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Polar ocean stratification in a cold climate

Abstract

The low-latitude ocean is strongly stratified by the warmth of its surface water. As a result, the great volume of the deep ocean has easiest access to the atmosphere through the polar surface ocean. In the modern polar ocean during the winter, the vertical distribution of temperature promotes overturning, with colder water over warmer, while the salinity distribution typically promotes stratification, with fresher water over saltier. However, the sensitivity of seawater density to temperature is reduced as temperature approaches the freezing point, with potential consequences for global ocean circulation under cold climates1,2. Here we present deep-sea records of biogenic opal accumulation and sedimentary nitrogen isotopic composition from the Subarctic North Pacific Ocean and the Southern Ocean. These records indicate that vertical stratification increased in both northern and southern high latitudes 2.7 million years ago, when Northern Hemisphere glaciation intensified in association with global cooling during the late Pliocene epoch. We propose that the cooling caused this increased stratification by weakening the role of temperature in polar ocean density structure so as to reduce its opposition to the stratifying effect of the vertical salinity distribution. The shift towards stratification in the polar ocean 2.7 million years ago may have increased the quantity of carbon dioxide trapped in the abyss, amplifying the global cooling.

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Competing interests

The authors declare that they have no competing financial interests.

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Acknowledgements

This work was supported by the US NSF, Schweizer Nationalfonds, Deutsche Forschungsgemeinschaft, and by British Petroleum and Ford Motor Company through the Carbon Mitigation Initiative at Princeton University. This research used samples provided by the ODP. The ODP is sponsored by NSF and participating countries under the management of Joint Oceanographic Institutions. We thank T. F. Pedersen and K. Gordon for isotope analyses, M. Soon for analytical assistance, and C.-D. Hillenbrand for providing data from ODP Site 1096. J. D. Hays focused our attention on M. Winton's work on the climate implications of the nonlinear dependence of density on temperature. D. P. Schrag encouraged its pursuit as a general mechanism, and J. F. Adkins contributed the thoughts about the role of sea ice formation on the Antarctic shelf.

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Competing interests

The authors declare that they have no competing financial interests.

Correspondence to Daniel M. Sigman.

Supplementary information

Supplementary Figure 1

Evidence from ODP Site 883 in the western Subarctic North Pacific for a correlation between warm climate and high opal accumulation during the mid-Cenozoic. (PDF 123 kb)

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Figure 1: Map of mean annual surface salinity5 illustrating the predominance of low-salinity surface waters at high latitudes, including the Subarctic North Pacific and the Antarctic.
Figure 2: Subarctic North Pacific (ODP Site 882) and Southern Ocean (ODP Site 1096)8 palaeoceanographic time series.
Figure 3: Density as a function of depth in the modern wintertime Antarctic and changes in this density structure for uniform changes in seawater temperature.

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