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43732-AC2
Testing Paleoenvironmental Models of the Cretaceous Western Interior Seaway via Stable Isotopes of Fossil Turtles and Fish

Matthew J. Kohn, University of South Carolina

Activities

This worked supported the activities of PhD candidate, Alan Coulson. We have initiated 2 key activities towards completing the proposed research. First, we have completed compilation of a large dataset on modern turtle bone oxygen isotope compositions and their relationship to local water compositions. This work required sampling different bones along flippers as well as shell material from different species of marine turtles (which are all endangered). We processed and analyzed this bone for oxygen isotope composition of the PO4 component. These data were compared with extant data from freshwater turtles and a few marine turtles. This work is now in review at Palaeogeography, Palaeoclimatology, and Palaeoecology. Second, we have received, processed, and begun analyzing a large suite of fossil turtle and fish fossils from the Cretaceous Western Interior Seaway. Now that we have firmly established the straightforward relationship between water and turtle isotope compositions, we have begun working towards our principal goal: establishing paleoceanographic circulation patterns within the seaway, specifically testing models of evaporative enrichment in the eastern part of the seaway and freshwater input in the western part. In this regard, we have received about 300 samples, mainly from Kansas, Mississippi, and Alabama; all samples have been processed and are ready to analyze, with one subsample for PO4 analysis (water composition) and a second split for CO3 analysis (temperature estimates). The estimation of temperature follows on research by the PI on use of diagenetic compositions (Kohn and Law, 2004, Geochimica et Cosmochimica Acta), funded by NSF.

Findings

Our work establishes a simple 1:1 relationship between the oxygen isotope composition of local water and turtle bone PO4, most importantly for marine taxa that have otherwise remained relatively unsamples. We have also shown that it does not matter which bone is used for inferring water compositions. All have indistinguishable compositions, regardless of proximity to the turtle's thermal core. Shell material, although more scattered compositionally, has a similar mean composition. These data suggest, firstly, that turtles more effectively thermoregulate in their flippers than other reptiles. Secondly, analysis of turtle bone PO4 can be made for paleoceanographic interpretations, as we propose doing for the Cretaceous Western Interior Seaway. Both results are now in review at Palaeo3, and several abstracts have been presented.

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