Constraints on the duration of the Paleocene-Eocene Thermal Maximum by orbitally-influenced fluvial sediments of the northern Bighorn Basin, Wyoming

Abstract

The addition of major amounts of carbon to the exogenic carbon pool caused rapid climate change and faunal turnover around 56 Ma. This event is termed the Paleocene-Eocene Thermal Maximum (PETM) and is recognized in sedimentary records around the world by a negative carbon isotope excursion (CIE). Polecat Bench in the Bighorn Basin, Wyoming, provides the best studied continental section spanning the PETM. The Polecat Bench redness record, previously used to construct a cyclostratigraphic age model for the PETM, is extended and complemented with other colour records and a soil development index. Based on the lithology and the soil development index, the Polecat Bench section is subdivided into intervals dominated by avulsion deposits and intervals dominated by overbank deposits, which alternate in a regular fashion. Correlation to the Big Sand Coulee section shows that the overbank-avulsion cycles are laterally consistent. Independent age control suggests a precession influence on overbank-avulsion cycles, which is strengthened by the presence of longer-period alternations related to modulation by the 100-kyr eccentricity cycle. The stable carbon isotope record, measured on pedogenic carbonate nodules, shows significant variation preceding the onset of the CIE. This is interpreted as climate destabilization before the PETM. An apparent discrepancy in initial recovery rate between continental and marine records is partly attributed to enhanced sedimentation rates in marine environments and partly to lowered sedimentation rates in continental environments, both characterized by lithological change. The CIE body encompasses one precession cycle less than previously determined using the redness, highlighting the importance of a multi-proxy and multi-dimensional approach in the analysis of cyclicity in fluvial deposits. Two options are possible for the duration of the CIE-recovery (~40 kyr and ~90 kyr), aligning the new chronology with either of two groups of estimates from marine age models. This study furthermore shows that the Paleogene sediments of the Bighorn Basin can potentially give insights into the absolute age and cause of the PETM.