4 March 2004

Oceans and ancient climate

New data have provided more evidence for a role of the ocean in ancient climate change. Deep-sea records of biogenic opal accumulation and bulk sediment nitrogen isotopic composition indicate that the Subarctic North Pacific and the Antarctic, the two major nutrient-bearing polar regions of the ocean, became more vertically stratified roughly 2.7 million years ago. This coincides with the transition from Pliocene warmth to Northern Hemisphere glaciation. Sigman et al. propose that the global drop in temperature at this time could have led to the surface stratification of high latitude oceans due to the reduced importance of temperature, as opposed to salinity, in establishing seawater density in cooler waters. The resulting stratification could have promoted further global cooling during the Cenozoic by trapping more carbon dioxide in deep waters.

Polar ocean stratification in a cold climate
DANIEL M. SIGMAN, SAMUEL L. JACCARD & GERALD H. HAUG
Nature 428, 59–63 (2004); doi:10.1038/nature02357
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Palaeoclimate: Cool stratification
ROGER FRANCOIS
The quirky relationship between seawater temperature and density is invoked to account for how, during past global cooling, the high-latitude oceans locked up atmospheric CO₂ and produced further cooling.
Nature 428, 31–32 (2004); doi:10.1038/428031a
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4 March 2004 table of contents