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Master of Science (MS)
The Paleocene Eocene Thermal Maximum (PETM) was a rapid global warming event that occurred approximately 56 million years ago and represents the largest and most abrupt warming event of the Cenozoic Era. The PETM caused mean annual temperatures to increase at least 5°C globally above the already warm, greenhouse climate state of the early Paleogene. The warming and associated perturbation of the carbon cycle had numerous consequences for paleoenvironments and paleobiologic systems. This study investigates the hydrologic response to the PETM within the interior of North America and presents a new d13C bulk organic record. This study generates reconstructions of floodplain drainage and paleo-precipitation in the Piceance Creek Basin of northwest Colorado (U.S.A.). A semi-quantitative soil morphology index was used to characterize floodplain drainage and whole-rock geochemistry in order to estimate mean annual precipitation from a 124-meter stratigraphic section in the western portion of the Piceance Creek Basin. The section can be roughly litho- and bio-stratigraphically correlated to isotopically, well-constrained PETM stratigraphic intervals. The new bulk organic d13C record exhibits a range of values with an average of -22.3‰ ± 0.9 (1s). Variability in d13C values does not appear to be related to the amount of carbon nor lithology sample. Up-section there is a 40-meter thick interval over which d13C values shift ~2‰ to lower values and then return to baseline values. This interval corresponds to the lateral equivalent of the lower portion of the Molina Member, which is known to correlate with PETM. This study suggests the new isotopic record documents the PETM, but additional isotopic and biostratigraphic work needs to be performed to confirm. Within the hypothesized PETM interval soil morphology indices double in the upper portion of the structured isotopic shift, which indicates transiently better drainage in the floodplain. Mean annual precipitation (MAP) estimates from soil geochemistry are ~1500 mm/year before and after the isotopic excursion, and values as low as ~500 mm/year associated with the upper portion of the isotopic shift. This represents a 40% to 60% decrease in MAP in the basin. These results document greater drying in the Piceance Creek Basin as compared to the well-studied Bighorn Basin in northwest Wyoming wherein previous studies documented an increase in the soil morphology by 80% and decrease in MAP by 30-40% using identical methodologies as well as paleofloral records. When combined with other regional proxy datasets the results are consistent with general circulation model outputs that indicate widespread drying within the continental interior of North America as well as increased variability within the hydrologic cycle during the PETM. Moreover, this study supports the hypothesis that an enhanced hydrologic cycle is a robust response by the climate system to elevated atmospheric pCO2 levels, whether the carbon is ultimately sourced from anthropogenic sources or otherwise.
climate, PETM, Paleocene Eocene Thermal Maximum
Western Washington University
Subject – LCSH
Paleohydrology--Colorado--Piceance Creek Basin; Paleoclimatology--Colorado--Piceance Creek Basin; Hydrologic cycle--Colorado--Piceance Creek Basin
Piceance Creek Basin (Colo.)
Copying of this thesis in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.
Lesko, Anna, "Increased Hydrologic Variability Near the Paleocene-Eocene Boundary (Piceance Creek Basin, Colorado, U.S.A.))" (2019). WWU Graduate School Collection. 889.
Available for download on Sunday, June 20, 2021