Sr/Ca Ratio of Skeletal Aragonites from the Recent Marine Biota at Palau and from Fossil Gastropods

The Sr/Ca ratios were determined from skeletal aragonites of the marine biota and sea water from Palau, South Pacific, where the temperature and Sr/Ca ratio of the waters are constant throughout the year. The Sr/Ca ratios of the aragonites from all groups of organisms except those of the higher Mollusca are similar or slightly higher than those in sea water. In samples from all phyla with high Sr/Ca ratios, the crystal chemistry is considered the primary control of the Sr/Ca ratios from their skeletal aragonites, whereas physiologic control by the organisms is considered as a second order effect. In agreement with data in the literature, the Sr/Ca ratios from the skeletal aragonites of the more advanced Mollusca were found to be low and indicate strong discrimination against Sr relative to Ca in the carbonate as compared to sea water, and this is related to physiologic control by the organisms. The measured Sr/Ca ratios from aragonitic shells of fossil gastropods from Cretaceous and Pennsylvanian deposits are shown to be higher the older the fossils. The increase in Sr/Ca ratios in the gastropod shells with increase in geologic age is toward the Sr/Ca ratio in present day sea water. It is suggested that this reflects biochemical evolutionary changes of increased discrimination by the gastropod against Sr relative to Ca from the late Paleozoic to the present