EFFECTS OF THE PETM ON CONTINENTAL WEATHERING: ANALYSIS OF ALLUVIAL PALEOSOLS, BIGHORN BASIN, WYOMING

Alluvial paleosols are one way to assess changes in continental weathering that accompanied the PETM. An excellent continental record of that episode is found in the Bighorn Basin, Wyoming, where paleosols that formed during the PETM are well exposed in different parts of the basin.

In the northern part of the basin, where sediment accumulation rates were rapid, the carbon isotope excursion (CIE) spans a stratigraphic interval ~40 m thick that includes at least 20 vertically stacked paleosols that are moderately to strongly developed. They are separated by heterolithic intervals that show weak pedogenesis. The paleosols consist of siltstones and are characterized by red B horizons with yellow-brown mottles. The paleosols resemble stratigraphically lower and higher paleosols in this part of the basin except for: abundant carbonate nodules, which suggest that PETM paleosols were drier; and stratigraphic spacing, which is markedly denser and suggests more intense weathering. Paleosols through the CIE interval also show changes. Paleosols in the lowest 10 m of the interval (initiation of the PETM) are the thinnest and most weakly developed. Paleosols in the middle 20 m of the interval are thicker, more strongly developed, and more densely spaced, features that suggest more intense pedogenesis. Paleosols in the uppermost 10 m of the interval (end of CIE) show abundant yellow-brown colors, suggesting a return to wetter climatic conditions.

The effects of the PETM were different in paleosols exposed in southern areas of the basin, which had slower sediment accumulation rates. In one representative section that was studied, the paleosols consist of mudstones and claystones rather than siltstones. The lower part of the section has vertically stacked, thin paleosols with red B horizons but uncommon carbonate nodules. The upper section is dominated by what appears to be a single 6m thick red paleosol. Trenching shows it actually consists of 3 red beds separated by thin gray mudstone beds; however, this interval is unique in the basin and shows the effects of very strong weathering. Comparing the two paleosol sections shows that the PETM strongly influenced development of paleosols in both parts of the basin, but its effects were modified by sediment accumulation rates.