FLORAL TURNOVER AFTER THE EARLY EOCENE CLIMATIC OPTIMUM IN NORTHERN PATAGONIA

How did Patagonian vegetation respond to global cooling after the Early Eocene Climatic Optimum (EECO, ~53–49 Ma) and increased tectonic separation of South America from Antarctica? The famously diverse Río Pichileufú (RP) flora of the Huitrera Fm., Río Negro, Argentina, is the only well-dated early middle Eocene (47.7 Ma) macroflora known from southern South America. Comparisons with the most complete EECO reference flora in South America, also from northern Patagonia, Laguna del Hunco (LH; Huitrera Fm., Chubut, Argentina; 52.2 Ma), over the last century have varied. Most physiologically water-dependent rainforest conifers known from LH survive at RP, but at reduced abundances. Still, in a preliminary analysis of a small field collection, RP was unexpectedly found to be as diverse as LH and had many entire-margined species, suggesting continued warmth. Here, we present interim results testing hypotheses related to post-EECO floral change in Patagonia. To test the idea that the climate at RP was more seasonally dry compared with the early Eocene LH flora, we assess species composition, relative abundance of drought-intolerant species, and angiosperm leaf morphologies. We compare diversity of our updated RP dataset to LH using rarefaction of the dicot leaf data. Using standard leaf margin analysis, we derive a relative temperature difference between LH and RP. To test for seasonality of rainfall, we compare leaf area data. We show only ~10% shared species between the floras. We use relative abundance based on leaf census data to assess changes in dominance and check for loss or reduction of Gondwanan and moisture-dependent taxa. Our hypothesis that the paleoenvironment of RP was warm, but drier than LH with seasonal moisture would be supported by the following results: (1) high floral diversity; (2) continued high proportions of entire-margined species, but with smaller leaf size and (3) compositional turnover, with the extinction or reduced abundances of many of the LH wet Gondwanan taxa, and emergence of more seasonal and dry-adapted taxa. This study will be the first modern, high-resolution paleoecological analysis of the middle Eocene Patagonian vegetation using macrofloras, which is currently only constrained by individual taxonomic studies and pollen records.