The ST22 chronology for the Skytrain Ice Rise ice core &ndash; part 2: an age model to the last interglacial and disturbed deep stratigraphy

Mulvaney, Robert; Wolff, Eric William; Grieman, Mackenzie; Hoffmann, Helene; Humby, Jack; Nehrbass-Ahles, Christoph; Rhodes, Rachael; Rowell, Isobel; Parrenin, Frédéric; Schmidely, Loïc; Fischer, Hubertus; Stocker, Thomas; Christl, Marcus; Muscheler, Raimund; Landais, Amaelle; Prié, Frédéric

doi:https://doi.org/10.5194/cp-2022-84

Preprints

https://doi.org/10.5194/cp-2022-84

Preprints

07 Nov 2022

| 07 Nov 2022

Status: a revised version of this preprint was accepted for the journal CP and is expected to appear here in due course.

The ST22 chronology for the Skytrain Ice Rise ice core – part 2: an age model to the last interglacial and disturbed deep stratigraphy

Robert Mulvaney, Eric William Wolff, Mackenzie Grieman, Helene Hoffmann, Jack Humby, Christoph Nehrbass-Ahles, Rachael Rhodes, Isobel Rowell, Frédéric Parrenin, Loïc Schmidely, Hubertus Fischer, Thomas Stocker, Marcus Christl, Raimund Muscheler, Amaelle Landais, and Frédéric Prié

Abstract. We present an age model for the 651 m deep Skytrain Ice Rise ice core. The top 2000 years have previously been dated using age markers interpolated through annual layer counting. Below this, we align the Skytrain core to the AICC2012 age model using tie points in the ice and air phase, and apply the Paleochrono program to obtain the best fit to the tie points and glaciological constraints. In the gas phase, ties are made using methane and, in critical sections, δ¹⁸O_air; in the ice phase ties are through ¹⁰Be across the Laschamps Event, and through ice chemistry related to long-range dust transport and deposition. This strategy provides a good outcome to about 108 ka (~605 m). Beyond that there are signs of flow disturbance, with a section of ice probably repeated. Nonetheless values of CH₄ and δ¹⁸O_air confirm that part of the last interglacial (LIG), from about 117–126 ka (617–628 m), is present and in chronological order. Below this there are clear signs of stratigraphic disturbance, with rapid oscillation of values in both the ice and gas phase at the base of the LIG section. Based on methane values, the warmest part of the LIG and the coldest part of the penultimate glacial are missing from our record. Ice below 631 m appears to be of age >150 ka.

Received: 25 Oct 2022 – Discussion started: 07 Nov 2022

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Robert Mulvaney et al.

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RC1:
'Review Mulvaney et al ST22', Anonymous Referee #1, 23 Nov 2022
Mulvaney et al. present a timescale for the Skytrain Ice Rise ice core which covers all ice older than 2 ka. The timescale to ~106 ka is continuous and based on methane matches using the continuous methane record supplemented by discreet measurements. They then identify an area of ice flow disturbance. Using discrete measurements of d18O of O2 and methane from the same depths, they confirm a flow disturbance, date a section below the disturbance to ~106-126ka and suggest a continuous climate record during this interval. Below this, the ice is again disturbed and likely from the penultimate glacial period. Interestingly, both the onset of the Last Interglacial and the Penultimate Glacial Maximum are missing, which the authors suggest is due to flow disturbances caused by contrasts in ice fabric.

The paper is important, well written and should be published with minor revisions. This paper will be foundational for what I image are future high impact papers on the climate and ice sheet interpretation for which a timescale is necessary. The authors have made a wide range of measurements and performed a thorough analysis of the core. The timescale is well developed with a lot of care taken to explain the approach. I have some recommendations for making this even clearer, but I appreciate the effort the authors have taken. I have only two area to suggest significant changes. The first is the timescale from about about 75 ka to 109 ka. The second is the uncertainty of the timescale.

I also want to specific note that I appreciate that the authors have made the data publicly available.

First, one initial thought on the introduction. The authors lead the introduction with a statement about the “intense interest” in the stability of WAIS and the need for paleoclimate records to constrain potential ice sheet changes. Later they write “old ice might be available … Berkner Island and Fletcher Promonotory, but there is no published age scale for these cores so far”. The lead author of this paper was the project leader for both of those ice cores. I hope that this work on Skytrain inspires those cores to be revisited and published.

