COMPARISON OF MODERN AND FOSSIL MOLLUSC SHELL COMPOSITIONS: IMPLICATIONS FOR PALEOECOLOGY INTERPRETATION

Stable isotopes liberated from fossil shell of molluscs have been used to make inferences about paleoecological conditions. Interpretations of isotopic analyses have been problematic due to a variety of processes responsible for altering original shell compositions during fossilization. This study used X-ray diffraction (XRD) of terrestrial shells from known taxa of modern and fossil gastropods and unknown bivalves. The goal of this analysis was to gain a better understanding of the relationship between fossilization and original shell composition. XRD analyses show that modern gastropod shells retain 100% of original aragonite composition. A modern bivalve contained 95% calcite and 5% vaterite, which is a less stable calcium carbonate polymorph than aragonite. Fossil mollusc shells collected from deposits of a buried stream channel dating approximately 10,000 BP were used for compositional analysis. Fossil gastropod shells range between 97-99% aragonite, and 1-3% calcite and unknown compounds. A fossil bivalve shell was comprised of 99% aragonite, and less than 1% calcite and unknown compounds. Fossil gastropod taxa including Physella integra, Discus whitneyi, and Gastrocopta similis all exhibited similar percentages of aragonite. Three individual Physellsa integra were tested showing little variability and were indistinguishable from compositions of other fossil taxa. The preliminary data reflect compositional changes among fossil molluscs. Therefore, compositions of fossil shells, especially those of older geologic ages, may have significant variability. Consequently, fossil shells used in stable isotopic analyses should be examined carefully after a more comprehensive understanding of the fossilization history.