Middle-Upper Jurassic seawater temperatures: clumped isotopes from Scottish belemnites

We present new seawater temperature estimates for the Callovian to Kimmerigian interval (Middle to Upper Jurassic), from clumped isotope analyses of pristinely preserved belemnites from Staffin Bay, Isle of Skye, Scotland. We compare two belemnite genera, finding that generally Pachyteuthis give warmer temperatures than Cylindroteuthis. Our clumped isotope temperatures for both genera are higher than previous stable isotope temperature estimates across this interval (from this and other localities). Whilst it is possible that there was some slight solid-state reordering of the belemnite calcite, these new data suggest that either or both δ18Osw estimates and the fractionation of 18O into belemnite rostral calcite are different to previously assumed values. Reconstructions of seawater δ18O (δ18Osw) using a number of published calcite temperature equations generally yield very high δ18Osw values. The results from the present study support previous evidence that belemnites may fractionate 18O differently to abiotic calcite, and other biotic calcites (e.g. foraminifera, brachiopod). This supports a growing body of evidence that a belemnite-specific δ18O temperature equation needs to be developed.