Glacial-interglacial Circumpolar Deep Water temperatures during the last 800,000 years: estimates from a synthesis of bottom water temperature reconstructions

Chandler, David M.; Langebroek, Petra M.

doi:https://doi.org/10.5194/egusphere-2023-850

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https://doi.org/10.5194/egusphere-2023-850

Preprints

09 May 2023

| 09 May 2023

Glacial-interglacial Circumpolar Deep Water temperatures during the last 800,000 years: estimates from a synthesis of bottom water temperature reconstructions

David M. Chandler and Petra M. Langebroek

Abstract. Future climate and sea-level projections depend sensitively on the response of the Antarctic Ice Sheet to ocean-driven melting and the resulting freshwater fluxes into the Southern Ocean. Incursion of Circumpolar Deep Water (CDW) across the Antarctic continental shelf, and into cavities beneath ice shelves, is increasingly recognised as a crucial heat source for ice shelf melt. Quantifying past changes in the temperature of CDW is therefore of great benefit for modelling ice sheet response to past warm climates, for validating paleoclimate models, and for putting recent and projected changes in CDW temperature into context. Here we synthesise the few available bottom water temperature reconstructions representative of CDW and its principal source water mass (North Atlantic Deep Water) over the past 800 kyr. Estimated CDW temperature anomalies consistently reached ca. −2 °C during glacial periods, warming to +0.1 to +0.5 °C during the strongest interglacials (marine isotope stages MIS 11, 9, 5, and 1). The temperature anomaly in MIS 7 was comparatively cooler at ca. −0.6 °C. Despite high variance amongst a small number of records, and poor (4 kyr) temporal resolution, we find persistent and close relationships between our estimated CDW temperature and Southern Ocean sea-surface temperature, Antarctic surface air temperature, and global ocean temperature reconstructions at glacial cycle time scales. Given the important role that CDW plays in connecting the world's three main ocean basins, and in driving Antarctic Ice Sheet mass loss, additional temperature reconstructions targeting CDW are urgently needed to increase temporal resolution and to decrease uncertainty in past CDW temperatures – whether for use as a boundary condition, model validation or in their own right.

Received: 28 Apr 2023 – Discussion started: 09 May 2023

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Journal article(s) based on this preprint

20 Sep 2024

Glacial–interglacial Circumpolar Deep Water temperatures during the last 800 000 years: estimates from a synthesis of bottom water temperature reconstructions

David M. Chandler and Petra M. Langebroek

Clim. Past, 20, 2055–2080, https://doi.org/10.5194/cp-20-2055-2024,https://doi.org/10.5194/cp-20-2055-2024, 2024

Short summary

David M. Chandler and Petra M. Langebroek

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-850', Anonymous Referee #1, 26 Jun 2023

This is an interesting premise for a paper. But ultimately it didn’t deliver.
The premise on the ice sheet melting is not really dealt with - I was expecting the authors to get back to it at the end and show how their new records might link to it. Furthermore it is unclear to me why they focussed on the last 800 ka - they didn't really try to interpret anything older than 400 ka for the interglacials. Why only 7 records - there are others in the literature for LCDW already published that go back to ~ 800 ka e.g. ODP 1168, ODP 1170, ODP 1171 from the South Tasman Rise, south of Tasmania (Nurnberg et al., 2004). I would suggest the authors look at the new dataset of d18O recently published by Mulitza et al., 2022 ESSD, which would have many more records covering the last 400 and even 800 ka. Given the lack of records that go back 800 ka why not focus on the shorter time periods – especially when there are no periods warmer than present between MIS11 and MIS19. They compare to several other datasets that do not cover the last 800 ka in their discussion.
Additionally, the new compilation shows strong agreement to past DWT compilations - so I'm not entirely sure that is really adds much to the literature. I was really hoping it would get into how it might influence the ice sheets or modelling so that it did add something new.
I also find the inclusion of the NADW records (4 out of the total of 7) a little odd given the focus on the LCDW and evidence in the literature of the shut down of AMOC and therefore I would not expect the NADW to be a strong contributor to LCDW during the glacials. The authors came to this eventually in the paper, but only after putting it all together and didn’t really discuss the implications on their records which have 4 records from NADW that may well swamp the LCDW datasets. Although I realise the authors are interested in the interglacials for the ice sheet melting. But then didn’t really come back to this and how the MIS5, 9 and 11 may have impacted the ice sheets.
Many sections there was insufficient detail on the methods and the issues - they came back to many of these in the discussion at the end - but they should have been upfront. There really was insufficient detail around the methods used for the Mg/Ca and the d18O. I realise these are in the original papers – but you need to provide a summary here so the reader doesn’t have to go back to the original papers.
I felt the oceanography background needed more information and detail on the CDW – and differentiate between the NADW, LCDW, UCDW and mCDW – it would have helped to have a 3D/Depth figure to show the links between these water masses.
It was unclear to me how the authors plotted up Figure 4 when you can’t look at BWT on both the NADW and the LCDW at the same time? How do you make plots like this? Also which cores had both benthic d18O and the Mg/Ca done on them to make the other X-Y plot.
The reason why there are not many DWT records using Mg/Ca for benthic foraminifera is that it is not a trivial thing to do. Firstly there are not always sufficient tests of the right species of benthic foraminifera. Secondly the method is time consuming and has some issues – some of which were outlined in the limitations. But one thing that has not been mentioned in the paper is the potential impact of dissolution and the Carbonate Compensation Depth - the LCDW of the Southern sits at or very close to the Carbonate Saturation Horizon which is around 3000 m and the CCD which sits around 4000 m- so not many cores actually preserve sufficient Carbonate organisms or the records can be compromised by period of dissolution due to the shoaling of the CCD - during the glacial cycle.

