Paleoceanography of the mid-Pleistocene

Abstract

The fossil record of the Pleistocene calcareous nannoplankton indicates that during the mid-Pleistocene (0.93–1.25 my) occurred an episode of overwhelming dominance of small Gephyrocapsa. During this episode normally abundant, large size specimens of this genus (mainly Gephyrocapsa oceanica) were virtually excluded from the phytoplankton of tropical and subtropical oceans. The best modern analog of this dominantly small Gephyrocapsa assemblage is the subpolar Emiliania huxleyi assemblage, which implies that nutrient content was significantly greater and water temperature was lower in the photic water column of the tropical oceans than they are today. Increased equatorial upwelling in the oceans, on a scale much greater than today, may explain the above pattern. To achieve such broad equatorial upwelling there must be a source and a drive for cold, dense water. The Arctic Ocean, which was probably seasonally free of ice during this interval of the mid-Pleistocene, is capable of providing the requisite source as well as a drive for the inferred equatorial upwelling. The energy balance of a predominantly ice-free Arctic Ocean requires an approximately three to seven fold increase of hydrospheric heat transport from the North Atlantic to the Arctic Ocean, which dictates a corresponding or even greater increase in the volume of warm water entering the Arctic Ocean at the surface and cold dense water exiting at depth to the North Atlantic. Such enhanced dense water formation in the Arctic Ocean could drive the intensified equatorial upwelling implied by the small Gephyrocapsa dominance interval. If the above scenario is correct then the climate of the earth's northern hemisphere during the mid-Pleistocene may have been very different from the younger Pleistocene climate. One manifestation of this difference may be the mid-Pleistocene shift in climatic cycle periodicity from 40 ky to 100 ky. Another important aspect is that the enhanced greenhouse effect expected during the next century because of an increase of atmospheric CO 2 is thought to lead directly to melting of the Arctic Ocean ice cover and of the Greenland ice sheet. Thus, the “greenhouse” Arctic Ocean and its attendant ocean circulation would resemble the inferred mid-Pleistocene conditions.