented studies based on advanced mineralogical (XRD, EBSD; SEM-EDS) and microtextural (SEM) analysis of rocks and sponge remnants verify the role of siliceous sponges in the formation of Cretaceous siliceous rocks, by studying successions deposited in similar marine environments, which contain abundant fossils of siliceous sponges associated with cherts and authigenic silica polymorphs and those without them. For the first time, the mineralogical and microtextural transformations of siliceous sponge loose spicules/rigid skeletal networks, which led to their preservation as siliceous or pyrite/marcasite infillings and also in form of limonite coatings, are presented. The data presented here about the diagenesis of siliceous sponges skeletons opens the discussion on the usefulness of stable isotopic studies of δ30Si in geological studies of fossils of silicifiers preserved as secondary silica polymorphs (opal-CT).
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CC1: 'Comment on egusphere-2024-2003', Shahab Varkouhi, 01 Aug 2024
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This manuscript is revolutionary in that it attributes a new, specific, and reliable combination of geochemical, mineralogical, and microfabric data documenting the transformation of Cretaceous diagenetic siliceous records from siliceous sponges. For the first time, the results reported in this study also open a new pathway towards application of silicon stable isotopic data for characterisation of the diagenetic products of transformed biosiliceous precipitates. Therefore, the publication of this great work is strongly recommended.
Shahab Varkouhi
Citation: https://doi.org/10.5194/egusphere-2024-2003-CC1 -
AC1: 'Reply on CC1', Agata Jurkowska, 02 Aug 2024
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Thank You for the constructive comment and great summary of our manuscript.
On behalf od Co-Authors,
Agata Jurkowska
Citation: https://doi.org/10.5194/egusphere-2024-2003-AC1
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AC1: 'Reply on CC1', Agata Jurkowska, 02 Aug 2024
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RC1: 'Comment on egusphere-2024-2003', Anonymous Referee #1, 26 Aug 2024
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General comments
The manuscript by Jurkowska et al. addresses Si burial in the oceans prior to the Ecocene. They performed some detailed microtextural and mineralogical analysis in Cretaceous siliceous rocks, then they concluded a relatively closed system for the decay and dissolution of siliceous sponges, thus a negligible role of sponges in regulating dissolved silica (dSi) in porewater. Jurkowska et al. question the current concepts regarding chert formation and present their study based on mineralogical and microtextural analysis of rock and sponge remains. In addition, they question studies using d30Si.
The authors are applying the concept that prior to the Eocene the biological Si cycle was dominated by sponges and radiolaria, and that diatoms became dominant at the beginning of the Eocene reducing dSi in the surface oceans to the low levels observed today (Siever 1991). This assumption has important implications for their model of Si burial in the pre-Ecocene time period. However, the literature on molecular clocks suggest that diatoms evolved over 200 Ma ago (Nakov et al. 2018, New Phytologist 219:462-473), although this result has been largely ignored because of the lack of diatoms in the geologic fossil record. Diatoms likely had a major impact on the Si cycle earlier than the Cenozoic (Conley et al. 2017, doi: 10.3389/fmars.2017.00397).
While it is true that there are more diatoms in the geologic record during the Cenozoic, there is more evidence that there were many species of diatoms in the Upper Cretaceous (review by Brylka et al. 2024, Marine Micropaleontology 190:102371). Diatoms dissolve in sediments with temperatures over 30oC and were likely dissolved as they were buried. Although the Lower Cretaceous record is limited, the global distribution of study sites and the diversity of the oldest diatoms point towards earlier dispersal and diversification events. Indirect evidence for the earlier evolution is also provided through molecular clocks. The taxonomic richness and geographical spread of these diatom communities suggest prior evolutionary events. Altogether the distribution of diatom deposits and their diversity in the Lower Cretaceous on both hemispheres, suggests that the earliest diatoms are yet to be discovered. Diatoms likely had been diversifying before 120 Ma to evolve into separate lineages and dispersed, adapted, and prevailed in new environments where they further diversified. Eventually, only a small proportion of these communities were preserved in sediments. Therefore, the assumption used by Jurkowska et al. that diatoms do not have a role in removing dSi from the oceans prior to the Eocene is likely incorrect.
