Evaluating Carbon Cycle Dynamics and Hydrologic Change during the Paleocene-Eocene Thermal Maximum, Bighorn Basin, Wyoming.

摘要

The Paleocene-Eocene Thermal Maximum (PETM), an episode of abrupt (≤20 kyr), transient (∼200 kyr), and large-scale (5 to 8°C) global warming ∼56 Ma, is the best-known natural analogue for anthropogenic climate change. This dissertation uses bulk soil organic carbon and compound-specific stable carbon and hydrogen isotopic records to evaluate terrestrial carbon cycle dynamics and hydrologic change during the PETM in the Bighorn Basin, Wyoming. The PETM carbon cycle perturbation is recorded as a prominent negative carbon isotope excursion (CIE) in carbon archive materials, and the characteristic 3-5 / CIE is the major criterion for global correlation of the Paleocene-Eocene boundary. However, spatial variation in the shape and position of the CIE has not been critically evaluated. This dissertation explores the utility of the CIE as a chemostratigraphic marker in the geologic record by measuring and correlating bulk soil organic carbon isotope ratios from six PETM sections across ∼16 km in the southeastern Bighorn Basin. Although the onset of the CIE occurs in the same lithostratigraphic position across the field area, the relative stratigraphic thickness of the CIE in bulk soil organic matter varies significantly. The bulk soil organic matter CIE is truncated relative to the n-alkane CIE and underrepresents the thickness of the body of the PETM by 30-80%. We hypothesize that discrepancies between the bulk soil organic matter and n-alkane carbon isotope records are due to allochthonous fossil carbon mixing with autochthonous PETM carbon. Using carbon isotope ratios of leaf-wax n-alkanes to infer the expected CIE in bulk soil organic matter, we calculate that ∼40-80% allochthonous carbon would be required to produce the observed truncation. n-Alkane deltaD ratios were also measured and used to produce a record of relative humidity. Relative humidity was low early in the PETM and then increased and remained high for the remainder of the PETM body. We suggest that the n-alkane deltaD record likely reflects an increase in the seasonality of precipitation during the PETM in the southeastern Bighorn Basin. Lastly, we generate PETM n-alkane records from a terrestrial sedimentary core and show that n-alkane proxy records from weathered surface outcrop are similar to those from less oxidized core material and reliably preserve paleoclimate information.