Insight into climate and hydrology of the late cretaceous western interior basin from clumped isotope Paleothermometry and strontium isotopes

• Δ 47 measurements indicate mean 30 ± 2.7 °C surface water temperature in the Campanian of North America. • δ 18 O water values are bimodal with higher absolute values than previous reports. • δ 18 O water and 87 Sr/ 86 Sr patterns are consistent with monsoonal precipitation. • δ 18 O water supports previous estimates of ≥3500 m paleoelevation. High sea levels in the Late Cretaceous led to the formation of vast seaways on every continent. These shallow seaways are without modern analogs and many fundamental aspects of their oceanography are poorly understood. In the Campanian (∼83–72 Ma), the Western Interior Seaway (WIS) of North America linked the proto-Gulf of Mexico and Arctic Ocean. Given its shallow depth, freshwater inputs to the WIS could have had a greater influence on conditions in the seaway compared to a deeper ocean and would have become increasingly important as the WIS regressed through the Maastrichtian. The isotopic composition of mollusk shells in freshwater facies can help constrain the composition and temperature of these inputs, improving our understanding of surface temperature, hydrological dynamics, and paleoelevation. Here we measure Δ 47 temperature, δ 18 O water , and 87 Sr/ 86 Sr in late Campanian (∼75 Ma) unionid bivalve shells from fluvial and pond deposits near the western shore of the WIS (∼42°N-56°N). Sample mean surface water temperatures spanned 22–44 °C, with a mean of 30 ± 2.7 °C. The latitudinal temperature gradient across this region is reduced compared to today, at ∼7 °C across these 14° of latitude based on stream sample means. These temperatures are outside the optimal growth conditions of modern unionids in North America, indicating a shift in niche. Following this finding, we recalculate δ 18 O water values from previously published δ 18 O carb values using new Δ 47 temperatures instead of assumed growth temperatures. Our findings support previous observations of a bimodal distribution in freshwater δ 18 O water values in this region although the absolute values shift higher. Spatial patterns of δ 18 O water are consistent with a Campanian Proto-North American Monsoon and the lowest δ 18 O water values we report are consistent with paleoelevation of >3500 m in the Proto-Cordillera. 87 Sr/ 86 Sr values broadly align with different facies, with more radiogenic values occurring in major trunk rivers draining the highlands and less radiogenic values in streams recharged by low-elevation precipitation. Predominance of a 87 Sr/ 86 Sr signature consistent with weathering Paleozoic carbonates could be consistent with seasonal increases in rock weathering associated with a monsoon.