Assessing the Variability of Stable Calcium Isotopes in Cretaceous Carbonate Oyster Fossils

Past ocean acidification events and their effects on ocean organisms have been previously studied through analyses of stable calcium isotope abundances in bulk sedimentary carbonate. Stable calcium isotopes can record global calcium cycle variation, changing carbonate precipitation rates, and diagenetic alteration. Bulk sedimentary carbonate records are complex because they comprise sediment from several sources, including inorganically precipitated calcite and shell material from multiple organisms with differing vital effects. To control for vital effects, this study aims to assess the fidelity and meaning of calcium isotope signatures in Pycnodonte oyster shells from the Cretaceous period. While bulk sediment can average the calcium isotope composition of carbonate accumulated over tens to thousands of years, oyster shells offer much higher temporal resolution because they record only a few years of organism growth. To use Pycnodonte oysters as long-term archives of calcium isotope change, we need to determine whether shells preserve primary or secondary signals and constrain the magnitude and direction of short-term fluctuations. Here, we focus on Pycnodonte kellumi oyster fossils collected from the Western Interior Seaway, USA, from approximately 95 million years ago. We use optical and SEM imaging as well as analyses of carbon and oxygen isotopes to distinguish between primary and secondarily altered calcite and guide sampling locations. Along with carbon and oxygen analyses, we use stable calcium isotopes to assess short-term fluctuations in growth conditions. If these fossils offer high-fidelity archives of calcium isotope signals, they can be used to reconstruct longer-term temporal records of calcification conditions, with the overall objective of assessing environmental change during candidate ocean acidification events.