the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Dec 2022
Cosmogenic 10Be in pyroxene: laboratory progress, production rate systematics, and application of the 10Be-3He nuclide pair in the Antarctic Dry Valleys
Abstract. Here, we present cosmogenic 10Be and 3He data from Ferrar dolerite pyroxenes in surficial rock samples and a bedrock core from the McMurdo Dry Valleys, Antarctica, with the goal of refining the laboratory methods for extracting beryllium from pyroxene, further estimating the 10Be production rate in pyroxene, and demonstrating the applicability of the 10Be-3He in mafic rock. The ability to routinely measure cosmogenic 10Be in pyroxene will open new opportunities for quantifying exposure durations and Earth surface processes in mafic rocks. We describe scalable laboratory methods for isolating beryllium from pyroxene, which includes a simple hydrofluoric acid leaching procedure for removing meteoric 10Be, and the addition of a pH 8 precipitation step to reduce the cation load prior to ion exchange chromatography. 10Be measurements in pyroxene from the surface samples have apparent 3He exposure ages of 1–6 Ma. We estimate a spallation production rate for 10Be in pyroxene, referenced to 3He, of 3.6 ± 0.2 atoms g−1 yr−1. 10Be and 3He measurements in the bedrock core yield initial estimates for parameters associated with 10Be and 3He production by negative muon capture (f10* = 0.00183 and f3* fC fD = 0.00337).
Next, we demonstrate that the 10Be-3He pair in pyroxene can be used to simultaneously resolve erosion rates and exposure ages, finding that the measured cosmogenic-nuclide concentrations in our surface samples are best explained by 2–8 Ma of exposure at erosion rates of 0–35 cm Myr−1. Finally, given the low 10Be in our laboratory blanks (average of 5.7 × 104 atoms), the reported measurement precision, and our estimated production rate, it should be possible to measure 2 g samples with 10Be concentrations of 6 × 104 atoms g−1 and 1.5 × 104 atoms g−1 with 5 and 15% uncertainty, respectively. With this level of precision, Last Glacial Maximum to Late Holocene surfaces can now be dated with 10Be in pyroxene. Application of 10Be in pyroxene, alone or in combination with 3He, will expand possibilities for investigating glacial histories and landscape change in mafic rock.
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RC1: 'Comment on egusphere-2022-1379', Shaun Eaves, 10 Feb 2023
General comments
This manuscript contains a multi-faceted study concerning the systematics and practical application of in situ cosmogenic 10Be and 3He production in pyroxene. The work presented makes several important advances, notably (i) advancing laboratory protocols for beryllium purification in pyroxene; (ii) confirming and refining previous spallation production estimates; and (iii) identifying and quantifying components of muonic production of both nuclides.
The study is clearly and concisely communicated, both in written and graphical forms, which is no mean feat given the multiple threads of research undertaken. I have only minor suggestions and a few typographical observations summarised below. Overall, the work presented here makes a significant contribution to the geochronological literature that I am sure will be influential for future applications of these geochronometers.
Specific comments
Quantitative assessment of the contaminant load against ion exchange resin capability is critical to effective beryllium isolation in mafic minerals (as well reasoned in lines 61-66). I would appreciate a fuller and more quantitative description of the decision making concerning measured contaminant loads (e.g. Table S3) and resin volumes within the paragraph covering lines 245-256. I am specifically interested in this matter given the described need to reprocess some samples based on visual assessment of Be(OH)2 purity via comparison of precipitates with the process blank. Were you ‘flying too close to the sun’ as it were, with cation loads for these columns? And if so, should the methods be more conservative? Given the specific focus of this manuscript on methodological refinement, it would be valuable for readers if the decision making was more quantitative and explicitly described. If you could also provide more precise information concerning the tolerance thresholds for the visual inspection of Be(OH)2 precipitates (beyond the rather vague ‘noticeably larger’ vs ‘similar in size’), this would also be a valuable addition. Ideally this would be photographic, but I appreciate that might not be possible after the fact.
There is immense (and rare) value in having the same samples both prepared and measured by multiple laboratories. In this regard, the differences in pyroxene purity identified in lines 287-297 are of fundamental importance. However the implications for mineral separation methodologies are left subject to reader interpretation. Could you add further detail as to what you think are the key causes of these differences and what therefore are the implications for best practice mineral separation?
The final paragraph of the discussion concerning the limits of this method is valuable and necessary, but at present it is restricted to specific end-member cases chosen by the authors. I wonder if there is a more effective way of communicating the wider applicability of this method, perhaps in graphical form, for non-cosmogenic experts that wish to know if this nuclide-mineral pair is suitable for their particular application. If you choose to revise this component, I would also suggest that you allow for higher process blanks – most laboratories are working with 10Be backgrounds several times higher!
Technical comments
L19 – check grammar ‘…of the 10Be-3He in mafic rock’. Remove ‘the’?
L29-33 – the low background is impressive but perhaps a more useful summary for the abstract would be what you think could be routine, rather than what are the limits of possibility. See also, final specific comment above.
L54 – awkward sentence construction, coupled with unusual use of ‘however’ in first sentence of a paragraph. Consider rewording to “Fewer cosmogenic nuclides are routinely measured in lithologies where quartz is absent.”
L64 isolating not isolated?
L104 – remove ‘ice-free’ – while there is far less ice than the immediate surroundings, ice-free is incorrect.
L182-3 – were (not ‘was’) similar – procedures is plural.
L209 – ‘home-design’ could be changed to custom-made
L239 – ‘very low 10Be’…abundance? Also – can you quantify here?
L337 – ‘first direct measurements of muon-produced 10Be in pyroxene’ – this makes it sound like you isolated the muonic component. Reword to something more accurate, e.g. first identification of in situ 10Be production in pyroxene by muons
L357 – remove ‘some’, it’s unneccessary
Table 1 – perhaps ‘Sample type’ for the core should read ‘core top’?
Table 2 – is superscript point #5 necessary?
Citation: https://doi.org/10.5194/egusphere-2022-1379-RC1 -
AC1: 'Reply on RC1', Allie Balter-Kennedy, 02 May 2023
RC1
We thank Shaun Eaves for his constructive review of the manuscript and his insightful comments.
Below, we address referee comments and describe additional, unsolicited changes that we’ve made to improve the manuscript. Referee comments are supplied in bold, with our responses in regular text.
Specific Comments
Quantitative assessment of the contaminant load against ion exchange resin capability is critical to effective beryllium isolation in mafic minerals (as well reasoned in lines 61-66). I would appreciate a fuller and more quantitative description of the decision making concerning measured contaminant loads (e.g. Table S3) and resin volumes within the paragraph covering lines 245-256. I am specifically interested in this matter given the described need to reprocess some samples based on visual assessment of Be(OH)2 purity via comparison of precipitates with the process blank. Were you ‘flying too close to the sun’ as it were, with cation loads for these columns? And if so, should the methods be more conservative? Given the specific focus of this manuscript on methodological refinement, it would be valuable for readers if the decision making was more quantitative and explicitly described. If you could also provide more precise information concerning the tolerance thresholds for the visual inspection of Be(OH)2 precipitates (beyond the rather vague ‘noticeably larger’ vs ‘similar in size’), this would also be a valuable addition. Ideally this would be photographic, but I appreciate that might not be possible after the fact.
We certainly understand the desire to include a more quantitative assessment of the cation exchange capacity. We based our decision-making concerning contaminant loads on the knowledge that the pH 8 precipitation should reduce or even completely remove Mg and Ca from our samples (Ochs and Ivy-Ochs, 1997), leaving Al as the primary contaminant removed during cation columns. Because the total Al in our samples was low (Table S3 in original submission), we opted to use 2 mL of cation exchange resin to isolate beryllium, following standard procedures in our lab (see references in manuscript). It is likely, however, that some Mg and/or Ca remained in the sample, "overloading" the column, possibly allowing Al to leak off with Be as happens with a large cation load, or perhaps Mg, which elutes immediately following Be, began eluting early. Nevertheless, a second cation column resulted in a final beryllium hydroxide precipitate of similar size to the blank.
In response to the reviewer’s comment, we will update some of the wording in Section 3.2.3 to make our decision making here clearer to the reader. We will also a photo of the Be(OH)2 precipitates in the supplement so that the reader can see the difference in precipitate size for the overloaded columns.
Lines 234-237 will read: “Here, we add a simple pH 8 precipitation step to reduce the cation load in our samples prior to ion exchange chromatography. At pH 8, Be, Al and Fe precipitate from solution as hydroxides, Be(OH)2, Al(OH)3 and Fe(OH)2, while Ca and Mg should remain in solution (Ochs and Ivy-Ochs, 1997).”
Lines 245-256 will read: “We isolated beryllium using ion chromatography methods described by Kohl & Nishiizumi (1992). Given that most Ca and Mg were likely removed in the pH 8 precipitation step, we opted to use 2 mL of BioRad-50W X8 200–400# mesh resin for cation exchange based on the amount of Al in our samples (Table S1). Following cation exchange columns, the beryllium fraction was evaporated to dryness and then taken up in 4 mL of 1% HNO3, transferred to 15 mL centrifuge tubes. From this solution, we precipitated Be(OH)2 by adjusting the pH to 9 with NH4OH. After pouring off the suprenate, this precipitation step was repeated. We then performed three rinses of the precipitate with milli-Q water adjusted to pH 8. The Be(OH)2 precipitates were noticeably larger than the blank for several samples (Fig. S3), indicating that we exceeded the capacity of the cation exchange resin and Al or another cation eluted with beryllium . For these samples, we performed a second round of cation exchange chromatography, also with 2 mL of resin. Following the second round of cation columns, the Be(OH)2 precipitates for all samples were similar in size to the blank, indicating that the second column step was successful in isolating beryllium.”
