the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jun 2024
Drastic changes in Depositional Environments at the Ross Sea Continental Margin since the Mid-Pleistocene: More evidence for West Antarctic Ice Sheet collapse
Abstract. This study investigates a sediment core (RS15-LC47) from the Ross Sea continental rise to elucidate the sea-ice interaction and resulting paleodepositional changes over the past 800 ka. By integrating whole-core Magnetic Susceptibility (MS), sediment biogenic components (TOC, CaCO3, and biogenic silica), sedimentological features, and the isotopic ratio of authigenic beryllium (10Be/9Be)reac, we unravel the paleoenvironmental changes and their influence on the sedimentary processes. The lower segment of the investigated interval (750–550 ka) exhibits distinct lithological characteristics, including parallel and cross laminations, along with millimeter-scale faults, suggestive of contourite depositional processes. This section also displays irregular trends in MS values due to poorly sorted sediments, characteristics feature of sediment slumping. The lowest (10Be/9Be)reac ratio in this interval suggests reduced Circumpolar Deep Water (CDW) inflow due to strengthened Antarctic Slope Current (ASC). Although the Total Organic Carbon (TOC) is highest in this interval, high Carbon-to-Nitrogen (C/N) ratio and low Barium excess (Baex) suggests reduced marine productivity due to increased terrestrial input likely from advancing ice sheets. Following the Mid-Pleistocene Transition (MPT), MS values remain consistently low until MIS 8 (~250 ka) and (10Be/9Be)reac relatively increases, indicating persistent lukewarm condition. We hypothesize this timeframe favorable for ice-shelf disintegration and possible collapse of the West Antarctic Ice Sheet (WAIS). Between 550 and 250 ka, TOC/TN levels resemble those observed in the euphotic layer of the Ross Sea, with relatively higher Baex and TOC, indicating higher productivity during an extended lukewarm condition. During the late Pleistocene (> 250 ka), coarser grain size and IRD-rich layers suggest strengthening of bottom currents. The upwelling of CDW facilitated a drastic increase in the (10Be/9Be)reac ratio during the late Pleistocene. OpalMAR and TOC % exhibit positive trends with (10Be/9Be)reac during the late Pleistocene interglacials, suggesting increased productivity during warmer periods.
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RC1: 'Comment on cp-2024-38', Christian Ohneiser, 02 Aug 2024
'Drastic changes in Depositional Environments at the Ross Sea Continental Margin since the Mid-Pleistocene: More evidence for West Antarctic Ice Sheet collapse.'
The title overstates what the manuscript is able to demonstrate in its current form.. or perhaps in general.
1. A lot of weight has ben placed on interpretation of the magnetic susceptibility but the assumptions made are incorrect. MS is a magnetic mineral concentration proxy and will provide insights into the ratios of ferri/ferro magnetic, paramagnetic and diamagnetic components in a sample. Ohneiser et al 2019 showed the relationship very clearly MS and magnetic mineral concentration at LC47.2. You need to reference the Ohneiser et al 2019 paper which provides the chronology for LC47. The B/M boundary at 11.65 m (773 ka) provides your chronological anchor from which you can correlate to LC42.
3. Carefully read the Bollen et al 2022 paper and see how their data and interpretations work with yours. In their work they develop a more complex chronology of ice advance and retreat than what is presented for LC47 which makes me think that some more detail can be extracted form this record.
4. There appears to be discussion in this manuscript on lower portions of the core than what is presented.
5. Line fits and statistics need to be presented so that we can see how strong the correlations are and in turn how valid the discussion and conclusions are.
6. Ad MIS labels to all figures os we can see what is being referred to.
7. There is no mention of MIS 12. This is a deep cold glacial which some speculate resulted in reorganisation in Antarctic more persistent sea-ice cover.
8. The discussion on CaCO3 preservation needs more development. Bonaccorsi et al., 2007 present a record of only the last few thousand years so this is not comparable with the LC47. No mention is made to carbonate production and its relationship to LC47. What is the carbonate? Is it foraminifera, coccoliths? Is it old recycled washed in from the shelf or new carbonate? Can assumptions on dilution made from CaCo3 data be corroborated with microscope images demonstrating varying dissolution? To discuss carbonate preservation more data are needed and a discussion on assumptions of production rates which will have been strictly during warm interglacials.
