the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Sep 2024
A thicker, rather than thinner, East Antarctic Ice Sheet plateau during the Last Glacial Maximum
Abstract. In this study, we present a surface-exposure chronology of past ice-thickness change derived from in-situ cosmogenic-14C dating at a site on the edge of the East Antarctic plateau, 380 km inland from the Antarctic coastline. Our knowledge of how the Antarctic ice sheet has responded to Quaternary climate change relies on a combination of geological data and ice-sheet modeling. At the Last Glacial Maximum (LGM), observations and models suggest that increased ice-sheet volume was accommodated by thicker ice near the coast and grounding-line advance towards the continental-shelf edge. In contrast, the ice sheet interior maintained a relatively stable thickness until present, with ice-core evidence even suggesting thinner ice relative to today. However, the magnitude of these thickness changes, and the location dividing thicker versus thinner ice at the LGM is poorly constrained. Geological reconstructions of past ice thickness in Antarctica mostly come from surface-exposure data using cosmogenic nuclides that are relatively insensitive records of ice-cover changes on timescales of tens of thousands of years. This can lead to inaccurate records of LGM ice thickness, particularly towards the East Antarctic plateau, where cold-based non-erosive ice may inhibit bedrock erosion. Samples saturated with 14C at 1912 m a.s.l. indicate that the summit of Nunatak 1921 was exposed during the LGM, while unsaturated samples indicate that thinning subsequently occurred, with some (25–45 %) post-LGM thinning recorded at ~15–11 ka and most (55–75 %) recorded during the Holocene. These results imply that at least part of the interior East Antarctic Ice Sheet (EAIS) was thicker at the LGM than it is now, and that gradual ice-sheet thinning began ~15 ka. Ice-sheet models that do not account for this thickness change would inaccurately characterize the LGM geometry of the EAIS and underestimate its contributions to deglacial sea-level rise.
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RC1: 'Comment on egusphere-2024-2674', Greg Balco, 23 Oct 2024
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This is a valuable and fairly short and straightforward paper that should be published in approximately its present form. There are a lot of sites in interior Antarctica with Be-10/Al-26 data that are ambiguous as regards the LGM thickness, the only real way to fix this is with C-14 data, and that is what this paper does. The data here are useful and important.
There are a few things that could be improved, as follows.
1. The diagrams in Figures 1 and 4 that are supposed to show ice surface slopes would (and should) be greatly improved by drawing them with parabolic ice sheet profiles. As drawn with straight lines, it doesn't represent the concept of an LGM ice sheet with an extended grounding line but a thinner interior. For example:
At the very least, the 'coastline' and 'interior' labels appear to be in the wrong place on Figure 1c and need to be corrected. But currently the figures are barely acceptable -- they should be redrawn with more realistic profiles so that readers can understand what they are talking about.2. The discussion of the blank corrections needs some attention. First, the stated uncertainty of 3000 atoms is extremely small. Is this really the standard deviation of many process blanks? Second, the statement '..continually updated mean of all process blanks run at TUCNL since 2016...' somewhat conflicts with data in other publications, for example the appendix in this paper:
https://tc.copernicus.org/articles/17/1787/2023/
In particular look at Figure 1 in this:
https://tc.copernicus.org/preprints/tc-2022-172/tc-2022-172-AC2-supplement.pdf
The Tulane blanks shown here may have a mean near 58,000, but they have a standard deviation that is substantially (like 10x) larger than 3000. Furthermore, they are not normally distributed, so it would be inappropriate to use the standard error instead of the standard deviation, or to divide by sqrt(n). This issue doesn't really matter very much for the present paper (except regarding the fairly minor point about whether nonzero cosmogenic C-14 was actually observed in GR15), so the authors can do whatever they want here, but they need to explain what they did in more detail -- as written, the description of the blank correction is not acceptable.
3. The discussion of 'stratigraphic order' in various places (e.g., caption to Fig. 2 near line 182) doesn't really make any sense and needs some work. First, there isn't really any 'stratigraphy' here; what is actually being talked about is just the geometric constraint that lower-elevation samples can't be deglaciated unless higher-elevation samples have already done so. Second, depending on how you define the correct 'order', you could say that GR15 is out of order (if the order is defined by GR04, GR03, GR13, and GR01), or you could say that GR13, GR01, and GR12 are out of order (if the order is defined by GR04, GR03, and GR15), or you could pick some other things to be 'out of order' if you wanted. What the authors are trying to say here is pretty simple -- if the general deglaciation trend is defined by all the samples that are not GR15, then GR15 is out of order -- but the discussion here needs some attention.
4. In figure 3, the sample names should just be put on the figure contours, instead of complicatedly referenced in the caption. Also, labeling the upper right corner 'burial histories inconsistent with post-LGM exposure' is very confusing, because really these are just impossible burial histories: if burial began 15 ka, the duration of burial can't be any longer than 15 ka. It would be much clearer to just mark this 'impossible' or something of that nature, or (preferred) just make the figure border the correct shape to exclude this area entirely.. More seriously, though, it is not clear to me exactly what purpose this figure serves in the text. What point is intended to be made here? The reference to the figure in the text (line 219) suggests that it is supposed to indicate that you can't get anywhere near saturation concentrations unless the burial took place a long time ago (lower left corner) or was very short (upper left corner). But nowhere is this really explained. In my opinion, this should be edited a bit to (i) move material from caption to text to make it clear what the point of this figure is; (ii) only include contours for the sample(s) that you are actually talking about, and (iii) generally clarify this discussion. Alternatively, this figure makes only a minor contribution to the discussion and can probably be removed entirely without significant loss of understanding.
