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Hydrologic Variability and Fluvial Responses to Increased Warming During the Paleocene-Eocene Thermal Maximum, Piceance Creek Basin, Colorado, USA


Climate is a key boundary condition that influences sediment transport systems. Changes in the hydrologic cycle in response to global warming can affect the volume, grain size, and frequency of sediment transport to sedimentary basins. It is suggested that anthropogenic climate change is already significantly altering the hydrologic cycle and potentially sediment transport systems, but it is difficult to separate a uniquely climatic control from direct human modification of waterways. Past climatic events offer ideal case studies to study how climate alters environmental conditions and sediment transport in isolation. A time period that shows parallel changes to anthropogenic warming is the Paleocene Eocene Thermal Maximum (PETM); a warming event that occurred 56 million years ago. Existing geochemical and fossil records suggest mean annual temperature increased by 5-8°C globally, coastal and polar areas experienced increased runoff and precipitation, and an isolated continental interior site suggests mean annual precipitation dropped by 30-40%. This study develops a detailed reconstruction of paleo-precipitation based on the soil morphology index and whole-rock geochemistry of alluvial deposits in the Piceance Creek Basin of northwest Colorado, U.S.A. The methodologies applied in this study have also been applied to the Bighorn Basin in northwest Wyoming, which serves as an important data set for comparison. Our results suggest that the Piceance Basin also became drier during the PETM, with decreases of up to 40% in mean annual rainfall and floodplains were better drained on average. When integrated with fluvial data sets, which suggest increased hydrograph variability, we infer greater interannual and, potentially seasonal rainfall in the region. These proxy-based conclusions compare favorably with general circulation model outputs of the PETM that predict drier, more seasonally variable rainfall within continental interiors. Importantly, this pattern appears to be a robust response of the climate system when subjected to an exogenic pulse of carbon regardless of the background climate state, whether the greenhouse state of the early Paleogene and perturbed by methane clathrates or an icehouse state of the recent and perturbed by fossil fuel release.