9. The Be data discussion needs to be reviewed by a specialist other than me. Figure 6 needs some statistics to see how good the line fits are so the reader can deduce how robust the discussion and conclusions are. At the moment I can't tell if the data support the discussion or conclusions.
10. More work is needed in clear data presentation and their statistical significance before discussions on CDW/ACC and ice sheet evolution can be evaluated.
Lines 24-25. From these data you can't propose a collapse of the WAIS or ice shelf collapse.Line 39. You need a reference to support the statement 'Ross Sea is the primary drainage...'
Line 42: What is meant by elucidating historical ice sheet dynamics? Ice sheet dynamics which have happened since the arrival of humans and documentation? Current ice sheet dynamics?
Line 54: Naish et al do not present organic geochemical data.
Line 55: Suggest referring to the Ohneiser et al 2023 Nat Geo paper here where we showed persistent ice advance and retreat paced by obliquity until 400 ka. Its the only other Pleistocene record from on the shelf which spans the last c 1 myr.
Line 97: You need to reference Ohneiser et al 2019. Ohneiser et al established the magnetostratigraphy and the placed the B-M boundary at 11.65 m (773 Ka) which provides the original age model for LC47 (and LC42). Bollen et al 2022 developed Ohneiser
Line 115. This age has been revised and is now 773 ka.
Line 120: You need to show the statistics associated with the line fits to see how strong the correlation is.
LIne 120: I suggest making cross plots of MS vs biogenic opal and other parameters. I suspect the correlation biogenic % and Ms will be stronger then grainsize and MS.
Line 125: Ohneiser et al 2023 showed that the LR04 does not reflect ice advance and retreat pacing in the Ross Sea. δ18O is a globally mixed record and not region specific.
Line 125:You need to acknowledge here that LC47 may be too far from the ice margin and will capture a mixed oceanographic and ice sheet advance record.
Line 140: Strictly speaking these are no organic geochemical proxies since organic geochemistry implies analysis of carbo base organic molecules (alkenones, alkenes/anes.. other lipids etc etc).
Line 149. It would be good to demonstrate whether the visual correlation is real.. i.e. statistically.
LIne 164. Need to provide a reference on where these broad climatic zones come from.
LIne 170. refer to figure
LIne 189 - poor sorting?
Line 193 - Ohneiser et al 2019 interpreted these thin gravel as lag surfaces which indicate enhanced current strength and winnowing. They likely provide a protective armoured surface once formed that prevents further erosion of underlying sediments.
LIne 197 - what is meant by 'this temporal span'? this interval?
LInes 205-215. This reads like a collection of bullet points. It should be revised into a coherent block of text with references.
Line 214. McKay et al 2012 do not discuss winnowing of magnetic susceptibility.
LIne 260. Im not sure this is a correct statement. The AND-1B site is in a very different setting because it was grounding zone proximal with multiple grounding zone advance and retreat cycles directly over the site. LC47 is oceanic and has never been near the grounding zone.
LIne 261 - where did temperatures increase by 1-2C°?
LInes 266-335- this discussion on Be data needs to be reviewed by someone other than me. Figure 6 needs some statistics to see how good the line fits are so the reader can deduce how robust the discussion and conclusions are.
Lines 338 - most agree that the MPT was complete at around 800 kyr.
LInes 340 etc. the presented interval in LC47 does not extend older than MIS19!
Line 395. The record presented here does not cover the mid-pleistocene transition which is generally agreed to be complete at around 800 kyr. Please revise this section.
Figures:
Fig 1. Why not use a blue colour ramp for regions below sea-level like in the inset Antarctic map? Map C and D need E/W and S° markers on the axes.
FIg 2 fonts are too small and figure is fuzzy. Please tidy this up.
Fig 3. You need to provide line fit data/statistics to support these line fits through your bivariate plots. Panel A should start at 0 unless you have negative values. It looks like these are not very good line fits.
Fig 4. Looks more pink than purple to me. The resolution of the figure is poor. In discontinuous data where there are relatively few points (everything except the MS and isotope record) I suggest marking your data points with a symbol.
Fig 5/6. show statistics!
There are numerous small grammatical errors which need correcting so please carry out a careful proof and get someone outside of the author group to review the manuscript with a fresh set of eyes.
