CERIUM REDOX CYCLES AND RARE-EARTH ELEMENTS IN THE SARGASSO SEA

Two profiles of the rare earth elements (REEs) are reported for the upper water column of the Sargasso Sea. The trivalent-only REEs have remarkably constant concentrations in the upper 500 m of an April 1989 profile and in the upper 200 m of a May 1989 profile. In contrast, Ce concentrations decrease smoothly with increasing depth. In April 1989 Ce decreases from 15.7 pmol/kg at 20 m to 5.1 pmol/kg at 750 m. Cerium, which has Redox transformations in seawater, behaves anomalously with respect to its REE(III) neighbors. While both dissolved Ce and Mn have elevated concentrations in the upper 200 m, their vertical gradients are distinctly different. In contrast to Mn, which reaches a minimum dissolved concentration near the zone (150-250 m) of a particulate Mn maximum, Ce is being removed both near this zone and to depths of at least 750 m. These new profiles indicate that Ce is involved in an upper ocean redox cycle. This interpretation is consistent with the MOFFETT (1990) incubation tracer experiments on the same May 1989 seawater. He showed that Ce(III) oxidation is biologically mediated, probably light inhibited, increases with depth, and 3-4 times slower than Mn(II) oxidation in the 100-200 m zone. "CERoclines" provide new information into the fine scale zonation of redox processes operating in the upper columns of oligothrophic oceans. While the trivalent-only REE have similar concentrations, striking differences exist between our Ce profiles and Ce anomalies and those of DE BAAR et al. (1983) from their August 1980 profile. The increase in their Ce concentration in the upper 150 m is ten times greater than ours. This leads to large positive Ce-anomalies while our samples show well-developed negative anomalies. After reanalyzing archived samples, DE BAAR (1991) has shown that the high Ce concentrations in DE BAAR et al. (1983) are too high due to an unknown analytical cause. His new results agree well with those reported here and rule out a previous interpretation regarding the formation of positive Ce anomalies in the Sargasso Sea.