The new Kr-86 excess ice core proxy for synoptic activity: West Antarctic storminess possibly linked to ITCZ movement through the last deglaciation

Buizert, Christo; Shackleton, Sarah; Severinghaus, Jeffrey P.; Roberts, William H. G.; Seltzer, Alan; Bereiter, Bernhard; Kawamura, Kenji; Baggenstos, Daniel; Orsi, Anaïs J.; Oyabu, Ikumi; Birner, Benjamin; Morgan, Jacob D.; Brook, Edward J.; Etheridge, David M.; Thornton, David; Bertler, Nancy; Pyne, Rebecca L.; Mulvaney, Robert; Mosley-Thompson, Ellen; Neff, Peter D.; Petrenko, Vasilii V.

doi:https://doi.org/10.5194/cp-2022-65

Preprints

https://doi.org/10.5194/cp-2022-65

Preprints

09 Sep 2022

Status: a revised version of this preprint was accepted for the journal CP.

The new Kr-86 excess ice core proxy for synoptic activity: West Antarctic storminess possibly linked to ITCZ movement through the last deglaciation

Christo Buizert¹, Sarah Shackleton², Jeffrey P. Severinghaus², William H. G. Roberts^3,4, Alan Seltzer^2,5, Bernhard Bereiter⁶, Kenji Kawamura⁷, Daniel Baggenstos⁶, Anaïs J. Orsi^8,9, Ikumi Oyabu⁷, Benjamin Birner², Jacob D. Morgan², Edward J. Brook¹, David M. Etheridge^10,11, David Thornton¹⁰, Nancy Bertler^12,13, Rebecca L. Pyne¹², Robert Mulvaney¹⁴, Ellen Mosley-Thompson¹⁵, Peter D. Neff^16,17, and Vasilii V. Petrenko¹⁷ Christo Buizert et al.

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¹College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA
²Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA
³Geography and Environmental Sciences, Northumbria University, Newcastle, UK
⁴BRIDGE, School of Geographical Sciences, University of Bristol, Bristol, UK
⁵Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
⁶Climate and Environmental Physics, Physics Institute, and Oeschger Center for Climate Research, University of Bern, 3012, Bern, Switzerland
⁷National Institute for Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan
⁸Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, l’Orme des merisiers, Gif-sur-Yvette, France
⁹Earth, Ocean and Atmospheric Sciences Department, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada
¹⁰CSIRO Oceans and Atmosphere, PMB 1, Aspendale, Victoria 3195, Australia
¹¹Australian Antarctic Program Partnership, Institute for Marine & Antarctic Studies, University of Tasmania, Hobart, Tasmania 7004, Australia
¹²Antarctic Research Centre, Victoria University of Wellington, Wellington, 6012, New Zealand
¹³GNS Science, Lower Hut 5010, New Zealand
¹⁴British Antarctic Survey, National Environment Research Council, Cambridge CB3 0ET, UK
¹⁵Byrd Polar and Climate Research Center, The Ohio State University, Columbus, OH 43210, USA
¹⁶Department of Soil, Water, and Climate, University of Minnesota, Saint Paul, MN 55108, USA
¹⁷Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA

¹College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA
²Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA
³Geography and Environmental Sciences, Northumbria University, Newcastle, UK
⁴BRIDGE, School of Geographical Sciences, University of Bristol, Bristol, UK
⁵Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
⁶Climate and Environmental Physics, Physics Institute, and Oeschger Center for Climate Research, University of Bern, 3012, Bern, Switzerland
⁷National Institute for Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan
⁸Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, l’Orme des merisiers, Gif-sur-Yvette, France
⁹Earth, Ocean and Atmospheric Sciences Department, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada
¹⁰CSIRO Oceans and Atmosphere, PMB 1, Aspendale, Victoria 3195, Australia
¹¹Australian Antarctic Program Partnership, Institute for Marine & Antarctic Studies, University of Tasmania, Hobart, Tasmania 7004, Australia
¹²Antarctic Research Centre, Victoria University of Wellington, Wellington, 6012, New Zealand
¹³GNS Science, Lower Hut 5010, New Zealand
¹⁴British Antarctic Survey, National Environment Research Council, Cambridge CB3 0ET, UK
¹⁵Byrd Polar and Climate Research Center, The Ohio State University, Columbus, OH 43210, USA
¹⁶Department of Soil, Water, and Climate, University of Minnesota, Saint Paul, MN 55108, USA
¹⁷Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA

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Received: 18 Aug 2022 – Discussion started: 09 Sep 2022

Abstract. Here we present a newly developed ice core gas-phase proxy that directly samples a component of the large-scale atmospheric circulation: synoptic-scale pressure variability. Surface pressure variability weakly disrupts gravitational isotopic settling in the firn layer, which is recorded in krypton-86 excess (⁸⁶Kr_xs). We validate ⁸⁶Kr_xs using late Holocene ice samples from eleven Antarctic and one Greenland ice core that collectively represent a wide range of surface pressure variability in the modern climate. We find a strong correlation (r = -0.94, p < 0.01) between site-average ⁸⁶Kr_xs and site synoptic variability from reanalysis data. The main uncertainties in the method are the corrections for gas loss and thermal fractionation, and the relatively large scatter in the data. We show ⁸⁶Kr_xs is linked to the position of the eddy-driven subpolar jet (SPJ), with a southern position enhancing pressure variability.

