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Alluvial Response to Climate Change at the Paleocene-Eocene Thermal Maximum (PETM), Bighorn Basin, Wyoming


The PETM interval in the Bighorn Basin provides a remarkable record of changes in the fluvial sedimentary record that can be tied to a high resolution climate record. The trends suggest that climate changes significantly influenced stratigraphic architecture in the northern Bighorn Basin. The middle 20m of the PETM is characterized by unusual deposits relative to the stratigraphic sections above and below: thick, welded paleosols in some locations and an abnormally thick channel-belt sandstone lateral to the welded paleosols. Comparison to the climate record, reconstructed qualitatively and quantitatively from paleosols, indicates that changes in alluvial architecture during the PETM are associated with overall drier conditions. In particular, the thick welded paleosols correspond to times when floodplains were well drained and mean annual precipitation (MAP) was reduced. Stratigraphic intervals below and above the main part of the PETM interval are dominated by thin paleosols, widely separated by thick avulsion deposits. Those intervals correspond to times of wetter floodplains and higher MAP. Paleosol complexity and spacing suggest that sediment flux to the basin varied due to precipitation fluctuations associated with PETM warming. Reduced floodplain accretion during the middle PETM indicates that sediment supply from the surrounding Laramide mountain ranges diminished in response to drier climates. Sediment supply depends on the balance between sediment availability and transport efficiency. We hypothesize that drier episodes caused reduced vegetation cover in source areas. Diminished plant cover promotes erosion and sediment yield by decreasing water infiltration and exposing more soil to overland flows. But, because precipitation was reduced, much of that sediment was stored in upstream reaches of the fluvial system rather than moving to the depositional basin. Welded paleosols formed because of diminished sediment supply, which caused lower accretion rates. With a return to wetter conditions after the middle PETM, upstream water flux increased, stored sediment was flushed to downstream areas, where rivers developed more prominent alluvial ridges, and avulsion/splay deposition on the floodplain was more common. Those intervals correspond to higher rates of accretion resulting in more widely spaced paleosols and thicker avulsion deposits. Our results show how paleosols provide information on the fluvial response to climate change.