1997 Theses Doctoral

The last interglacial in the North Atlantic: deep-sea records of climate variability and sediment flux

McManus, Jerry F.

While the orbital theory of the ice ages successfully accounts for much of the pacing of Pleistocene climate cycles, there is increasing evidence that significant variability in the past occurred on relatively short time scales. Most of this evidence comes from studies of the last glacial interval. In a series of high resolution studies using deep-sea sediment cores from the North Atlantic Ocean, the last interglacial interval (70,000-130,000 years ago) was examined, extending the existing records of suborbital variability throughout the entire last large climate cycle of the Pleistocene. A multi-proxy approach, incorporating planktonic foraminiferal assemblages, abundances of ice-rafted debris, and the stable isotopic composition of foraminiferal calcite, was used to establish the history of sea surface conditions. In addition, measurements of excess 230Th in the sediments were used to establish bulk and constituent absolute burial rates for the entire climate cycle.

Three types of abrupt or high frequency behavior evident in these studies are glacial terminations, episodes of deposition by icebergs, and repeated millennial-scale coolings. The rapid deglaciation that immediately preceded the last interglacial is one of the most dramatic climatic transitions of the Pleistocene. It is accompanied by a massive ice-rafting event and associated meltwater, linked first to a large transient and then a major reorganization of the thermohaline circulation, followed by rapid regional warming. The ice-rafting event is equivalent to the Heinrich events that punctuated the last glacial interval. Our flux data indicate that the events were indeed times of increased deposition by icebergs, but that they were also intervals of decreased burial of foraminifera. A similar pattern of ice-rafting episodes occurred north of Iceland, beyond the likely influence of icebergs derived from the North American Laurentide ice sheet. Millennial-scale cooling cycles, equivalent to the Dansgaard/Oeschger oscillations evident in Greenland ice cores, and accompanied by increased ice-rafting, occurred throughout the region subsequent to the end of the last peak interglacial period 110,000 years ago. Less dramatic variability characterized the North Atlantic region during the peak interglacial.