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Migration of the subtropical front as a modulator of glacial climate


Ice cores extracted from the Antarctic ice sheet suggest that glacial conditions, and the relationship between isotopically derived temperatures and atmospheric have been constant over the last 800,000 years of the Late Pleistocene epoch1. But independent lines of evidence, such as the extent of Northern Hemisphere ice sheets2, sea level3 and other temperature records4, point towards a fluctuating severity of glacial periods, particularly during the more extreme glacial stadials centred around 340,000 and 420,000 years ago (marine isotope stages 10 and 12). Previously unidentified mechanisms therefore appear to have mediated the relationship between insolation, CO2 and climate. Here we test whether northward migration of the subtropical front (STF) off the southeastern coast of South Africa acts as a gatekeeper for the Agulhas current5,6, which controls the transport of heat and salt from the Indo-Pacific Ocean to the Atlantic Ocean. Using a new 800,000-year record of sea surface temperature and ocean productivity from ocean sediment core MD962077, we demonstrate that during cold stadials (particularly marine isotope stages 10 and 12), productivity peaked and sea surface temperature was up to 6 °C cooler than modern temperatures. This suggests that during these cooler stadials, the STF moved northward by up to 7° latitude, nearly shutting off the Agulhas current. Our results, combined with faunal assemblages from the south Atlantic7,8 show that variable northwards migration of the Southern Hemisphere STF can modulate the severity of each glacial period by altering the strength of the Agulhas current carrying heat and salt to the Atlantic meridional overturning circulation. We show hence that the degree of northwards migration of the STF can partially decouple global climate from atmospheric partial pressure of carbon dioxide, , and help to resolve the long-standing puzzle of differing glacial amplitudes within a consistent range of atmospheric .

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Figure 1: SeaWiFS (Sea-viewing Wide Field-of-view Sensor) image of ocean colour during austral summer with the highly productive STF to the south of core MD962077 (red star).
Figure 2: Long-term trends in SST and productivity-sensitive parameters from core MD962077 compared to the stable baseline glacial temperatures recorded in EPICA Dome C.
Figure 3: Mechanistic link between the migration of the STF, orbital eccentricity and ocean overturning.


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We thank D. P. Schrag and E. Goddard for their help with analyses in the early stages of this work. We thank C. Sonzogni, F. Rostek, N. Thouveny and J. Carignan for help with analyses and age model, and D. Sansom for help with the figures. We also thank H. Gildor, Y. Ashkenazy, R. Toggweiler, M. Meredith, M.-F. Loutre, P. Huybers and N. Edwards for comments and discussion on an earlier version of this manuscript. R.E.M.R. is grateful to the Royal Society for the International Outgoing Visit award for the exchange visits to CEREGE, which facilitated the development of the ideas and the manuscript. Palaeoclimate work at CEREGE is supported by grants from the Gary Comer Foundation, the CNRS and the Collège de France. Core MD962077 was collected by the RV Marion Dufresne supported by the Institut Polaire Français (IPEV).

Author Contributions E.B. and R.E.M.R. contributed equally to this work.

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Correspondence to Edouard Bard or Rosalind E. M. Rickaby.

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This file contains the dataset for core MD962077. (XLS 29 kb)

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Bard, E., Rickaby, R. Migration of the subtropical front as a modulator of glacial climate. Nature 460, 380–383 (2009).

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