Citation: https://doi.org/10.5194/egusphere-2023-850-RC1
- AC1: 'Reply to RC1 and RC2', David Chandler, 25 Aug 2023
  
  We would like to thank both reviewers for their comments, and have provided our responses in the attachment.
  
  Citation: https://doi.org/10.5194/egusphere-2023-850-AC1
RC2:
'Comment on egusphere-2023-850', Anonymous Referee #2, 04 Jul 2023

Chandler and Langebroek compiled 7 deep water records (3 benthic Mg/Ca, 1 ostracod Mg/Ca, and 3 benthic d18O) during the last 800 ka, aiming to understand the glacial-interglacial temperature variability of Circumpolar Deep Water. However, there are several problems with data selection, data interpretation, and presentation, I have to say that this study, at least in the current form, is not to the standard of the CP.

Here are my main concerns.
1) The premise of using a compilation of BWT at selected sites to reflect CDW BWT change.
Including NADW sites in the compilation is not appropriate because 1) NADW sites could be bathed in different water masses during glacials and interglacials due to circulation change, as already mentioned by the authors in Section 5.3, 2) shallow NADW sites (e.g. ODP 980) would have minimal influence on CDW that can potentially upwell underneath the Antarctic Ice shelves, due to the lower seawater density. On the contrary, Pacific sites at relatively deep depths, though downstream of CDW, may more reliably record the CDW temperature as those could be on the same isopycnal as CDW upwelling underneath the Antarctic Ice shelves. More benthic d18O records from the Pacific included in Bates et al (2014) thus might be included in the compilation as well. But ultimately, it is not most convincing to include sites not bathed in CDW to infer CDW changes. There may be more SO sites available if the time span of the compilation can be shortened.

2) Using a compiled CDW T to inform ice shelf melting triggered by CDW.
The authors set out to use a compiled CDW T record to infer potential ice shelf melting triggered by CDW during previous warm interglacials. It is noted that such an event can be triggered by ~1 degC warming in a short period (a year) in the modern ocean (e.g., Jenkins et al., 2018), which is a really small difference challenging for any given record reconstructed by any proxy to resolve with confidence, let alone a compilation smoothing multiple records. So the 0.1-0.5degC warming during previous interglacials mentioned in the abstract must be interpreted in the context of the uncertainty range, which is not reported but can be expected to be large enough to make the reported warming statistically insignificant.

3) Hydrographic settings of CDW.
As a manuscript on CDW, there are lots of inaccurate statements about CDW. While NADW is the heat source of the modified CDW, it is not the principal source water mass of CDW (Line 7). Deep waters from the Indian and Pacific sectors, as well as deep water formed around Antarctica, are also key sources of CDW (refer to Talley 2011 for detailed hydrography). Also, UCDW and LCDW are not defined properly. And modified CDW is often used in the context of melting under ice shelves, but it is not a counterpart of UCDW and LCDW.
4) Lack of details about their method.
how the BWT is calculated? How is the sampling done? Which types of uncertainties are included in the uncertainty envelope shown in Figure 4? These questions are not trivial. Details, such as choice of d18O-T equation, treatments of ice volume change, and local effects on seawater d18O, are lacking for benthic d18O calculation. For the sampling method, no original data is shown, so one would have no way to assess if the compilation is potentially biased by one particular site.