Finally, I assume this figure assumes initial saturation at the time of burial -- that doesn't appear to be stated anywhere either.
Also a couple of minor comments:
Abstract, line 18. The 'relatively insensitive' here doesn't really make any sense. Be-10 and Al-26 are perfectly sensitive to short periods of exposure; the difference isn't the 'sensitivity', but the half-lives. It seems like what the authors are trying to say here is more like '...mostly come from surface exposure dating using cosmogenic nuclides with long half-lives, such as Be-10 and Al-26. These often record a cumulative exposure history extending over many glacial-interglacial cycles, rather than reflecting a single period of exposure after the most recent deglaciation....."
Line 142. It would probably be helpful to mention that only the gas released at 1100 C is analyzed; the 500 C step is for cleaning.
Line 150. "run mass" doesn't really make sense here. I see that 'run' is supposed to be a passive voice verb, but it reads like a noun. Simply 'sample mass' would be much better.
Near line 204. The statement 'most post-LGM thinning is recorded during the Holocene' does not seem to be very meaningful, because most of the time since deglaciation is also in the Holocene (so this is kind of like saying that most of the numbers less than 10 are also less than 9). What is the point the authors are trying to make here? This could use some clarification.
Citation: https://doi.org/10.5194/egusphere-2024-2674-RC1 -
RC2: 'Comment on egusphere-2024-2674', Allie Balter-Kennedy, 01 Nov 2024
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This paper contributes to an important discussion about EAIS thickness during the LGM by adding new C-14 exposure ages, which generally circumvent the common issue of 10Be and 26Al inheritance in Antarctica. With these new data, the authors identify thicker-than-present LGM ice at a location previously thought to not have been covered by ice at that time. This finding has implications for EAIS volume during the LGM and the EAIS contribution to deglacial sea-level rise. The presence of samples with unsaturated C-14 samples also allows the authors to determine a post-LGM thinning history at this site, which couldn't be done with prior 10Be and 26Al measurements. Overall, I agree with the authors’ treatment and interpretation of their data. I really enjoyed reading this manuscript – it is well written and nicely presented. There were a few points, however, that I think could be clarified with relatively minor revisions.
General comments
- Interpretation of saturated samples: The authors spend some time with the question of whether the two saturated samples at/near the nunatak could have been covered by ice at some point during the LGM, which is certainly worthwhile as this determines whether they’re able to put an upper bound on ice thickness during the LGM. They conclude that the answer is no, or if they were, it must have been for a short duration or by very thin ice (L217-219; L233-236) and use Figure 3 to support this conclusion. I really struggled, however, to digest Figure 3. There is a lot of information in this figure, so perhaps some of my questions below could be addressed with a slightly longer discussion in the main text and some updates to the figure.
- The caption says the C-14 concentrations shown result from glacial histories with one episode of burial. I assume then followed by exposure so that the total history is equal to burial date? What are the starting conditions? I assumed saturation?
- I was confused by the labeling of the black region as “Inconsistent with post-LGM exposure.” Isn’t the black region just above the 1:1 line for burial start and burial duration? Like, if burial started at 15 ka, and the burial duration was 20 kyr, the site would not only be buried today but also 5 kyr into the future? Since we know the sites aren’t covered by ice today, they can’t have the exposure histories that fall on that line, or anything in the black area because that requires future burial. In addition, the caption says that the black is scenarios that require burial after 15 ka, but I think there are scenarios that are not in the black area that also require burial after 15 ka? For example, 5 kyr of burial starting at 15 ka, or 10 kyr of burial starting at 20 ka.
- What are the two different greys? Is this the region for burial histories producing unsaturated samples (stated in caption), and if so can the label be moved there?
- Is the entire white area the zone of saturation (as stated in the caption) or is it just the area to the left of the 7.3 x 10^5 atoms/g line (where the arrow and label point to)? If I’m reading Figure 2b correctly, it seems like saturation concentrations at 1921 m would span from the 7 x 10^5 atoms/g to the left side of the diagram?
- “Only the lesser end of the saturation window is consistent with any significant degree of burial under enough ice to effectively stop production (~10 m)” – I found this sentence confusing, probably in part because I was unsure what the bounds of the saturation window were (see bullet above). Is the “lesser” end just the lowest concentrations that are still considered saturated? If the entire white area produces saturated samples, then it looks to me that there’s a lot of burial allowed. This sentence sort of makes it sound like the ice thickness needed to stop production was explored here, but I don’t think it was? Maybe this just needs explanation in the first sentence of the caption – “…one episode of burial assuming that ice was thick enough (~10 m) that nuclide production in the sampled rock surfaces is negligible on these timescales.”
