ALLUVIAL RESPONSE TO CLIMATE CHANGE INTERPRETED FROM PALEOSOLS ACROSS THE PALEOCENE-EOCENE THERMAL MAXIMUM, BIGHORN BASIN, WYOMING

The stratigraphic interval that spans the Paleocene-Eocene Thermal Maximum (PETM) in the northern Bighorn Basin, WY provides a remarkable record of changes in the alluvial record that can be tied to a high resolution climate record. The study interval shows changes in the complexity and stratigraphic spacing of paleosols. Comparison to the climate record, reconstructed from paleosols and ichnofaunas, indicates that the changes are linked to PETM climatic fluctuations. In particular, the middle of the PETM interval contains thick, welded paleosols that correspond to times of well-drained floodplains and lower mean annual precipitation (MAP). Stratigraphic intervals below and above the main part of the PETM interval correspond to times of less well drained floodplains and higher MAP. These strata contain thinner paleosols separated by thick intervals showing minimal pedogenesis.

Differences in paleosol complexity and spacing suggest that sediment flux to the depositional site varied in response to precipitation fluctuations associated with the PETM. Welded paleosols indicate reduced floodplain accretion during deposition of the middle of the PETM interval. Intervals with widely spaced paleosols indicate more rapid rates of accretion. We hypothesize that drier episodes associated with PETM warming caused reduced vegetation cover 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 PETM, upstream water flux increased, stored sediment moved to the basin, and vertically spaced and thinner paleosols developed.

Whereas tectonic changes take time to propagate through a fluvial system, climate can impact an entire drainage basin at the time scale (10⁴ to 10⁵ yr) seen in the study section. Our results demonstrate how vertical sections of alluvial paleosols can provide information on how climate fluctuated through time as well as how the fluvial system responded to that climate change.