Porewater cadmium geochemistry and the porewater cadmium:. delta. sup 13 C relationship

Results from two continental margin cores collected off Pt. Sur, California, make possible a direct determination of the relationship between dissolved cadmium and the carbon isotopic composition of dissolved inorganic carbon in the interstitial water of suboxic marine sediments. The {delta}{sup 13}C values of dissolved inorganic carbon decrease nearly 1{per thousand} in the top 0.5 cm of the sediments and display a more gradual decrease through the top 20 cm of the sediments. Dissolved cadmium concentrations increase by 0.6 to 1.5 nmol/kg in the top 0.5 cm of the sediments, and then rapidly decrease to values of approximately 0.12 nmol/kg between 5 and 10 cm into the sediments, apparently as a result of scavenging onto iron oxide surfaces. This pattern contrasts with porewater cadmium profiles previously reported for pelagic sediments, which no evidence of cadmium scavenging by the sediments. However, porewater cadmium concentrations are poorly correlated with porewater {delta}{sup 13}C values in both oxic and suboxic sediments. These results suggest that the cadmium/calcium ratios and {delta}{sup 13}C values of infaunal benthic foraminifera need not be tightly linked, and that the relationship between these two aspects of foraminiferal shell chemistry may vary in response to changes in sediment geochemistry.