The distribution of proxy nutrient-like tracers such as 13C and Cd provides information on past deep ocean circulation patterns and the processes controlling atmospheric C02. These tracers are particularly valuable when they are measured in the same samples. It is now certain that the water mass structure of the North Atlantic Ocean was very different during glacial times: the present vertically-homogeneous North Atlantic nutrient structure changed into one in which deep waters had higher nutrient levels and shallow waters had lower nutrient levels. Drs. Boyle and Keigwin propose to determine the most recent glacial maximum hydrography of the Atlantic in detail, and to do several long time series over the last 300, 000 years that will provide evidence on the response of deep ocean circulation to Milankovitch orbital forcing. We also propose to examine the 13C and Cd content of high-latitude planktonic foraminifera as a guide to the nutrient content of polar oceans. Polar nutrients play heavily in some models of why glacial atmospheric C02 was lower than at present, and it is vital that it be shown how they might have changes over time. The P.I.'s also propose to develop barium as a new paleoceanographic tracer. Barium is more silica-like in its distribution in the open ocean, so it provides more useful information on past variations in Pacific hydrography than do 13C and Cd. Because barium is enriched in the Mediterranean Sea, paleo-barium studies will allow for a better evaluation of the relative roles of North Atlantic Intermediate Water and Mediterranean Outflow in determining the intermediate depth nutrient depletions observed in the North Atlantic Ocean. The P.I.s also propose to continue systematic evaluation of the reliability of benthic foraminifera as paleochemical tracers.