- Is it possible to indicate where the saturated sample concentrations are on the diagram, rather than just referring to them being off the lower left corner of the diagram in the caption? Can the concentrations also be labeled with the sample id?
- This figure actually opened a question for me about whether the unsaturated samples had inheritance, which I wasn’t concerned about before seeing this figure. Considering an extreme example, the measured C-14 concentration in sample GR06 (highest unsaturated sample), could be achieved if burial started at 15 ka, with ~6 yr of burial and 8 kyr of exposure, meaning a true deglaciation age of 8 kyr. That scenario seems implausible, but is consistent with the data. On the other hand, as long as burial started before ~30–35ka, the apparent exposure age should be roughly equal to the true deglaciation age. I’m not sure if the authors were trying to make this point, but it came up for me in trying to understand this figure.
Stepping back a bit, even if LGM ice-cover were compatible with the saturated C-14 concentrations, the authors' main conclusions still stand. The conclusion that ice was thicker at Nunatak 1921 during the LGM is still true and the unsaturated samples still record the thinning history after 15 ka. So maybe the level of detail in Figure 3, at least as presented, is just overcomplicating things a bit.
- MWP-1a discussion: I actually find the sentences on L285-288 a more impactful way to end the Discussion, given the dataset and conclusions, than the discussion of MWP-1a, which I think could be shortened and simplified. There seems to be a tension between the fact that this chronology shows thinning during MWP-1a and its consistency with the EAIS as a whole being a minor MWP-1a contributor. I agree with the sentence on L290-292 that the work here suggests a modest additional ice volume for MWP-1a. I also agree that the chronology presented here suggests, although does not require, some thinning during MWP-1a (although “likely less than half of post-LGM ice loss” (L293-294) sounds like a lot of ice loss, maybe a nominal thickness loss (<20 m?) is a better reference here). However, this is one nunatak in one part of the EAIS, so I’m not sure it’s necessary to extrapolate to the EAIS (L288-290) or Antarctica (L295) as a whole to the extent that’s done here.
Minor Comments
- Figure 2 caption: I wasn’t sure exactly what is meant by “error envelope” – is this determined by the typical measurement uncertainty, production rate uncertainty, or both (i.e., above this concentration there is no discernable change in the in the nuclide concentration beyond uncertainty)?
- L194–195: I might be careful about extrapolating to the covering of nunatak summits throughout the Grove Mountains as a whole, at least at this point in the paper, because I’m guessing neither the elevation difference between each summit and the local ice surface, nor the change in LGM ice thickness, is uniform across the Grove Mountains. I also found the parenthetical statement here (neither lengthily nor deeply enough…” ) slightly confusing. Does this mean that if ice did override the summit, it wasn’t thick enough to shield the sampled surfaces from the cosmic-ray flux?
- L201–205: How were the percent thinning calculations made?
- L274-279: “Deglaciation thus possibly started and likely finished earlier downstream” - Are the Prince Charles Mountains actually downstream of the Grove Mountains (it doesn’t look like it to me in Figure 1a)? I was also wondering if it is expected that the glacial history in the Grove Mountains is so different than in the Prince Charles Mountains, and if so, why? It looks like the White data are from Al-26 and Be-10, so is it possible they have some inheritance?
Line edits
- L10-11: “380 km inland from the Antarctic coastline” – which sector of Antarctica?
- L20–21: “above 1912 m asl”? Or, “from 1912 m asl to the nunatak summit at 1921 m asl”? Could this sentence also include an indication of how much thicker these findings require that the ice was during the LGM? Also, there is no mention anywhere in the abstract where Nunatak 1921 is – add reference to Grove mountains somewhere?
- L59–59: Mention the half-lives of Be-10 and Al-26?
- Figure 1 caption: move the sentence now on lines 84–85, which cites White et al. (2011) and Lilly et al. (2010), up to L68–69 to make it clear where this placement of the potential hinge zone comes from?
- Figure 1c: “interior” and “coastline” are switched.
- Line 87–88: “testing previously measured samples at a key site in the ice sheet interior” – maybe just state what you are testing and what the key site is?
- Section 1.1: Make it clear that Nunatak 1921 is named for the altitude of its peak and also state the ice surface elevation at this site specifically? I think it’s mentioned later but it would be helpful to have it here.
- Table 2 caption (L160): 10^5 is missing when stating blank value.
- L167: 10Be and 26Al exposure ages, not concentrations.
- L182: GR12, not GR21, is out of order?
- L202: Add timing of MWP-1a since this is the first mention? Also, should the reference be to figure 2d, not 2b?
- Line 210: Maybe specify at Nunatak 1921, instead of in the Grove Mountains generally? As consistent with a few comments above, this could be done more often throughout the paper.
- Line 212: “contrary to previous ice-thickness data” – this isn’t really true, it’s contrary to previous interpretations of 26Al and 10Be data.
- L216: “indicate that ice cover occurred at this site [up to x m above the present ice surface]?”
- L219: “re-saturated during the Holocene” – or during the deglacial / late glacial and Holocene, if it must have been uncovered before GR06?
