Rapid change in high-elevation precipitation patterns of Western North America during the Middle Eocene Climatic Optimum (MECO)

We present Eocene terrestrial oxygen (δ¹⁸O), carbon (δ¹³C), and strontium (⁸⁷Sr/⁸⁶Sr) isotope lake and paleosol records from two high elevation sites on the western North American plateau. These records represent terrestrial stable isotope evidence of the Middle Eocene Climatic Optimum (MECO) an enigmatic and rapid global warming event that interrupted protracted Eocene global cooling at around 40.0 Ma. Revised stratigraphy based on 15 ⁴⁰Ar/³⁹Ar ages from air-fall and water-lain ash in the Elko basin (Nevada) places evaporative lake conditions followed by rapid (<150 ka) freshening and a large (14-15 ‰) negative shift in δ¹⁸O of lake water between 40.2 Ma and 39.4 Ma, right at the climax of MECO. Prior to MECO high δ¹⁸O and δ¹³C values in well-laminated organic-rich shales are consistent with high evaporation-to-precipitation ratios, lack of lake overturn, persistent lake stratification, and moderate to high lake salinity. However, a step-wise large magnitude (>20‰) decrease in δ¹³C values already during the pre-MECO cooling phase, requires that in contrast to pre-MECO climate conditions, MECO temperature seasonality in western North America was sufficiently strong to (re-)establish regular lake overturn. The rate and magnitude of the rapid negative lacustrine δ¹⁸O shift during MECO in the Elko lake basin is inconsistent with a simple scenario of regional surface uplift affecting decrease of δ¹⁸O in precipitation and associated lake freshening. We consider the overall negative shift in δ¹⁸O of non-evaporatively ¹⁸O-enriched lake waters (-9.3 ±1.8 ‰) between the 42 to 43 Ma Elko Formation and the ca. 38 Ma base of the Indian Well Formation to be composed of a ca. 3 to 4 permil decrease in δ¹⁸O of lake carbonate as a consequence of post-MECO cooling and an additional ca. 5 to 6 permil decrease in δ¹⁸O of riverine lake input as a response to 43 to 38 Ma surface uplift. When combined with isotope-enabled global circulation model results, the Elko lake data further suggest that once critical elevations were attained a change in upstream moisture transport including strengthening of monsoonal summer rainfall on the (south-)eastern flanks of the Cordillera and a larger fraction of air parcel trajectories to the (eastern) lee of the Cordilleran highlands that had passed over the (western) continental interior were responsible for the rapid middle Eocene decrease in δ¹⁸O of precipitation in the central Cordilleran hinterland.