δ18O SCHLEROCHRONOLOGY OF LATE CRETACEOUS BIVALVES AND PRESERVATION OF SEASONALITY IRRESPECTIVE OF ALTERATION

The Late Cretaceous Western Interior Seaway (WIS) was home to a flourishing community of marine invertebrates. Among other geochemical proxies, the oxygen isotope composition of marine bivalves has been used for paleoclimate reconstructions. We investigate if the oxygen isotope composition of Campanian (~ 83 Ma to ~72 Ma) bivalves could be used as a paleoclimate indicator and to what degree diagenesis altered original oxygen isotope compositions. We measured δ¹⁸O of a variety of well-preserved calcitic and aragonitic marine bivalves (Inoceramus, Mactra, Cucullaea, and Lucina) that spanned a wide geographic range from Montana to New Mexico. In addition, select specimens were chosen for serial analysis to determine if annual cycles commonly observed in modern shells are also preserved in Campanian bivalves. Although bulk δ¹⁸O values of marine bivalves show a wide range of variation between +0.4‰ and -13.5‰, most values cluster in a much narrower interval, between -1.0‰ and -3.0‰ (vs. PDB). Heavier values likely represent unaltered oxygen isotope compositions as calculated paleo temperatures are in agreement with other proxies of Late Cretaceous climate. However, temperatures calculated from these δ¹⁸O values, do not support a significant temperature gradient over a 20º latitudinal range. In addition to marine specimens, Unionoida bivalves, collected from the non-marine Fruitland Formation of New Mexico, ranged from -5.0‰ to -6.4‰, and are 3.0‰ to 5.0‰ lower than average bulk δ¹⁸O marine values. High-resolution serial analyses of well-preserved inoceramid bivalves reveal cyclicity in δ¹⁸O values. These cycles may reflect seasonal variations as they match interpreted annual shell growth bands. Remarkably, even specimens with very light δ¹⁸O values that strongly suggest digenetic alteration, preserve seasonal cycles. Our data suggest that conservative diagenesis may preserve seasonal cycles and shift original δ¹⁸O values lower instead of completely resetting the oxygen isotope regime.