- Figure 3: 14.9 kyr stated for GR06 in caption, but table and text say 14.6 kyr. Also, the caption says GR21 = 7 x 10^5 atoms/g, which I don’t think is right?
- L247: Rather than the coast being representative of the interior, could this be simplified to “the zone of thicker-than-present LGM ice extends further inland than previously thought”?
Citation: https://doi.org/10.5194/egusphere-2024-2674-RC2 - Interpretation of saturated samples: The authors spend some time with the question of whether the two saturated samples at/near the nunatak could have been covered by ice at some point during the LGM, which is certainly worthwhile as this determines whether they’re able to put an upper bound on ice thickness during the LGM. They conclude that the answer is no, or if they were, it must have been for a short duration or by very thin ice (L217-219; L233-236) and use Figure 3 to support this conclusion. I really struggled, however, to digest Figure 3. There is a lot of information in this figure, so perhaps some of my questions below could be addressed with a slightly longer discussion in the main text and some updates to the figure.
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RC3: 'Comment on egusphere-2024-2674', Anonymous Referee #3, 10 Jan 2025
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GENERAL COMMENTS
This manuscript presents in situ 14C measurements from an altitude transect of bedrock and erratic samples from the Grove Mountains in East Antarctica that had previously been measured for in situ 10Be and 26Al by Lilly et al. (2010). That earlier study found large inherited inventories of the longer-lived nuclides. In situ 14C provides a means of seeing through that inherited signal to try to discern evidence of deglaciation since the Last Glacial Maximum (LGM) at ca. 21 ka. The results could indicate whether ice sheet models that predict coastal thickening and inland thinning of the East Antarctic Ice Sheet at the LGM are consistent with empirical evidence. This study constrains ice thickness history at a location significantly inland from the East Antarctic coast, but in my view requires significant additional data and discussion in two main areas before publication would be appropriate.
Most importantly, in my view the manuscript suffers from sloppiness and inconsistencies in the descriptions of the procedures used, as well as in presentation of the data and assumptions made in their interpretation. Clarity and transparency in those areas is critical to allow comparison with other work. It’s clear that procedures have changed since the original Goehring et al. (2019) Tulane laboratory study; however, I could not find supporting data for those changes in the literature (see Specific Comments below). Much of the interpretation (e.g., production rates, saturation concentrations, etc.) is based on measurements from the Goehring et al. (2019) study. Without detailing potential effects of the subsequently modified procedures on measured 14C concentrations from more recent replicate analyses of CRONUS-A or other intercomparison materials (or otherwise documenting that there are none), this leads me to question the implied robustness of the results presented here. It’s not enough just to state that there are no effects from the procedural changes – that conclusion should be documented. Neglecting to discuss potential additional uncertainties (if any) arising from these procedural modifications and calculator dependencies (see Specific Comments) gives the impression of in situ 14C ages that are more robust than they actually are in my view – particularly for pre-Holocene ages. I’m happy to be proven wrong about this impression but the effects of procedural changes on measurements and interpretations should be well documented.
Second, I think that the study would have benefited from at least one bedrock-erratic pair being analyzed from the original paper by Lilly et al. (2010), with more justification for sample selection (if there was no sample material left for paired analyses, fine, but that should be stated explicitly). Only three bedrock samples were analyzed, and not in conjunction with their respective erratics from the earlier Lilly et al. (2010) study. Analyses in that original study were split between 16 bedrock and 10 erratic samples, with several bedrock-erratic pairs – most contained significant inherited 10Be/26Al signals relative to the 14C results here. Given the analytical focus here on erratics and the fact that the Grove Mountains comprise an isolated small group of nunataks with varying compositions (GeoMap - https://data.gns.cri.nz/ata_geomap/index.html) subject to W-NW ice flow toward the Lambert Glacier and Amery Ice Shelf (e.g., Mouginot et al., 2019, Geophysical Research Letters, 46(16), 9710–9718), a more detailed discussion of the site setting in terms of ice flow and broadly up-gradient lithologies would be useful. In particular, a discussion of rock types comprising the local nunatak group shown in Fig 1b would be very helpful to place the erratic lithologies in context to assess the possibility of exposure prior to deposition – particularly for those nunataks broadly up-gradient from the sampled nunatak (see Specific Comments). In my opinion, the authors need to demonstrate to the extent possible that the erratics are indeed truly erratic, and not locally derived lithologies that may have been exposed supraglacially during transport or recently on hillslopes prior to deposition on bedrock at the sampled locations, given the clear long-term inherited signals. For example, were there striations or other evidence of subglacial transport associated with the cobbles? If there is none, that’s fine, but then that should be stated and clarified that the authors are then simply assuming that they are true erratics. Descriptions of each sample (with photos if possible) in the supplement would be useful information in this context. Analysis of bedrock-erratic pairs from the same locations would have gone a long way to support the authors’ arguments. Given the remoteness of the study area and 20-20 hindsight from a decade and a half ago, I’m willing to cut the authors some slack on these points, but I believe that at least some non-trivial discussion of ice flow and local bedrock lithology from the local nunataks and the potential for prior erratic exposure over the late Pleistocene/Holocene is warranted.