We present a ⁸⁶Kr_xs record covering the last 24 ka from the WAIS Divide ice core. West Antarctic synoptic activity is slightly below modern levels during the last glacial maximum (LGM); increases during the Heinrich Stadial 1 and Younger Dryas North Atlantic cold periods; weakens abruptly at the Holocene onset; remains low during the early and mid-Holocene, and gradually increases to its modern value. The WAIS Divide ⁸⁶Kr_xs record resembles records of monsoon intensity thought to reflect changes in the meridional position of the intertropical convergence zone (ITCZ) on orbital and millennial timescales, such that West Antarctic storminess is weaker when the ITCZ is displaced northward, and stronger when it is displaced southward. We interpret variations in synoptic activity as reflecting movement of the South Pacific SPJ in parallel to the ITCZ migrations, which is the expected zonal-mean response of the eddy-driven jet in models and proxy data. Past changes to Pacific climate and the El Niño Southern Oscillation (ENSO) may amplify the signal of the SPJ migration. Our interpretation is broadly consistent with opal flux records from the Pacific Antarctic zone thought to reflect wind-driven upwelling.

We emphasize that ⁸⁶Kr_xs is a new proxy, and more work is called for to confirm, replicate and better understand these results; until such time, our conclusions regarding past atmospheric dynamics remain tentative. Current scientific understanding of firn air transport and trapping is insufficient to explain all the observed variations in ⁸⁶Kr_xs.

How to cite. Buizert, C., Shackleton, S., Severinghaus, J. P., Roberts, W. H. G., Seltzer, A., Bereiter, B., Kawamura, K., Baggenstos, D., Orsi, A. J., Oyabu, I., Birner, B., Morgan, J. D., Brook, E. J., Etheridge, D. M., Thornton, D., Bertler, N., Pyne, R. L., Mulvaney, R., Mosley-Thompson, E., Neff, P. D., and Petrenko, V. V.: The new Kr-86 excess ice core proxy for synoptic activity: West Antarctic storminess possibly linked to ITCZ movement through the last deglaciation, Clim. Past Discuss. [preprint], https://doi.org/10.5194/cp-2022-65, in review, 2022.

Christo Buizert et al.

Status: final response (author comments only)

CC1:
'Comment on cp-2022-65', Aymeric Servettaz, 15 Sep 2022

The paper presents a new proxy directly tracking atmospheric pressure variability, which until now had been largely unexplored in paleoclimate studies, and provides an application on the interpretation of this proxy in the past from a West Antarctic ice core. The manuscript is richly detailed and clearly understandable, yet I found that some points could be further clarified, and have some open questions. I understand that the Kr-86 excess proxy is emerging, and questions asked in this comment may remained unanswered until further studies. I also noted minor typos. Please refer to the comments in the attached document.

Citation: https://doi.org/10.5194/cp-2022-65-CC1
- AC1: 'Response to Aymeric Servettaz', Christo Buizert, 05 Dec 2022
  
  The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2022-65/cp-2022-65-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/cp-2022-65-AC1
RC1:
'Comment on cp-2022-65', Anonymous Referee #1, 06 Oct 2022

The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2022-65/cp-2022-65-RC1-supplement.pdf

Citation: https://doi.org/10.5194/cp-2022-65-RC1
- AC2: 'Response to reviewer #1', Christo Buizert, 05 Dec 2022
  
  The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2022-65/cp-2022-65-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/cp-2022-65-AC2
RC2:
'Comment on cp-2022-65', Anonymous Referee #2, 13 Oct 2022
This manuscript presents a compilation of isotopic composition of krypton which has been obtained over the past years in different ice cores from Greenland and Antarctica.

Using the present-day understanding of the drivers of gas repartition in the firn as well as correlation with ERA products, the authors propose that the 86Kr_excess can be used for synoptic activity.

The manuscript is generally well written and well documented. The authors also explained in details the numerous limitations associated with this interpretation both in section 3.3 as well as in the supplements.

Because of the limitations in the interpretation of the 86Kr_excess, the authors should be more cautious in the proposed interpretation. While the scientists in the filed of ice cores will get the limitations and use the results with caution, it may be different for people who do not understand the complexity of processes affecting air elementar and isotopic repartition in the firn. I thus suggest to modify the abstract and the conclusion to insist on the speculative interpretation of the 86Kr_excess and on the additionnal measurements to be done to better quantify the effect of gas loss, thermal diffusion (including rectifier effect) and possible existence of a convective zone.

146 : it is strange to refer to Fig 6 here. Moreover, it is strange to prefer one or the other since it is shown later that gas loss and thermal effect are the most important corrections to take into account (d40Ar being sensitive to gas loss and d15N being the most sensitive to thermal fractionation). There is no obvious reason to prefer one notation compared to another.

