Barium and Molybdenum Records in Bivalve Shells: Geochemical Proxies for Phytoplankton Dynamics in Coastal Environments?

Barium:calcium and molybdenum:calcium ratios were investigated in shells of the tropical scallop Comptopallium radula. Three juvenile specimens were harvested alive in the southwest lagoon of New Caledonia after a 1-yr hydrological survey. Calcite samples representing a few hours of biomineralization were laser-ablated along the maximal growth axis and analyzed for Ba and Mo content with an inductively coupled plasma mass spectrometer. Absolute dates of shell precipitation assigned on the basis of periodic formation of shell growth patterns led to the accurate reconstruction of ontogenetic variations of elemental ratios with subweekly resolution. Interindividual variability of Ba:Ca and Mo:Ca time series was low, indicating an environmental control on the incorporation of these elements within shells. Both profiles were characterized by a background level punctuated by sharp peaks. The ingestion of diatoms enriched in Ba (adsorbed on iron oxyhydroxides associated with the frustules) is the most likely cause of the formation of Ba:Ca peaks. Some contribution of diatom-associated barite is also possible. In every instance, Ba:Ca would possibly be a proxy for the timing and magnitude of diatom blooms. Among all the theories that could be advanced to explain the occurrence of Mo:Ca peaks, the most plausible appears to be the ingestion of phytoplankton cells grown on $NO_3^ - $ , and therefore containing high levels of Mo required for the activity of nitrate reductase. If this is so, then Mo:Ca could be a new proxy for nitrate uptake by phytoplankton in coastal ecosystems, helping to reconstruct the balance between new and regenerated production in paleoenvironments.