Carbon isotope signature and precession-scale chronology of the PETM in a terrestrial setting

The Bighorn Basin in Wyoming provides expanded terrestrial sections that span the PETM and lack the carbonate dissolution hampering many marine records. Here we provide carbon isotope records at unprecedented resolution for the Polecat Bench and Head of Big Sand Coulee sections, parallel sites in the northern Bighorn Basin. Cyclostratigraphic analyses of these fluvial sediment records using descriptive sedimentology and proxy records allow subdivision into intervals dominated by
avulsion deposits and intervals dominated by overbank deposits. These sedimentary sequences alternate in a regular fashion, related to climatic precession. Correlation of the circa 7-km-spaced sections shows that the avulsion-overbank cycles are laterally consistent. The presence of longer-period alternations, related to modulation by the 100-kyr eccentricity cycle, corroborates the precession influence on the sediments. The cyclicity is then used to develop a floating precession-scale age model for the PETM carbon isotope excursion (CIE). We find a CIE body encompassing about 100 kyrs, in line with marine cyclostratigraphic age models. The CIE initial recovery is a 2 to 3 per mille step and the recovery lasts circa 40 kyrs.