STABLE ISOTOPE RECORDS AND THE PALEOECOLOGY OF JURASSIC BELEMNITES

Belemnites are one of the oldest known fossils and widely used in evaluating temperature variation via stable isotope analysis. They are widely distributed and are seen in both Arctic and Antarctic settings. Characterisation of their mode of life is of key importance with respect to climate reconstruction. On the basis of stable isotope data the paleoecology of belemnites has been suggested to have been nektobenthonic. This interpretation contrasts with earlier ideas, which suggested that belemnites were nektonic surface dwelling organisms with habitats ranging from surface waters and near shore environments. This study examines a range of macrofossils (belemnites, Gryphaea and ammonites) from the Callovian-Oxfordian boundary (Jurassic) at Redcliff Point, Weymouth, UK. The samples have been examined using petrographic, isotopic, and geochemical methods to investigate the conditions within which they were formed. The belemnites (Hibolithes hastata), Gryphaea and ammonites (Cardioceras sp.) attain the petrographic and geochemical criteria for being well preserved. Within an environmental setting where continental ice volume is at a minimum and evaporation or freshwater input are assumed to be minor factors oxygen isotope values for Gryphaea provide paleotemperatures ranging from ~ 9.8°C – 14.1°C (mean 11.8°C); belemnite paleotemperatures ranging from ~ 10.8°C – 15.8°C (mean 12.8°C) and cardioceritid ammonites paleotemperatures ranging from ~ 12.7°C – 19.6 °C (mean 15.4°C). The data define a predictable pattern of depth related temperature stratification. The estimated range of paleotemperatures 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 evasion from predators. The carbon isotope profile revealed is less clear. Different carbon isotope values of the each of the fossil groups point to them not co-existing in the same waters and could result from either carbonate precipitated in reduced salinity surface waters characterised by more negative carbon isotopes or reflects the effects of non- equilibrium fractionation.