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Southern Ocean during the ice ages: A review of the Antarctic surface isolation hypothesis, with comparison to the North Pacific

Sigman, Daniel M. and Fripiat, François and Studer, Anja S. and Kemeny, Preston C. and Martínez-García, Alfredo and Hain, Mathis P. and Ai, Xuyuan and Wang, Xingchen and Ren, Haojia and Haug, Gerald H. (2021) Southern Ocean during the ice ages: A review of the Antarctic surface isolation hypothesis, with comparison to the North Pacific. Quaternary Science Reviews, 254 . Art. No. 106732. ISSN 0277-3791.

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The Southern Ocean is widely recognized as a potential cause of the lower atmospheric concentration of CO₂ during ice ages, but the mechanism is debated. Focusing on the Southern Ocean surface, we review biogeochemical paleoproxy data and carbon cycle concepts that together favor the view that both the Antarctic and Subantarctic Zones (AZ and SAZ) of the Southern Ocean played roles in lowering ice age CO₂ levels. In the SAZ, the data indicate dust-driven iron fertilization of phytoplankton growth during peak ice age conditions. In the ice age AZ, the area-normalized exchange of water between the surface and subsurface appears to have been reduced, a state that we summarize as “isolation” of the AZ surface. Under most scenarios, this change would have stemmed the leak of biologically stored CO₂ that occurs in the AZ today. SAZ iron fertilization during the last ice age fits with our understanding of ocean processes as gleaned from modern field studies and experiments; indeed, this hypothesis was proposed prior to evidentiary support. In contrast, AZ surface isolation is neither intuitive nor spontaneously generated in climate model simulations of the last ice age. In a more prospective component of this review, the suggested causes for AZ surface isolation are considered in light of the subarctic North Pacific (SNP), where the paleoproxies of productivity and nutrient consumption indicate similar upper ocean biogeochemical changes over glacial cycles, although with different timings at deglaciation. Among the proposed initiators of glacial AZ surface isolation, a single mechanism is sought that can explain the changes in both the AZ and the SNP. The analysis favors a weakening and/or equatorward shift in the upwelling associated with the westerly winds, occurring in both hemispheres. This view is controversial, especially for the SNP, where there is evidence of enhanced upper water column ventilation during the last ice age. We offer an interpretation that may explain key aspects of the AZ and SNP observations. In both regions, with a weakening in westerly wind-driven upwelling, nutrients may have been “mined out” of the upper water column, possibly accompanied by a poleward “slumping” of isopycnals. In the AZ, this would have encouraged declines in both the nutrient content and the formation rate of new deep water, each of which would have contributed to the lowering of atmospheric CO₂. Through several effects, the reduction in AZ upwelling may have invigorated the upwelling of deep water into the low latitude pycnocline, roughly maintaining the pycnocline’s supply of water and nutrients so as to (1) support the high productivity of the glacial SAZ and (2) balance the removal of water from the pycnocline by the formation of Glacial North Atlantic Intermediate Water. The proposed return route from the deep ocean to the surface resembles that of Broecker’s (1991) “global ocean conveyor,” but applying to the ice age as opposed to the modern ocean.

Item Type:Article
Related URLs:
URLURL TypeDescription
Sigman, Daniel M.0000-0002-7923-1973
Fripiat, François0000-0002-2591-6301
Studer, Anja S.0000-0001-7354-8497
Kemeny, Preston C.0000-0003-1693-4142
Martínez-García, Alfredo0000-0002-7206-5079
Hain, Mathis P.0000-0002-8478-1857
Ai, Xuyuan0000-0003-1353-6114
Wang, Xingchen0000-0001-5316-789X
Haug, Gerald H.0000-0001-7534-4420
Additional Information:© 2021 Princeton University. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Received 20 July 2019, Revised 15 November 2020, Accepted 16 November 2020, Available online 26 December 2020. The research was supported by the National Science Foundation through grant PLR-1401489 (D. M. S.), by the Max Planck Society (G. H. H.), by ExxonMobil through the Andlinger Center for Energy and the Environment at Princeton University (D. M. S.), and by the Grand Challenges Program of Princeton University (D. M. S.). M. P. H. acknowledges the NERC Independent Research Fellowship NE/K00901X/1. P. C. K. was supported by the Princeton Environmental Institute’s Undergraduate Research Fund for senior thesis research. We thank J.R. Toggweiler and J.F. Adkins for discussions. The manuscript was improved substantially by reviews from H.C. Bostock and J.W.B. Rae. Author statement: Daniel Sigman: Writing- Original draft preparation; Anja Studer, Ellen Ai: Drafted data-based figures; François Fripiat: Drafted conceptual figures; All authors: Writing- Reviewing and Editing. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funding AgencyGrant Number
Max Planck SocietyUNSPECIFIED
Princeton UniversityUNSPECIFIED
Natural Environment Research Council (NERC)NE/K00901X/1
Subject Keywords:Paleoclimatology; Paleoceanography; Pleistocene; Ice ages; Stable isotopes; Nitrogen; Atmospheric CO2; Southern Ocean; North Pacific; Ocean circulation; Biological pump
Record Number:CaltechAUTHORS:20210311-135522011
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Official Citation:Daniel M. Sigman, François Fripiat, Anja S. Studer, Preston C. Kemeny, Alfredo Martínez-García, Mathis P. Hain, Xuyuan Ai, Xingchen Wang, Haojia Ren, Gerald H. Haug, The Southern Ocean during the ice ages: A review of the Antarctic surface isolation hypothesis, with comparison to the North Pacific, Quaternary Science Reviews, Volume 254, 2021, 106732, ISSN 0277-3791, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:108405
Deposited By: Tony Diaz
Deposited On:12 Mar 2021 18:58
Last Modified:12 Mar 2021 18:58

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