THE GEOCHEMISTRY OF RARE-EARTH ELEMENTS IN THE SEASONALLY ANOXIC WATER COLUMN AND POREWATERS OF CHESAPEAKE BAY

A twelve cruise time-series study of a seasonally anoxic basin in Chesapeake Bay was carried out between February 1988 and February 1989. Data from filtered bottom water and upper (0-1 cm) porewater samples are presented. This is the first study in which time-series measurements have been employed to understand the low temperature geochemistry of rare earth elements (REEs). The focus is on the coupling between REDOX (Reduction-Oxidation) variations and REE cycling, fractionation of trivalent REEs and development of cerium anomalies. The time-series results demonstrate that REEs have large seasonal cycles in both the water column and porewaters in response to the development of anoxia in the spring and reoxygenation in the fall. The transition from oxic to suboxic to anoxic conditions results in the release of REEs into the upper porewaters and bottom waters. The release of REEs and Fe are coincident in the bottom waters, while the release of REEs lags Fe by approximately 50 days in the upper porewaters. This decoupling is explained in terms of a REDOX front which moves upward from the sediments to bottom waters during the development of seasonal anoxia. The release of REEs to the upper porewaters and bottom water are accompanied by (1) fractionation across the trivalent REE series and by (2) the preferential input of Ce relative to its trivalent-only REE neighbors. REE(III) fractionation is such that the relative rate of release increases from Lu to La (light REE > middle REE > heavy REE). During reoxygenation, removal of dissolved REEs occurs from both the water column and upper porewaters and follows the same relative rates, LREE > MREE > HREE. Cerium removal occurs faster than its neighbor REE(III), indicating a rapid oxidation of Ce(III) to Ce(IV).