A paleopedological approach to understanding Eocene environmental conditions in southern Patagonia, Argentina

Abstract

The Eocene Las Flores and Koluel-Kaike formations in southern Patagonia (~48° S, Golfo San Jorge Basin, Argentina) are pedogenically modified fluvial and fluvio-eolian successions, respectively, which document early-middle Eocene environments at mid-paleolatitudes in the Southern Hemisphere. In order to reconstruct the paleoenvironment for the Las Flores and Koluel-Kaike formations, we performed a multiproxy and coordinated study of sediments and paleosols of both units, considering abiotic components. Using detailed sedimentology and paleopedology (macro- and micromorphology), bulk paleosol geochemistry and clay mineralogy, and organic carbon concentrations and stable isotope (δ13Corg) compositions, we show that the Las Flores and Koluel-Kaike paleosols are overall Ultisol-like paleosols, mineralogically and chemically consistent with a high to high-moderate degree of weathering, and developed on different parent materials (sedimentary with sandy and silty texture vs. silty volcaniclastic). Climate proxies and a comparison with modern Ultisols with similar features suggest that these paleosols formed under a broadly tropical-temperate and humid-subhumid climate with distinct seasonality. Overall, these combined data record long-term environmental conditions during the Paleogene (early-middle Eocene), and preserve a record of Eocene terrestrial climate in the Southern Hemisphere. This research is relevant for understanding latitudinal climatic gradients during warm periods like the Eocene, a key knowledge gap for future predictions, and these sites are particularly important because mid-latitude reconstructions in the Southern Hemisphere are the poorest resolved.