BIOLOGICAL CONTROLS ON THE STABLE ISOTOPE AND TRACE ELEMENT COMPOSITION OF MAASTRICHTIAN AGE BIVALVE SHELLS FROM SEYMOUR ISLAND, ANTARCTICA

Since the early days of oxygen isotope paleothermometry, it has been known that biological processes can cause shell material to precipitate out of isotopic equilibrium with ambient water. So-called “vital effects” have complicated paleoenvironmental interpretations using many species of cnidarians, echinoderms, brachiopods, and mollusks. However, within mollusks, bivalves are thought to be minimally influenced by vital effects and are therefore widely used as proxies in paleoclimatology and paleoceanography. Here, we present new evidence for differences in δ¹⁸O, δ¹³C, and Sr/Ca values between discrete shell layers in four out of five studied bivalve genera (Cucullaea, Lahillia, Eselaevitrigonia, Nordenskjoldia, Dozyia) from the Maastrichtian (69-66 Ma) of Seymour Island, Antarctica. The outer layers of these shells appear to be faithful recorders of environmental conditions, but the interior layers can be either enriched or depleted in ¹⁸O, ¹³C, and/or Sr relative to the outer layers. The compositional differences between layers of these well-preserved shells are unable to be explained by diagenesis, mineralogy, differing life modes, or time-dependent environmental change. Therefore, we explore the possibility that this chemical differentiation is the result of biological processes, including the modulation of internal fluid composition and ontogenetic changes in metabolic carbon incorporation. While studies of modern bivalves have revealed much about their biomineralization, these processes and their ramifications for the chemistry of fossil shell material remain poorly understood. Interrogating vital effects in the isotopic and elemental record of fossil bivalves may reveal new information about the physiology of these ancient organisms, providing insight into the selectivity of an under-studied dimension of biodiversity across mass extinctions.