Chemical tracers of past deep ocean circulation

This chapter reviews the major geochemical methods used to reconstruct the past circulation of the deep ocean, which is important for the distribution of heat, carbon, and nutrients. Some tracers, such as the carbon isotopic composition and trace metal concentrations in the calcite tests of benthic foraminifera, are used to reconstruct the nutrient content of deep water masses. These tracers are non-conservative, reflecting both biological cycling and physical circulation and mixing in the oceans. We also discuss conservative water mass tracers which reflect circulation and mixing alone. These are proxies for temperature, salinity, and air-sea exchange of carbon dioxide. We also examine methods for inferring the rates of past ocean circulation, including radiocarbon, the accumulation of the decay products of uranium in sediments, and geostrophic shear estimates. Throughout, we discuss the ocean circulation inferred for the last glacial maximum based on these techniques.