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

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 giganteus
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.