Paleoceanographic records extracted from a global array of sediment cores obtained by the Ocean Drilling Program (ODP) can be used to elucidate differences between oceanographic conditions during the early Pliocene warm period (~4.5 to 3.0 million years ago [Ma]) and the late Pliocene and Pleistocene cool ice age period (3.0 Ma to present). Oxygen isotope gradients derived by laboratory analysis of calcareous microfossil shells from low-latitude sites are used to reconstruct tropical surface hydrographic (i.e., temperature and/or salinity) gradients and to examine the role of tropical oceans on global climate over the last 5 million years, including the factors that caused the warm to cold climate transition, commonly referred to as the onset of significant Northern Hemisphere Glaciation (NHG). We find that a small west-east temperature gradient across the Pacific Ocean, similar to El Niño conditions, accompanied and perhaps played a critical role in determining early Pliocene global warmth; steeper temperature gradients, more typical of the modern ocean, were established during the cool, ice-age climatic state by ~1.5 Ma.

Ravelo, A.C., and M.W. Wara. 2004. The role of the tropical oceans on global climate during a warm period and a major climate transition. Oceanography 17(3):32–41, https://doi.org/10.5670/oceanog.2004.28.

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