GREEN RIVER ANTIPODE: PALEOCLIMATIC SIGNIFICANCE OF A DIVERSE MACROFLORA FROM THE EARLY EOCENE CLIMATIC OPTIMUM IN SOUTHERN SOUTH AMERICA

Rich fossil floras tied to high-resolution stratigraphy are useful for interpreting past climates but are rare in the early Cenozoic of the Southern Hemisphere. Here, we investigate an exceptionally diverse macroflora from caldera-lake beds exposed at Laguna del Hunco, Patagonia, Argentina, paleolatitude ~47.5 deg. S. We found 132 types of plant organs from an estimated 107 species, far exceeding previous reports for the site, as well as three datable tuffs and six paleomagnetic reversals. Results indicate that deposition of the most fossiliferous beds occurred from 52.4-51.6 Ma, near the base of magnetic polarity Chron 23. This places the flora within the early Eocene climatic optimum, known for the warmest sustained temperatures of the Cenozoic. Leaf-margin analysis indicates a mean annual temperature near 16 deg. C, in agreement with estimates of coeval sea surface temperatures from the South Atlantic. Terrestrial and marine temperature estimates from the same time interval and comparable paleolatitudes of the Northern Hemisphere are significantly warmer (>20 deg. C), suggesting that the meridional temperature gradient may have been steeper in the Southern Hemisphere.