FRESHWATER VARIABILITY ALONG THE WESTERN SHORELINE OF THE MAASTRICHTIAN WESTERN INTERIOR SEAWAY

With growing concern over the effects of current global warming on the water cycle, there is increased interest in understanding precipitation patterns during past ‘greenhouse’ intervals to better constrain potential future climate scenarios. The Upper Cretaceous geological, paleontological, and paleoenvironmental records of the Western Interior Seaway (WIS) are an excellent natural archive that can be utilized to better understand such phenomena during ‘greenhouse’ intervals. The marine dynamics of the WIS have been well documented, however the chemistry of its freshwater input remains poorly constrained. To address this issue, this study sclerochonologically analyzed well-preserved freshwater unionid bivalves to examine trends in the stable oxygen isotope (δ¹⁸O) composition of freshwater systems during the Late Cretaceous ‘greenhouse’ interval. Specifically, the seasonal variability and long-term stability of stable oxygen isotopes (δ¹⁸O) in river water were examined by reconstructing freshwater conditions at a locality in south-central Wyoming that records four successive aggradational parasequences, which were deposited within marginal marine and terrestrial environments along the western shoreline of the WIS during the Maastrichtian. Preliminary δ¹⁸O_CaCO3 values show a seasonal fluctuation from -6.1‰ to -8.9‰, which suggests general stability in annual precipitation patterns, and likely records effects from seasonally reversing wind patterns resulting from the position of the Nevadaplano to the west. Additional unionid δ¹⁸O values will augment the existing data set to further identify precipitation endmembers and recurring patterns and/or variability in longer term δ¹⁸O precipitation records from a past ‘greenhouse’ interval. While Late Cretaceous pCO₂ levels were likely in excess of modern short-term predictions, we will be increasing the understanding of climate trends under a near-future ‘greenhouse’ climatic mode forced by high pCO₂ levels.