Stable Isotope Sclerochronology of the Late Miocene Oyster Pycnodonte heermanni from the Salton through region, California

In the Miocene, southern California experienced extension on a long and relatively straight rift basin parallel to the pre-existing continental margin. This extension and the subsequent detachment of Baja California from North America formed the Gulf of California. By the Late Miocene, approximately 6.5 Ma, marine waters of the proto-Gulf (the present-day Salton Trough region) had extended north to the San Gorgonio Pass (Winker and Kidwell 1996). The sediments found in the Salton Trough represent the first marine faunal deposits from when Baja separated from the mainland. Studying the marine fauna of the Salton Trough region will provide insight into the paleoecology and environment of the proto-Gulf of California in this region during the Miocene. Oysters secrete the building material CaCO3 in incremental bands. Using sclerochronology, the marine equivalent of dendrochronology, I can examine these growth bands on the hinge of the oyster. Because shells incorporate ions from the surrounding water into their shells, oyster shells contain a biogeochemical record of the environmental and climatic conditions experienced throughout their lifetimes (Jones 1985, Jones and Quitmyer 1996). Previous work on fossil oysters of the genus Crassostrea has yielded robust isotopic results (Kirby 2000, Surge et al. 2008, Durham et al. 2017) suggesting that fossil oysters are suitable for isotopic research. Although some other organisms do not precipitate their shells in equilibrium with seawater (Brenchley and Harper 1998), the vital effects (physiologic effects) among mollusks are minimal (Jones 1985). The aim of this study is to conduct a growth rate analysis of the Miocene oyster Pycnodonte heermanni. I will test the hypotheses that: (1) the large shell size of the P. heermanni was the result of rapid ontogenic growth and not due to increase longevity; and (2) the pycnodontids ceased growth during the warmest season of the year.