A Bayesian age-depth model of the Wilkins Peak Member of the Eocene Green River Formation

Abstract:

Deep-time geologic records of warming climate, such as the Early Eocene Climatic Optimum (EECO) provide analogues of how Earth systems will respond to rising temperatures in the future. Our understanding of the timing and intensity of the hyperthermal events that characterized the EECO is impeded by ocean acidification-driven dissolution in the marine record. The Wilkins Peak Member (WPM) of the Green River Formation, WY, USA, records deposition by an alkaline lake spanning the EECO, less likely to be degraded by dissolution. WPM deposits contain dozens of volcanic tuffs, and abundant lacustrine and paleosol-derived climate proxies, including eccentricity-paced alternations between lacustrine carbonate and fluvial siliciclastic deposits. By integrating astrochronologic analyses with radioisotopic, magnetostratigraphic and paleoclimate data, we can examine how temperatures, seasonality, hydrologic cycling, and weathering on land evolved during the EECO.

In order to integrate multiple chronometers with obliquity to precession-level (± 20-40 kyr) uncertainty, we employ a Bayesian approach to combine new suites of ⁴⁰Ar/³⁹Ar sanidine and isotope dilution U-Pb zircon ages with weighted mean ages across the WPM ranging from 49.796 ± 0.016 Ma to 51.576 ± 0.007 Ma. Unlike prior models which rely on linear interpolation, this Bayesian approach produces an age-depth model which conservatively estimates uncertainty between dated horizons. Our model exhibits a two to ten-fold increase in temporal resolution compared to previous radioisotopic age models for the WPM, and as a result clearly constrains a decrease in sedimentation rates from ~164 mm/kyr in the lower portion of the WPM, to ~87 mm/kyr in the upper portion, coincident with the disappearance of bedded trona from the WPM, suggesting a shift in basin response to climatic and/or tectonic forcing. Constraining sedimentation rates for these intervals will help refine future astrochronological analysis of the WPM, allow correlation between terrestrial and marine archives, and ultimately aid in radioisotopically and astronomically anchoring the GPTS for a critical part of the Eocene time scale.