Sensitivity of the Phosphate-Cadmium-Carbon Isotope Relation in the Ocean to Cadmium Removal by Suboxic Sediments

Reconstructions of past deep ocean nutrient concentrations rely largely on the carbon isotopic composition and cadmium content of shells of benthic foraminifera. When comparing records of these two paleonutrient tracers, a relation similar to the phosphate-cadmium-carbon isotope relation in the water column today is generally assumed. We present new water column, sediment, and pore water data from the California continental margin which demonstrate that Cd accumulation is enhanced in suboxic sediments. These results suggest that the oceanic Cd inventory is sensitive to changes in the areal extent and redox intensity of suboxic sediments and thus that the phosphate-cadmium relation in the ocean could have been different in the past. The magnitude of this effect and its evolution through time is presently hard to quantify because the oceanic Cd budget is poorly constrained.