5) The structure of the paper.
A reader is left with the impression that the work done here did not at all contribute to solving the research question raised in the introduction, i.e., the potential influence of CDW T on ice shelf melting during previous interglacials. The introduction sets the expectation of a reader very high and by finishing reading I felt a little disappointed. Perhaps, the authors could put something that can be achieved by this work in the introduction. Also, I think it would be better to provide some background on the limitations of the employed proxies in the introduction.

Citation: https://doi.org/10.5194/egusphere-2023-850-RC2
- AC2: 'Reply to RC1 and RC2', David Chandler, 25 Aug 2023
  
  We would like to thank both reviewers for their comments, and have provided our responses in the attachment.
  
  Citation: https://doi.org/10.5194/egusphere-2023-850-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-850', Anonymous Referee #1, 26 Jun 2023

This is an interesting premise for a paper. But ultimately it didn’t deliver.
The premise on the ice sheet melting is not really dealt with - I was expecting the authors to get back to it at the end and show how their new records might link to it. Furthermore it is unclear to me why they focussed on the last 800 ka - they didn't really try to interpret anything older than 400 ka for the interglacials. Why only 7 records - there are others in the literature for LCDW already published that go back to ~ 800 ka e.g. ODP 1168, ODP 1170, ODP 1171 from the South Tasman Rise, south of Tasmania (Nurnberg et al., 2004). I would suggest the authors look at the new dataset of d18O recently published by Mulitza et al., 2022 ESSD, which would have many more records covering the last 400 and even 800 ka. Given the lack of records that go back 800 ka why not focus on the shorter time periods – especially when there are no periods warmer than present between MIS11 and MIS19. They compare to several other datasets that do not cover the last 800 ka in their discussion.
Additionally, the new compilation shows strong agreement to past DWT compilations - so I'm not entirely sure that is really adds much to the literature. I was really hoping it would get into how it might influence the ice sheets or modelling so that it did add something new.
I also find the inclusion of the NADW records (4 out of the total of 7) a little odd given the focus on the LCDW and evidence in the literature of the shut down of AMOC and therefore I would not expect the NADW to be a strong contributor to LCDW during the glacials. The authors came to this eventually in the paper, but only after putting it all together and didn’t really discuss the implications on their records which have 4 records from NADW that may well swamp the LCDW datasets. Although I realise the authors are interested in the interglacials for the ice sheet melting. But then didn’t really come back to this and how the MIS5, 9 and 11 may have impacted the ice sheets.
Many sections there was insufficient detail on the methods and the issues - they came back to many of these in the discussion at the end - but they should have been upfront. There really was insufficient detail around the methods used for the Mg/Ca and the d18O. I realise these are in the original papers – but you need to provide a summary here so the reader doesn’t have to go back to the original papers.
I felt the oceanography background needed more information and detail on the CDW – and differentiate between the NADW, LCDW, UCDW and mCDW – it would have helped to have a 3D/Depth figure to show the links between these water masses.
It was unclear to me how the authors plotted up Figure 4 when you can’t look at BWT on both the NADW and the LCDW at the same time? How do you make plots like this? Also which cores had both benthic d18O and the Mg/Ca done on them to make the other X-Y plot.
The reason why there are not many DWT records using Mg/Ca for benthic foraminifera is that it is not a trivial thing to do. Firstly there are not always sufficient tests of the right species of benthic foraminifera. Secondly the method is time consuming and has some issues – some of which were outlined in the limitations. But one thing that has not been mentioned in the paper is the potential impact of dissolution and the Carbonate Compensation Depth - the LCDW of the Southern sits at or very close to the Carbonate Saturation Horizon which is around 3000 m and the CCD which sits around 4000 m- so not many cores actually preserve sufficient Carbonate organisms or the records can be compromised by period of dissolution due to the shoaling of the CCD - during the glacial cycle.

Citation: https://doi.org/10.5194/egusphere-2023-850-RC1
- AC1: 'Reply to RC1 and RC2', David Chandler, 25 Aug 2023
  
  We would like to thank both reviewers for their comments, and have provided our responses in the attachment.
  