The timescale for this period seems more uncertain than the authors imply and could use more description. I realize much of the emphasis of this paper is on the LIG, but I think it is important to discuss where the end of the continuous climate record is reached.

The methane matches are not particularly robust given how different the shapes are, like the wider width of the peak centered on ~82 ka. The subpeak at 80 ka is also distinctly different. The nssMg does seem to help in this period.

The nssMg match is less robust in the period 95 ka to 109ka. At 95 ka, the methane rise at Skytrain looks substantially larger than the EPICA composite. Since there are folds in this area, I think more explanation should be given about why the methane features at ~95 ka and ~106ka are not a repeat of younger events.

I think a new section which discusses this interval would be help. With more description, I might agree that the timescale is continuous to 109 ka.

The uncertainty is not discussed substantially in the manuscript. The uncertainties are given as part of the paleochrono output in the supplement, but I think these are likely too low. They assume a continuous climate record with well behaved ice-dynamics and confident tie points. I think this is likely only justified to about 75 ka. Given the meter to decameter scale folding, there is almost certainly smaller scale folding as well. Thus, the assumption of continuously increasing age even in sections that are primarily intact is not a given (hopefully the stratigraphy is being looked at for a future paper because it could be fascinating). And the confidence in the tie points seems to drop. The 82 ka methane peak is a good example. The uncertainty is given as <300 years but it looks like there is a 1000 year offset between the midpoints of the rise. I suggest two things:

the uncertainty be manually adjusted to be greater for the older portions of the core. This can be a qualitative uncertainty. It would serve as a warning to users in a way that text in the manuscript would not because it would travel with the timescale data file

Add a section and a figure which specifically discusses the uncertainty and how it changes through the core.

The figures and discussion of the timescale older than 106 ka can be revised to make it even easier to follow. Many of these figures should be integrated with subpanels because trying to find and compare the figures was challenging. At one point, I had a printed version open to three different pages and two electronic versions open on my screen. Here’s a couple of suggestions:

Provide a plot of the methane and d18O measurements below 600m by depth. The methane variations are not visible in Figure 3 because it spans the full core. When the methane is already matched and put on the timescale (e.g. Figure 5), the record can be deceiving.

The methane-d18O cross plots should be combined into fewer figures. I suggest combining Figures 8 and 9 so that is much easier to compare the reference records with the cross plot. The cross-plot should also be extended to start at ~60 ka, such that the two intervals at ~80ka and ~100 ka, which have similar d18O-O2 and ch4 values similar to the LIG, are shown. Figures 10 and 11 should be combined with subpanels and have the same length of time shown – i.e. both run from 100 to 130 ka so that the colors remain the same in the two cross plots.

L503 – Can you rule out ice from the Ellsworth mountains flowing to Skytrain ice rise at a time when Skytrain was lower in elevation (and not an ice rise)? Are the elevations Ellsworths and its glaciers not sufficient to flow onto the bedrock beneath Skytrain? Or are the water isotope values in the Ellsworth mountains too cold? You have demonstrated that there is a stratigraphic disturbance, but I think stating that the “only plausible” explanation is due to contrasting rheology is too strong – particularly since there are no ice fabric measurements presented in the manuscript.
Citation: https://doi.org/10.5194/cp-2022-84-RC1
- AC1: 'Reply on RC1', Eric Wolff, 13 Jan 2023
  
  See attached file
  
  Citation: https://doi.org/10.5194/cp-2022-84-AC1

RC2: 'Comment on cp-2022-84', Anonymous Referee #2, 13 Dec 2022

Review of

Authors: Robert Mulvaney, Eric W. Wolff, et al

General Comments: The manuscript presents the age-scale for the deep section of the Skytrain Ice Rise, below the depths where annual layer counts are evident. The methodology chosen by the authors is robust and is similar to that chosen for other Antarctic ice cores. The manuscript is well written with generally good presentation. Relevant work is properly cited. I recommend publishing pending minor changes.

