Mid-high latitude forest expansions across the Early Eocene Climatic Optimum in South America: phytolith records from the San Jorge Basin of Argentine Patagonia

Abstract

Cenozoic temperatures and pCO2 levels reached a maximum during the Early Eocene Climatic Optimum(EECO), during which temperature latitude gradients were significantly shallower than today. Globalvegetation reconstruction models based on paleofloral records broadly suggest the expansion of tropical ormixed tropical-temperate forests to mid-high latitudes and evergreen floras in polar latitudes in response to the EECO. However, global and regional models show significant discrepancies regarding South Americanvegetation responses during this interval. The inconsistency is likely due to limited Southern Hemisphererecords. We present a regional study of phytolith records across the late Paleocene-middle to late Eocene inthe San Jorge Basin of Argentine Patagonia. Our analyses show predominately palm-dominated assemblagesbefore and after the EECO and a significant increase in forest indicators (e.g., dicot trees) relative to palmphytoliths during the EECO. We interpret this pattern as a spread of woody dicot-dominated forests duringthe early Eocene, followed by a shift towards palm-dominated shrublands in the mid-late Eocene, consistentwith hypotheses associated with other regional paleobotanical and geochemical records. We find no evidenceof savannah-like conditions; grass phytoliths in our samples are rare and represent groups currently occupyingthe forest understory.