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