43732-AC2
Testing Paleoenvironmental Models of the Cretaceous Western Interior Seaway via Stable Isotopes of Fossil Turtles and Fish
Activities. This work supported the activities of PhD candidate, Alan Coulson. The last year has witnessed major advances in our research. We have published a paper on modern turtle bone isotopic systematics (Coulson et al., 2008; Palaeogeography, Palaeoclimatology, Palaeoecology, v. 264, pp. 78-84), showing a simple, 1:1 relationship between the isotope composition of the PO4 component of turtles and seawater compositions. And we have now collected all requisite isotopic data to interpret the paleoceanography of the Cretaceous Western Interior Seaway (KWIS). Mr. Coulson is well on his way to completion of his degree, and we anticipate final submission of his dissertation in February or March 2009. Preliminary results indicate our approach has been astonishingly successful, and will allow us to place firm constraints on water temperatures, amounts of freshwater input, and likely circulation patterns. Our results, discussed below, will have major impact on models of KWIS circulation. We are currently in the process of writing up 2 papers, a short article on atmospheric and oceanic circulation patterns for Science/Nature, and a longer one exploring paleobiological implications. Findings. Note that results for modern turtles, published in Coulson et al. (2008) were described in detail in last year's report. Findings described here focus on our new results from fossils. Our most important finding is a strong temperature gradient from south to north along the KWIS, accompanied by a strong isotopic gradient. Atmospheric and oceanic circulation models for the late Cretaceous infer rapid heat advection through the seaway, implying that there should have been minimal temperature and isotopic gradients between the sites we studied. Simply put, strong south to north transport advects warm, isotopically enriched marine water farther north. Some have even argued that enhanced hurricane activity promoted such transport as far north as Montana, ultimately leading to fossil-rich dinosaur bone beds through flooding. Our PO4 data unequivocally refute these concepts by showing both a strong difference among north-south sites in turtle bone composition (indicating major freshwater input in the north), and in the offset between fish and turtle compositions (indicating substantially cooler temperatures in the north). The strong temperature and salinity gradients we measured instead indicate that any hurricanes would have lost power long before reaching Kansas. This in turn indicates minimal flow from south to north. The strong gradients further explain the strong north-south marine faunal gradients that have long been observed. One reason for the disparity in the interpretations from previous studies vs. ours lies in the interpretation of paleotemperatures. Past studies of carbonate fossils have indicated high temperatures, essentially throughout the KWIS. Our analysis of the CO3 component of fossil bone helps explain the disparity. The offset between PO4 and CO3 isotopic compositions can be used to infer diagenetic temperatures (e.g., Kohn and Law, 2004; Zanazzi et al., 2007). Our analysis of fossil bone CO3 unequivocally indicates major diagenetic overprinting to the CO3 component, at temperatures ranging up to 50 °C. That is, carbonate appears to have been diagenetically altered during burial, so isotopic compositions are unrepresentative of original ocean temperatures. Such temperatures, while unrealistically high for the oceans, are nonetheless too low to alter the isotope composition of the PO4 component on which our main paleoceanographic interpretations rely.
