OXYGEN AND CARBON STABLE ISOTOPES FROM BIOGENIC CALCITES OF LATE EOCENE FRESHWATER BIVALVES FROM THE FLORISSANT FOSSIL BEDS NATIONAL MONUMENT, CO

The Eocene-Oligocene boundary is marked by a rapid shift in global temperatures with an average global temperature drop of ~7^oC, recorded from extensive deep sea data sets. In comparison to the well-studied marine record, little work has been done on the terrestrial climate signals across this boundary with very few published studies on the freshwater biota. Current evidence from terrestrial based studies in North America points to a lag in the temperature drop of ~400ky, and a larger temperature shift during this interval. Current work on paleo-terrestrial environments show that the use of stable isotopes of carbon and oxygen are a viable means of analyzing and understanding past environments. We applied these methodologies to biogenic calcites from aquatic biota from the lagerstatten locality at Florissant Fossil Beds National Monument, Colorado. Over the 2010-2012 field seasons, collections were made of freshwater bivalves, gastropods and ostracods from mudstones, paper shales and conglomeratic units of the Florissant Formation, which has been dated at 34.07 ± 0.10 Ma. X-ray diffraction analyses of multiple specimens of the aquatic bivalve, Sphaerium sp., have shown that the bivalve shells consist of original unaltered aragonite. δ¹³C and δ¹⁸O data from the aragonite shells show a positive correlation with a consistent range and repeatability. The δ¹⁸O results suggest a high evaporation to precipitation effect on the lake system, while the negative values in the δ¹³C suggest that the lake had high biological activity and atmospheric exchange that enriched the system in ¹³C. Together, the positive correlation in the stable isotope data, and the results taken individually, suggest that the lake system was shallow and possibly stagnant. These results indicate that unaltered biogenic calcites from freshwater localities can indeed be used as proxies for paleoecology studies in terrestrial environments, and the study is being expanded to other paleolacustrine localities that span the Eocene-Oligocene boundary in the Rocky Mountains.