Citation: https://doi.org/10.5194/cp-2024-38-RC1 -
AC1: 'Reply on RC1', Yeong Bae Seong, 22 Sep 2024
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2024-38/cp-2024-38-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Yeong Bae Seong, 22 Sep 2024
-
RC2: 'Comment on cp-2024-38', Anonymous Referee #2, 16 Aug 2024
The authors present a valuable dataset from the Ross Sea region that offers insights into the dynamics of the West Antarctic Ice Sheet over the past ~780,000 years. While they provide a substantial amount of data, there are several issues that need to be addressed.
- General Comments
1.1 Age Model:
In Lines 90-99, the authors state that the age model for RS15-LC47 is based on correlating magnetic susceptibility (MS) to a nearby core referenced in Bollen et al. (2022). They assert a "noteworthy" similarity between their MS records and those from Bollen et al. (2022) in Line 119 and mention a "comprehensive approach utilizing 12 tie points" (Line 114) to align their records with the Bollen et al. (2022) record. However, the authors do not clearly demonstrate how they tested this "similarity" or explain the methods and rationale behind correlating the MS records.
For instance, in Figure 2C, the patterns older than 250 ka, according to the current age model, appear quite different. Bollen et al. (2022) show a relatively muted signal during ~300-550 ka, yet the LC42 record still displays numerous high-frequency peaks. Given the potential for bioturbation and hiatus/event layers in LC47, how can the authors confidently assert that these correlations are "robust"?
This issue is fundamental since an unreliable age constraint undermines the entire discussion. The authors are encouraged to meticulously re-examine their sedimentation records, incorporating more biostratigraphic controls as in Bollen et al. (2022), before making further interpretations.
- Typos and Other Comments:
2.1 The manuscript frequently repeats full names and acronyms. For instance, "Middle-Pleistocene Transition (MPT)" is repeated in Lines 50, 229, 399, 449, and 457, and numerous other terms (AABW, MS, CDW, ASC, etc.) are also redundantly mentioned. The authors should carefully review the manuscript for such repetitions before resubmission.
2.2 The discussion on the MPT event is also perplexing since the record in this manuscript only covers a very brief portion of the later part of the MPT. The authors should consider comparing their records with well-dated ice core records instead. Although the introduction suggests a lack of records for the past few hundred thousand years, especially in the Ross Sea, many high-quality records exist that are not referenced in this manuscript.
2.3 Another point of confusion is the extensive discussion on the importance of Be isotope measurement and principles in Lines 65-75. By the end of the manuscript, Be isotopes do not seem to play a significant role in the model the authors propose, instead serving a supportive role that requires support from other evidence.
2.4 The correlation presented in Figure 6 lacks clarity and statistical robustness due to the limited data points for each time period. The authors should consider grouping some of the time periods and providing clear descriptive statistical tests to support their arguments. A similar issue is present in Figure 3, where there is no clear evidence indicating which correlations are strong and which are not.
Finally, Figure 4 is difficult to read due to an overload of poorly organized information, and the color labeling is inconsistent with Figure 2.
-
AC2: 'Reply on RC2', Yeong Bae Seong, 22 Sep 2024
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2024-38/cp-2024-38-AC2-supplement.pdf
Status: closed
-
RC1: 'Comment on cp-2024-38', Christian Ohneiser, 02 Aug 2024
'Drastic changes in Depositional Environments at the Ross Sea Continental Margin since the Mid-Pleistocene: More evidence for West Antarctic Ice Sheet collapse.'
The title overstates what the manuscript is able to demonstrate in its current form.. or perhaps in general.
1. A lot of weight has ben placed on interpretation of the magnetic susceptibility but the assumptions made are incorrect. MS is a magnetic mineral concentration proxy and will provide insights into the ratios of ferri/ferro magnetic, paramagnetic and diamagnetic components in a sample. Ohneiser et al 2019 showed the relationship very clearly MS and magnetic mineral concentration at LC47.2. You need to reference the Ohneiser et al 2019 paper which provides the chronology for LC47. The B/M boundary at 11.65 m (773 ka) provides your chronological anchor from which you can correlate to LC42.
