The use of external micro-PIXE to investigate the factors determining the Sr:Ca ratio in the shells of fossil aragonitic molluscs

The ratio of strontium to calcium in the shells of fossil marine molluscs has been proposed as an indicator of seawater temperature in past epochs, since this is less affected by other environmental conditions (such as variation of salinity) than the more conventional seawater palaeotemperature proxy δ18O. However, in this paper, we demonstrate that the Sr:Ca ratio is influenced by several factors including temperature and growth rate. Two fossil molluscs, a large spiral gastropod, Clavilithes macrospira, and a large bivalve, Venericarda planicosta, from the Eocene-aged deposits of Southern England were selected for study. The variation of the Sr:Ca ratio as a function of distance along the growth direction was measured using PIXE in the Oxford external microbeam facility and the δ18O was measured using small samples removed by drilling at points along the same direction. The Sr:Ca profiles show a significant increase of Sr with age, as well as seasonal, possibly temperature-related variations. Comparison of the Sr:Ca profiles with those for δ18O and the spacing of the dark annual growth bands suggests that Sr incorporation is controlled primarily by metabolic activity, which in turn is influenced by factors such as temperature, salinity, age and growth rate