M. Elliot, M.T. McCulloch, M.K. Gagan, G. Cabioch* and J. Chappell
*Institut de Recherche pour le Developpement, Nouméa, New Caledonia

We measured the trace element composition (Sr/Ca) and oxygen isotope ratio (18O) of a modern Tridacna maxima shell collected live in December 2001 from New Caledonia. Bivalves secrete daily layers of carbonate which can be sub-sampled to obtain information with seasonal resolution. T. maxima are found in tropical reef environments and can live for over one-hundred years. In contrast with corals, they precipitate their shell in isotopic equilibrium with the water in which they grow and thus have the potential to be valuable tools in paleo-environmental studies. Sr/Ca and Mg/Ca of marine carbonates are commonly used to reconstruct past temperatures. However previous studies have not succeeded in establishing a clear relationship between water temperature and the Sr/Ca of T. maxima bivalves. Our approach was to measure skeletal Sr/Ca using high resolution by laser ICP-MS and the 18O of a modern specimen which has grown in a controlled environment.

A section of the giant T. maxima shell reveals 3 distinct zones. The hinge and the outer zones present well-defined successive dark and light yearly increments, whereas the inner layer is homogenous and reveals no obvious annual banding. The Sr/Ca of different zones was analysed in order to assess any differences in the distribution throughout the shell. We obtained high-resolution profiles of Sr/Ca from the hinge and inner zones, which were unaltered relative to the outer zone. Two parallel scans perpendicular to the growth layers were performed in order to check the reproducibility of the laser ICP-MS Sr/Ca measurements which were obtained every 50 microns. 18O was measured on samples of carbonate powder extracted from the hinge area with an average resolution of 300-400 microns using a high precision microdrill.

The profiles of Sr/Ca in each zone all exhibit seasonal variations but with different amplitude (Figure 12). The 18O profile along the hinge area also shows seasonal variations which have been used to develop an annual chronology. High Sr/Ca and 18O values are associated with the dark-winter increments in the hinge zone which is similar to what has been observed in corals. We have thus been able to obtain seasonal variability of trace elements which covary with the 18O record. This observation supports the possibility of a temperature-controlled uptake of trace elements. However, our results show that the distribution of the trace elements throughout the shell varies significantly. The biological and physical processes controlling the uptake of trace elements from the surrounding water and their distribution within the shell of T. maxima will have to be better understood before obtaining a reliable palaeothermometer.

Figure 12: Sr/Ca variations measured by laser ICP-MS along the inner layer (top) and along the hinge area (bottom) of a modern T. maxima collected live in December 2002, in New Caledonia. The estimated age of this specimen is approximately 49 years. The transect of the inner layer covers the last ~33 years, whereas the transect along the hinge area covers the period 1943-1985.