REDOX processes influence the geochemistry of many elements in the ocean, but attributing the distribution of these elements in the water column and sediments to specific redox processes is difficult because they are also influenced by non-redox processes and by other inputs that are poorly constrained. Cerium provides an opportunity to study redox processes in the ocean1–5 because its redox chemistry leads to its enrichment or depletion ('cerium anomalies') with respect to its lanthanide neighbours. A detailed understanding of Ce geochemistry is lacking, however, because of the paucity of knowledge of the redox rates and mechanisms in natural waters. Here I report measurements of Ce(III) oxidation rates using radiotracers in seawater samples collected in the Sargasso Sea and in Vineyard Sound, Massachussetts, which are much faster than previous, indirect estimates1. The data indicate that the negative Ce anomaly in sea water is the result of microbial oxidation followed by preferential scavenging of Ce(IV); no abiotic oxidation was detectable. This suggests that inhibition by sunlight of microbial oxidation and scavenging of Ce and Mn contribute to the pronounced surface maxima observed for these elements.
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