Abstract
To precisely reconstruct environmental changes from high-resolution bivalve shell geochemistry, the shell layers must represent environmental conditions at the time of deposition. Therefore, calcium carbonate along growth lines that formed at the same time should show the same geochemical signature. We test this assumption by evaluating the geochemical spatial homogeneity along growth layers (i.e., shell secreted at a given time) on shell cross-sections of modern shells of the bivalve Protothaca thaca, for ontogenitically young and old specimens. Analyses were carried out using laser ablation ICP-MS and solution nebulization ICP-MS. Scanning electron microscope images showed signs of early diagenesis of old shell sections (close to the umbo) compared with recent (close to the ventral margin). The spatial distribution of Sr/Ca, Mg/Ca and Ba/Ca along growth layers differed consistently between old and recent shell sections. Elemental distribution at the ventral margin of young specimens was similar for the two specimens analyzed and similar to the element distribution in the older specimens at a similar ontogenic stage (closer to the umbo). If a balancing of the shell composition with the surrounding seawater between the old and recent part of the shell, supported by diagenesis, cannot be excluded, we assume that trace element incorporation process at a given time varied with the age of the specimen, in connection with changes of mantle characteristics. Whereas Ba/Ca and Sr/Ca were almost constant for shell secreted at a given time close to the current ventral margin, all individuals showed significant Mg/Ca heterogeneities, with reproducible patterns. We assumed that this would be due to a zonation of the outer epithelium of the mantle that would favor or inhibit the transport of Mg into the extrapallial fluid, and subsequently into the shell. Time-series obtained crossing successive growth layers using laser ablation ICP-MS and solution nebulization ICP-MS on a same-shell growth section were similar for Sr/Ca and Ba/Ca ratios and not for Mg/Ca, which reflected both the different spatial resolutions of the analytical techniques and the heterogeneous distribution of Mg/Ca. In the P. thaca shells, Sr and Ba seemed to be most promising as potential high-resolution environmental proxies. Our study showed that the location of high-resolution analyses points in shell cross-section must be i/ precisely defined and ii/ consistently followed along the shell section to provide accurate environmental records; i.e., without bias related to shell layer heterogeneities.