3. Carefully read the Bollen et al 2022 paper and see how their data and interpretations work with yours. In their work they develop a more complex chronology of ice advance and retreat than what is presented for LC47 which makes me think that some more detail can be extracted form this record.
4. There appears to be discussion in this manuscript on lower portions of the core than what is presented.
5. Line fits and statistics need to be presented so that we can see how strong the correlations are and in turn how valid the discussion and conclusions are.
6. Ad MIS labels to all figures os we can see what is being referred to.
7. There is no mention of MIS 12. This is a deep cold glacial which some speculate resulted in reorganisation in Antarctic more persistent sea-ice cover.
8. The discussion on CaCO3 preservation needs more development. Bonaccorsi et al., 2007 present a record of only the last few thousand years so this is not comparable with the LC47. No mention is made to carbonate production and its relationship to LC47. What is the carbonate? Is it foraminifera, coccoliths? Is it old recycled washed in from the shelf or new carbonate? Can assumptions on dilution made from CaCo3 data be corroborated with microscope images demonstrating varying dissolution? To discuss carbonate preservation more data are needed and a discussion on assumptions of production rates which will have been strictly during warm interglacials.
9. The Be data discussion needs to be reviewed by a specialist other than me. Figure 6 needs some statistics to see how good the line fits are so the reader can deduce how robust the discussion and conclusions are. At the moment I can't tell if the data support the discussion or conclusions.
10. More work is needed in clear data presentation and their statistical significance before discussions on CDW/ACC and ice sheet evolution can be evaluated.
Lines 24-25. From these data you can't propose a collapse of the WAIS or ice shelf collapse.Line 39. You need a reference to support the statement 'Ross Sea is the primary drainage...'
Line 42: What is meant by elucidating historical ice sheet dynamics? Ice sheet dynamics which have happened since the arrival of humans and documentation? Current ice sheet dynamics?
Line 54: Naish et al do not present organic geochemical data.
Line 55: Suggest referring to the Ohneiser et al 2023 Nat Geo paper here where we showed persistent ice advance and retreat paced by obliquity until 400 ka. Its the only other Pleistocene record from on the shelf which spans the last c 1 myr.
Line 97: You need to reference Ohneiser et al 2019. Ohneiser et al established the magnetostratigraphy and the placed the B-M boundary at 11.65 m (773 Ka) which provides the original age model for LC47 (and LC42). Bollen et al 2022 developed Ohneiser
Line 115. This age has been revised and is now 773 ka.
Line 120: You need to show the statistics associated with the line fits to see how strong the correlation is.
LIne 120: I suggest making cross plots of MS vs biogenic opal and other parameters. I suspect the correlation biogenic % and Ms will be stronger then grainsize and MS.
Line 125: Ohneiser et al 2023 showed that the LR04 does not reflect ice advance and retreat pacing in the Ross Sea. δ18O is a globally mixed record and not region specific.
Line 125:You need to acknowledge here that LC47 may be too far from the ice margin and will capture a mixed oceanographic and ice sheet advance record.
Line 140: Strictly speaking these are no organic geochemical proxies since organic geochemistry implies analysis of carbo base organic molecules (alkenones, alkenes/anes.. other lipids etc etc).
Line 149. It would be good to demonstrate whether the visual correlation is real.. i.e. statistically.
LIne 164. Need to provide a reference on where these broad climatic zones come from.
LIne 170. refer to figure
LIne 189 - poor sorting?
Line 193 - Ohneiser et al 2019 interpreted these thin gravel as lag surfaces which indicate enhanced current strength and winnowing. They likely provide a protective armoured surface once formed that prevents further erosion of underlying sediments.
LIne 197 - what is meant by 'this temporal span'? this interval?
LInes 205-215. This reads like a collection of bullet points. It should be revised into a coherent block of text with references.
Line 214. McKay et al 2012 do not discuss winnowing of magnetic susceptibility.
LIne 260. Im not sure this is a correct statement. The AND-1B site is in a very different setting because it was grounding zone proximal with multiple grounding zone advance and retreat cycles directly over the site. LC47 is oceanic and has never been near the grounding zone.
LIne 261 - where did temperatures increase by 1-2C°?
LInes 266-335- this discussion on Be data needs to be reviewed by someone other than me. Figure 6 needs some statistics to see how good the line fits are so the reader can deduce how robust the discussion and conclusions are.
