COMPARISON OF BELEMNITE AND FORAMINIFER STABLE ISOTOPE COMPOSITIONS FROM SEYMOUR ISLAND, ANTARCTICA

Although compilation of foraminiferal oxygen isotope records has greatly shaped our understanding of the evolution of sea surface temperatures (SST) in the geologic past, Jurassic and Cretaceous SST reconstructions often rely upon belemnite d¹⁸O when foraminifer d¹⁸O data are sparse. Unfortunately, because belemnites are an extinct group of organisms, their fractionation of oxygen isotopes cannot be constrained through modern calibration and little is known of their paleobiology or paleoecology. Disparity between cool paleotemperatures derived from nektonic belemnites and warm paleotemperatures derived from planktic foraminifers at high-latitudes in the Cretaceous has sparked debate over the appropriateness of using belemnite d¹⁸O as a proxy for SST.

Excellent preservation of shelfal marine invertebrate fossils on Seymour Island, Antarctica allows the opportunity to make a direct comparison between d¹⁸O of belemnites and both benthic and planktic foraminifers. No evidence of diagenetic alteration was detected based upon cathodoluminescence and trace element composition of belemnite rostra. The belemnites were cut and polished and sampled at high-resolution along growth bands to assess geochemical variability within each specimen. Belemnite d¹⁸O ranges from 0.3 to 1.2‰, while d¹⁸O of benthic foraminifers spans 0.4 to 1.1‰ and planktic forams record d¹⁸O values between -1.0 and -0.3‰. The strong similarity between d¹⁸O values of belemnites and benthic foraminifers suggests that belemnite d¹⁸O is an excellent proxy for benthic water temperature in this high-latitude neritic paleoenvironment. Inferences of belemnite paleoecology based on this result include two possibilities: a) belemnites calcify their shells in equilibrium with benthic water temperatures, or b) belemnites calcify their shells in surface waters, but have a vital effect offset in d¹⁸O. To test these hypotheses, further comparisons between belemnite and foraminifer d¹⁸O are needed at different latitudes.