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 h as been proposed as an indicator of seawater temperature in past epochs, si nce this is less affected by other environmental conditions (such as variat ion of salinity) than the more conventional seawater palaeotemperature prox y delta O-18. However, in this paper, we demonstrate that the Sr:Ca ratio i s influenced by several factors including temperature and growth rate. Two fossil molluscs, a large spiral gastropod, Clavilithes macrospira, and a la rge bivalve, Venericarda planicosta, from the Eocene-aged deposits of South ern 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 delta O-18 was measured usin g 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 seas onal, possibly temperature-related variations. Comparison of the Sr:Ca prof iles with those for delta O-18 and the spacing of the dark annual growth ba nds suggests that Sr incorporation is controlled primarily by metabolic act ivity, which in turn is influenced by factors such as temperature, salinity , age and growth rate [1]. (C) 2001 Elsevier Science B.V. All rights reserv ed.