COMBINED TRIPLE OXYGEN AND CLUMPED ISOTOPE MEASUREMENTS TO RECONSTRUCT PALEOENVIRONMENTS OF THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

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 δ¹⁸O_seawater 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 δ¹⁸O_seawater values ranging between -3 and + 0‰.