T160 Geochronology of the American West: In Honor of the Careers of Bill Cobban and John Obradovich and the Roots of EARTHTIME

Tuesday, 2 November 2010
8:00 AM - 12:00 PM
Room 205

SEARCHING FOR THE PETM IN CARBON ISOTOPE VALUES OF BULK ORGANIC MATTER FROM THE PALEOSOL INTERVAL OF THE D2 SYNOROGENIC STRATA OF THE DENVER BASIN, COLORADO

Paper number: T160-10
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

The Paleocene-Eocene Thermal Maximum (PETM) is a time of rapid global warming that occurred at the Paleocene-Eocene (P-E) boundary (~55 Ma). The PETM is coincident with an abrupt negative carbon isotope excursion (CIE) of 4-8‰ in terrestrial basins that is superimposed on a longer-term negative shift in the global carbon isotope (δ13C) record of about 3‰. The Denver Basin is a foreland basin in the Rocky Mountains that contains strata that may preserve the Paleocene-Eocene boundary. According to current age constraints, the P-E boundary of the Denver Basin lies within a 28-meter interval that includes the contact of two units, informally termed the D1 and D2 sequences. These sequences are characterized by mudstone, sandstone, and siltstone. The base of the D2 sequence includes a ~10 m thick interval of intense paleosol development. Given the similarity of this paleosol interval to amalgamated paleosols that preserve the P-E boundary in the Bighorn Basin, Wyoming, it is plausible that this paleosol interval preserves the CIE of the PETM. To test this hypothesis, we generated δ13C records from bulk organic matter through a section of the Kiowa core and two outcrop sections exposed in the Bijou Creek Escarpment that contain the paleosol interval at the base of the D2 sequence.

We employed multiple treatment methods to remove siderite and/or carbonate bound within goethite that was not initially removed with the standard 10% hydrochloric acid and Dichloromethane treatments. After these treatments, δ13C values for the samples range from ~ -23‰ to ~ -27‰, which are reasonable values for a pure C3 ecosystem. Haas Ranch section 1 of the outcrop sections has an invariant δ13C record, while Haas Ranch section 2 preserves a δ13C record with a negative excursion of up to ~2‰. The δ13C record from the Kiowa core gradually decreases up section by ~2‰. Based on these three records, it is uncertain, though not impossible, that the PETM and concurrent negative CIE are present within the paleosol interval in the Denver Basin. This ambiguity may be clarified by additional age constraint of the upper D1 unit and the paleosol interval within the lower D2 unit.