Oxygen isotope evidence for high-altitude snow in the Laramide Rocky Mountains of North America during the Late Cretaceous and Paleogene

The δ 18 O values of river waters in Late Cretaceous and Paleogene basins of Alberta, Montana, Wyoming, and Colorado are calculated from the isotopic composition of unaltered aragonite mollusk fossils. The isotopic composition of these river waters has significant implications for the uplift and climate history of the mountains surrounding the basins. Our data show that the δ 18 O values of river water varied dramatically, ranging from −23‰ to −5‰ relative to Vienna standard mean ocean water, with an uncertainty of ±2‰. Close stratigraphic association of river deposits with very different isotopic values indicates two different water sources: rainfall in the warm, low-elevation basins and snowfall in surrounding mountains. The δ 18 O difference between basinal and highland precipitation suggests that local relief was similar to today9s, on the order of 2.5–3 km. Seasonal variation in shell δ 18 O values also indicates that there are times when large snow fields in the Maastrichtian and Paleocene mountains of Montana and Wyoming survived summer melting and were multiannual features.