Once these significant issues (and Specific Comments below) are addressed, I look forward to reviewing this paper again.
SPECIFIC COMMENTS
Line 1: Thicker and thinner are relative to something. Suggest changing the title to ‘A thicker than present East Antarctic Ice Sheet plateau during the Last Glacial Maximum’
Line 9: in situ is not hyphenated as it is Latin – should just be italicized throughout
Line 14: On the other hand, ‘ice sheet’ SHOULD be hyphenated, as it modifies the word ‘interior.’ The rule is that compound words that act as a noun such as 'ice sheet' should not be hyphenated, but should be hyphenated when they modify another noun (serving as an adjective), like 'ice-sheet interior', 'ice-sheet history', 'ice-sheet model', etc. Be consistent throughout.
Line 20: ‘Saturated’ is colloquial - better to avoid colloquial phrasing without first defining. Better to say “Samples with 14C concentrations at a secular equilibrium between production and decay (saturation)...”
Line 43: Should use Oxford commas throughout in lists – ‘radar, ice-sheet, and geological…’. Suggest replacing ‘sparse’, with ‘rare’ since 'sparsity' is used later in the sentence
Line 44: Active voice is almost always better in writing: ‘'often disagree' instead of ‘are often in disagreement…’
Line 51: Replace ‘indicate with ‘suggest’. Less definitive since an interpolated value is referenced
Line 58: Replace ‘Yet’ with ‘However,’.
Line 60: Replace ‘nuclides’ with ‘nuclide inventories’. More precise wording
Line 62: Reference should just be to Goehring et al. (2019) – no Balco. If there are multiple Goehring et al. (2019) citations for different papers, then specify 2019a, 2019b, etc.
Line 65: Fig 1c appears to have the Interior and Coastline labels reversed, based on the caption
Line 73: Shading looks more pink than red
Line 77: Fig 1b: Would be useful to indicate the general direction of ice flow on this image - what nunataks are upstream of the sampled nunatak, if any
Line 89: Instead of ‘be saturated with’ it would be more accurate and succinct to say here 'have concentrations of in situ 14C at secular equilibrium between production and decay (saturation) - a state that requires at ca. 5 half-lives of continuous exposure (Dunai, 2010). Need to make clear that it's the concentrations that indicate secular equilibrium.
Line 90: Delete ‘exposed’
Line 91: Replace ‘covered at some time since the LGM’ with ‘likely covered for some duration post-LGM’. More precise wording.
Line 107: This is the situation I referenced in my General Comments. Do the rock types of the cobbles occur in the Grove Mountains (or is bedrock all just orthogneiss throughout)? The GeoMAP site, (https://data.gns.cri.nz/ata_geomap/index.html) indicates felsic plutonic rock types are actually quite common in the vicinity of the sampled nunatak. While that map is at quite a large scale, this suggests that at least some of the cobbles could well be locally derived erratics and thus have the potential for some subaerial exposure either on the land surface (e.g., rockfall and downslope transport) and/or on the ice surface (e.g., rockfalls onto the ice) before being deposited at the sampled locations. I’m not convinced by the single declaration that they are not locally derived, without any other discussion. Are the erratics striated or otherwise have evidence of subglacial transport? The rock types in Table 1 indicate orthogneiss for the bedrock, but all the erratics could totally be associated with felsic plutonics in the vicinity, lacking any more detailed description of the rocks. If the quartzites are sedimentary, state that as that is evidence of a true erratic. Metamorphic quartzites could potentially be associated with the gneiss. My point here is that uncertainty is fine but one needs to be up front about it. Statements of certainty when in fact significant uncertainty exists is a common theme I find in this manuscript.
Line 114: Indicate the half-life for each nuclide considered so that the reader can assess the duration required for saturation for each.
Line 123: Looks as though from Fig 2 that 4 of the erratics were part of bedrock-erratic pairs from the original paper? Why were the corresponding bedrock or erratic samples not analyzed here - at least one or two of them? In those cases, the higher altitude erratics generally indicated longer apparent exposures than the corresponding bedrock. It would have been very useful to have that perspective for this dataset as well. State the reason(s) both members of the pairs were not analyzed to clarify for the reader. See comment on Line 107.
Line 128: Is the lithology actually ‘Unknown’ or just not recorded and no unprocessed sample remaining? If the latter it’s better to just say that. If the latter is not the case then it should be possible to ascertain some sort of rock type for the sample.
Line 141: ‘Li-flux-containing’ is awkward wording. Clarify to indicate that the flux had been previously fused and degassed of contaminants and cooled prior to loading.
Crucibles are typically round – these should be referred to as Pt combustion boats.