  Citation: https://doi.org/10.5194/egusphere-2023-850-AC1
RC2:
'Comment on egusphere-2023-850', Anonymous Referee #2, 04 Jul 2023

Chandler and Langebroek compiled 7 deep water records (3 benthic Mg/Ca, 1 ostracod Mg/Ca, and 3 benthic d18O) during the last 800 ka, aiming to understand the glacial-interglacial temperature variability of Circumpolar Deep Water. However, there are several problems with data selection, data interpretation, and presentation, I have to say that this study, at least in the current form, is not to the standard of the CP.

Here are my main concerns.
1) The premise of using a compilation of BWT at selected sites to reflect CDW BWT change.
Including NADW sites in the compilation is not appropriate because 1) NADW sites could be bathed in different water masses during glacials and interglacials due to circulation change, as already mentioned by the authors in Section 5.3, 2) shallow NADW sites (e.g. ODP 980) would have minimal influence on CDW that can potentially upwell underneath the Antarctic Ice shelves, due to the lower seawater density. On the contrary, Pacific sites at relatively deep depths, though downstream of CDW, may more reliably record the CDW temperature as those could be on the same isopycnal as CDW upwelling underneath the Antarctic Ice shelves. More benthic d18O records from the Pacific included in Bates et al (2014) thus might be included in the compilation as well. But ultimately, it is not most convincing to include sites not bathed in CDW to infer CDW changes. There may be more SO sites available if the time span of the compilation can be shortened.

2) Using a compiled CDW T to inform ice shelf melting triggered by CDW.
The authors set out to use a compiled CDW T record to infer potential ice shelf melting triggered by CDW during previous warm interglacials. It is noted that such an event can be triggered by ~1 degC warming in a short period (a year) in the modern ocean (e.g., Jenkins et al., 2018), which is a really small difference challenging for any given record reconstructed by any proxy to resolve with confidence, let alone a compilation smoothing multiple records. So the 0.1-0.5degC warming during previous interglacials mentioned in the abstract must be interpreted in the context of the uncertainty range, which is not reported but can be expected to be large enough to make the reported warming statistically insignificant.

3) Hydrographic settings of CDW.
As a manuscript on CDW, there are lots of inaccurate statements about CDW. While NADW is the heat source of the modified CDW, it is not the principal source water mass of CDW (Line 7). Deep waters from the Indian and Pacific sectors, as well as deep water formed around Antarctica, are also key sources of CDW (refer to Talley 2011 for detailed hydrography). Also, UCDW and LCDW are not defined properly. And modified CDW is often used in the context of melting under ice shelves, but it is not a counterpart of UCDW and LCDW.
4) Lack of details about their method.
how the BWT is calculated? How is the sampling done? Which types of uncertainties are included in the uncertainty envelope shown in Figure 4? These questions are not trivial. Details, such as choice of d18O-T equation, treatments of ice volume change, and local effects on seawater d18O, are lacking for benthic d18O calculation. For the sampling method, no original data is shown, so one would have no way to assess if the compilation is potentially biased by one particular site.

5) The structure of the paper.
A reader is left with the impression that the work done here did not at all contribute to solving the research question raised in the introduction, i.e., the potential influence of CDW T on ice shelf melting during previous interglacials. The introduction sets the expectation of a reader very high and by finishing reading I felt a little disappointed. Perhaps, the authors could put something that can be achieved by this work in the introduction. Also, I think it would be better to provide some background on the limitations of the employed proxies in the introduction.

Citation: https://doi.org/10.5194/egusphere-2023-850-RC2
- AC2: 'Reply to RC1 and RC2', David Chandler, 25 Aug 2023
  
  We would like to thank both reviewers for their comments, and have provided our responses in the attachment.
  
  Citation: https://doi.org/10.5194/egusphere-2023-850-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (09 Sep 2023) by Erin McClymont

Thank you for addressing the detailed comments provided by the two reviewers.

It is clear that the reviewers' expectations from the Abstract/Introduction were not met, but you have indicated how the emphasis and detail in the early parts of the manuscript can be changed to make your aims more clear.

The reviewers have major concerns over which sites were selected and the assumptions and uncertainties which result. Concerns about NADW in particular were highlighted, and will need careful discussion in the text. I agree with you that the use of benthic d18O records introduces issues for temperature reconstructions, due to the time-varying influences of temperature and seawater d18O on glacial-interglacial timescales (c.f. Elderfield et al., 2012, Science): whilst a test of benthic d18O records is a useful comparison these uncertainties may also introduce further issues into the assessment. If you can flag these concerns and show the analysis with and without d18O (as you suggest) then a more comprehensive discussion will be possible and may address the reviewers' concerns.

