HIGH-RESOLUTION STABLE-ISOTOPE ANALYSIS OF THE MODERN CUTTLEFISH (SEPIA): IMPLICATIONS FOR BELEMNITE PALEOTEMPERATURES, PALEODIET AND PALEOECOLOGY

The modern cuttlefish, Sepia, sequentially secretes an internal aragonitic skeleton (cuttlebone) over the course of its life, thus preserving environmental information in the stable-isotope composition of their cuttlebone. This study aims to evaluate cuttlebone stable-isotope geochemistry, with a view to comparing this to the now extinct relative the belemnite. Five cuttlebone specimens were collected from Jersey, English Channel and subsequently sampled at high-resolution and analyzed for carbonate carbon and oxygen isotopic ratios. For an organic investigation we samples 5 layers per sample in order to generate enough for duplicate carbon and nitrogen isotopic analyses. δ¹⁸O within each cuttlebone show clear cyclicity, and when converted to temperature based on Grossman and Ku, it shows excellent agreement with sea surface temperatures measured by satellite for the Jersey area: suggesting oxygen isotopes are secreted in isotopic equilibrium with seawater. Carbonate δ¹³C values do not show any cyclicity but increase linearly to more positive values with age: a similar trend is also recorded in δ¹⁵N. Organic δ¹³C does not show such a clear trend. Since the linear trend in carbonate δ¹³C is very similar in each cuttlebone analyzed, we interpret this as a kinetic effect that relates to age. δ¹⁵N on the other hand would relate to an increase in trophic level with age. Notwithstanding the paucity of other Sepia data to compare with, we believe that cuttlebone stable-isotope ratios are recording environmental information such as, seasonality, diet and ecology. Preliminary analysis of belemnites using a very similar sampling strategy also indicates significant cyclicity in δ¹³C and δ¹⁸O: such variability may be a source of the scatter in the belemnite isotopic curve in the Jurassic and Cretaceous.