Did Holocene climate changes drive West Antarctic  grounding line retreat and readvance?

Neuhaus, Sarah U.; Tulaczyk, Slawek M.; Stansell, Nathan D.; Coenen, Jason J.; Scherer, Reed P.; Mikucki, Jill A.; Powell, Ross D.

doi:https://doi.org/10.5194/tc-15-4655-2021

Articles | Volume 15, issue 10

https://doi.org/10.5194/tc-15-4655-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-15-4655-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 10

Research article

|

05 Oct 2021

Research article |

| 05 Oct 2021

Did Holocene climate changes drive West Antarctic grounding line retreat and readvance?

Sarah U. Neuhaus, Slawek M. Tulaczyk, Nathan D. Stansell, Jason J. Coenen, Reed P. Scherer, Jill A. Mikucki, and Ross D. Powell

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Final revised paper (published on 05 Oct 2021)
Supplement to the final revised paper
Preprint (discussion started on 19 Nov 2020)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review for "Did Holocene climate changes drive West Antarctic grounding line retreat and re-advance?"', Anonymous Referee #1, 31 Dec 2020
- SC1: 'Reply on RC1', Sarah Neuhaus, 04 Jan 2021
- AC1: 'Reply on RC1', Sarah Neuhaus, 03 May 2021
RC2: 'Review for Neuhaus et al - Did Holocene climate changes....?', Anonymous Referee #2, 15 Mar 2021
- SC2: 'Reply on RC2', Sarah Neuhaus, 20 Mar 2021
- AC2: 'Reply on RC2', Sarah Neuhaus, 03 May 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Publish subject to revisions (further review by editor and referees) (06 May 2021) by Delphine Lannuzel

AR by Sarah Neuhaus on behalf of the Authors (17 Jun 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (28 Jun 2021) by Delphine Lannuzel

RR by Anonymous Referee #1 (11 Jul 2021)

Suggestions for revision or reasons for rejection

In the revised manuscript entitled, “Did Holocene climate changes drive West Antarctic grounding line retreat and readvance”, Neuhaus and colleagues have addressed most of the concerns presented in my first review. The authors have included uncertainties in their modeled chronological constraints on grounding line retreat and re-advance, along with significant differences in timing from the first version of this manuscript. As the objective of this manuscript is to shift away from the glaciological forcing presented by Kingslake et al. (2018) and toward a climate forcing, the timing and inclusion of uncertainties is very important. I note that though the peak model matches presented in the first version of this manuscript may be loosely correlated with modest changes in climate, a correlation between revised timing (with uncertainty) and changes in Holocene climate requires a more rigorous explanation than what is currently presented. My primary concerns remain:

1. Whillans Ice Stream is the only ice stream in the study with two sites along a flowline. This serves as an opportunity to test whether the model produces realistic results. I note that the upstream site (WIS) resulted in an earlier retreat time (4700 years ago) than the downstream site (SLW; 4300 years ago). Because this distribution of timing does not make physical sense, I am wondering if these were typed in backwards? Should SLW be 4700 years ago and upstream WIS be 4300 years ago? If this was not a mistake, this distribution of timing must be further explained. Additionally, there should be further discussion of why WIS and KIS/BIS sites result in significantly different retreat timing, as they are quite close together and would be experiencing similar changes in climate during the Holocene. The addition of Halberstadt et al. (2016)'s marine-based model indicates that the authors have considered differences in underlying geology/bathymetry. It should be stated clearly whether this is the assumed reason for differences in retreat.

2. Do the results support a claim of a climate forcing? Retreat timings computed at BIS and KIS sites significantly overlap with the re-advance timings. The explanation for re-advance is slight cooling observed in the region over the last 2000 years, however, I note that the re-advance timing of these two sites falls closely in line with warm periods in the Ross Sea (Hall et al., 2006; grey shaded box in figure 8) that the authors state forced retreat at WIS and SLW earlier in the Holocene. The full range of uncertainty for retreat encompasses 4000 years, a range that exceeds any of the associated changes in climate being used to explain the observed retreat and re-advance.

3. Assumptions of the model: Hodson et al. (2016) demonstrated that sediment transport occurs in a cm’s thick layer of deformable till beneath Whillans Ice Stream. This mechanism could result in the transport of old (14C-free) carbon to (and deposition at) sites as ice streams flow over them. This point was overlooked in the manuscript and the response to review.

As the authors state in their concluding paragraph, this study presents an interesting and new use of old data. However, it is too strong of a claim to state that the modeled chronological constraints for retreat and re-advance can be explained by a climate forcing when viewed through the lens of the full model output. I recommend that acknowledgement of conjecture come before the last paragraph so that the efforts herein are not oversold as a new/alternative explanation for observed Holocene changes. I would suggest that the future version of this manuscript be reframed to focus on the methodological advances so that sufficient explanation for model assumptions, sensitivity, and uncertainty be presented in the main text.