I am not sure of the claim that Skytrain is the only WAIS ice core that extends into MIS5 (the last interglacial period). Siple Dome extends to at least 100 ka (Severinghaus et al, 2009) and maybe to 118 Ka (Dunbar et al, 2011). RICE likely contains ice from ~90 ka. Taylor Glacier, which is on the border between East and West Antarctic similar to Hercules Dome, has been measured for MIS5-4 transition (Menking et al, 2019).
Beyond establishing the ST22 age scale, the authors hypothesize that ice from the last interglacial period is inherently prone to stratigraphic disturbances (e.g. folding). The Skytrain ice core is poorly suited for testing this hypothesis since this age range is within the bottom 50 m of the core, near the basal interface and stratigraphic disturbances may be expected. The authors do not convincingly demonstrate that the discontinuities observed in the Skytrain chronology are due to processes related to the age of the ice.
Several of the figures could be combined into panels such as figures 4-7, all of which show how tie points were matched using reference records, and figures 9-12, which all show CH4 and d18O cross plots.

Specific Comments:

Line
22	“… Skytrain ice rise ice core.” There should be some description of where this ice core is (Greenland vs Antarctica, Ross Sea Basin or Atlantic side of WAIS…)
32	“base of LIG section” is confusing. Please clarify.
38	“Antarctic Ice Sheet” ï “Ice sheets of Antarctica”. “Antarctic Ice Sheet” is a misnomer.
56-57	“The Siple Dome core reached the bed at 1004 m, but again data have only been presented as far back as 90 ka (Brook et al., 2005; Saltzman et al., 2006)” Severinghaus et al (2009) presents d18Oatm data back to 100 ka (973 m)
58	In Lee et al (2020), there appear to have MIS5 ice near the base as identified by negative d18Oatm that are unique to interglacial periods.
87	“(J. Kingslake, pers. Comm.)” Personal communication is not an acceptable reference. (https://www.climate-of-the-past.net/submission.html#references)
123	“as has been observed in other ice core records including those of the LIG in Greenland”. See general comments.
145	CFA has not yet been defined.
154	“discrete samples analysed in Bern” ï add something like “discussed below” or a reference for the discrete sample system.
175	“ginput” MATLAB function. This function returns the [x,y] coordinates of a point plotted on a graph. It does not describe a method of identifying and removing outlier data. Do you mean, “manual identification”?
Fig 3	It is hard to tell how well the CFA and discrete data represent each other. You could try a cross-plot as presented in Chappellaz et al (2013) Figure 6 and 7. Is it possible to “correct” the cfa data as they did for the NEEM record?
212	There should be a reference to a paper describing the system.
Fig 4	Why do the well known 5.2 and 8.2 ka methane variations not line up with the reference methane record?
Fig 5	Why are there such large age offsets between when nssMg is observed to increase in Skytrain at 90 and 105ka compared to EDML?
355-356	“…with some thousands of years potentially missing from our record” Are they missing or smoothed out? The reference record show d18Oatm approaching values of -0.4 permil at the peak of MIS5e. Based on the compressed age scale at this part of the core, a sample of 5-10 cm length could represent a significant amount of time. 5-10 cm is the length of typical discrete CH4 samples (Schmidley et al 2020) and 5-6 cm was typical system response of CFA CH4 (Rhodes 2017). This smoothing is on top of uncertainty of interpolation caused by measuring d18Oatm on adjacent samples, not on identical depths. Basically, I question whether uncertainty in age horizons are properly handled and suggest that the authors provide more discussion.
Figure 9	This should probably show the cross-plot for age range 60-140 ka as the authors need to prove that the ice is not just folded duplicates of MIS3, 4, 5a.
Figure 13	This would be nice to also show age uncertainty (as is output by ICECHRONO) and annual layer thickness as is provided in Buiron et al. 2011, Lee et al 2020). These are important metrics in understanding an ice core chronology.
515	Were there d15N-N2 measurements made in parallel to the d18Oatm? Wouldn’t this give you some idea of the accumulation rate?
518	“bubble-free ice” could be confused with clathrated ice

Add'l References:

