GSA Connects 2021 in Portland, Oregon

Paper No. 45-3
Presentation Time: 2:05 PM


WOSTBROCK, Jordan1, WITTS, James2, GAO, Yang3, MYERS, Corinne4, HENKES, Gregory3 and SHARP, Zachary D.4, (1)Department of Earth and Planetary Sciences, Yale University, New Haven, CT 06511, (2)School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Clifton, Bristol, BS8 1RJ, United Kingdom, (3)Geosciences, Stony Brook University, Earth and Space Sciences Building, Stony Brook, NY 11794-2100, (4)Earth and Planetary Sciences, University of New Mexico, Northrop Hall, 221 Yale Blvd NE, Albuquerque, NM 87131

Alteration of a rock or fossil after deposition is a constant concern when using geologic samples as proxies for paleoenvironmental conditions. Traditional δ18O analyses of carbonates are under-constrained because the temperature, δ18O value of the water, and the degree of alteration are all variables. Clumped isotope (Δ47) analyses are independent of the water oxygen isotope composition, but the degree of alteration and its effect on the ‘clumping’ of carbon and oxygen isotope atoms in carbonate samples are unconstrained. However, during diagenesis, triple oxygen isotope values change in a predictable manner, allowing the estimation of the ‘pristine’ triple oxygen isotope composition of the carbonate. Thus, by including the δ17O value of a carbonate, we can better constrain original paleoenvironmental conditions and determine the likelihood of alteration affecting the primary oxygen isotope composition.

In this study, we analyzed concretions and macrofossil samples (ammonites and bivalves) from the North American Late Cretaceous (Campanian-Maastrichtian) Western Interior Seaway. Samples representing a range of paleoenvironments were taken from the B. compressus, H. birkelundae, H. nicolletii, and H. nebrascensis biozones. Carbonate shell material and void filling sparry calcite, when available, were analyzed. Both the Δ47 and triple oxygen isotope values of the void filling calcite indicate alteration from a low δ18O fluid. The effect of alteration on the triple oxygen isotope values of the carbonate shells varied. Five shells retained primary triple oxygen isotope compositions, forming at a temperature between 10 and 30 °C in an ocean with δ18Oseawater values between -3 to 0‰. Temperature estimates from Δ47 measurements were slightly higher (25-40°C). These differences could be due to minor clumped alteration of the shells at a very low fluid-rock ratio. Clumped isotope values may be a better recorder of alteration temperature in this scenario. We used this alteration temperature to ‘see through’ the altered samples and calculate the initial temperature of formation (5-15°C). We conclude that the Late Cretaceous Western Interior Seaway ranged in temperature from 5-30°C with δ18Oseawater values ranging between -3 and + 0‰.