ISOTOPE PALEOTHERMOMETRY OF BELEMNITES FROM THE JURASSIC SUNDANCE SEA OF WESTERN NORTH AMERICA

The Sundance Sea covered much of western North America during the Middle to Late Jurassic Period. Deposits from this vast epeiric sea are now widely exposed across the region, including the Stump Formation in northeastern Utah, which consists of sandstones and shales reflecting shallow marine deposition. Well-preserved belemnites (Pachyteuthis densus, Oxfordian Stage, ~156 Ma) collected from this unit preserve stable isotope data (d¹⁸O and d¹³C ) that can be used to better understand the paleoceanography and paleoclimatology of the area, as well as possibly some paleobiologic characteristics such as migration patterns through the life cycle and age at sexual maturity and death. Accretion of internal shell (rostrum) of belemnites created incremental growth bands that record isotopic values through various life stages, thus potentially providing information about the life history traits of these organisms, in addition to seasonal temperature variations. Preliminary results suggest our material is consistent with previous reports from other locations in the region, with paleotemperatures in the 17 to 20° C range. In addition, there is some indication of seasonal variations. However, analyses of more samples and further evaluation of potential diagenetic alteration is necessary before more robust conclusions can be drawn.