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RR by Mike Bentley (12 Jul 2021)

Suggestions for revision or reasons for rejection

Comments on revised Neuhaus et al

The revised paper is substantially improved from the original version I reviewed and I believe that the authors have addressed almost all of my comments. The figures in particular are clearer, the captions explain them better and uncertainty is better represented throughout the figures. As I noted previously I think the paper is a novel contribution to the debate and provides important constraints for understanding Holocene grounding line retreat in the Ross Sea. I also would point out again the importance seeing novel work using innovative approaches to archived samples: this sort of study is very welcome.

In the abstract I think that it would be helpful to include the word ‘primarily’ or ‘dominantly’ when referring to climatic controls. The authors have not ruled out GIA processes, only that they suggest climate was dominant. Indeed in the introduction the authors only say that their results are ‘consistent with a climatic forcing..’. So last sentence of abstract could read: Based on these results, we propose that the Siple Coast grounding line motions in the mid- to late-Holocene were primarily driven by relatively modest changes in regional climate, rather than by ice sheet dynamics and glacioisostatic rebound, as hypothesized previously (Kingslake et al., 2018).

A similar edit is likely to be needed in discussion. This is important and scientifically prudent because the authors do not find a complete match and they cannot rule out some GIA control on readvance.

In my first review I noted that the fields on Fig 9 (now Fig 3) were plotted in Lamb et al (2006) using wt%:wt% not atom:atom (molar) ratios. My point was that there is a conversion between these two units (x1.17) and this needed to be applied if the authors were to plot their data from the atim:atiom measurements. The response claims that Lamb et al uses atom:atom and that therefore the plot in Fig 3 is unmodified. This is not my specialism but I have to note again I have checked and Lamb et al (2006) note very clearly in the text on their p.30 that they use wt% ratios unless stated otherwise (and their field diagram does not state otherwise):
“The weight ratio of organic carbon to total nitrogen (C/N) is normally measured alongside d13C, and can also help to distinguish carbon sources. Occasionally, the weight ratio is converted into an atomic/molar ratio by multiplying by 1.17; however, the ratio used is not always stated. Here, C/N refers to the weight ratio unless stated otherwise.” Lamb et al (2006)
I am not a specialist in this field but unless I have missed something, the response implies that the Lamb et al (2006) fields and the data from this study continue to be plotted on different x-scales in Fig 3 and therefore are offset and the plot is in error. This may need someone more expert to take a look or to ask Angela Lamb directly.

Line 645: Additional

Symbology on Fig 9 - the captions in the revised paper are much richer and more helpful than in the original submission but fig 9 still needs some further explanation of the shaded colour bars at the base of the figure - what does the shading mean, what are the red (?) boxes towards their younger ends and so on. Would be helpful to more clearly and explicitly match the symbology to the plots in Fig 8 e.g. by placing a dot at the optimal GL retreat timing (along with the lines for uncertainty) and a box and whisker for the readvance timing. This might need a small zoom in inset for this part of the figure

The discussion of saloon door and swinging gate is now much better: it is nuanced, and points out clearly that the model results presented here cannot distinguish between these two canonical models. The discussion of bathymetric controls is good but I would note, however, that the dependence of retreat on bathymetry has been discussed in many of John Anderson’s papers on Antarctic deglaciation for several decades and he should be cited here. The most recent paper, and perhaps most generally applicable is perhaps this one: Anderson,John B. et al.. 2019. Seismic and geomorphic records of Antarctic Ice Sheet evolution in the Ross Sea and controlling factors in its behaviour, Geological Society, London, Special Publications(2019),475(1):223 https://doi.org/10.1144/SP475.5 but there are several earlier examples.

Mike Bentley, July 2021

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ED: Publish subject to revisions (further review by editor and referees) (16 Jul 2021) by Delphine Lannuzel

AR by Sarah Neuhaus on behalf of the Authors (27 Aug 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (14 Sep 2021) by Delphine Lannuzel

AR by Sarah Neuhaus on behalf of the Authors (14 Sep 2021) Manuscript

Short summary

We estimate the timing of post-LGM grounding line retreat and readvance in the Ross Sea sector of Antarctica. Our analyses indicate that the grounding line retreated over our field sites within the past 5000 years (coinciding with a warming climate) and readvanced roughly 1000 years ago (coinciding with a cooling climate). Based on these results, we propose that the Siple Coast grounding line motions in the middle to late Holocene were driven by relatively modest changes in regional climate.