Abstract
Oxygen isotopes from ocean sediments (δ18O) used to reconstruct past continental ice volumes additionally record deep water temperatures (DWTs). Traditionally, these are assumed to be coupled (ice-volume changes cause DWT changes). However, δ18O records during peak Middle Miocene warmth (~16–15 million years ago) document large rapid fluctuations (~1–1.5‰) difficult to explain as huge Antarctic ice sheet (AIS) volume changes. Here, using climate modelling and data comparisons, we show DWTs are coupled to AIS spatial extent, not volume, because Antarctic albedo changes modify the hydrological cycle, affecting Antarctic deep water production regions. We suggest the Middle Miocene AIS had retreated substantially from previous Oligocene maxima. The residual ice sheet varied spatially more rapidly on orbital timescales than previously thought, enabling large DWT swings (up to 4 °C). When Middle Miocene warmth terminated (~13 million years ago) and a continent-scale AIS had stabilized, further ice-volume changes were predominantly in height rather than extent, with little impact on DWT. Our findings imply a shift in ocean sensitivity to ice-sheet changes occurs when AIS retreat exposes previously ice-covered land; associated feedbacks could reduce the Earth system’s ability to maintain a large AIS. This demonstrates ice-sheet changes should be characterized not only by ice volume but also by spatial extent.
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Data availability
The climate model output data are available for analysis and download at https://www.paleo.bristol.ac.uk/ummodel/scripts/papers/Bradshaw_et_al_2021.html. It is possible to reproduce the information in Figs. 2, 3, 5 and 6 via this interface as well as download the data itself and the ancillary information (palaeogeography and ice-sheet configuration).
Code availability
The UK Met Office made available the source code of HadCM3 via the Ported Unified Model release (https://www.metoffice.gov.uk/research/approach/collaboration/unified-model/partnership). Enquiries regarding the use of HadCM3 should be directed in the first instance to the UM Partnership team, who can be contacted at um_collaboration@metoffice.gov.uk. The main repository for the Met Office Unified Model (UM) version corresponding to the model presented here can be viewed at http://cms.ncas.ac.uk/code_browsers/UM4.5/UMbrowser/index.html (registration required). The code detailing the changes required to update HadCM3 to HadCM3LB-M2.1 are available as a supplement to Valdes et al.28.
Change history
23 June 2021
In the online PDF version of this Article originally published, owing to a technical issue Fig. 5 was appearing incorrectly as a grey box; this has now been corrected. The HTML and print versions were unaffected.
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Acknowledgements
C.D.B. and D.J.L. were supported by NERC grant NE I006281/1. C.H.L. and S.M.S. were supported by NERC grant NE/I006427/1. A.M.deB. and C.D.B. gratefully acknowledge support from the Swedish Research Council project [2016-03912]. This work was carried out using the computational facilities of the Advanced Computing Research Centre, University of Bristol: http://www.bristol.ac.uk/acrc/. We thank D. Suri, chief operational meteorologist, Met Office; G. Guentchev, senior scientist, Met Office; S. Feakins; and T. Naish for feedback on earlier drafts of this manuscript. We also thank E. Gasson for providing a plot of his ice-sheet model results13.
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C.D.B., C.H.L. and D.J.L. conceived the project and directed the research with the assistance of A.M.deB.; C.D.B. conducted and interpreted the modelling with the assistance of D.J.L., A.M.deB. and P.M.L.; C.D.B compiled and interpreted the proxy records with the assistance of C.H.L., A.M.deB., H.K.C. and S.M.S.; C.D.B. led the writing of the paper. All authors contributed to writing the manuscript.
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Peer review information Nature Geoscience thanks Sarah Feakins, Tim Naish and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: James Super.
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Bradshaw, C.D., Langebroek, P.M., Lear, C.H. et al. Hydrological impact of Middle Miocene Antarctic ice-free areas coupled to deep ocean temperatures. Nat. Geosci. 14, 429–436 (2021). https://doi.org/10.1038/s41561-021-00745-w
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DOI: https://doi.org/10.1038/s41561-021-00745-w
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