NITROGEN ISOTOPES IN MOLLUSKS AS A PROXY FOR MARINE PRODUCTIVITY

Nitrogen stable isotope analysis of mollusk shells is a potential method for assessing marine productivity. Deep ocean water is relatively enriched ¹⁵N compared to shallow water due to higher biomass of phytoplankton in the photic zone, which preferentially incorporate light isotopes. Inputs of ¹⁵N-depleted deep water to a shallow marine system may be detected in local marine organisms such as mollusks that subsequently feed on the ¹⁵N-depleted phytoplankton biomass. We expect the shell of a mollusk that experiences such deepwater inputs to record the shift in N stable isotope source, based on the fact that the isotopes present in a marine organism are largely a factor of the isotopic content of its environment. Despite the potential, comparatively little research has been done in this area as nitrogen analysis in mollusk shells is a relatively new method. In this study, a group of Trachycardium procerum shells were collected off of the coast of Peru after the 1982-1983 El Niño, when a decrease in upwelling resulted in the presence of less phytoplankton rich (thus ¹⁵N-enriched) deeper waters to coastal areas. The T. procerum shells contain a visible growth anomaly that provides an independent measure of the portion of the shell that grew during the event. One of these shells was sequentially analyzed for nitrogen isotope variation. Extensive analysis has already been done on this shell (and other shells from the same bed and collection time), including radiocarbon, stable carbon and oxygen, and trace element analyses. These data constrain upwelling and other environmental parameters which will aid in determining unknown biologic processes and effects, including the incorporation of nitrogen into the shell material. When paired with independent radiocarbon upwelling data from the 1982-1983 El Niño event, nitrogen isotope analysis of the Trachycardium procerum shell should begin to elucidate the links between marine productivity and upwelling flux.