Cadmium: Chemical tracer of deepwater paleoceanography

Abstract

The oceanic distribution of cadmium resembles that of phosphorus. Because the cadmium content of foraminiferal shells is governed by the cadmium content of seawater, planktonic and benthic fossil shells can be used to infer nutrient distributions within ancient oceans. Empirical studies demonstrate that cadmium in benthic foraminiferal shells is related to the bottom water composition through a proportionality constant D ≃ 2.9. This constant is the same for each of the species studied: Cibicidoides wuellerstorfi, Cibicidoides kullenbergi, Nuttallides umbonifera, and Uvigerina.spp. Downcore cadmium data from high‐quality Pacific and Atlantic sediment cores suggest that the cadmium inventory of the ocean did not change significantly between the most recent glacial maximum and the present. Hence changes in the cadmium content of fossils at a site directly reflect changes in nutrient distributions due to altered oceanic circulation patterns. Studies of cadmium in Pleistocene sediments show that deep ocean circulation patterns were significantly different during the most recent glacial maximum. In the western North Atlantic Ocean, the nutrient content of waters from 2500 to 3500 m was twice as high during glacials as during interglacial periods, signifying an increase in the proportion of waters of Antarctic origin in this depth interval and a decrease (but not a cessation) in the flux of North Atlantic Deep Waters through this depth range. Below 3500 m, the nutrient increase was greater, indicating a higher proportion of Antarctic Bottom Water. Above 2500 m, the nutrient content was lower during glacial periods than it has been during interglacial times. The Cd content of high‐latitude North Atlantic and southern ocean surface waters does not show a significant glacial/interglacial change, which argues against theories for changes atmospheric carbon dioxide that require changes in high‐latitude nutrient concentrations.