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The residence time of Southern Ocean surface waters and the 100,000-year ice age cycle

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Science  08 Mar 2019:
Vol. 363, Issue 6431, pp. 1080-1084
DOI: 10.1126/science.aat7067

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Resetting the glacial timer

The periodicity of glacial cycles changed from 100,000 to 41,000 years during the middle of the Pleistocene epoch. Why? Hasenfratz et al. measured the oxygen isotope composition and magnesium/calcium ratio in benthic and planktonic foraminifera from the Antarctic in order to reconstruct changes in the rate of transfer of ocean water from the depths to the surface over the past 1.5 million years (see the Perspective by Menviel). The emergence of the 100,000-year cycle coincided with a reduction in deep-water supply and a freshening of the surface ocean. This slowing may have caused more prolonged ice ages by making the Antarctic less responsive to orbitally paced drivers of carbon dioxide release.

Science, this issue p. 1080; see also p. 1040

Abstract

From 1.25 million to 700,000 years ago, the ice age cycle deepened and lengthened from 41,000- to 100,000-year periodicity, a transition that remains unexplained. Using surface- and bottom-dwelling foraminifera from the Antarctic Zone of the Southern Ocean to reconstruct the deep-to-surface supply of water during the ice ages of the past 1.5 million years, we found that a reduction in deep water supply and a concomitant freshening of the surface ocean coincided with the emergence of the high-amplitude 100,000-year glacial cycle. We propose that this slowing of deep-to-surface circulation (i.e., a longer residence time for Antarctic surface waters) prolonged ice ages by allowing the Antarctic halocline to strengthen, which increased the resistance of the Antarctic upper water column to orbitally paced drivers of carbon dioxide release.

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