Lines 338 - most agree that the MPT was complete at around 800 kyr.
LInes 340 etc. the presented interval in LC47 does not extend older than MIS19!
Line 395. The record presented here does not cover the mid-pleistocene transition which is generally agreed to be complete at around 800 kyr. Please revise this section.
Figures:
Fig 1. Why not use a blue colour ramp for regions below sea-level like in the inset Antarctic map? Map C and D need E/W and S° markers on the axes.
FIg 2 fonts are too small and figure is fuzzy. Please tidy this up.
Fig 3. You need to provide line fit data/statistics to support these line fits through your bivariate plots. Panel A should start at 0 unless you have negative values. It looks like these are not very good line fits.
Fig 4. Looks more pink than purple to me. The resolution of the figure is poor. In discontinuous data where there are relatively few points (everything except the MS and isotope record) I suggest marking your data points with a symbol.
Fig 5/6. show statistics!
There are numerous small grammatical errors which need correcting so please carry out a careful proof and get someone outside of the author group to review the manuscript with a fresh set of eyes.
Citation: https://doi.org/10.5194/cp-2024-38-RC1 -
AC1: 'Reply on RC1', Yeong Bae Seong, 22 Sep 2024
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2024-38/cp-2024-38-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Yeong Bae Seong, 22 Sep 2024
-
RC2: 'Comment on cp-2024-38', Anonymous Referee #2, 16 Aug 2024
The authors present a valuable dataset from the Ross Sea region that offers insights into the dynamics of the West Antarctic Ice Sheet over the past ~780,000 years. While they provide a substantial amount of data, there are several issues that need to be addressed.
- General Comments
1.1 Age Model:
In Lines 90-99, the authors state that the age model for RS15-LC47 is based on correlating magnetic susceptibility (MS) to a nearby core referenced in Bollen et al. (2022). They assert a "noteworthy" similarity between their MS records and those from Bollen et al. (2022) in Line 119 and mention a "comprehensive approach utilizing 12 tie points" (Line 114) to align their records with the Bollen et al. (2022) record. However, the authors do not clearly demonstrate how they tested this "similarity" or explain the methods and rationale behind correlating the MS records.
For instance, in Figure 2C, the patterns older than 250 ka, according to the current age model, appear quite different. Bollen et al. (2022) show a relatively muted signal during ~300-550 ka, yet the LC42 record still displays numerous high-frequency peaks. Given the potential for bioturbation and hiatus/event layers in LC47, how can the authors confidently assert that these correlations are "robust"?
This issue is fundamental since an unreliable age constraint undermines the entire discussion. The authors are encouraged to meticulously re-examine their sedimentation records, incorporating more biostratigraphic controls as in Bollen et al. (2022), before making further interpretations.
- Typos and Other Comments:
2.1 The manuscript frequently repeats full names and acronyms. For instance, "Middle-Pleistocene Transition (MPT)" is repeated in Lines 50, 229, 399, 449, and 457, and numerous other terms (AABW, MS, CDW, ASC, etc.) are also redundantly mentioned. The authors should carefully review the manuscript for such repetitions before resubmission.
2.2 The discussion on the MPT event is also perplexing since the record in this manuscript only covers a very brief portion of the later part of the MPT. The authors should consider comparing their records with well-dated ice core records instead. Although the introduction suggests a lack of records for the past few hundred thousand years, especially in the Ross Sea, many high-quality records exist that are not referenced in this manuscript.
2.3 Another point of confusion is the extensive discussion on the importance of Be isotope measurement and principles in Lines 65-75. By the end of the manuscript, Be isotopes do not seem to play a significant role in the model the authors propose, instead serving a supportive role that requires support from other evidence.
2.4 The correlation presented in Figure 6 lacks clarity and statistical robustness due to the limited data points for each time period. The authors should consider grouping some of the time periods and providing clear descriptive statistical tests to support their arguments. A similar issue is present in Figure 3, where there is no clear evidence indicating which correlations are strong and which are not.
Finally, Figure 4 is difficult to read due to an overload of poorly organized information, and the color labeling is inconsistent with Figure 2.
-
AC2: 'Reply on RC2', Yeong Bae Seong, 22 Sep 2024
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2024-38/cp-2024-38-AC2-supplement.pdf
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