Alluvial response to the Paleocene–Eocene Thermal Maximum

Alluvial response to the Paleocene–Eocene Thermal Maximum climatic event, Polecat Bench, Wyoming (U.S.A.)

Authors:

Kraus et al

Abstract:

The stratigraphic interval spanning the Paleocene–Eocene Thermal Maximum in the northern Bighorn Basin, Wyoming shows changes in the alluvial record that can be tied to a high resolution climate record. The complexity and stratigraphic spacing of paleosols change through the study section. Comparison to the climate record, reconstructed from paleosols, indicates the changes correspond to Paleocene–Eocene Thermal Maximum climatic fluctuations. In particular, the middle of the section contains thick, welded paleosols and thin avulsion deposits that link to times of well-drained floodplains and lower mean annual precipitation. Stratigraphic intervals below and above the main part of the Paleocene–Eocene Thermal Maximum interval correspond to times of less well drained floodplains and higher mean annual precipitation. These strata contain thinner paleosols and thick avulsion deposits.

Differences in paleosol complexity and spacing suggest that sediment flux to the depositional site varied in response to precipitation fluctuations associated with the Paleocene–Eocene Thermal Maximum. Welded paleosols and thin avulsion deposits indicate reduced floodplain accretion during deposition of the middle of the Paleocene–Eocene Thermal Maximum. Intervals with widely spaced paleosols indicate more rapid accretion. We hypothesize that drier episodes associated with warming caused reduced vegetation in source areas and promoted erosion and increased sediment yield. 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 to the basin. With a return to wetter conditions during the recovery phase of the Paleocene–Eocene Thermal Maximum, upstream water flux increased, stored sediment moved to the basin, and vertically spaced, thinner paleosols developed. The results demonstrate how vertical sections of alluvial paleosols can provide information on how climate fluctuated through time and how the fluvial system responded to climate change.