Coupled CO2-climate response during the Early Eocene Climatic Optimum

The Early Eocene Climatic Optimum (EECO) has been characterized as a prolonged warm event indicated by increased atmospheric pCO2, temperatures, precipitation, and biological turnover. A new paleoenvironmental reconstruction using integrated pedological, geochemical, and isotopic data from the Green River Basin (Green River/Great Divide region) provides a high-resolution record of environmental and climatic change throughout the EECO. Our reconstruction indicates that this region, and likely much of the margin of paleolake Gosiute, was a stable, fluvially-controlled floodplain environment with evidence of large scale continuous soil development and features comparable with modern Alfisols (temperate forest soils). Regional climate data from multiple proxies indicates that the period was warm-temperate and semi-arid to sub-humid, with a peak interval from about 51.5–50.9 Ma that exhibits significantly warmer (~ 7 °C) and wetter (~ 750 mm yr− 1) conditions, resulting in major changes to the local weathering regime. Isotopic analyses also indicate a rapid increase to high atmospheric pCO2 values (~ 1700 ppmV) and a shift in the δ13C composition of pedogenic carbonates during this peak interval that appear to define and provide a cause for this significant regional response to global climatic change. The new data, when combined with foraminiferal δ13C records, are consistent with CO2 ventilation from a deep marine reservoir source. This multi-proxy reconstruction suggests that the EECO may have had a superimposed “peak” of climatic and ecological change on land.