Buiron, D., Chappellaz, J., Stenni, B., Frezzotti, M., Baumgartner, M., Capron, E., Landais, A., Lemieux-Dudon, B., Masson-Delmotte, V., Montagnat, M., Parrenin, F., and Schilt, A.: TALDICE-1 age scale of the Talos Dome deep ice core, East Antarctica, Clim. Past, 7, 1–16, https://doi.org/10.5194/cp-7-1-2011, 2011

Chappellaz, J., Stowasser, C., Blunier, T., Baslev-Clausen, D., Brook, E. J., Dallmayr, R., Faïn, X., Lee, J. E., Mitchell, L. E., Pascual, O., Romanini, D., Rosen, J., and Schüpbach, S.: High-resolution glacial and deglacial record of atmospheric methane by continuous-flow and laser spectrometer analysis along the NEEM ice core, Clim. Past, 9, 2579–2593, https://doi.org/10.5194/cp-9-2579-2013, 2013.

Dunbar, Nelia W., and Andrei V. Kurbatov. "Tephrochronology of the Siple Dome ice core, West Antarctica: correlations and sources." Quaternary Science Reviews 30.13-14 (2011): 1602-1614.

Menking, J. A., Brook, E. J., Shackleton, S. A., Severinghaus, J. P., Dyonisius, M. N., Petrenko, V., McConnell, J. R., Rhodes, R. H., Bauska, T. K., Baggenstos, D., Marcott, S., and Barker, S.: Spatial pattern of accumulation at Taylor Dome during Marine Isotope Stage 4: stratigraphic constraints from Taylor Glacier, Clim. Past, 15, 1537–1556, https://doi.org/10.5194/cp-15-1537-2019, 2019.

Severinghaus, Jeffrey P., et al. "Oxygen-18 of O2 records the impact of abrupt climate change on the terrestrial biosphere." Science 324.5933 (2009): 1431-1434.

Citation: https://doi.org/10.5194/cp-2022-84-RC2

AC2: 'Reply on RC2', Eric Wolff, 13 Jan 2023

See attached file

Citation: https://doi.org/10.5194/cp-2022-84-AC2

Status: closed

RC1:
'Review Mulvaney et al ST22', Anonymous Referee #1, 23 Nov 2022
Mulvaney et al. present a timescale for the Skytrain Ice Rise ice core which covers all ice older than 2 ka. The timescale to ~106 ka is continuous and based on methane matches using the continuous methane record supplemented by discreet measurements. They then identify an area of ice flow disturbance. Using discrete measurements of d18O of O2 and methane from the same depths, they confirm a flow disturbance, date a section below the disturbance to ~106-126ka and suggest a continuous climate record during this interval. Below this, the ice is again disturbed and likely from the penultimate glacial period. Interestingly, both the onset of the Last Interglacial and the Penultimate Glacial Maximum are missing, which the authors suggest is due to flow disturbances caused by contrasts in ice fabric.

The paper is important, well written and should be published with minor revisions. This paper will be foundational for what I image are future high impact papers on the climate and ice sheet interpretation for which a timescale is necessary. The authors have made a wide range of measurements and performed a thorough analysis of the core. The timescale is well developed with a lot of care taken to explain the approach. I have some recommendations for making this even clearer, but I appreciate the effort the authors have taken. I have only two area to suggest significant changes. The first is the timescale from about about 75 ka to 109 ka. The second is the uncertainty of the timescale.

I also want to specific note that I appreciate that the authors have made the data publicly available.

First, one initial thought on the introduction. The authors lead the introduction with a statement about the “intense interest” in the stability of WAIS and the need for paleoclimate records to constrain potential ice sheet changes. Later they write “old ice might be available … Berkner Island and Fletcher Promonotory, but there is no published age scale for these cores so far”. The lead author of this paper was the project leader for both of those ice cores. I hope that this work on Skytrain inspires those cores to be revisited and published.

The timescale for this period seems more uncertain than the authors imply and could use more description. I realize much of the emphasis of this paper is on the LIG, but I think it is important to discuss where the end of the continuous climate record is reached.

The methane matches are not particularly robust given how different the shapes are, like the wider width of the peak centered on ~82 ka. The subpeak at 80 ka is also distinctly different. The nssMg does seem to help in this period.

