The oxygen isotopic composition of seawater during the Last Glacial Maximum

High-resolution oxygen and hydrogen isotope measurements were made on pore fluids from deep-sea sediments from sites in the North and South Atlantic. The data provide direct measurements of changes in the isotopic composition of bottom waters during the Last Glacial Maximum (LGM). Results from Ocean Drilling Program (ODP) Site 981 in the North Atlantic, currently bathed in North Atlantic Deep Water (NADW) reproduces previous results from the Ceara and Bermuda Rises, constraining the glacial-interglacial change in delta(18)O of the deep Atlantic to be 0.7-0.8parts per thousand. Results from Site 984, which is located north of Site 981 and at a shallower water depth, yield a similar value (0.8parts per thousand), providing insight into the properties of Glacial North Atlantic Intermediate Water (GNAIW). Sites from ODP Leg 177 in the South Atlantic span the modern boundary between northern and southern sources of deep water. Data from the northern site (1088) yield a similar result to sites in the tropical and North Atlantic (0.7parts per thousand). At the southern site (1093), located south of the polar front, the change is substantially larger (1.1parts per thousand), representing the change in delta(18)O of southern source waters since the LGM. These results confirm previous estimates that the global average change in delta(18)O of seawater is 1.0+/-0.1parts per thousand. Hydrogen isotopes measured on pore fluids from three sites are consistent with the oxygen isotopes from these locations, giving further support to these results. At all sites studied, the temperature of the deep ocean during the LGM, calculated by combining the pore fluid results with oxygen isotope data from benthic foraminifera, was within VC of the freezing point of seawater. (C) 2001 Elsevier Science Ltd. All rights reserved.