A carbon and oxygen isotopic analysis of molluscan faunas from the Callovian-Oxfordian boundary at Redcliff Point, Weymouth, Dorset: implications for belemnite behaviour

Macrofossils (belemnites, Gryphaea and ammonites) from the Callovian-Oxfordian boundary at Redcliff Point, Weymouth, Dorset were examined using petrographic, isotopic and geochemical methods to investigate the environmental conditions within which they were formed. The belemnites (Hibolithes hastata), Gryphaea and ammonites (Cardioceras sp.) exhibit the petrographic and geochemical criteria for being well preserved. Values of δ18O for Gryphaea provide palaeotemperature ranges of c. 9.8–14.1 °C (mean 11.8°C); belemnite palaeotemperature ranges of c. 10.8–15.8°C (mean 12.8°C); and cardioceritid ammonites palaeotemperature ranges of c. 12.7–19.6°C (mean 15.4°C). The data define a predictable depth-related temperature gradient. The estimated range of palaeotemperatures derived from the belemnites straddle both Gryphaea and ammonite ranges. Hence, rather than being strictly nektobenthonic, belemnites may have migrated vertically within the water column in search of food, warmth, or for evasion from predators. The δ13C profile revealed is less clear and may result from either carbonate precipitated in reduced salinity surface waters characterized by more negative carbon isotopes than open-marine conditions or it may reflect the effects of non-equilibrium fractionation.