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Orbital control, climate seasonality, and landscape evolution in the Quaternary Rocky Mountains

Sewall, Jacob O., Riihimaki, Catherine A., Kadegis, Jeffrey
Geomorphology 2015 v.250 pp. 89-94
General Circulation Models, basins, climate, data collection, glaciers, landscapes, periodicity, rivers, runoff, summer, watersheds, Montana, Rocky Mountain region, Wyoming
While climate has long been implicated in the extensive erosion of Eocene through Miocene-aged basin fills in the Rocky Mountains, lack of precise, high temporal-density datasets of landform ages has made it difficult to detail the mechanisms by which climate increased relief. A dense dataset of (U–Th)/He dates from the Powder River Basin, Wyoming and Montana, USA, indicates correspondence between elevated exhumation and peaks in orbital eccentricity. Here we use an atmospheric general circulation model to investigate the potential role of eccentricity in enhancing erosion in the Rocky Mountains. We find that with high orbital eccentricity (0.05767), elevated seasonality (the moving vernal equinox of perihelion [MVELP]=270°) results in 10–100% more summer precipitation and surface runoff than low seasonality (MVELP=90°). Under low orbital eccentricity (0.0034), precipitation and runoff changes across a precession cycle are negligible. These results suggest that elevated eccentricity could, indeed, be associated with more intense summer precipitation and runoff, which could then drive higher landscape erosion rates. This finding could explain the occurrence of ~100-kyr cyclicity in Powder River Basin landform ages and provides a clear, non-glacial, link between climate variability and landscape evolution in the Rocky Mountains. In this, and other low-to-mid-latitude sedimentary basins, runoff volume and not glacier dynamics may be the variable that exerts primary control on landscape evolution.