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Deep Arctic Ocean warming during the last glacial cycle


In the Arctic Ocean, the cold and relatively fresh water beneath the sea ice is separated from the underlying warmer and saltier Atlantic Layer by a halocline. Ongoing sea ice loss and warming in the Arctic Ocean1,2,3,4,5,6,7 have demonstrated the instability of the halocline, with implications for further sea ice loss. The stability of the halocline through past climate variations8,9,10 is unclear. Here we estimate intermediate water temperatures over the past 50,000 years from the Mg/Ca and Sr/Ca values of ostracods from 31 Arctic sediment cores. From about 50 to 11 kyr ago, the central Arctic Basin from 1,000 to 2,500 m was occupied by a water mass we call Glacial Arctic Intermediate Water. This water mass was 1–2 °C warmer than modern Arctic Intermediate Water, with temperatures peaking during or just before millennial-scale Heinrich cold events and the Younger Dryas cold interval. We use numerical modelling to show that the intermediate depth warming could result from the expected decrease in the flux of fresh water to the Arctic Ocean during glacial conditions, which would cause the halocline to deepen and push the warm Atlantic Layer into intermediate depths. Although not modelled, the reduced formation of cold, deep waters due to the exposure of the Arctic continental shelf could also contribute to the intermediate depth warming.

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Figure 1: Core site location map.
Figure 2: Holocene and MIS3 Mg/Ca ratios.
Figure 3: Deep Arctic Ocean temperature history.


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We thank E. Brouwers, C. Callicott, L. Gemery, T. Ichinose, R. Kihl, T. Nettleton,and N. Seevers for sample processing, E. Klein for use of the atomic emission spectrometer, R. Poirier for chronology, T. Rasmussen, D. Darby, P. De Deckker and R. Marzen for graphics, and C. Merz, C. Vogt and R. DeConto for helpful discussions. T.M.C., J.F. and G.S.D. are financially supported by the US Geological Survey Global Change Program; W.M.B. by National Science Foundation Grant OPP-9400,255; H.A.B. and R.F.S. received financial support from the Academy of Sciences, Humanities, and Literature Mainz through the ‘Akademienprogramm’. The Swedish contribution was supported by a grant from the Knut and Alice Wallenberg Foundation.

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Authors and Affiliations



T.M.C. coordinated the study, analysed the data and wrote most of the paper, G.S.D. and J.F. conducted the Mg/Ca and Sr/Ca analyses, R.F.S. contributed chronology and interpretation of MIS3-1 results, M.J. contributed material, age model for HLY0503-18TC and oceanographic interpretation, J.N. developed the oceanographic model, Krithe identified by W.M.B., from P1-AR-94 cores, A.S. and H.A.B. provided Krithe and chronology from PS51-154, H.A.B. contributed Krithe, proxy records and chronology from PS1243, and oceanographic interpretation.

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Correspondence to T. M. Cronin.

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Cronin, T., Dwyer, G., Farmer, J. et al. Deep Arctic Ocean warming during the last glacial cycle. Nature Geosci 5, 631–634 (2012).

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