VERTICAL AND LATERAL VARIATION OF CARBONATE ISOTOPE RATIOS WITHIN PALEOSOL PROFILES OF THE WILLWOOD FORMATION, WYOMING

Isotope values preserved in authigenic paleosol carbonate can record paleoclimate and paleoenvironmental information that can help us understand Earth’s history. Models predict that C and O isotope ratios of carbonate should vary vertically and perhaps laterally within paleosol beds. Although a large number of studies have used paleosol carbonates, few have documented this isotopic variability or its effect on paleo reconstructions. We studied a well-developed paleosol profile in the Willwood Formation at Polecat Bench (Big Horn Basin, Wyoming). The studied soil formed within sediments deposited during the Paleocene-Eocene thermal maximum (PETM), an interval of great interest because it documents the climate effects of carbon release to the ocean and atmosphere. We collected 36 individual nodules from a 1.6 x 0.6 meter vertical section of the paleosol exposed by mass wasting in a small wash. The position of each nodule within the paleosol was referenced on a Cartesian grid. In the lab, nodules were cleaned, described, photographed and measured to document their morphology. Microsamples of micritic and sparry calcite were drilled from polished faces and analyzed for their stable C and O isotopic composition. The measured isotope ratios show clear patterns of horizontal and vertical variability within the bed. In general, C isotope values were lower, and O isotope ratios higher, for nodules sampled deeper within the paleosol. The data also show a steeper gradient for both C and O isotopes on the right side of the bed. We used a 1-dimensional respiration-diffusion model for soil CO₂ isotope ratios to understand the processes that might lead to lateral variation in the pedogenic carbonate isotope profiles. These results show that systematic variation in isotope fractionation can be observed within a single paleosol bed, and confirms the importance of sampling multiple nodules, preferably from deep within paleosol profiles, in paleoclimate studies.