Line 142: Goehring et al. (2019, Nuclear Instruments and Methods in Physics Research B, 455, 284–292) indicate combustion for 1 hr at 500 C and extraction for 3 hr at 1100 C. However, Nichols and Goehring (2019, Geochronology, 1(1), 43–52) subsequently indicated a procedure of 30 min at 500 C followed by 3 hr at 1100 C – this change in combustion procedure that potentially has implications for the extracted 14C results (e.g., Lifton et al., 2023, Geochronology, 5, 361–375) was presented without supporting experiments. Analyses presented in Balco et al. (2023, The Cryosphere, 17(4), 1787–1801), coeval with the samples from this study, indicate combustions of 30 min at 500 C with extractions of 2 hr at 1100 C, again without experimental data supporting the procedural change. It’s thus unclear to me what procedure was actually followed here in these analyses, as the authors cite only Goehring et al. (2019). Given that these procedural changes have the potential to affect the measured 14C concentrations significantly in either direction, I think it's crucial to clarify what procedures were actually used in the analyses here, and to the extent that they deviate from the procedures in Goehring et al. (2019), to document that the modified procedures had no significant effect on the resulting measurements.
Line 143: Specify what is meant by ‘hot’, and describe the quartz for the reader - single crystal, gravel, sand, etc. - provide citation
Line 144: Specify to what equivalent mass of C the sample is typically diluted to.
Line 146: NOSAMS measures the 14/12 isotope ratios, not the concentrations. Concentrations are derived from those ratios - reference the data tables. Clarify.
Line 148: As noted in Balco et al. (2023), and in Greg Balco's review comments, this is not necessarily representative of the system blanks at the time of extraction for this study's samples. The blank data included in Balco et al. (2023) show wide temporal variability - the 3110-atom uncertainty is not representative – the standard deviation of the data is about 10x that, as indicated by Balco's comments. Also indicated by Balco, if this is standard error, that is not appropriate for a non-Gaussian distribution such as that of the blanks. Comparing the sample numbers (TUCNL) from this manuscript’s supplement with those from the Balco et al. (2023) supplement, it appears that these data were coeval with some of the Balco et al. data. In my opinion the most defensible approach is to utilize only the blanks from the time of these analyses. See my comments on the Supplemental data for more details.
Line 155: Again, see the Balco et al. (2023) 14C supplement. The measurements of CRONUS-A since those included in Goehring et al. (2019) scatter quite a bit more than what is quoted here and show a significant uptick in concentration in the most recent two values in that paper (but which are still well before these analyses). Given that the Goehring et al. (2010) CRONUS-A measurements are used as the basis for the default 14C production rate in the University of Washington v3 online calculator (UWv3 - Balco et al., 2008), and particularly in light of the last two higher concentrations in Balco et al. (2023), the authors should present any subsequent CRONUS-A measurements spanning the time that the Grove mountains samples were run to demonstrate that the production rates assumed are appropriate. If the high concentrations of more recent samples are more representative of results when these samples were run, then a production rate consistent with those should be used, with associated changes in ages and predicted saturation concentrations.
The authors need to be fully transparent about their data and underlying procedures/measurements since the production rates used (and subsequent exposure age implications) depend on the significantly (15-20%) lower measured concentrations of CRONUS-A in Goehring et al. (2019) as compared to most other labs (e.g., Lifton et al, 2023).
Also, the authors should post the code being used to calculate the ages - running the concentrations and site parameters through the UWv3 calculator gives quite different ages for the older ages, and larger uncertainties as well than what are presented here. Any conclusions based on ages thus need to be approached very cautiously as pre-Holocene ages appear less robust than the late Holocene ages.
Line 156: It is not clear where this claimed 6% uncertainty comes from. The CRONUS-A concentrations in Balco et al. (2023) have a standard deviation of about 8-9% - if one includes the last two from that paper which are much higher than most of the others, it's over 10%. Also, the 6% uncertainty in 14C concentrations is almost certainly concentration-dependent - see Balco et al. (2023) for example with low-concentration samples' % reproducibility - this should be stated as such. CRONUS-A is a high concentration sample - some of these concentrations are high as well but many are much lower, so that uncertainty is likely a minimum value in my opinion. Clarify.
Line 160: A blank of 0.58 ± 0.31 atoms is inconsistent with the text – also see above comments. Make sure the units and values are correct and clearly stated, and reference supplemental data tables for complete information.
Line 161: What level of uncertainty is being cited here and throughout? 1 sigma, 2 sigma? An early statement as to what level all uncertainty values represent will take care of them for the whole paper, unless otherwise noted.
Line 165: 0.02 ± 0.01 ka? 20 years? I don't believe it. Running the data through the UW v3 calculator yields ca. 200 yr. Proofread all numbers in tables and text.
Line 169: Is there field evidence to support the claim of sediment or boulder cover that moved recently? Pretty speculative. Also, is there evidence of drifting snow in the sample vicinity currently? State clearly that this is speculative. Again, it’s fine to speculate or have uncertainty just be up front about it clearly. Particularly when you already said there’s no evidence of past snow or sediment cover.
Line 174: Again, what significance (or sigma uncertainty) level is indicated by the error bars? 1 sigma? 2 sigma?
‘Approximately’ is a more precise term than ‘roughly’
Line 176: As implemented in this figure, the highlighting makes the numbers harder to read. It would be better to make the sample numbers from this study in a different font, color, bold or italic, etc., to set them apart more clearly. You could also make the photo larger and more legible if you move the legend into panel d, for example, or otherwise rearrange the legend.