There is immense (and rare) value in having the same samples both prepared and measured by multiple laboratories. In this regard, the differences in pyroxene purity identified in lines 287-297 are of fundamental importance. However the implications for mineral separation methodologies are left subject to reader interpretation. Could you add further detail as to what you think are the key causes of these differences and what therefore are the implications for best practice mineral separation?
We will make a few changes to clarify the implications of mineral separation methods for the reader. First, we will re-word the sentence on Lines 287-290 to provide more definitive support for the presence of plagioclase: “Higher-than-expected Al concentrations measured by ICP-OES in the CRPG-prepared pyroxene suggest that plagioclase remained in the CRPG-prepared samples (Tables S1 and S2).” [see unsolicited changes section – the Supplementary table numbers will be updated to reflect their order in the updated text]
We will also add a sentence near Line 294 of the submitted manuscript that reads “The observation that plagioclase-contaminated pyroxene samples yielded lower 3He concentrations further supports the use of HF etching as an effective method for producing pure pyroxene separates, as plagioclase is readily dissolved in HF (Bromley et al., 2014).” This sentence summarizes the best practice for pyroxene mineral separation based on our results. We note that the full procedure for HF leaching of pyroxenes is described in Bromley et al. (2014), and to avoid redundancy with that paper we do not go into further detail here.
The final paragraph of the discussion concerning the limits of this method is valuable and necessary, but at present it is restricted to specific end-member cases chosen by the authors. I wonder if there is a more effective way of communicating the wider applicability of this method, perhaps in graphical form, for non-cosmogenic experts that wish to know if this nuclide-mineral pair is suitable for their particular application. If you choose to revise this component, I would also suggest that you allow for higher process blanks – most laboratories are working with 10Be backgrounds several times higher!
It is true that our discussion of the limits of this method are based on the results from our own lab, where process blanks contain relatively low abundances of 10Be and appreciate the reviewer’s suggestion that we therefore widen the breadth of this discussion. We think it most appropriate to base this discussion on the dataset presented in this paper, as users in other laboratories will need to assess the limits of this method given their own process blank data, which varies from lab to lab. To aid in that exploration, we have moved the former Figure S3, which provides the reader with a comparison of number of atoms measured to relative uncertainty (which comes mostly from measurement uncertainty although does contain blank uncertainty), from the Supplemental Information to the main text. Readers will then be able to determine what applications are feasible in their own lab given their process blank values. We have also added a parenthetical statement on Line 580 that reads “other users will need to evaluate the limits of these methods for exposure dating given the average blank values from their own laboratories.”
Technical comments
L19 – check grammar ‘…of the 10Be-3He in mafic rock’. Remove ‘the’?
Good catch, we will fix this.
L29-33 – the low background is impressive but perhaps a more useful summary for the abstract would be what you think could be routine, rather than what are the limits of possibility. See also, final specific comment above.
While we agree that the backgrounds presented here are lower than in many other laboratories, we note that these values are unique to each laboratory and therefore can only speak to the blank values in our own lab. In addition, the blanks should be similar to those processed alongside quartz samples, and even might be smaller because of the lower reagent volumes used for the small pyroxene sample sizes. We hope that the addition of Figure 9 to the main text will help readers evaluate the limits of this method in their own laboratories.
L54 – awkward sentence construction, coupled with unusual use of ‘however’ in first sentence of a paragraph. Consider rewording to “Fewer cosmogenic nuclides are routinely measured in lithologies where quartz is absent.”
We will make this change.
L64 isolating not isolated?
We will update this.
L104 – remove ‘ice-free’ – while there is far less ice than the immediate surroundings, ice-free is incorrect.
Good point. We will remove “ice-free”.
L182-3 – were (not ‘was’) similar – procedures is plural.
We will update this.
L209 – ‘home-design’ could be changed to custom-made
We will change this to custom-made.
L239 – ‘very low 10Be’…abundance? Also – can you quantify here?
We acknowledge that it’s possible that the LDEO carrier contains virtually zero 10Be, but at the very least it must be lower than the process blanks, which average 5.7 x 103 10Be atoms.
In response to this comment, we will update this sentence to read “For beryllium extraction from pure pyroxene separates we first weighed and spiked 100-200 mg of pyroxene with ~180 µg of 9Be using LDEO carrier (Schaefer et al., 2009). The addition of 9Be carrier contributed less than 5.7 x 103 atoms of 10Be, which is the average value of our process blanks.”
L337 – ‘first direct measurements of muon-produced 10Be in pyroxene’ – this makes it sound like you isolated the muonic component. Reword to something more accurate, e.g. first identification of in situ 10Be production in pyroxene by muons
Agreed! We will update the sentence to read “we present the first identification of in situ 10Be production by muons, and further confirm the importance of quantifying muon production of 3He”
L357 – remove ‘some’, it’s unneccessary
We will remove this.
Table 1 – perhaps ‘Sample type’ for the core should read ‘core top’?
We will make this change.
Table 2 – is superscript point #5 necessary?
It’s probably not necessary, good point. We will remove superscript #5.
Additional unsolicited changes to the manuscript
Main Text
- L30 – In the original submission file, we erroneously listed the average blank value as 5.7 x 104 atoms in the abstract, rather than the true value of 5.7 x 103 We will update this to 5.7 x 103 atoms and note that it was already correct elsewhere in the manuscript.
- L45 – will update “serve” to “serves” to be consistent with the sentence subject, “concentration”.
- L50 – will add hyphen to “103–106 year” timescales so that it reads “103–106-year timescales”.
- L61-66 – we will update the description of pyroxene compositions to be more consistent with the cited reference. We will also update the next sentence to better reflect the challenges related to isolating beryllium from pyroxene. These sentences will now read: “First, the mineral composition of pyroxene [XYSi2O6, where X and Y are both divalent cations (primarily Ca, Fe, or Mg), or X is a monovalent cation (Na, Li) and Y is a trivalent cation (Al, Fe); Nespolo, 2020)] is highly variable. In contrast to quartz (SiO2), the high cation quantities in pyroxenes present a significant challenge for isolating beryllium using ion chromatography, limiting the feasible sample size.”
- L138 – Will remove “the Dais” from sentence describing the bedrock core location, as the core was not collected from the Dais itself, rather a different (unnamed) erosional surface of the Labyrinth.
- L163 – will remove extra “the”
- L271 – The 3He production rate stated in the original submission was outdated. We will update this to read 120 atoms g-1 yr-1, not 124 atoms g-1 yr-1.
- L240 - For clarity, we will update sentence from “The small sample sizes were sufficient for these samples with high cosmogenic nuclide inventories and minimized the overall ion load” to “The small sample sizes were sufficient for these samples with high cosmogenic nuclide inventories while minimizing the overall ion load”.
- L248 – Will remove “and diluted with 10 mL of milli-Q water” as this shouldn’t have been included here.
- L375 – Will add a definition to first instance of “SLHL”: “sea level high latitude (SLHL)”
- L406, L 408 – Will fix erroneous equation references
- In some places, “dolerite”, in “Ferrar dolerite” was capitalized, but should not have been. We will remove capitalization where necessary.
Tables
- Table 2 – will add reference to Schaefer et al. (2006), which is where the 3He measurements for the surface samples were originally reported. This is already discussed in the text but will be added to the table for clarity and completeness.
- Will update supplementary table numbers to be consistent with the order they are now presented in text.
Equations
- Equation 15 – Will add Equation number as it was missing
- Equation 13 – The variable “Sthick” was not defined in the text. We will add a definition near Line 505 of the submitted manuscript that reads, “Where is the sample thickness correction (dimensionless)”.
Citation: https://doi.org/10.5194/egusphere-2022-1379-AC1
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AC1: 'Reply on RC1', Allie Balter-Kennedy, 02 May 2023
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RC2: 'Comment on egusphere-2022-1379', Mark Kurz, 04 Apr 2023
General Comments:
This is an excellent paper that definitely should be published in this journal. It will be of great interest to specialists and to new users of cosmogenic nuclide methods. It presents a methodological study of 10Be in pyroxenes, describing a leaching method to remove meteoric 10Be, which is new and important practical information. It also presents new and important data sets: 3He and 10Be data from pyroxenes in a number of surficial Antarctic samples, and as a function of depth in an Antarctic dolerite core. The depth profile is particularly useful because it yields new information on cosmogenic 3He and 10Be production rates and mechanisms, including muon production rates, which are very important.
The paper compares results from several different laboratories, on some of the same samples, which is rare, but attests to the high quality of the data presented here. This is a large and valuable analytical effort. The paper is forthcoming in discussing (some of the) uncertainties, such as core depth issues related to drilling, and problems with sample preparation and removal of plagioclase. Overall, this is a really impressive paper. I congratulate Balter-Kennedy and co-authors.
