Oxygen isotope analysis of authigenic minerals from the Rocky Mountain region as an indicator of Cenozoic paleoclimate and paleoelevation

The oxygen isotope composition of goethite from ferricrete deposits is a useful new paleoclimate proxy. In order to investigate Holocene climate change in the Rocky Mountains, we collected goethite from ferricrete deposits in Montana and Colorado. Previous paleoclimate studies have suggested these two areas have had different Holocene climate histories. In the Rockies of Montana, the relative amount of summer precipitation has increased since the early Holocene. This increase in the amount of summer precipitation is recorded in the oxygen isotope compositions of goethite from the region. δ 18O of goethite samples from the northern Rockies increase linearly approximately 3.5‰ from the early Holocene to the present, which is a result of increased, isotopically heavy summer precipitation. In the southern Rockies, the Holocene climate history is highly dependant on geographic position and local topography. δ18O of Holocene goethite from the southern Rocky Mountains increases ∼2.3‰ abruptly at ∼6000bp, then decreases ∼2.3‰ between ∼2000bp and the present. These results agree with some, but not all Holocene paleoclimate proxies from the southern Rockies. This suggests spatial variability of Holocene climate change in the southern Rocky Mountains.; Traditionally, the Rocky Mountains were thought to have been uplifted in the Late Neogene. However, many recent studies have suggested high elevations in the Rockies have existed since the early Tertiary. In order to investigate if an elevation gradient from existed in the region before the Late Miocene, the oxygen isotope composition of 57 Tertiary authigenic smectite samples was determined. These smectite samples were collected from volcanic airfall deposits along a traverse extending from western Nebraska to western Wyoming. The δ18O value of these smectites decreases by ∼10‰ from east to west. This trend very minics the δ 18O values of theoretical smectites in isotopic equilibrium with modern precipitation. This suggests modern atmospheric circulation patterns have not been significantly modified since the early Tertiary. Because the Rocky Mountains are an integral part of the modern climate system of North America, the lack of a major modification of atmospheric circulation since the early Tertiary suggests the Rocky Mountains have been a topographic barrier since at least that time.