Stable isotope (δ18O and δ13C) sclerochronology of Callovian (Middle Jurassic) bivalves (Gryphaea (Bilobissa) dilobotes) and belemnites (Cylindroteuthis puzosiana) from the Peterborough Member of the Oxford Clay Formation (Cambridgeshire, England): Evidence of palaeoclimate, water depth and belemnit
Abstract Incremental δ 18 O and δ 13 C signals were obtained from three well-preserved specimens of Cylindroteuthis puzosiana and from three well-preserved specimens of Gryphaea ( Bilobissa ) dilobotes from the Peterborough Member of the Oxford Clay Formation (Cambridgeshire, England). Through-ontogeny (sclerochronological) δ 18 O data from G. ( B. ) dilobotes appear to faithfully record seasonal temperature variations in benthic Callovian waters of the study area, which range from c . 14 °C to c . 17 °C (arithmetic mean temperature c . 15 °C). Water depth is estimated to have been in the region of c . 50 m, based upon comparisons between these data, previously published non-incremental sea surface δ 18 O values, and a modern analogue situation. Productivity in Callovian waters was comparable with that in modern seas, based upon δ 13 C data from G. ( B. ) dilobotes , with 13 C depletion occurring during warmer periods, possibly related to an interaction between plankton blooms and intra-annual variations in mixing across a thermocline. Incremental δ 18 O data from C. puzosiana provide temperature minima of c . 11 °C for all specimens but with maxima varying between c . 14 °C and c . 16 °C for different individuals (arithmetic mean values c . 13 °C). Temperatures for late ontogeny, when the C. puzosiana individuals must have been living close to the study site and hence the analysed specimens of G. ( B .) dilobotes , are closely comparable to those indicated by the latter. However, for significant portions of ontogeny C. puzosiana experienced temperatures between c . 2 °C and c . 3 °C cooler than the winter minimum as recorded by co-occurring G. ( B. ) dilobotes . Comparisons with modern seas suggest that descent to a depth of c . 1000 m would be necessary to explain such cool minimum temperatures. This can be discounted due to the lack of deep waters locally and due to estimates of the depth tolerance of belemnites. The most likely cause of cool δ 18 O signals from C. puzosiana is a cosmopolitan lifestyle including migration to more northerly latitudes. Mean δ 13 C values from C. puzosiana are comparable with those from G. ( B. ) dilobotes . However, the incrementally acquired data are highly variable and probably influenced by metabolic effects. The probable identification of migratory behaviour in C. puzosiana calls into question the reliability of some belemnite species as place-specific palaeoenvironmental archives and highlights the benefits of adopting a sclerochronological approach.