I do have some suggestions outlined below, many of which are optional, and all of which should be relatively easy to address. I think the authors should include more discussion of the uncertainties wherever possible, as outlined below, particularly with respect to the geologic setting of the core, the depth dependence, and analytical methods.
Main scientiific comments:
The geologic setting of the core. Since the Labyrinth dolerite core has never been described (is that correct?), I recommend adding more information, either in the text or supplement. The paper cites Lewis for basic background and gives a brief summary of the geologic setting, but only from a fairly simple glacial history perspective. It never mentions that the Ferrar dolerite is Jurassic (~180Ma), igneous intrusive, and does not describe the mineralogy of the outcrop. Is it typical of the Ferrar, which has variable mineralogy? Are the minerals analyzed clinopyroxenes, as I assume (are they augite?). Is the mineralogy homogeneous within the core? The text mentions that the average density is 2.94 g/cc, but how variable is the density within the core? That could provide some uncertainty for the depth profile, if it varies. This is all fairly basic information that could be easily added (if there is no paper to cite).
Depth dependence. The calculation of muon production rates depends strongly on the assumed spallation attenuation length. The paper assumes 140 g/cm2 and cites Gosse and Phillips (2001) for this value, but this is not explained or justified. I looked at Gosse and Phillips, and could not see any justification for this attenuation length, for either 3He and 10Be, in Antarctica or elsewhere. In addition, the 3He and 10Be depth data, in figures 3 and 4, all seem to fall to the right of the 140 g/cm2 lines (and note that this is a log scale). At the shallowest depths, the production should be dominated by spallation, so it looks like a longer attenuation length might be more appropriate for both 3He and 10Be. For example, all the data below 20 cm depth. in figure 3, deviate from the line. Could the data be fit to a longer attenuation length? Gosse and Phillips present a table that summarizes the experimentally determined values, and they are highly variable, and as mentioned above the use of 140 g/cm2 is not clear. How uncertain is this value and how would it impact the muon production rate estimates? There is an existing depth profile for 10Be in quartz which yielded 145 g/cm2, probably from a similar altitude (Brown et al., 1992), which should be cited in my opinion).
It is also conceivable that the shallow data do not fit the curve because the core area had a complex exposure/erosion history. A complex exposure history at the core site (which seems likely to me) could have an impact on the muon production rate estimates, i.e. long exposure at depth. This should be discussed.
Analytical information. Given that this paper has a methodological emphasis, there should be more complete analytical details. For example, in section 3.2.2. describing the helium measurements, the LDEO and BGC blanks are not given (they are given for CRPG), and there is no information given on sample sizes (this is not tabulated), or on the size of the primary standards used, or on the reproducibility of primary standards. The running standards are mentioned for LDEO and CRPG but not BGC. I recommend providing full information, somewhere, for all the labs, rather than make the reader try to track it down. It is also unclear how the standards can be used to obtain absolute 3He abundances, i.e. are the Yellowstone and Matsuda standards compared to air?
The tables provide total 3He in atoms/gram, but there is no mention of any “initial” or “inherited” 3He in the Ferrar dolerite. It is an unstated assumption that inherited 3He is insignificant, which is probably true near the surface for these old samples, but maybe not at depth. At least mention that this is an assumption, which cannot necessarily be made in all cases (i.e. basaltic pyroxenes), and could be misleading to some readers. What is the initial/inherited 3He/4He of the Ferrar dolerite pyroxenes? Does the core data provide constraints on this?
In section 3.2.3. it is mentioned that the pH 8 precipitation reduces cation load, which seems like a good idea, but is there no loss of Be or Al? Was this experimentally verified?
Other suggestions:
Line 56: I recommend that you cite cite Kurz (1986) and Blard (2021), old and new, in mentioning that 3He is widely used, including in olivine and pyroxene (no citation given here).
Line 68: the paper should also cite Brown et al. (1991) for HF leaching to remove meteoric 10Be, in addition to the paper cited here. Since this paper discusses HF leaching for pyroxenes, it should at least cite the first paper to describe HF leaching (for quartz).
Line 88: the paper should cite Goehring et al. (2010) for 3He production rates, in addition to Borchers et al.
Line 167-168. Are the samples sonicated or shaken for 5-6 hours? Be specific.
Line 196. Give units of atoms/gram here.
Line 445 and 449. Replace the word “advecting” here with “exposed” or similar. The rock is not advecting, it is exposed by erosion.
Table 4. In the table title add “at the Labyrinth core” or something to be more specific.
Line 525. Add “clino” to pyroxene here and elsewhere?
Line 539 add Sea Level High Latitude to the 3.6 at/g/yr production rate, for clarity.
Figure 7 caption: add “Ferrar dolerite” and “clino” before pyroxene to be more specific.
Citation: https://doi.org/10.5194/egusphere-2022-1379-RC2 -
AC2: 'Reply on RC2', Allie Balter-Kennedy, 02 May 2023
RC2
We thank Mark Kurz for his thorough review and constructive comments that have helped improve this manuscript.
Below, we address referee comments and describe additional, unsolicited changes that we’ve made to improve the manuscript. Referee comments are supplied in bold, with our responses in regular text.
Main scientific comments:
The geologic setting of the core. Since the Labyrinth dolerite core has never been described (is that correct?), I recommend adding more information, either in the text or supplement. The paper cites Lewis for basic background and gives a brief summary of the geologic setting, but only from a fairly simple glacial history perspective. It never mentions that the Ferrar dolerite is Jurassic (~180Ma), igneous intrusive, and does not describe the mineralogy of the outcrop. Is it typical of the Ferrar, which has variable mineralogy? Are the minerals analyzed clinopyroxenes, as I assume (are they augite?). Is the mineralogy homogeneous within the core? The text mentions that the average density is 2.94 g/cc, but how variable is the density within the core? That could provide some uncertainty for the depth profile, if it varies. This is all fairly basic information that could be easily added (if there is no paper to cite).
That is correct, the Labyrinth core has not been described elsewhere and this is a great suggestion to add more information about the geologic setting. As a further description of the Ferrar dolerite, we will add a sentence near Line 105 that states, “Local basement rock and overlying sedimentary rocks are intruded by Jurassic Ferrar dolerite sills (~180 Ma; Burgess et al., 2015; McKelvey & Webb, 1962).”
As the reviewer notes, Ferrar dolerite has variable mineralogy, and we agree it is important to highlight the particular mineralogy of the bedrock core. The core was collected from a fairly “typical” outcrop of fine-to-medium grained Ferrar dolerite with ~50% each pyroxene and plagioclase. In response to the reviewer’s suggestion, we will add a sentence on lines 159-160 explaining this: “The rock type is fine-to-medium-grained Ferrar dolerite, with roughly equal parts pyroxene and plagioclase.”
This study did not entail detailed investigation of pyroxene compositions, although the ICP-OES data (Tables S3 and S4 in original submission) reveal that pyroxene compositions were fairly consistent in the surface samples and throughout the Labyrinth core. As the reviewer suggests, the pyroxenes analyzed appear to be clinopyroxenes (augite). Therefore, we will add a sentence the that reads: “Based on ICP-OES data (Tables S1 and S2), analyzed pyroxene separates were primarily clinopyroxene (augite).”
Finally, in response to this comment, we will also add more detail about the rock density measurements: “The core was split into sections at the University of Washington and measured for rock density. Four rock density measurements from 0.5–1.5 m depth in the core gave consistent values from 2.93 ± 0.02 g cm-3 to 2.96 ± 0.02 g cm-3, averaging 2.94 ± 0.03 g cm-3.”
Depth Dependence. The calculation of muon production rates depends strongly on the assumed spallation attenuation length. The paper assumes 140 g/cm2 and cites Gosse and Phillips (2001) for this value, but this is not explained or justified. I looked at Gosse and Phillips, and could not see any justification for this attenuation length, for either 3He and 10Be, in Antarctica or elsewhere. In addition, the 3He and 10Be depth data, in figures 3 and 4, all seem to fall to the right of the 140 g/cm2 lines (and note that this is a log scale). At the shallowest depths, the production should be dominated by spallation, so it looks like a longer attenuation length might be more appropriate for both 3He and 10Be. For example, all the data below 20 cm depth. in figure 3, deviate from the line. Could the data be fit to a longer attenuation length? Gosse and Phillips present a table that summarizes the experimentally determined values, and they are highly variable, and as mentioned above the use of 140 g/cm2 is not clear. How uncertain is this value and how would it impact the muon production rate estimates? There is an existing depth profile for 10Be in quartz which yielded 145 g/cm2, probably from a similar altitude (Brown et al., 1992), which should be cited in my opinion). It is also conceivable that the shallow data do not fit the curve because the core area had a complex exposure/erosion history. A complex exposure history at the core site (which seems likely to me) could have an impact on the muon production rate estimates, i.e. long exposure at depth. This should be discussed.