Line 178: Again, what significance (or sigma uncertainty) level is indicated by the error envelope? 1 sigma? 2 sigma? Based on what production rate? Specify
Line 194: ‘Lengthily’ is not a word. Suggest rephrasing to something like '...but not thick enough to override the summits for a long enough duration to allow the 14C to decay below saturation'
Line 201: 14.9 ± 1.0 is inconsistent with what is listed in Table 2. And as noted previously, pre-Holocene ages appear to be calculator-dependent, so any correlations with well-defined events such as MWP-1a should be appropriately couched in language emphasizing the uncertainties.
Line 214: Delete ‘do not show saturation’ and instead reference that many of the samples from the original Lilly et al. (2010) paper show evidence of complex exposure over long time frames (and then state the range of minimum exposure durations consistent with each sample)
Line 216: Suggest ‘Our 14C data indicate that this site was covered by ice > ca. 10 m thick for long enough to allow in situ 14C concentrations in the samples to decay.’ To what level, though? Measurement background – what is that for this case? Specify. It depends on how long the shielding lasted, and starting from what concentration? What are you assuming here - justify that.
Line 218: What do you mean specifically by ‘covered briefly’? Put a value on this - if the summit was covered by over 10 m of ice, how long could it have been covered during the LGM or subsequently and still yield saturated concentrations today?
Line 219: Suggest ‘… nunataks were progressively re-exposed through the late Holocene.’ More succinct.
Line 223: What is the starting point for these calculations - do you assume all samples were saturated before burial? What effect does starting from a non-saturated concentration have on the predictions here? Describe for the reader.
Line 230: Again, the 14.9 ka age is calculator-dependent. Provide specifics and discuss effects of different calculated ages for that sample (a different production rate and calculation scheme would also likely affect this plot overall)
There appear to be three or four slightly different shades of gray on the graph, ‘gray-shaded’ could be more specific. Might be more obvious for the reader if colors were also used instead of just grayscale.
Line 231: ‘…being unsaturated with 14C’ is awkwardly worded. Suggest ‘having a concentration below saturation for 14C’
‘The unshaded portion of the graph…’: As above, define where this is - it's not clear from the figure in the PDF - does everything have some degree of shading except for concentrations >7.3e5? Suggest modifying the shading (colors or something more obvious to the reader than really light grays - bigger steps between grayscale values would help). Maybe hatchures instead of black in forbidden region. Describe what is meant by 'uncertainty window' - it should reflect Fig 2b, but does not appear to with this shading scheme. Are you indicating any concentration > 6e5, per Fig 2? Make sure all figures and discussion/descriptions are internally consistent.
Line 238/39: Be skeptical of all 10/26 ages in cold-based regimes such as many places in Antarctica. It obviously would be good to have 14C from these datasets if possible now as well to confirm there's no significant inherited inventory (not 10s-100s of ka, obviously, but perhaps a few ka worth). Be up front as to the potential pitfalls of relying on long-lived nuclide chronologies in these environments - it may be that the ages are fine once all recalculated using the same methods/assumptions (e.g., UWv3 calculator, LSDn), but to me there is always going to be some uncertainty in 10/26 ages for LGM and younger time frames in Antarctica, without 14C confirmation. Future 14C work, yes, but point out the potential for even low levels of Be inheritance.
The authors should also provide code for calculations through GitHub/Zenodo or similar, as noted previously, so that interested readers can work to reproduce all calculations and results here.
Line 243/44: See above comment - rephrase this paragraph to clarify the possibility of even low levels of inheritance in longer lived nuclides.
Line 248: Prefer ‘geomorphic’ to ‘geomorphological’
Line 257: Fig 4: 'Present-day ice surface' should be located along the actual line for clarity - above the LGM surfaces on the right side would work. Alternatively, have a legend on the figure identifying each dashed line separately from the lines themselves. If the present-day ice surface continues below the previous LGM surfaces, then it should continue into the gray area at the bottom of the figure.
Line 261: Is the ice surface slope upstream of the hinge zone constrained by the Prince Charles Mountains data as you have drawn this, or could it just be subparallel to the previous LGM surface upstream of the hinge? Clarify in the caption and discuss further in the text.
Line 268: Reword - this is a confusing sentence. Suggest '...EAIS being thicker than previously suggested at the LGM is that...' Any leads and lags should be evaluated with 14C in coast and interior locations to reduce the possibility of minor inheritance in longer-lived nuclides. At least you should qualify any discussion with that possibility so the reader is clear on that.
Line 271: Clarify which White et al. (2011) you're citing in each case - should be 2011a and 2011b to differentiate. The authors cite two.
Line 274: As noted earlier - be careful tying this 15 ka and other 14C ages to other events as especially the older ones are calculator dependent. Make sure all previously published ages in this paper (14C, 10Be, 26Al, etc.) are re-calculated using the same underlying assumptions and algorithms, and make sure to state that that is what has been done. The UW v3 calculator and ICE-D Antarctica is quite useful for that. And specify which production rate datasets you’re using for each nuclide.
Line 276: Again, my take on these 10Be ages is that they should be viewed with caution as they can easily skew a bit old - hence the importance of 14C. Any comparison between 14C results and 10/26 results from other sites should be appropriately qualified in the discussion - that there is the potential for 10Be/26Al ages (even post LGM deglaciation ages) to skew older due to an inherited component. I don't think you can get away from that possibility. And especially since the 14C ages and uncertainties are calculator-dependent to some extent, the authors need to dial back strong correlations.
