57-4 Tracking Petrogenic Organic Carbon Oxidation during the Paleocene-Eocene Thermal Maximum using the Rhenium Proxy and Provenance Tracers

Abstract:

The Paleocene-Eocene Thermal Maximum (PETM) was a climate change event where global temperatures increased by 5-8°C in ~20,000 years and were sustained for an additional ~200,000 years before recovering. This duration is long compared to other climate events of similar magnitude in the Paleogene; one speculated cause is enhanced oxidation of petrogenic (or rock-bound) organic carbon (OC_petro). OC_petro is stored in siliciclastic sedimentary rocks (i.e., shale), which can be oxidized during rock weathering, releasing CO₂ that can keep atmospheric temperatures elevated. Few geologic carbon cycle models directly account for OC_petro oxidation, partly because few deep-time records of this process exist. 

In this study, we produce records of OC_petro oxidation in fluvial deposits spanning the PETM using the novel trace element proxy rhenium (Re), a transition metal that strongly associates with marine organic matter during deposition and mobilizes during oxidation of OM-bearing lithologies. We focus on 3 localities in the Bighorn Basin (Wyoming, USA) containing extensive PETM sediments but cropping out at varying distances from the basin axis. We hypothesize that landscape position modulates the provenance and proportion of shale- and igneous-bedrock-derived sediment each locality received from the hinterland, impacting the magnitude of OC_petro oxidation. Immobile element tracers (Rb/Zr, Ti/Al) show distinct inputs of igneous-bedrock- and shale-derived sediment, with the location near the basin axis (Sand Coulee) having a greater proportion of igneous sediment than the more distal location (Polecat Bench). Re concentrations in sediment spanning the PETM (n = 9) at Sand Coulee range from 14 to 44 ppb, with post-PETM sediments generally having higher Re concentrations than pre-onset samples, perhaps due to lower temperatures after the event and consequently lower organic matter breakdown. Known Re concentrations of felsic igneous rocks (~2 ppb) and black shales (~150 ppb) suggest that Sand Coulee sediments must have received a significant proportion of sediment from basin-bounding shale, but the degree of oxidation they experienced will depend on measurements of Bighorn Basin bedrock samples. A July 2026 sampling campaign yielded a complete transect of the Sand Creek Divide locality, which is positioned farthest from the basin axis and contains greater proportion of fine sediment. Additional Re, trace element, and isotopic measurements (e.g., ⁸⁷Sr/⁸⁶Sr) will help clarify the local controls and magnitudes of OC_petro oxidation during the PETM.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.