THE STABLE-ISOTOPE COMPOSITION OF SEPIA: A POTENTIAL ANALOGUE TO THE MESOZOIC BELEMNITE?

It has long been a matter of speculation in the literature that the modern day cuttlefish (Sepia) is the closest living analogue to the now extinct free-swimming cephalopods, belemnites. The rostrum, or guard (bullet shaped calcitic fossil) is commonly preserved in the Jurassic and Cretaceous fossil record. As belemnites secrete a rostrum of low magnesium calcite, it is resistant to diagenesis, and the isotopic nature (primarily δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr) of this calcite has been used extensively in reconstructing palaeoceanographic and palaeoclimatic conditions of the Jurassic and the Cretaceous. However, several assumptions have been made about belemnites in order to interpret their geochemical signatures: they are internally homogeneous, and the calcite is secreted in equilibrium with seawater. Isotopic analysis of modern Sepia from South Australia suggests that they are relatively homogeneous in oxygen but not in carbon. Temperatures calculated from Sepia δ¹⁸O values using the Grossman and Ku (1986) equation suggests a range of ~3.5°C (1‰) with an average of 20.7°C. Based on modern SSTs around South Australia this would indicate that Sepia precipitated dominantly during the spring/summer season. Carbon isotopes showed a range of ~3‰, with more positive values towards the end of its life. This is in contrast with belemnite data, which show greatest variation in oxygen rather than carbon isotopes. Therefore, stable-isotope ratios in belemnites require a re-evaluation in light of its modern living relative, Sepia, which suggests different life mechanisms, modes, fractionation factors and/or diagenesis to explain the disparity in isotopic trends. Further research on high-resolution laser-ablation trace-element ratios in Sepia and belemnites will also be discussed, since they are becoming more widely used in ancient geochemical investigations to reconstruct palaeoceanographic and palaeoenvironmental conditions.