The reference to Gosse and Phillips here was misleading and we will remove it. The 140 g/cm2 value is from fits to 10Be, 26Al, and 21Nedata from the nearby, although slightly higher elevation, Beacon Heights core. This is documented in Balco et al. (2019) for 21Ne and in the supplement of Borchers et al. (2016) for 10Be and 26Al, although it is actually quite deep in the supplement and not immediately evident. We will change the citations here to reflect this. The sentence on lines 467-469 will now read: “The fitted value for is close to other estimates of the spallation attenuation length in Antarctica (~140 g cm-2; Borchers et al. 2016; Balco et al., 2019; Brown et al., 1992), although the attenuation length is expected to vary slightly for different nuclides and lithologies (Argento et al., 2015).”
As the reviewer states, the calculation of muon production rates depends in part on the spallation attenuation length. Therefore, when we estimated values for parameters associated with muon production of 3He and 10Be, we did not assume an attenuation length for spallation, rather left this as a free parameter in our model. As shown in Figures 5 and 6 and discussed in Lines 467-469 and 484-485 of the text, we found a spallation attenuation length of 142-144 g cm-2 through our fitting exercises. In response to this comment, we will also add the best-fitting spallation attenuation length for each end member scenario to Table 4.
The the plotted spallation curves on Figures 3 and 4, which are calculated using an attenuation length of 140 g/cm2, are meant to contextualize the 10Be and 3He measurements for the reader. Because these figures are presented earlier in the paper than are the fitting exercises, we employ the attenuation length of 140 g/cm2 for reasons described above. We emphasize that the plotted lines are not meant to be quantitative estimates of 10Be or 3He production either by spallation or muons, rather to demonstrate that the measured 3He and 10Be concentrations likely cannot be explained by spallation alone, motivating us to fit the parameters associated with production by negative muon capture later in the paper.
In response to this aspect of the reviewer’s comment, we will make the purpose of the plotted spallation curves on Figures 3 and 4 more clear. We will edit a sentence in the Figure 3 caption to read: “The solid black line is an exponential curve showing expected spallation-produced 3He concentrations calculated using the surface 3He concentration and an attenuation length of 140 g cm-2.” A similar sentence in the Figure 4 caption will read: “The black line is an exponential curve showing the expected concentration of spallation-produced 10Be, calculated using the measured 10Be concentration at 24 cm depth and an attenuation length of 140 g cm-2.”
We did use an attenuation length of 140 g cm-2 in the spallation production rate estimation and to make the two-nuclide diagram (Figure 8). In response to this comment, we performed both tasks with an attenuation length of 145 g cm-2 and found that there is no discernable change to the presented spallation production rate or the results from the two-nuclide diagram.
Finally, we agree with the reviewer that the coring site may have experienced a complex exposure/erosion history. Because of this, we explored two endmember scenarios in our fitting exercise (zero erosion and steady erosion) that should capture the range of possible production rates given what is known about the exposure history at the coring site. As shown in Table 4, Figures 5 and 6 and Lines 471-474-540 and 487-493, there is a difference between the best-fitting production rate parameters under each end member assumption, which is consistent with the expectations the reviewer lays out in this comment. On lines 557-560 of the submitted manuscript, we summarize the results of the model fitting exercise by stating, “The fitted values for P10,sp,SLHL, and for the steady-erosion endmember are closer to the expected values than for the zero-erosion endmember, and there is no geomorphic evidence against steady erosion taking place at the Labyrinth core site. Therefore, going forward, we assume steady erosion for the Labyrinth core and use the muon cross-sections derived under this assumption for calculating muon production rates.”
In response to the reviewer’s comment, we have reviewed the manuscript again and believe our decisions regarding the implemented exposure histories are well justified on Lines 431-455 and that the resulting range of production-rate parameters is thoroughly explored. Therefore, we do not plan to make any significant updates in the manuscript in response to the last point in this comment.
Analytical Information. Given that this paper has a methodological emphasis, there should be more complete analytical details. For example, in section 3.2.2. describing the helium measurements, the LDEO and BGC blanks are not given (they are given for CRPG), and there is no information given on sample sizes (this is not tabulated), or on the size of the primary standards used, or on the reproducibility of primary standards. The running standards are mentioned for LDEO and CRPG but not BGC. I recommend providing full information, somewhere, for all the labs, rather than make the reader try to track it down. It is also unclear how the standards can be used to obtain absolute 3He abundances, i.e. are the Yellowstone and Matsuda standards compared to air?
First, for conciseness in this paper, we intentionally referenced some of the methods to other papers in which they were already described. For example, methods for 3He analysis at BGC, including standardization and QC information, are exactly as described in detail in Balter-Kennedy et al. (2020). Here we defer to the editors for guidance on whether we should copy that material into this paper, or leave it as is.
Second, we did omit some of the material requested here, which we will add in the revision. Specifically, we will add sample weights for 3He analysis to Table 3 in the main text and add supplementary table to detail the analytical information for the CRONUS-P measurements at each lab. We will also provide more complete information about process blanks in Section 3.2.2 of the main text.
The tables provide total 3He in atoms/gram, but there is no mention of any “initial” or “inherited” 3He in the Ferrar dolerite. It is an unstated assumption that inherited 3He is insignificant, which is probably true near the surface for these old samples, but maybe not at depth. At least mention that this is an assumption, which cannot necessarily be made in all cases (i.e. basaltic pyroxenes), and could be misleading to some readers. What is the initial/inherited 3He/4He of the Ferrar dolerite pyroxenes? Does the core data provide constraints on this?
This is an important point that “inherited/initial” 3He is present in Ferrar dolerite and not only warrants acknowledgement but is likely important at depth in the bedrock core. On Lines 361-365 of the submitted manuscript, we discuss the presence of (3.3 ± 1.1) x 106 atoms g-1 of non-cosmogenic 3He in Ferrar pyroxenes (see Balco (2020) blog post at https://cosmognosis.wordpress.com/2020/08/22/noncosmogenic-helium-3-in-pyroxene-and-antarctic-exposure-dating/). The non-cosmogenic 3He constitutes <1% of total 3He in the surface samples, but as much as 10% of measured 3He at depth. For this reason, we have included non-cosmogenic 3He in our forward model, as shown in Equations 1 and 10. For completeness, non-cosmogenic 3He is also included in the spallation production rate fitting exercise for the surface samples in Section 5.1.2. In revisiting the manuscript in response to this comment, we realized that we had left out non-cosmogenic 3He in Equation 13 in the manuscript, although it was included in our original calculations. We will update Equation 13 accordingly.
We recognize that this topic was somewhat buried in the modeling section, and in response to this comment will move most of this text to the Methods section so that it is clearer for the reader. Specifically, Section 3.3 will have a paragraph that reads: “Ferrar dolerite pyroxenes are known to contain non-cosmogenic 3He, which is most likely produced by neutron capture on Li via the reaction 6Li(n, 𝛼)3He (Ackert & Kurz, 2004; Andrews & Kay, 1982). In Ferrar Dolerite, the total concentration of non-cosmogenic 3He has been measured in samples that are shielded from the cosmic-ray flux (Ackert, 2000; Kaplan et al., 2017; Margerison et al., 2005). Together, these measurements converge on (3.3 ± 1.1) x 106 atom g-1 of non-cosmogenic 3He throughout this lithology (see discussion in Balco, 2020). Non-cosmogenic 3He therefore constitutes <1% of the measured 3He in the surface samples, so we do not consider non-cosmogenic 3He significant when calculating apparent exposure ages or erosion rates. At depth in the Labyrinth core, however, non-cosmogenic 3He comprises as much as 10% of the measured 3He. Therefore, we do include non-cosmogenic 3He when performing production rate calculations in Sects. 5.1.1 and 5.1.2.“
To avoid redundancy later in the paper, we will simplify the description of non-cosmogenic 3He in the model set-up in section 5.1.1. to read: (atoms g-1) is non-cosmogenic 3He, which we consider to be (3.3 ± 1.1) x 106 atom g-1 in Ferrar pyroxenes (see Sect. 3.3 and discussion in Balco, 2020).”
In section 3.2.3. it is mentioned that the pH 8 precipitation reduces cation load, which seems like a good idea, but is there no loss of Be or Al? Was this experimentally verified?
We did not experimentally verify that there was no loss of Be in this step. However, given that Be forms an insoluble hydroxide at pH 8, we have no reason to believe that Be was lost during this additional precipitation step. We note for other readers that loss of Be in this step would not affect the measurement because 10Be should have been isotopically equilibrated with the carrier during initial dissolution.
Other suggestions:
Line 56: I recommend that you cite cite Kurz (1986) and Blard (2021), old and new, in mentioning that 3He is widely used, including in olivine and pyroxene (no citation given here).
Great suggestion, we will include these citations.
Line 68: the paper should also cite Brown et al. (1991) for HF leaching to remove meteoric 10Be, in addition to the paper cited here. Since this paper discusses HF leaching for pyroxenes, it should at least cite the first paper to describe HF leaching (for quartz).
We will include this citation.
Line 88: the paper should cite Goehring et al. (2010) for 3He production rates, in addition to Borchers et al.
We will add this reference.
Line 167-168. Are the samples sonicated or shaken for 5-6 hours? Be specific.
We will update this to include that the leaching took place on a shaker table.
Line 196. Give units of atoms/gram here.
Great catch, we will add the units.
Line 445 and 449. Replace the word “advecting” here with “exposed” or similar. The rock is not advecting, it is exposed by erosion.
Good point. We will update these sentences to replace the word advecting.
Table 4. In the table title add “at the Labyrinth core” or something to be more specific.
