T190 Cenozoic Terrestrial Environmental Change in North America: From Paleosols to Paleobotany (Posters)

Tuesday, 2 November 2010
8:00 AM - 6:00 PM
Hall D

SEDIMENTARY RESPONSE TO CLIMATE CHANGE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM, SOUTHEASTERN BIGHORN BASIN, WY

Paper number: T190-3
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

The Paleocene-Eocene Thermal Maximum (PETM) is well exposed in alluvial deposits of the Bighorn Basin of Wyoming. In the southeastern basin, the ~50 m thick PETM interval can be divided into three parts. The lowest interval is dominated by red and red-purple paleosols that are < 1 m thick and separated by thick avulsion deposits. The middle interval is dominated by avulsion deposits and weakly developed paleosols. The uppermost interval includes as many as ten red and purple paleosols that are closely spaced; many of the paleosols are > 1 m thick and strongly developed. Multiple paleosol proxies indicate that the floodplain became better drained at onset of the PETM. The driest or most seasonally dry conditions characterize the middle interval. Carbon isotope ratios show that the middle interval corresponds to the plateau phase of the carbon isotope excursion when temperatures were warmest. Paleosols in the upper interval, where isotopes become more positive again, show a return to wetter conditions.

Changes in the degree of soil development through the section indicate that sediment supply was higher during deposition of the middle interval. Grain size analyses also show that floodplain deposits in the middle interval are coarser grained (< 40% clay) than floodplain deposits in the lower and upper intervals (clay percents commonly 60% or more). Thus, more rapid rates of sediment accumulation and coarser grain sizes correspond to a time of warmer temperatures and drier or more seasonally dry conditions. We hypothesize that the climatic conditions that characterized the middle interval caused reduced vegetation cover in the sediment source area and depositional basin. Reduction of vegetation, in turn, led to an increase in both the size and the quantity of sediment that was delivered to and transported by the fluvial system. In addition, greater seasonality of precipitation may have enhanced sediment yield and particle size. Other factors that can influence sediment supply and grain size – tectonic activity and source area lithology – are highly unlikely given the short time span (~104 years) over which the sediment changed character. This study demonstrates how climate change can impact fluvial systems over relatively short time scales.