There is also an underlying concern from the reviewers about how you outline and explain your uncertainties, which will need to be carefully written.

Reviewer 2 has concerns about the description of the oceanography. It will be important to fully address this concern because it also feeds into the decisions you are making about which sites you will include in your compilations.

Minor comment in connection to your final comment in reply to reviewer 2.2. I think that the reviewer was indicating that today we see impacts due to very short timescale changes, so that these might be expected to be smoothed out in a longer term / 4 kyr resolution record. I don’t think they were requesting to generate outputs at very high temporal resolution. You might address this comment in the text by flagging that short-term variability (which is known today) will not be identified by this study, and may even be smoothed by the long timescales or timesteps over which the data and the compilation are operating.

Although your replies to the reviewer indicate that many of the comments can be addressed, the large-scale changes to the project outline, its assumptions, and the resulting evaluation of the outputs mean that I am recommending that we will reconsider the manuscript after you have undertaken these major revisions to the text as it currently stands.

Hide

AR by David Chandler on behalf of the Authors (28 Feb 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (18 Mar 2024) by Erin McClymont

ED: Publish subject to minor revisions (review by editor) (12 Jul 2024) by Erin McClymont

I thank the authors for their careful and detailed responses to the constructive comments provided by the two reviewers. They have revised the compilation of sites to ensure that the reviewers concerns about including North Atlantic data has been addressed. Furthermore, they provide a comparison between the new compilation and the North Atlantic perspective.

The reviewers were also concerned about an apparent lack of discussion about the impact of CDW on ice sheet melting; the authors have clarified that this was not the aim of the study and have revised the text to ensure there is no similar misunderstanding in this draft. The comparison with different ice margins gives some context for the impact of the new analysis however.

The uncertainties in the approach are better explained in this revised version. Some points require some clarification (all line numbers refer to the tracked changes document):
- line 77 “considered less reliable…” Can you clarify that you mean “less reliable as temperature proxies” rather than that the oxygen isotopes are less reliable in general?
- Line 180 refers to “local hydrographic controls” perhaps influencing d18Osw but not at glacial interglacial timescales. But given the formation and transport processes of LCDW as described in section 2 (E.g. lines 105-110) is there a chance that mixing with Antarctic waters, which might been more strongly influenced by local impacts on temperature and salinity, could impact LCDW properties?
- Related to the previous comment, lines 111-120 there is discussion of the modification of LCDW before it reaches the ice front. In the discussion (line 694) the authors flag the caveat that they reconstruct temperature and not transport: but would they also expect a potential cooling during cross-shelf transport which would also mean their reconstructed temperatures lie towards the maximum expected?
- Line 392 describes the number of sites used. Two of the sites have two proxies for Tcdw being used, so are they being treated as four separate records (4 sites?). What would happen if only Mg/Ca was used for the two sites where it was available, along with d18O where it was not? Although the potential bias between proxies or regions was explored, the potential bias caused by two records coming from the same site was not.
- Line 415 the authors note the Mg/Ca records tend to give cooler glacials and warmer interglacials, so a wider range than suggested by their d18O reconstruction. But the stack is dominated by d18O, so should we treat the stack as a conservative prediction of Tcdw? (Or might this observation explain the quadratic nature of the relationship with other archives due to systematic bias or underestimation of the lowest temperatures?)
- Figure 5: I can’t see the vertical shading described in the text, whether printed or on screen.

Hide

AR by David Chandler on behalf of the Authors (17 Jul 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (06 Aug 2024) by Erin McClymont

AR by David Chandler on behalf of the Authors (07 Aug 2024) Manuscript

Journal article(s) based on this preprint

20 Sep 2024

Glacial–interglacial Circumpolar Deep Water temperatures during the last 800 000 years: estimates from a synthesis of bottom water temperature reconstructions

David M. Chandler and Petra M. Langebroek

Clim. Past, 20, 2055–2080, https://doi.org/10.5194/cp-20-2055-2024,https://doi.org/10.5194/cp-20-2055-2024, 2024

Short summary

David M. Chandler and Petra M. Langebroek

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Sea-level rise and global climate change caused by ice melt in Antarctica is a puzzle of feedbacks between the climate, ocean and ice sheets, over tens to thousands of years. Antarctic Ice Sheet melting is caused mainly by warm deep water from the Southern Ocean. Here, we analyse close relationships between deep water temperatures and global climate in the last 800,000 years. This knowledge can help us to better understanding how climate and sea-level are likely to change in the future.


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