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
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
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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