We will add this to the table title.
Line 525. Add “clino” to pyroxene here and elsewhere?
As discussed above, we did not do a detailed mineralogical study so prefer to refrain from adding further specifics throughout the manuscript. In response to the comment above, however, we will add that most of the pyroxenes analyzed were likely clinopyroxene (specifically, augite), which we feel provides specifics for interested readers.
Line 539 add Sea Level High Latitude to the 3.6 at/g/yr production rate, for clarity.
We will update this to read: “reference production rate of 3.6 ± 0.8 atoms g-1 yr-1 at sea level high latitude”.
Figure 7 caption: add “Ferrar dolerite” and “clino” before pyroxene to be more specific.
We will add “Ferrar dolerite” to the figure caption in response to this comment. Please see comment above about L525 regarding the addition of “clino”.
Citation: https://doi.org/10.5194/egusphere-2022-1379-AC2
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AC2: 'Reply on RC2', Allie Balter-Kennedy, 02 May 2023
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-1379', Shaun Eaves, 10 Feb 2023
General comments
This manuscript contains a multi-faceted study concerning the systematics and practical application of in situ cosmogenic 10Be and 3He production in pyroxene. The work presented makes several important advances, notably (i) advancing laboratory protocols for beryllium purification in pyroxene; (ii) confirming and refining previous spallation production estimates; and (iii) identifying and quantifying components of muonic production of both nuclides.
The study is clearly and concisely communicated, both in written and graphical forms, which is no mean feat given the multiple threads of research undertaken. I have only minor suggestions and a few typographical observations summarised below. Overall, the work presented here makes a significant contribution to the geochronological literature that I am sure will be influential for future applications of these geochronometers.
Specific comments
Quantitative assessment of the contaminant load against ion exchange resin capability is critical to effective beryllium isolation in mafic minerals (as well reasoned in lines 61-66). I would appreciate a fuller and more quantitative description of the decision making concerning measured contaminant loads (e.g. Table S3) and resin volumes within the paragraph covering lines 245-256. I am specifically interested in this matter given the described need to reprocess some samples based on visual assessment of Be(OH)2 purity via comparison of precipitates with the process blank. Were you ‘flying too close to the sun’ as it were, with cation loads for these columns? And if so, should the methods be more conservative? Given the specific focus of this manuscript on methodological refinement, it would be valuable for readers if the decision making was more quantitative and explicitly described. If you could also provide more precise information concerning the tolerance thresholds for the visual inspection of Be(OH)2 precipitates (beyond the rather vague ‘noticeably larger’ vs ‘similar in size’), this would also be a valuable addition. Ideally this would be photographic, but I appreciate that might not be possible after the fact.
There is immense (and rare) value in having the same samples both prepared and measured by multiple laboratories. In this regard, the differences in pyroxene purity identified in lines 287-297 are of fundamental importance. However the implications for mineral separation methodologies are left subject to reader interpretation. Could you add further detail as to what you think are the key causes of these differences and what therefore are the implications for best practice mineral separation?
The final paragraph of the discussion concerning the limits of this method is valuable and necessary, but at present it is restricted to specific end-member cases chosen by the authors. I wonder if there is a more effective way of communicating the wider applicability of this method, perhaps in graphical form, for non-cosmogenic experts that wish to know if this nuclide-mineral pair is suitable for their particular application. If you choose to revise this component, I would also suggest that you allow for higher process blanks – most laboratories are working with 10Be backgrounds several times higher!
Technical comments
L19 – check grammar ‘…of the 10Be-3He in mafic rock’. Remove ‘the’?
L29-33 – the low background is impressive but perhaps a more useful summary for the abstract would be what you think could be routine, rather than what are the limits of possibility. See also, final specific comment above.
L54 – awkward sentence construction, coupled with unusual use of ‘however’ in first sentence of a paragraph. Consider rewording to “Fewer cosmogenic nuclides are routinely measured in lithologies where quartz is absent.”
L64 isolating not isolated?
L104 – remove ‘ice-free’ – while there is far less ice than the immediate surroundings, ice-free is incorrect.
L182-3 – were (not ‘was’) similar – procedures is plural.
L209 – ‘home-design’ could be changed to custom-made
L239 – ‘very low 10Be’…abundance? Also – can you quantify here?
L337 – ‘first direct measurements of muon-produced 10Be in pyroxene’ – this makes it sound like you isolated the muonic component. Reword to something more accurate, e.g. first identification of in situ 10Be production in pyroxene by muons
L357 – remove ‘some’, it’s unneccessary
Table 1 – perhaps ‘Sample type’ for the core should read ‘core top’?
Table 2 – is superscript point #5 necessary?
Citation: https://doi.org/10.5194/egusphere-2022-1379-RC1 -
AC1: 'Reply on RC1', Allie Balter-Kennedy, 02 May 2023
RC1
We thank Shaun Eaves for his constructive review of the manuscript and his insightful comments.
Below, we address referee comments and describe additional, unsolicited changes that we’ve made to improve the manuscript. Referee comments are supplied in bold, with our responses in regular text.
Specific Comments
Quantitative assessment of the contaminant load against ion exchange resin capability is critical to effective beryllium isolation in mafic minerals (as well reasoned in lines 61-66). I would appreciate a fuller and more quantitative description of the decision making concerning measured contaminant loads (e.g. Table S3) and resin volumes within the paragraph covering lines 245-256. I am specifically interested in this matter given the described need to reprocess some samples based on visual assessment of Be(OH)2 purity via comparison of precipitates with the process blank. Were you ‘flying too close to the sun’ as it were, with cation loads for these columns? And if so, should the methods be more conservative? Given the specific focus of this manuscript on methodological refinement, it would be valuable for readers if the decision making was more quantitative and explicitly described. If you could also provide more precise information concerning the tolerance thresholds for the visual inspection of Be(OH)2 precipitates (beyond the rather vague ‘noticeably larger’ vs ‘similar in size’), this would also be a valuable addition. Ideally this would be photographic, but I appreciate that might not be possible after the fact.
We certainly understand the desire to include a more quantitative assessment of the cation exchange capacity. We based our decision-making concerning contaminant loads on the knowledge that the pH 8 precipitation should reduce or even completely remove Mg and Ca from our samples (Ochs and Ivy-Ochs, 1997), leaving Al as the primary contaminant removed during cation columns. Because the total Al in our samples was low (Table S3 in original submission), we opted to use 2 mL of cation exchange resin to isolate beryllium, following standard procedures in our lab (see references in manuscript). It is likely, however, that some Mg and/or Ca remained in the sample, "overloading" the column, possibly allowing Al to leak off with Be as happens with a large cation load, or perhaps Mg, which elutes immediately following Be, began eluting early. Nevertheless, a second cation column resulted in a final beryllium hydroxide precipitate of similar size to the blank.
In response to the reviewer’s comment, we will update some of the wording in Section 3.2.3 to make our decision making here clearer to the reader. We will also a photo of the Be(OH)2 precipitates in the supplement so that the reader can see the difference in precipitate size for the overloaded columns.
Lines 234-237 will read: “Here, we add a simple pH 8 precipitation step to reduce the cation load in our samples prior to ion exchange chromatography. At pH 8, Be, Al and Fe precipitate from solution as hydroxides, Be(OH)2, Al(OH)3 and Fe(OH)2, while Ca and Mg should remain in solution (Ochs and Ivy-Ochs, 1997).”
Lines 245-256 will read: “We isolated beryllium using ion chromatography methods described by Kohl & Nishiizumi (1992). Given that most Ca and Mg were likely removed in the pH 8 precipitation step, we opted to use 2 mL of BioRad-50W X8 200–400# mesh resin for cation exchange based on the amount of Al in our samples (Table S1). Following cation exchange columns, the beryllium fraction was evaporated to dryness and then taken up in 4 mL of 1% HNO3, transferred to 15 mL centrifuge tubes. From this solution, we precipitated Be(OH)2 by adjusting the pH to 9 with NH4OH. After pouring off the suprenate, this precipitation step was repeated. We then performed three rinses of the precipitate with milli-Q water adjusted to pH 8. The Be(OH)2 precipitates were noticeably larger than the blank for several samples (Fig. S3), indicating that we exceeded the capacity of the cation exchange resin and Al or another cation eluted with beryllium . For these samples, we performed a second round of cation exchange chromatography, also with 2 mL of resin. Following the second round of cation columns, the Be(OH)2 precipitates for all samples were similar in size to the blank, indicating that the second column step was successful in isolating beryllium.”
There is immense (and rare) value in having the same samples both prepared and measured by multiple laboratories. In this regard, the differences in pyroxene purity identified in lines 287-297 are of fundamental importance. However the implications for mineral separation methodologies are left subject to reader interpretation. Could you add further detail as to what you think are the key causes of these differences and what therefore are the implications for best practice mineral separation?