Line 277: Clarify which White et al. (2011) citation – there are 2 such papers cited.
Line 286: Replace ‘thinned’ with ‘thinned more’
Line 288: Replace ‘thicker-than-at-present’ with ‘thicker-than-present’
Replace ‘hundreds of’ with ‘hundred’
Overall this sentence is the sort of thing I'm talking about in my earlier comments - yes using the calculator you are employing gives something close to MWP-1A, but UWv3 gives a significantly older age (although overlapping with much larger uncertainty). Just be transparent and up-front about the limitations of the data. This is also why it's good to provide the code so anyone can see what's happening.
Line 298: Suggest starting with ‘Our new in situ 14C results provide improved constraints…’
Line 300: Again, ‘thicker’ and ‘thinner’ are relative terms - say relative to what. Suggest just saying 'thicker than present at the LGM'
Line 302: ‘between thinner ice in the interior and thicker ice at the coast relative to today’ is confusing to me as worded - suggest rewording to clarify that the hinge zone separates the thinner-than-present LGM ice in the interior from the thicker-than-present LGM ice at the coast.
Line 306: Please provide all relevant code used for the calculations in this paper.
SUPPLEMENT
Line 1: Split Table S1 into at least two tables: 14C measurement data and ages and 10/26 measurement data and ages - all ages should be recalibrated from the original paper using UWv3 or publicly available code from the authors.
For clarity I would suggest combining value and uncertainty columns (i.e., x.xxx±y.yyy) and have them at the same exponent and a common significant figure level (e.g., both should be 10^3 at/g or 10^4 at/g, not one at 10^3 and one at 10^4)
Use the symbol for permil for the units in the stable C column
Quartz column: GR01 should read 0.6034. Have all values in this column at 4 decimal places.
All values in Carbon yield and diluted carbon columns should have one decimal place
I would recommend 4 decimal places for all scientific notations, and combine value ± uncertainty in a single column with a single exponent common to the entire column. Pay attention to significant figures. Carry as many as possible through each column so the reader can arrive at the same value as the authors. But no need to carry extra, as in the 10/9 ratio column. No way we know those numbers better than the nearest 10-100e-15 values.
Line 3: ‘Table of sample measurement details’ should be "Notes" below the table.
‘0.58 ± 0.31’ 1 sigma? 2 sigma? Standard deviation? Seems like something like standard deviation from the blank data in Balco et al. (2023). Present all blank data from the time of the extractions, or reference blanks presented in Balco et al (2023) for that period if that is a complete record of that time period. If that dataset is valid, the blank fluctuated by about a factor of 4 over the period covering the TUCNL numbers represented here.
‘Where the 1 sigma…’: So, are all measurement uncertainties in this paper quoted as 1 sigma? At any rate the 6% value is on a concentration – that is not applicable to any of the other measurement columns. Clarify that. As noted earlier I'm also dubious about the 6% value since the CRONUS-A data in Balco et al. (2023) has a standard deviation of ca. 8.3%, and if you just look at the subset of measurements from Goehring et al. (2019), that value is ca. 8.7%. And the last two CRONUS-A measurements in Balco et al. (2023) are significantly higher than any of the previous values, and stop quite a bit earlier than the sample numbers here (60-100 samples later than the last of the ones listed in Balco et al.) – standard deviation of the whole dataset is over 10% if those are included. Include any additional CRONUS-A measurements from the time period spanning the measurements here to demonstrate either that the two high values are just scatter significantly outside the mean of the other samples, or that they represent a new mean if there was some sort of procedural change that happened to cause them to be higher from that point onward. In which case the default production rate in UWv3 is incorrect. If there was a procedural change at that time, that should also be clearly described and justified.
Line 12: If you need to break this 14C table across two pages you should just make a second table with 14C concentrations and ages, but also have a column on the left with the sample IDs for each page. Same for a separate table with 10Be and 26Al measurements – each should have the IDs.
Blanks for the system need to be presented for the time frame of the extractions here. The authors need to demonstrate that they are consistent with what they claim for the long-term blank, which was calculated from earlier data. As noted earlier, in Balco et al. (2023) there were periods in which the blanks deviated from that mean value by quite a bit.
Define 'effective blank'. This should be listed next to the blank-corrected total 14C columns. Also consider having just a % uncertainty column as with 10Be and 26Al.
Notes for this table should indicate how the ages were calculated, and which production rate dataset was used.
Line 21: In general it is clearer in tables to have the units entirely below the column heading, in a different size or typeface (bold, italic, etc.), instead of running on to the end of the heading without any typographic differences.
The 27 in 27Al should be a superscript
What sigma level is represented by the uncertainties? Are they treated similarly to the 14C, with comparison to replicate CRONUS-A or another repeat measurement?
The [26Al] % uncertainty column does not reflect the uncertainties and measurements in the previous column of atoms/g
Citation: https://doi.org/10.5194/egusphere-2024-2674-RC3
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