T45 High-Resolution Geochemical Bioarchives: Recognition of Signals and Implications for Evolution, Paleoecology, and Paleoclimatology

Monday, 5 November 2001
1:30 PM - 5:30 PM

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

Paper number: T45-8
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

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.