46858-AC8
Biotic Change in the Bighorn Basin, Wyoming, Before, During, and After the Paleocene-Eocene Thermal Maximum (PETM): Consistent with Orbital Forcing?
One of the primary goals of our study is to determine whether or not observed cycles in the sedimentary sequences and stable carbon isotopes of the early Eocene Willwood Formation are consistent with orbitally-forced climate change. Over 675 meters of stratigraphic section have been measured, logged, and sampled for dispersed organic carbon (DOC) to produce lithologic and stable isotope records that span 1.45 million years, an interval sufficient to detect precessional (19/21 kyr), obliquity (41 kyr), and eccentricity (100 kyr) scale periodicities. DOC isotope samples were taken every ~ 1 meter, or every ~ 2 kyr, based on an average depositional rate of 467 meters per million years. Fifteen percent of the collected DOC stable isotope samples have been analyzed thus far, producing 200 kyr of isotopic values. Although a 200 kyr interval is insufficient to rigorously detect eccentricity-scale periodicities, there is an observed trend occurring at a 100-125 kyr scale. The 200 kyr interval, however, is sufficient to detect precessional- and obliquity-scale periodicities via spectral analysis.
Preliminary spectral analyses do not detect either precessional- or obliquity-scale periodicities, but instead detect a 50 kyr periodicity in the DOC stable isotope record that corresponds to fluvial facies changes. Specifically, the 50 kyr cycle coincides with the deposition of a fluvial channel sandstone onto a relatively thick sequence of floodplain deposits, including well-developed paleosol horizons. The fact that cycles in the DOC stable isotope record correspond to fluvial sedimentary sequences suggests an autocyclic, lithological control on DOC stable isotope values. We are now working on understanding the lithologic controls on the DOC stable isotope values in hopes of removing the lithologic effects on stable isotope values. Removing lithologic effects will help us better detect changes in DOC stable isotopes that may be the result of orbitally-forced climate change.
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