We will make a few changes to clarify the implications of mineral separation methods for the reader. First, we will re-word the sentence on Lines 287-290 to provide more definitive support for the presence of plagioclase: “Higher-than-expected Al concentrations measured by ICP-OES in the CRPG-prepared pyroxene suggest that plagioclase remained in the CRPG-prepared samples (Tables S1 and S2).” [see unsolicited changes section – the Supplementary table numbers will be updated to reflect their order in the updated text]
We will also add a sentence near Line 294 of the submitted manuscript that reads “The observation that plagioclase-contaminated pyroxene samples yielded lower 3He concentrations further supports the use of HF etching as an effective method for producing pure pyroxene separates, as plagioclase is readily dissolved in HF (Bromley et al., 2014).” This sentence summarizes the best practice for pyroxene mineral separation based on our results. We note that the full procedure for HF leaching of pyroxenes is described in Bromley et al. (2014), and to avoid redundancy with that paper we do not go into further detail here.
The final paragraph of the discussion concerning the limits of this method is valuable and necessary, but at present it is restricted to specific end-member cases chosen by the authors. I wonder if there is a more effective way of communicating the wider applicability of this method, perhaps in graphical form, for non-cosmogenic experts that wish to know if this nuclide-mineral pair is suitable for their particular application. If you choose to revise this component, I would also suggest that you allow for higher process blanks – most laboratories are working with 10Be backgrounds several times higher!
It is true that our discussion of the limits of this method are based on the results from our own lab, where process blanks contain relatively low abundances of 10Be and appreciate the reviewer’s suggestion that we therefore widen the breadth of this discussion. We think it most appropriate to base this discussion on the dataset presented in this paper, as users in other laboratories will need to assess the limits of this method given their own process blank data, which varies from lab to lab. To aid in that exploration, we have moved the former Figure S3, which provides the reader with a comparison of number of atoms measured to relative uncertainty (which comes mostly from measurement uncertainty although does contain blank uncertainty), from the Supplemental Information to the main text. Readers will then be able to determine what applications are feasible in their own lab given their process blank values. We have also added a parenthetical statement on Line 580 that reads “other users will need to evaluate the limits of these methods for exposure dating given the average blank values from their own laboratories.”
Technical comments
L19 – check grammar ‘…of the 10Be-3He in mafic rock’. Remove ‘the’?
Good catch, we will fix this.
L29-33 – the low background is impressive but perhaps a more useful summary for the abstract would be what you think could be routine, rather than what are the limits of possibility. See also, final specific comment above.
While we agree that the backgrounds presented here are lower than in many other laboratories, we note that these values are unique to each laboratory and therefore can only speak to the blank values in our own lab. In addition, the blanks should be similar to those processed alongside quartz samples, and even might be smaller because of the lower reagent volumes used for the small pyroxene sample sizes. We hope that the addition of Figure 9 to the main text will help readers evaluate the limits of this method in their own laboratories.
L54 – awkward sentence construction, coupled with unusual use of ‘however’ in first sentence of a paragraph. Consider rewording to “Fewer cosmogenic nuclides are routinely measured in lithologies where quartz is absent.”
We will make this change.
L64 isolating not isolated?
We will update this.
L104 – remove ‘ice-free’ – while there is far less ice than the immediate surroundings, ice-free is incorrect.
Good point. We will remove “ice-free”.
L182-3 – were (not ‘was’) similar – procedures is plural.
We will update this.
L209 – ‘home-design’ could be changed to custom-made
We will change this to custom-made.
L239 – ‘very low 10Be’…abundance? Also – can you quantify here?
We acknowledge that it’s possible that the LDEO carrier contains virtually zero 10Be, but at the very least it must be lower than the process blanks, which average 5.7 x 103 10Be atoms.
In response to this comment, we will update this sentence to read “For beryllium extraction from pure pyroxene separates we first weighed and spiked 100-200 mg of pyroxene with ~180 µg of 9Be using LDEO carrier (Schaefer et al., 2009). The addition of 9Be carrier contributed less than 5.7 x 103 atoms of 10Be, which is the average value of our process blanks.”
L337 – ‘first direct measurements of muon-produced 10Be in pyroxene’ – this makes it sound like you isolated the muonic component. Reword to something more accurate, e.g. first identification of in situ 10Be production in pyroxene by muons
Agreed! We will update the sentence to read “we present the first identification of in situ 10Be production by muons, and further confirm the importance of quantifying muon production of 3He”
L357 – remove ‘some’, it’s unneccessary
We will remove this.
Table 1 – perhaps ‘Sample type’ for the core should read ‘core top’?
We will make this change.
Table 2 – is superscript point #5 necessary?
It’s probably not necessary, good point. We will remove superscript #5.
Additional unsolicited changes to the manuscript
Main Text
- L30 – In the original submission file, we erroneously listed the average blank value as 5.7 x 104 atoms in the abstract, rather than the true value of 5.7 x 103 We will update this to 5.7 x 103 atoms and note that it was already correct elsewhere in the manuscript.
- L45 – will update “serve” to “serves” to be consistent with the sentence subject, “concentration”.
- L50 – will add hyphen to “103–106 year” timescales so that it reads “103–106-year timescales”.
- L61-66 – we will update the description of pyroxene compositions to be more consistent with the cited reference. We will also update the next sentence to better reflect the challenges related to isolating beryllium from pyroxene. These sentences will now read: “First, the mineral composition of pyroxene [XYSi2O6, where X and Y are both divalent cations (primarily Ca, Fe, or Mg), or X is a monovalent cation (Na, Li) and Y is a trivalent cation (Al, Fe); Nespolo, 2020)] is highly variable. In contrast to quartz (SiO2), the high cation quantities in pyroxenes present a significant challenge for isolating beryllium using ion chromatography, limiting the feasible sample size.”
- L138 – Will remove “the Dais” from sentence describing the bedrock core location, as the core was not collected from the Dais itself, rather a different (unnamed) erosional surface of the Labyrinth.
- L163 – will remove extra “the”
- L271 – The 3He production rate stated in the original submission was outdated. We will update this to read 120 atoms g-1 yr-1, not 124 atoms g-1 yr-1.
- L240 - For clarity, we will update sentence from “The small sample sizes were sufficient for these samples with high cosmogenic nuclide inventories and minimized the overall ion load” to “The small sample sizes were sufficient for these samples with high cosmogenic nuclide inventories while minimizing the overall ion load”.
- L248 – Will remove “and diluted with 10 mL of milli-Q water” as this shouldn’t have been included here.
- L375 – Will add a definition to first instance of “SLHL”: “sea level high latitude (SLHL)”
- L406, L 408 – Will fix erroneous equation references
- In some places, “dolerite”, in “Ferrar dolerite” was capitalized, but should not have been. We will remove capitalization where necessary.
Tables
- Table 2 – will add reference to Schaefer et al. (2006), which is where the 3He measurements for the surface samples were originally reported. This is already discussed in the text but will be added to the table for clarity and completeness.
- Will update supplementary table numbers to be consistent with the order they are now presented in text.
Equations
- Equation 15 – Will add Equation number as it was missing
- Equation 13 – The variable “Sthick” was not defined in the text. We will add a definition near Line 505 of the submitted manuscript that reads, “Where is the sample thickness correction (dimensionless)”.
Citation: https://doi.org/10.5194/egusphere-2022-1379-AC1
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AC1: 'Reply on RC1', Allie Balter-Kennedy, 02 May 2023
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RC2: 'Comment on egusphere-2022-1379', Mark Kurz, 04 Apr 2023
General Comments:
This is an excellent paper that definitely should be published in this journal. It will be of great interest to specialists and to new users of cosmogenic nuclide methods. It presents a methodological study of 10Be in pyroxenes, describing a leaching method to remove meteoric 10Be, which is new and important practical information. It also presents new and important data sets: 3He and 10Be data from pyroxenes in a number of surficial Antarctic samples, and as a function of depth in an Antarctic dolerite core. The depth profile is particularly useful because it yields new information on cosmogenic 3He and 10Be production rates and mechanisms, including muon production rates, which are very important.
The paper compares results from several different laboratories, on some of the same samples, which is rare, but attests to the high quality of the data presented here. This is a large and valuable analytical effort. The paper is forthcoming in discussing (some of the) uncertainties, such as core depth issues related to drilling, and problems with sample preparation and removal of plagioclase. Overall, this is a really impressive paper. I congratulate Balter-Kennedy and co-authors.
I do have some suggestions outlined below, many of which are optional, and all of which should be relatively easy to address. I think the authors should include more discussion of the uncertainties wherever possible, as outlined below, particularly with respect to the geologic setting of the core, the depth dependence, and analytical methods.
Main scientiific comments:
The geologic setting of the core. Since the Labyrinth dolerite core has never been described (is that correct?), I recommend adding more information, either in the text or supplement. The paper cites Lewis for basic background and gives a brief summary of the geologic setting, but only from a fairly simple glacial history perspective. It never mentions that the Ferrar dolerite is Jurassic (~180Ma), igneous intrusive, and does not describe the mineralogy of the outcrop. Is it typical of the Ferrar, which has variable mineralogy? Are the minerals analyzed clinopyroxenes, as I assume (are they augite?). Is the mineralogy homogeneous within the core? The text mentions that the average density is 2.94 g/cc, but how variable is the density within the core? That could provide some uncertainty for the depth profile, if it varies. This is all fairly basic information that could be easily added (if there is no paper to cite).
