T119 Halo-Dash: The Deep and Shallow History of Aquatic Life’s Passages between Marine and Freshwater Habitats

Monday, 10 October 2022
1:30 PM - 5:30 PM
Mile High Ballroom 4A

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

Paper number: T119-15
Program: Technical Sessions
Time: 5:00 PM – 5:15 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-381265

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