Carbon and oxygen isotope records from Paleosols spanning the Paleocene-Eocene boundary, Bighorn Basin, Wyoming

The isotopic composition of paleosol carbonate and organic matter were investigated in the Bighorn Basin, Wyoming to explore changes in the carbon cycle and climate across the Paleocene-Eocene boundary. In three different measured sections, soil carbonate δ 13 C values change in phase with marine surface water carbonates on both long (∼7 m.y.) and short (∼100 k.y.) time scales. The carbon cycle perturbations at the Paleocene-Eocene Boundary Thermal Maximum (PETM) and the Eocene Warm Interval (EWI) are recorded in multiple sections, providing unambiguous links between marine and continental deposits. The PETM and EWI δ 13 C excursions in the Bighorn Basin are larger than those in the surface ocean, but the reasons for this amplification are unclear. Organic matter samples from the Bighorn Basin yield noisy δ 13 C records that do not mirror global changes, perhaps due to diagenetic alteration or postformational contamination. The δ 18 O values of soil carbonate are subject to multiple climatic influences that are often antagonistic. Although the δ 18 O shifts at the PETM and EWI are small, the shift at the PETM is statistically significant in two of the measured sections. Assuming a plausible range of values for the meteoric water δ 18 O/mean annual temperature relationship, the perturbation in soil carbonate δ 18 O at the PETM is consistent with an increase in meteoric water δ 18 O of ∼2‰ and changes in local temperature of 3–7 °C.

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