TERTIARY CLIMATE AND VEGETATION OF SOUTHERN SOUTH AMERICA

Currently there are few quantitative data on the Tertiary terrestrial paleoclimate of southern South America. Yet such data are critical, because they can be used to better understand the evolution of the flora and fauna, to provide boundary conditions for tectonic models, and to evaluate the output of general circulation models. In this study, we analyzed the foliar physiognomy and systematics of 15 Tertiary floras from Chile, Argentina, and Bolivia in order to reconstruct temperature, precipitation, and vegetation cover. In general, the results are consistent with global climate trends as recorded by marine isotope data.

The paleofloras suggest that in the late Paleocene and early Eocene, the mid-latitudes in South America were covered by tropical-humid forest (Gondwanica flora). We estimate mean annual temperatures over 25 °C, mean annual precipitation around 200 cm, and low seasonality. The climate then shifted towards more temperate and dry conditions, and the tropical-humid forest was replaced by subtropical-humid forest (Subtropical Gondwanica flora) and then warm-temperate forest (Mixed flora). This trend culminated in the late Eocene/Oligocene deterioration, a marked drop of temperature that occurred around 33 Mya. In the early Oligocene, mean annual temperature was around 15 °C and mean annual precipitation was less than 50 cm at mid-latitudes, and a cold-temperate dry flora dominated by Nothofagus prevailed (Mixed flora). The climate then warmed and became wetter, culminating in the mid-Miocene climatic optimum, during which the cold-temperate dry forest of the early Oligocene was replaced by subtropical dry forest (Subtropical Neogene flora). In central Chile, we estimate mean annual temperatures of 21-25 °C and mean annual precipitation of around 100 cm. If the higher range of our temperature estimate is accurate, then the floral data would contrast with marine isotope records, which show significantly cooler temperatures during the mid-Miocene climatic optimum than the early Eocene climatic optimum. In the late Miocene, the climate again shifted towards cooler and drier conditions.