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Barium/calcium profiles of bivalve shells are characterized,by,flat background,signals periodically interrupted by sharp peaks, with the background signals correlated with water Ba/Ca. To test if the peaks are an environmental signal related to productivity, we analyzed high-resolution Ba/Ca profiles in bivalve shells that grew adjacent to one another. Two aragonitic Saxidomus gigan- teus show remarkable similarity over a decade of growth, clearly indicating an environmental,forcing. Four calcitic Pecten maximus,shells also record synchronous,Ba/Ca peaks, again indicating an exogenous control. The Ba/Ca peaks, however, start ~40 days after the crash of a bloom, while sedimentation takes place immediately following the bloom. Barite formation in settling phytoplankton flocs, as has been previously proposed, is clearly not the cause of these peaks. Other possible causes, such as dissolved Ba in ambient water, spawning, shell organic matter content, and kinetic growth rate effects are also discussed, but none provide,satisfactory explanations. Background,shell Ba partition coefficients (Ba/Cacarbonate/Ba/Cawater) for both the calcitic shells (0.18) and aragonitic shells (0.16) are similar to that previously reported for the calcitic Mytilus edulis (~0.1). We suggest that Ba/Ca peaks in bivalve shells are caused,by an as yet undetermined,environmental forcing, while background Ba/Ca levels are a good indication of dissolved Ba/Ca in the water; both are independent of shell mineralogy.

Synchronous barium peaks in high-resolution profiles of calcite and aragonite marine bivalve shells