Jurkowska et al. also question the “Usefulness of stable isotopic studies of d30Si in geological studies of fossils.” in lines 27-39. The transformation from opal-a to opal-CT is a dissolution precipitation reaction and these diagenesis reactions are well known to fractionate Si isotopes. This does not invalidate all uses of Si isotopes in geologic studies. What should be done is that the material should be measured by x-ray diffraction to determine if the material is opal-a or has been transformed to opal-CT. The literature using Si isotopes from sponge spicules that are opal-a shows that during the last 90 Ma dSi bottom water dSi concentrations in different parts of the ocean range from 0-70 um (Conley et al. 2027; Dai et al. 2022; Doering et al. 2024; Fontorbe et al. 2016, 2017, 2020; Störling et al. 2024).
The idea that chert formation is governed by “volcano-hydrothermal Si sources” is primarily supported by Jurkowska’s publications. In the GBC 2021 paper by Jurkowska suggest that the dSi is released into seawater by volcano-hydrothermal sources and then transported by ocean currents into the basin. In today’s oceans only 11% of the total dSi inputs comes from volcano-hydrothermal sources (Treguer et al. 2021). The inputs are only a small amount of dSi relative to the total burden of dSi in the water of the entire oceans. I fail to see how this could be a major factor. The other complicating factor is the diffusion of these supposedly high dSi concentrations from the water column into the sediment. How far into the sediment do you envision the dSi penetrating? Usually dSi is high in sediments and the diffusion will be working against a concentration gradient.
Overall, the structure is not well organized. For example, Section 2 and Section 3 are essentially literature reviews without new work, so it should be included into the Introduction. Section 5.1 is a description of results, while Section 5.2-5.3 is mostly discussion. It will be more readable to describe the observations of sponge spicules, organic matter and minerals in this Section, and move subjective investigations to the following Section Discussion. The authors even wrongly labeled section 5.3.3 (Line 595), which I assume should be 5.4? The manuscript is currently too confusing and wordy in the present version.
The manuscript includes repeated discussion and sometimes self-contradictory statements. For example, in lines 427-428, they state “taking into account that during the Cretaceous the seawater dSi concentration was high (Siever, 1991)”, in lines 439-443 they also claim “similar to those... under relatively high dSi... contradicts the diminished seawater dSi”, but then in lines 707-708 they state “low seawater dSi concentration is very probable”. I got lost several times when reading the text, a few concluding sentences in each section may help to understand their main idea.
I agree that the dSi concentration of porewater is important for silica precipitation, but how to exclude the dissolution of spicules as part of the porewater after burial? The dissolution pits on the spicule surface and the voids left by spicules indicate that dissolution is happening, and dSi can diffuse to the surrounding matrix. The reprecipitation of silica (opal-CT or quartz) in some voids may consume comparable or a bit higher amount of original silica, but what about the voids occupied by pyrite, barite or not occupied at all? Where is the original spicule Si? Isn’t this a dSi source for porewater? Finally, how should one distinguish if the dSi origins are from seawater or hydrothermal or spicules? How can you estimate the share of each dSi source? How is it reflected in silicon isotopes?
Specific comments
Line 1. The title. Can you please define what time period is “pre-Ecocene”? What does “perspective rock mineralogy” mean? Why is “perspective” in the title?
Lines 13-15. Why use “Both ideas”? Isn’t “chert formation” “part of the marine Si cycle”?
Lines 78-114, 333-373, 400-474, 517-577, 638-671. These paragraphs are excessively large sometimes covering 3 pages of text and should be rewritten.
Line 150. What is the definition of “Earth history”?
Line 210. A map or lithological column, as well as some outcrop photos can be added in this section to help readers understand where the samples are collected from and what the samples look like. Moreover, I suggest adding some background information about other silicifiers such as diatoms or radiolarians in the study area.
Lines 343-348. I can not follow the logic here. Why would the chert nodules overlap with sponges if sponge spicules serve as a dSi source? The different morphology of opal-CT in spicules and the matrix may refer to differences in the microenvironment, and may indicate their independence in the precipitation process, but not the dissolution.
Line 350. Does the “seawater” here mean the overlying seawater? To which depth can it react with porewater? How to recognize in which stage of diagenesis, the dissolution or precipitation of spicules happen in the sediments?
Lines 464-465. The absence of OM does not necessarily mean no decomposition, instead it might indicate complete decay of OM in the oxic zone.
Lines 603-604. What kind of transformation?
Line 614. Which silica polymorph?
Citation: https://doi.org/10.5194/egusphere-2024-2003-RC1 -
AC2: 'Reply on RC1', Agata Jurkowska, 31 Aug 2024
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2003/egusphere-2024-2003-AC2-supplement.pdf
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AC2: 'Reply on RC1', Agata Jurkowska, 31 Aug 2024
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