ORBITAL ECCENTRICITY, CLINKER FORMATION, AND THE CLIMATE-LANDSCAPE EVOLUTION LINK IN THE NORTH AMERICAN ROCKIES AND HIGH PLAINS

While surface processes are sensitive to changes in the moisture balance of drainage basins, proxies for paleo-hydrology are rare in the geologic record. Consequently, the observed correlation between orbital eccentricity and high rates of landscape evolution in the Powder River Basin of Wyoming and Montana, U.S.A. is difficult to explain with only empirical data. High orbital eccentricity, particularly if enhanced by precession, could lead to a highly seasonal climate with strong mid-continental summer warming and increased precipitation that would drive an increase in local incision rates and the exhumation of coal. We test this hypothesis with the National Center for Atmospheric Research, Community Atmosphere Model v.3 at a spectral resolution of T85 (~1.4° lat. x 1.4° lon.). Orbital eccentricity in our simulations was 0.05767 or 0.0034 with each eccentricity value paired with a moving vernal equinox latitude of perihelion (MVELP) of 270° and 90° for four total atmosphere-only simulations. Planetary obliquity was constant at a maximum of 24.5°; all other boundary conditions were held static at modern values. Simulations were integrated for 30 years with the final 10 years averaged for analyses. Comparisons across the simulations suggest that, under eccentricity maxima, increasing seasonality (MVELP=270°) substantially increases summer (June, July, and August average) precipitation totals in much of Wyoming, western Nebraska, and northeastern Colorado (32-48 cm; >14 cm more than with MVELP=90°). Under minimum eccentricity, summer precipitation is ~33% lower than under maximum eccentricity and precession’s impact is negligible (~4 cm change over MVELP=270° to 90°). By linking high values of planetary eccentricity and precession to enhanced mid-continental precipitation, these results provide a possible mechanism to explain the observed association between increased erosion, clinker formation, and orbital eccentricity in sediments of the Powder River Basin and, thus, a direct link between orbital parameters and landscape evolution in this region. We predict similar relationships between surface processes and insolation across much of the central Rocky Mountains and High Plains.