A radiotracer study of cerium and manganese uptake onto suspended particles in Chesapeake Bay

Abstract

The oxidation kinetics of Ce (III) and Mn(II) were studied in Chesapeake Bay in March and July 1990 to establish the role of water column redox processes in contributing to Ce anomalies observed in this estuary (Sholkovitz and Elderfield, 1988; Sholkovitz et al., 1992). Oxidation was measured by adding Mn(II) and Ce (III) to freshly collected water samples as radiotracers and measuring their uptake onto the ambient suspended particle assemblage. Mn(II) oxidation was measured by following the uptake of 54Mn(II) onto suspended particles and utilizing protocols established by other workers to distinguish oxidation from Mn(II) adsorption. The same protocols were applicable to Ce (III), using 139Ce(III)and were supported by the use of 152Eu(III) as a nonredox reactive control. Specific rates of Ce (III) and Mn(II) oxidation measured at a station in the North Bay (depth = 4 m) in July were 2016% per day and 4032% per day, respectively. In March at the same station, the specific rate of Mn(II) oxidation was only 10% per day and Ce (III) oxidation was undetectable. Both Ce (III) and Mn(II) oxidation processes were inhibited by azide, indicating that they were microbially mediated. The seasonal differences probably reflect strong seasonal variation in the abundance of Mn oxidizing bacteria. No Ce (III) oxidation occurred in samples collected below the oxic/anoxic interface in July. The specific rates of oxidation for both elements were over 1000 times higher than those measured in the Sargasso Sea. However, the specific rates for Ce (III) and Mn(II) were very similar to each other. This fact, coupled with similar spatial and temporal trends for specific oxidation rates, suggests a common mechanism of oxidation of both elements which may be significant in a wide range of marine environments.