Trace Element Uptake in Marine Bivalve Shells Constraints from Field- and Laboratory Studies

There is an increasing interest in the use of the trace element signatures recorded in calcium carbonate skeletons of marine organisms as archives of past and present environmental conditions, such as temperature, salinity or nutrition level. Because of their global occurrence in the modern and ancient oceans, the trace element chemistry of bivalve shells might be used as a potential proxy for present and past environmental conditions. If the composition of bivalve shells, for instance, can be shown to represent the environment in which they lived, then shells can be used to investigate conditions in the lifetime of the animal. And as the shell material is sequentially deposited, an understanding of the internal shell structure will enable time- resolution of the analyses. Therefore, the trace element signature of bivalve shells may provide an important record of climate changes and global geochemical cycles. One of the difficulties of using the trace element signatures of bivalve shells as proxies for environmental conditions is that little is known about the mechanisms by which the trace elements are incorporated into the shells. There has been quite an amount of research into the use of bivalve shell chemistry as proxy for one or more environmental parameters, but there are relatively few datasets in which both bivalve shells and the water in which the animals lived have been analysed. It is as yet not clear to what extent the trace element incorporation into bivalve shells is governed by biological processes, like growth rate and metabolism of the animals, or by physical and crystal chemical parameters. An added difficulty is that the existing data do suggest that trace element uptake in bivalve shells may be species specific. Therefore, studies that investigate the relationships between the content of these elements in the shells and the ambient water and the possible incorporation mechanisms are needed if the potential that bivalve shells offer as environmental proxies should be realised. To this end, we have initiated a long-term field and laboratory study. Bivalves of the species Mytilus edulis and Arctica islandica are cultured in the Dutch Wadden Sea. Physico-chemical water parameters, such as temperature, salinity and water chemistry are monitored. Shells and ambient seawater have been collected periodically in order to constrain seasonal effects on shell growth and trace element uptake. Both shells and seawater are analysed using bulk chemical methods as well as in situ analysis of the shells by LA-ICP-MS. In this contribution, we will present the first important results of this on-going study.