Depth dependence. The calculation of muon production rates depends strongly on the assumed spallation attenuation length. The paper assumes 140 g/cm2 and cites Gosse and Phillips (2001) for this value, but this is not explained or justified. I looked at Gosse and Phillips, and could not see any justification for this attenuation length, for either 3He and 10Be, in Antarctica or elsewhere. In addition, the 3He and 10Be depth data, in figures 3 and 4, all seem to fall to the right of the 140 g/cm2 lines (and note that this is a log scale). At the shallowest depths, the production should be dominated by spallation, so it looks like a longer attenuation length might be more appropriate for both 3He and 10Be. For example, all the data below 20 cm depth. in figure 3, deviate from the line. Could the data be fit to a longer attenuation length? Gosse and Phillips present a table that summarizes the experimentally determined values, and they are highly variable, and as mentioned above the use of 140 g/cm2 is not clear. How uncertain is this value and how would it impact the muon production rate estimates? There is an existing depth profile for 10Be in quartz which yielded 145 g/cm2, probably from a similar altitude (Brown et al., 1992), which should be cited in my opinion).
It is also conceivable that the shallow data do not fit the curve because the core area had a complex exposure/erosion history. A complex exposure history at the core site (which seems likely to me) could have an impact on the muon production rate estimates, i.e. long exposure at depth. This should be discussed.
Analytical information. Given that this paper has a methodological emphasis, there should be more complete analytical details. For example, in section 3.2.2. describing the helium measurements, the LDEO and BGC blanks are not given (they are given for CRPG), and there is no information given on sample sizes (this is not tabulated), or on the size of the primary standards used, or on the reproducibility of primary standards. The running standards are mentioned for LDEO and CRPG but not BGC. I recommend providing full information, somewhere, for all the labs, rather than make the reader try to track it down. It is also unclear how the standards can be used to obtain absolute 3He abundances, i.e. are the Yellowstone and Matsuda standards compared to air?
The tables provide total 3He in atoms/gram, but there is no mention of any “initial” or “inherited” 3He in the Ferrar dolerite. It is an unstated assumption that inherited 3He is insignificant, which is probably true near the surface for these old samples, but maybe not at depth. At least mention that this is an assumption, which cannot necessarily be made in all cases (i.e. basaltic pyroxenes), and could be misleading to some readers. What is the initial/inherited 3He/4He of the Ferrar dolerite pyroxenes? Does the core data provide constraints on this?
In section 3.2.3. it is mentioned that the pH 8 precipitation reduces cation load, which seems like a good idea, but is there no loss of Be or Al? Was this experimentally verified?
Other suggestions:
Line 56: I recommend that you cite cite Kurz (1986) and Blard (2021), old and new, in mentioning that 3He is widely used, including in olivine and pyroxene (no citation given here).
Line 68: the paper should also cite Brown et al. (1991) for HF leaching to remove meteoric 10Be, in addition to the paper cited here. Since this paper discusses HF leaching for pyroxenes, it should at least cite the first paper to describe HF leaching (for quartz).
Line 88: the paper should cite Goehring et al. (2010) for 3He production rates, in addition to Borchers et al.
Line 167-168. Are the samples sonicated or shaken for 5-6 hours? Be specific.
Line 196. Give units of atoms/gram here.
Line 445 and 449. Replace the word “advecting” here with “exposed” or similar. The rock is not advecting, it is exposed by erosion.
Table 4. In the table title add “at the Labyrinth core” or something to be more specific.
Line 525. Add “clino” to pyroxene here and elsewhere?
Line 539 add Sea Level High Latitude to the 3.6 at/g/yr production rate, for clarity.
Figure 7 caption: add “Ferrar dolerite” and “clino” before pyroxene to be more specific.
Citation: https://doi.org/10.5194/egusphere-2022-1379-RC2 -
AC2: 'Reply on RC2', Allie Balter-Kennedy, 02 May 2023
RC2
We thank Mark Kurz for his thorough review and constructive comments that have helped improve this manuscript.
Below, we address referee comments and describe additional, unsolicited changes that we’ve made to improve the manuscript. Referee comments are supplied in bold, with our responses in regular text.
Main scientific comments:
The geologic setting of the core. Since the Labyrinth dolerite core has never been described (is that correct?), I recommend adding more information, either in the text or supplement. The paper cites Lewis for basic background and gives a brief summary of the geologic setting, but only from a fairly simple glacial history perspective. It never mentions that the Ferrar dolerite is Jurassic (~180Ma), igneous intrusive, and does not describe the mineralogy of the outcrop. Is it typical of the Ferrar, which has variable mineralogy? Are the minerals analyzed clinopyroxenes, as I assume (are they augite?). Is the mineralogy homogeneous within the core? The text mentions that the average density is 2.94 g/cc, but how variable is the density within the core? That could provide some uncertainty for the depth profile, if it varies. This is all fairly basic information that could be easily added (if there is no paper to cite).
That is correct, the Labyrinth core has not been described elsewhere and this is a great suggestion to add more information about the geologic setting. As a further description of the Ferrar dolerite, we will add a sentence near Line 105 that states, “Local basement rock and overlying sedimentary rocks are intruded by Jurassic Ferrar dolerite sills (~180 Ma; Burgess et al., 2015; McKelvey & Webb, 1962).”
As the reviewer notes, Ferrar dolerite has variable mineralogy, and we agree it is important to highlight the particular mineralogy of the bedrock core. The core was collected from a fairly “typical” outcrop of fine-to-medium grained Ferrar dolerite with ~50% each pyroxene and plagioclase. In response to the reviewer’s suggestion, we will add a sentence on lines 159-160 explaining this: “The rock type is fine-to-medium-grained Ferrar dolerite, with roughly equal parts pyroxene and plagioclase.”
This study did not entail detailed investigation of pyroxene compositions, although the ICP-OES data (Tables S3 and S4 in original submission) reveal that pyroxene compositions were fairly consistent in the surface samples and throughout the Labyrinth core. As the reviewer suggests, the pyroxenes analyzed appear to be clinopyroxenes (augite). Therefore, we will add a sentence the that reads: “Based on ICP-OES data (Tables S1 and S2), analyzed pyroxene separates were primarily clinopyroxene (augite).”
Finally, in response to this comment, we will also add more detail about the rock density measurements: “The core was split into sections at the University of Washington and measured for rock density. Four rock density measurements from 0.5–1.5 m depth in the core gave consistent values from 2.93 ± 0.02 g cm-3 to 2.96 ± 0.02 g cm-3, averaging 2.94 ± 0.03 g cm-3.”
Depth Dependence. The calculation of muon production rates depends strongly on the assumed spallation attenuation length. The paper assumes 140 g/cm2 and cites Gosse and Phillips (2001) for this value, but this is not explained or justified. I looked at Gosse and Phillips, and could not see any justification for this attenuation length, for either 3He and 10Be, in Antarctica or elsewhere. In addition, the 3He and 10Be depth data, in figures 3 and 4, all seem to fall to the right of the 140 g/cm2 lines (and note that this is a log scale). At the shallowest depths, the production should be dominated by spallation, so it looks like a longer attenuation length might be more appropriate for both 3He and 10Be. For example, all the data below 20 cm depth. in figure 3, deviate from the line. Could the data be fit to a longer attenuation length? Gosse and Phillips present a table that summarizes the experimentally determined values, and they are highly variable, and as mentioned above the use of 140 g/cm2 is not clear. How uncertain is this value and how would it impact the muon production rate estimates? There is an existing depth profile for 10Be in quartz which yielded 145 g/cm2, probably from a similar altitude (Brown et al., 1992), which should be cited in my opinion). It is also conceivable that the shallow data do not fit the curve because the core area had a complex exposure/erosion history. A complex exposure history at the core site (which seems likely to me) could have an impact on the muon production rate estimates, i.e. long exposure at depth. This should be discussed.
The reference to Gosse and Phillips here was misleading and we will remove it. The 140 g/cm2 value is from fits to 10Be, 26Al, and 21Nedata from the nearby, although slightly higher elevation, Beacon Heights core. This is documented in Balco et al. (2019) for 21Ne and in the supplement of Borchers et al. (2016) for 10Be and 26Al, although it is actually quite deep in the supplement and not immediately evident. We will change the citations here to reflect this. The sentence on lines 467-469 will now read: “The fitted value for is close to other estimates of the spallation attenuation length in Antarctica (~140 g cm-2; Borchers et al. 2016; Balco et al., 2019; Brown et al., 1992), although the attenuation length is expected to vary slightly for different nuclides and lithologies (Argento et al., 2015).”
As the reviewer states, the calculation of muon production rates depends in part on the spallation attenuation length. Therefore, when we estimated values for parameters associated with muon production of 3He and 10Be, we did not assume an attenuation length for spallation, rather left this as a free parameter in our model. As shown in Figures 5 and 6 and discussed in Lines 467-469 and 484-485 of the text, we found a spallation attenuation length of 142-144 g cm-2 through our fitting exercises. In response to this comment, we will also add the best-fitting spallation attenuation length for each end member scenario to Table 4.
The the plotted spallation curves on Figures 3 and 4, which are calculated using an attenuation length of 140 g/cm2, are meant to contextualize the 10Be and 3He measurements for the reader. Because these figures are presented earlier in the paper than are the fitting exercises, we employ the attenuation length of 140 g/cm2 for reasons described above. We emphasize that the plotted lines are not meant to be quantitative estimates of 10Be or 3He production either by spallation or muons, rather to demonstrate that the measured 3He and 10Be concentrations likely cannot be explained by spallation alone, motivating us to fit the parameters associated with production by negative muon capture later in the paper.
