SEASONALLY RESOLVED CARBON AND OXYGEN ISOTOPE RECORDS FROM LATE CRETACEOUS TO EARLY PALEOGENE FRESHWATER UNIONID BIVALVES

The Judith River, Hell Creek, and Fort Union formations of central and western Montana preserve a record of terrestrial environmental conditions during the Late Cretaceous and earliest Paleogene. Fossils of vertebrates, plants, and invertebrates are common throughout these units, and are often very well-preserved, both physically and geochemically. Freshwater bivalves, particularly unionids, build their shells by accretion, potentially providing an archive of environmental conditions throughout the length of their growing season. Their calcium carbonate (aragonite) shells allow sampling for both δ¹³C and δ¹⁸O, which can provide information about the hydrology and temperature of their habitat. Previous studies have recorded an increase of ~9‰ in δ¹⁸O from North American bivalves from the Campanian into the Eocene, but δ¹⁸O values measured from the Hell Creek and Fort Union formations typically show two clearly distinct populations, with values very near -16‰ or -8‰ (VPDB).

Here we present results for high-spatial resolution sequential sampling of unionid bivalves from these formations. Most specimens show clear seasonal variation in δ¹⁸O and δ¹³C that corresponds with visible growth lines on the shell. The magnitude of seasonal variation in δ¹⁸O is largely consistent across specimens and would equate to ~20°C seasonal temperature variation if all variation was explained by temperature, with coldest temperatures recorded at the growth lines, indicating cessation in growth during winter. The absolute δ¹⁸O values are consistent with the previously observed δ¹⁸O populations, and changes in growth period or seasonality of source water conditions are insufficient to explain this δ¹⁸O distribution. Both absolute δ¹³C values and the magnitude of their seasonal range were more variable across specimens, but they are consistent with increased oxidation of organic matter in summer months. Within specimens, δ¹³C and δ¹⁸O are not correlated, but the mean specimen δ¹³C and δ¹⁸O values are correlated, with lower δ¹⁸O values correlated with higher δ¹³C values. We have not yet observed a correlation between isotopic values and bivalve taxon or depositional environment.