The nssMg match is less robust in the period 95 ka to 109ka. At 95 ka, the methane rise at Skytrain looks substantially larger than the EPICA composite. Since there are folds in this area, I think more explanation should be given about why the methane features at ~95 ka and ~106ka are not a repeat of younger events.

I think a new section which discusses this interval would be help. With more description, I might agree that the timescale is continuous to 109 ka.

The uncertainty is not discussed substantially in the manuscript. The uncertainties are given as part of the paleochrono output in the supplement, but I think these are likely too low. They assume a continuous climate record with well behaved ice-dynamics and confident tie points. I think this is likely only justified to about 75 ka. Given the meter to decameter scale folding, there is almost certainly smaller scale folding as well. Thus, the assumption of continuously increasing age even in sections that are primarily intact is not a given (hopefully the stratigraphy is being looked at for a future paper because it could be fascinating). And the confidence in the tie points seems to drop. The 82 ka methane peak is a good example. The uncertainty is given as <300 years but it looks like there is a 1000 year offset between the midpoints of the rise. I suggest two things:

the uncertainty be manually adjusted to be greater for the older portions of the core. This can be a qualitative uncertainty. It would serve as a warning to users in a way that text in the manuscript would not because it would travel with the timescale data file

Add a section and a figure which specifically discusses the uncertainty and how it changes through the core.

The figures and discussion of the timescale older than 106 ka can be revised to make it even easier to follow. Many of these figures should be integrated with subpanels because trying to find and compare the figures was challenging. At one point, I had a printed version open to three different pages and two electronic versions open on my screen. Here’s a couple of suggestions:

Provide a plot of the methane and d18O measurements below 600m by depth. The methane variations are not visible in Figure 3 because it spans the full core. When the methane is already matched and put on the timescale (e.g. Figure 5), the record can be deceiving.

The methane-d18O cross plots should be combined into fewer figures. I suggest combining Figures 8 and 9 so that is much easier to compare the reference records with the cross plot. The cross-plot should also be extended to start at ~60 ka, such that the two intervals at ~80ka and ~100 ka, which have similar d18O-O2 and ch4 values similar to the LIG, are shown. Figures 10 and 11 should be combined with subpanels and have the same length of time shown – i.e. both run from 100 to 130 ka so that the colors remain the same in the two cross plots.

L503 – Can you rule out ice from the Ellsworth mountains flowing to Skytrain ice rise at a time when Skytrain was lower in elevation (and not an ice rise)? Are the elevations Ellsworths and its glaciers not sufficient to flow onto the bedrock beneath Skytrain? Or are the water isotope values in the Ellsworth mountains too cold? You have demonstrated that there is a stratigraphic disturbance, but I think stating that the “only plausible” explanation is due to contrasting rheology is too strong – particularly since there are no ice fabric measurements presented in the manuscript.
Citation: https://doi.org/10.5194/cp-2022-84-RC1
- AC1: 'Reply on RC1', Eric Wolff, 13 Jan 2023
  
  See attached file
  
  Citation: https://doi.org/10.5194/cp-2022-84-AC1

RC2: 'Comment on cp-2022-84', Anonymous Referee #2, 13 Dec 2022

Review of

Authors: Robert Mulvaney, Eric W. Wolff, et al

I am not sure of the claim that Skytrain is the only WAIS ice core that extends into MIS5 (the last interglacial period). Siple Dome extends to at least 100 ka (Severinghaus et al, 2009) and maybe to 118 Ka (Dunbar et al, 2011). RICE likely contains ice from ~90 ka. Taylor Glacier, which is on the border between East and West Antarctic similar to Hercules Dome, has been measured for MIS5-4 transition (Menking et al, 2019).
Beyond establishing the ST22 age scale, the authors hypothesize that ice from the last interglacial period is inherently prone to stratigraphic disturbances (e.g. folding). The Skytrain ice core is poorly suited for testing this hypothesis since this age range is within the bottom 50 m of the core, near the basal interface and stratigraphic disturbances may be expected. The authors do not convincingly demonstrate that the discontinuities observed in the Skytrain chronology are due to processes related to the age of the ice.
Several of the figures could be combined into panels such as figures 4-7, all of which show how tie points were matched using reference records, and figures 9-12, which all show CH4 and d18O cross plots.