From l. 223 : the preparartion of the samples is different for DE08-OH than for the other samples. May this explain the different slopes associated with gas loss in figure A1-B.

241 : How are the samples flagged for drill liquid contamination ? How is it possible to detect the drill fluid contamination ? From which measurements ?

245 – 247 : Can you explain the error propagation explaining why the 2 sigma is larger for 86Kr than for 86Kr_excess ?

253 : I do not see why it is useful to present these data to remove them immediatly after. In this case, the 86_Kr data from EDC samples affected by drill fluid should also be displayed with an explanation on how they were discarded.

Section 3.1 : The Phi parameter exhibits strong seasonal and interannual variabilities and I do not understand how this variability is taken into account in the « calibration » of the 86Kr_excess. Such sensitivity should be studied or implemented in Figure 3 since this is crucial for the interpretation of the 86Kr_excess proposed here.

Section 3.3 : this section is interesting in providing the limitations of the interpretation of 86Kr_excess and strongly suggest that further study should be performed for a robust interpretation such as firn air pumping study at different site with a correct determination of the thermal gradient (it is really surprising to find such temperature gradient at DE08 and EDC) + analyses of ice not affected by gas loss, etc… this is the reason why the authors should be much more cautious in their conclusions and better suggest concrete perspectives on how to progress with such proxy if it is reallt promising. Actually, the concluding paragraph of section 3.3 should also be summarized in both the abstract and conclusion of the manuscript to clearly state the limit of this interpretation which is now speculative.

In section 4, I feel that a discussion on the seasonal variability and its possible impact is missing.

Section 5 : I understand that the authors do their best with the poor data quality but it would be nice to comment on the strong scattering for the data at « present-day » ? Can this scattering be used to estimate the uncertainty as the authors mention that « no true replicate to assess the reproducibility » …

20 and 21 : the discussion is quite long for such speculative interpretation. I would suggest shorten it to stay on the safe side of the interpretation.

725 : I am not sure that the authors really « calibrate » the proxy – let’s say that this is a first proposition of interpretation. A calibration would require more dedicated studies as mentionned in the concluding paragraphe of section 3.3.

Figure 3 : What is the origin of the uncertainty bars for the different sites ? Do the sites with more data have more scattering hence a larger uncertainty bar ? It would be useful to mention the number of points used for each sites in this calibration and how the error bar is calculated. A table may be useful to exactly describe the number of samples for each site, depth range, conditions of storage, etc…

Figure A2 : The displayed results show very depleted samples in dO2/N2 and dAr/N2 – are these results really relevant for this paper ? What is the origin of these samples ? core top ? Bottom ice ?

Figure B1 shows that there may be a large scattering with depth of 86Kr-excess. I am sure that this is taken into account in this study but it would be nice to explain a little bit more how it is done (also for the other cores). Probably again a table explaining the number of samples considered for present-day for each core, the depth range and individual values would help.
Citation: https://doi.org/10.5194/cp-2022-65-RC2
- AC3: 'Response to reviewer #2', Christo Buizert, 05 Dec 2022
  
  The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2022-65/cp-2022-65-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/cp-2022-65-AC3
EC1:
'Comment on cp-2022-65', Hubertus Fischer, 08 Nov 2022

Dear authors
the manuscript has been seen by two referees and some additional comments hve been added. Both evaluation were overall quite positive but stress the still explorative stage of this new proxy. I invite you to provide your author responses to the comments with special focus on the discussion of limitations of the method, the data and potential other effects that may influence this proxy.
Looking forward to your replies
Hubertus Fischer (CP editor)

Citation: https://doi.org/10.5194/cp-2022-65-EC1
- AC4: 'Response to editor', Christo Buizert, 05 Dec 2022
  
  Dear Hubertus,
  We have provided a response to the comments by two anonymous reviewers and by Aymeric Servettaz. All three have provided very valuable feedback that will improve our manuscript. We look forward to your response, and hope we will get the opportunity to submit a revised manuscript.
  Thank you for your kind consideration,
  the authors.
  
  Citation: https://doi.org/10.5194/cp-2022-65-AC4

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Co-editor-in-chief

This paper presents a new proxy application of permanent gas isotopes in ice core research based on the kinetic fractionation of gases in the firn column of polar ice sheets (offset from the diffusive equilibrium profile) which may be induced by synoptic atmospheric pressure variations at the surface. Although still exploratory in nature, this new proxy would allow to reconstruct changes in mean synoptic activity for different climate states based on ice core data.

Short summary

It is unclear how different components of the global atmospheric circulation, such as the El Niño effect, respond to large-scale climate change. We present a new ice core gas proxy, called Krypton-86 excess, that reflects past storminess in Antarctica. We present data from 11 ice cores that suggest the new proxy works. We present a reconstruction of changes in West Antarctic storminess over the last 24,000 years, and suggest these are caused by north-south movement of the tropical rain belt.


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