In response to this aspect of the reviewer’s comment, we will make the purpose of the plotted spallation curves on Figures 3 and 4 more clear. We will edit a sentence in the Figure 3 caption to read: “The solid black line is an exponential curve showing expected spallation-produced 3He concentrations calculated using the surface 3He concentration and an attenuation length of 140 g cm-2.” A similar sentence in the Figure 4 caption will read: “The black line is an exponential curve showing the expected concentration of spallation-produced 10Be, calculated using the measured 10Be concentration at 24 cm depth and an attenuation length of 140 g cm-2.”
We did use an attenuation length of 140 g cm-2 in the spallation production rate estimation and to make the two-nuclide diagram (Figure 8). In response to this comment, we performed both tasks with an attenuation length of 145 g cm-2 and found that there is no discernable change to the presented spallation production rate or the results from the two-nuclide diagram.
Finally, we agree with the reviewer that the coring site may have experienced a complex exposure/erosion history. Because of this, we explored two endmember scenarios in our fitting exercise (zero erosion and steady erosion) that should capture the range of possible production rates given what is known about the exposure history at the coring site. As shown in Table 4, Figures 5 and 6 and Lines 471-474-540 and 487-493, there is a difference between the best-fitting production rate parameters under each end member assumption, which is consistent with the expectations the reviewer lays out in this comment. On lines 557-560 of the submitted manuscript, we summarize the results of the model fitting exercise by stating, “The fitted values for P10,sp,SLHL, and for the steady-erosion endmember are closer to the expected values than for the zero-erosion endmember, and there is no geomorphic evidence against steady erosion taking place at the Labyrinth core site. Therefore, going forward, we assume steady erosion for the Labyrinth core and use the muon cross-sections derived under this assumption for calculating muon production rates.”
In response to the reviewer’s comment, we have reviewed the manuscript again and believe our decisions regarding the implemented exposure histories are well justified on Lines 431-455 and that the resulting range of production-rate parameters is thoroughly explored. Therefore, we do not plan to make any significant updates in the manuscript in response to the last point in this comment.
Analytical Information. Given that this paper has a methodological emphasis, there should be more complete analytical details. For example, in section 3.2.2. describing the helium measurements, the LDEO and BGC blanks are not given (they are given for CRPG), and there is no information given on sample sizes (this is not tabulated), or on the size of the primary standards used, or on the reproducibility of primary standards. The running standards are mentioned for LDEO and CRPG but not BGC. I recommend providing full information, somewhere, for all the labs, rather than make the reader try to track it down. It is also unclear how the standards can be used to obtain absolute 3He abundances, i.e. are the Yellowstone and Matsuda standards compared to air?
First, for conciseness in this paper, we intentionally referenced some of the methods to other papers in which they were already described. For example, methods for 3He analysis at BGC, including standardization and QC information, are exactly as described in detail in Balter-Kennedy et al. (2020). Here we defer to the editors for guidance on whether we should copy that material into this paper, or leave it as is.
Second, we did omit some of the material requested here, which we will add in the revision. Specifically, we will add sample weights for 3He analysis to Table 3 in the main text and add supplementary table to detail the analytical information for the CRONUS-P measurements at each lab. We will also provide more complete information about process blanks in Section 3.2.2 of the main text.
The tables provide total 3He in atoms/gram, but there is no mention of any “initial” or “inherited” 3He in the Ferrar dolerite. It is an unstated assumption that inherited 3He is insignificant, which is probably true near the surface for these old samples, but maybe not at depth. At least mention that this is an assumption, which cannot necessarily be made in all cases (i.e. basaltic pyroxenes), and could be misleading to some readers. What is the initial/inherited 3He/4He of the Ferrar dolerite pyroxenes? Does the core data provide constraints on this?
This is an important point that “inherited/initial” 3He is present in Ferrar dolerite and not only warrants acknowledgement but is likely important at depth in the bedrock core. On Lines 361-365 of the submitted manuscript, we discuss the presence of (3.3 ± 1.1) x 106 atoms g-1 of non-cosmogenic 3He in Ferrar pyroxenes (see Balco (2020) blog post at https://cosmognosis.wordpress.com/2020/08/22/noncosmogenic-helium-3-in-pyroxene-and-antarctic-exposure-dating/). The non-cosmogenic 3He constitutes <1% of total 3He in the surface samples, but as much as 10% of measured 3He at depth. For this reason, we have included non-cosmogenic 3He in our forward model, as shown in Equations 1 and 10. For completeness, non-cosmogenic 3He is also included in the spallation production rate fitting exercise for the surface samples in Section 5.1.2. In revisiting the manuscript in response to this comment, we realized that we had left out non-cosmogenic 3He in Equation 13 in the manuscript, although it was included in our original calculations. We will update Equation 13 accordingly.
We recognize that this topic was somewhat buried in the modeling section, and in response to this comment will move most of this text to the Methods section so that it is clearer for the reader. Specifically, Section 3.3 will have a paragraph that reads: “Ferrar dolerite pyroxenes are known to contain non-cosmogenic 3He, which is most likely produced by neutron capture on Li via the reaction 6Li(n, 𝛼)3He (Ackert & Kurz, 2004; Andrews & Kay, 1982). In Ferrar Dolerite, the total concentration of non-cosmogenic 3He has been measured in samples that are shielded from the cosmic-ray flux (Ackert, 2000; Kaplan et al., 2017; Margerison et al., 2005). Together, these measurements converge on (3.3 ± 1.1) x 106 atom g-1 of non-cosmogenic 3He throughout this lithology (see discussion in Balco, 2020). Non-cosmogenic 3He therefore constitutes <1% of the measured 3He in the surface samples, so we do not consider non-cosmogenic 3He significant when calculating apparent exposure ages or erosion rates. At depth in the Labyrinth core, however, non-cosmogenic 3He comprises as much as 10% of the measured 3He. Therefore, we do include non-cosmogenic 3He when performing production rate calculations in Sects. 5.1.1 and 5.1.2.“
To avoid redundancy later in the paper, we will simplify the description of non-cosmogenic 3He in the model set-up in section 5.1.1. to read: (atoms g-1) is non-cosmogenic 3He, which we consider to be (3.3 ± 1.1) x 106 atom g-1 in Ferrar pyroxenes (see Sect. 3.3 and discussion in Balco, 2020).”
In section 3.2.3. it is mentioned that the pH 8 precipitation reduces cation load, which seems like a good idea, but is there no loss of Be or Al? Was this experimentally verified?
We did not experimentally verify that there was no loss of Be in this step. However, given that Be forms an insoluble hydroxide at pH 8, we have no reason to believe that Be was lost during this additional precipitation step. We note for other readers that loss of Be in this step would not affect the measurement because 10Be should have been isotopically equilibrated with the carrier during initial dissolution.
Other suggestions:
Line 56: I recommend that you cite cite Kurz (1986) and Blard (2021), old and new, in mentioning that 3He is widely used, including in olivine and pyroxene (no citation given here).
Great suggestion, we will include these citations.
Line 68: the paper should also cite Brown et al. (1991) for HF leaching to remove meteoric 10Be, in addition to the paper cited here. Since this paper discusses HF leaching for pyroxenes, it should at least cite the first paper to describe HF leaching (for quartz).
We will include this citation.
Line 88: the paper should cite Goehring et al. (2010) for 3He production rates, in addition to Borchers et al.
We will add this reference.
Line 167-168. Are the samples sonicated or shaken for 5-6 hours? Be specific.
We will update this to include that the leaching took place on a shaker table.
Line 196. Give units of atoms/gram here.
Great catch, we will add the units.
Line 445 and 449. Replace the word “advecting” here with “exposed” or similar. The rock is not advecting, it is exposed by erosion.
Good point. We will update these sentences to replace the word advecting.
Table 4. In the table title add “at the Labyrinth core” or something to be more specific.
We will add this to the table title.
Line 525. Add “clino” to pyroxene here and elsewhere?
As discussed above, we did not do a detailed mineralogical study so prefer to refrain from adding further specifics throughout the manuscript. In response to the comment above, however, we will add that most of the pyroxenes analyzed were likely clinopyroxene (specifically, augite), which we feel provides specifics for interested readers.
Line 539 add Sea Level High Latitude to the 3.6 at/g/yr production rate, for clarity.
We will update this to read: “reference production rate of 3.6 ± 0.8 atoms g-1 yr-1 at sea level high latitude”.
Figure 7 caption: add “Ferrar dolerite” and “clino” before pyroxene to be more specific.
We will add “Ferrar dolerite” to the figure caption in response to this comment. Please see comment above about L525 regarding the addition of “clino”.
Citation: https://doi.org/10.5194/egusphere-2022-1379-AC2
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AC2: 'Reply on RC2', Allie Balter-Kennedy, 02 May 2023
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Allie Balter-Kennedy
Joerg M. Schaefer
Roseanne Schwartz
Jennifer L. Lamp
Laura Penrose
Jennifer Middleton
Bouchaïb Tibari
Pierre-Henri Blard
Gisela Winckler
Alan J. Hidy
Greg Balco
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