Specific Comments:

Line
22	“… Skytrain ice rise ice core.” There should be some description of where this ice core is (Greenland vs Antarctica, Ross Sea Basin or Atlantic side of WAIS…)
32	“base of LIG section” is confusing. Please clarify.
38	“Antarctic Ice Sheet” ï “Ice sheets of Antarctica”. “Antarctic Ice Sheet” is a misnomer.
56-57	“The Siple Dome core reached the bed at 1004 m, but again data have only been presented as far back as 90 ka (Brook et al., 2005; Saltzman et al., 2006)” Severinghaus et al (2009) presents d18Oatm data back to 100 ka (973 m)
58	In Lee et al (2020), there appear to have MIS5 ice near the base as identified by negative d18Oatm that are unique to interglacial periods.
87	“(J. Kingslake, pers. Comm.)” Personal communication is not an acceptable reference. (https://www.climate-of-the-past.net/submission.html#references)
123	“as has been observed in other ice core records including those of the LIG in Greenland”. See general comments.
145	CFA has not yet been defined.
154	“discrete samples analysed in Bern” ï add something like “discussed below” or a reference for the discrete sample system.
175	“ginput” MATLAB function. This function returns the [x,y] coordinates of a point plotted on a graph. It does not describe a method of identifying and removing outlier data. Do you mean, “manual identification”?
Fig 3	It is hard to tell how well the CFA and discrete data represent each other. You could try a cross-plot as presented in Chappellaz et al (2013) Figure 6 and 7. Is it possible to “correct” the cfa data as they did for the NEEM record?
212	There should be a reference to a paper describing the system.
Fig 4	Why do the well known 5.2 and 8.2 ka methane variations not line up with the reference methane record?
Fig 5	Why are there such large age offsets between when nssMg is observed to increase in Skytrain at 90 and 105ka compared to EDML?
355-356	“…with some thousands of years potentially missing from our record” Are they missing or smoothed out? The reference record show d18Oatm approaching values of -0.4 permil at the peak of MIS5e. Based on the compressed age scale at this part of the core, a sample of 5-10 cm length could represent a significant amount of time. 5-10 cm is the length of typical discrete CH4 samples (Schmidley et al 2020) and 5-6 cm was typical system response of CFA CH4 (Rhodes 2017). This smoothing is on top of uncertainty of interpolation caused by measuring d18Oatm on adjacent samples, not on identical depths. Basically, I question whether uncertainty in age horizons are properly handled and suggest that the authors provide more discussion.
Figure 9	This should probably show the cross-plot for age range 60-140 ka as the authors need to prove that the ice is not just folded duplicates of MIS3, 4, 5a.
Figure 13	This would be nice to also show age uncertainty (as is output by ICECHRONO) and annual layer thickness as is provided in Buiron et al. 2011, Lee et al 2020). These are important metrics in understanding an ice core chronology.
515	Were there d15N-N2 measurements made in parallel to the d18Oatm? Wouldn’t this give you some idea of the accumulation rate?
518	“bubble-free ice” could be confused with clathrated ice

Add'l References:

Dunbar, Nelia W., and Andrei V. Kurbatov. "Tephrochronology of the Siple Dome ice core, West Antarctica: correlations and sources." Quaternary Science Reviews 30.13-14 (2011): 1602-1614.

Severinghaus, Jeffrey P., et al. "Oxygen-18 of O2 records the impact of abrupt climate change on the terrestrial biosphere." Science 324.5933 (2009): 1431-1434.

Citation: https://doi.org/10.5194/cp-2022-84-RC2

AC2: 'Reply on RC2', Eric Wolff, 13 Jan 2023

See attached file

Citation: https://doi.org/10.5194/cp-2022-84-AC2

Robert Mulvaney et al.

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https://doi.org/10.5194/cp-2022-84-supplement

Robert Mulvaney et al.

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Short summary

We present an age scale for a new ice core drilled at Skytrain Ice Rise, an ice rise facing the Ronne Ice Shelf in Antarctica. Various measurements in the ice and air phases are used to match the ice core to other Antarctic cores that have already been dated. The 651 m ice core includes ice that is confidently dated to the last interglacial, 126,000 years ago. Older ice is found deeper down, but there are flow disturbances in the deeper ice.


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