Climatic and ecologic changes during Miocene surface uplift in the Southern Patagonian Andes

The up to ∼4 km high southern Patagonian Andes form a pronounced topographic barrier to atmospheric circulation in the southern hemisphere westerlies, and cause one of the most drastic orographic rain shadows on earth. Geologic data imply that this climatic pattern has been established or significantly enhanced during Miocene surface uplift of this Andean segment. We report evidence for important climatic and ecologic changes in the eastern foreland of the Patagonian Andes that appear to be the result of this uplift. To provide constraints on Miocene plant ecosystems and precipitation in the eastern (leeward) foreland of the Patagonian Andes, we determined carbon and oxygen isotope values of pedogenic carbonate nodules from a ∼500 m thick section of the continental Santa Cruz Formation. The age of these deposits was constrained by Ar/Ar dating of intercalated tuffs, which range from ∼22 to 14 Ma. At ∼16.5 Ma, the δ13C values increase by ∼3‰, the δ18O values decrease by >2‰, and the scatter in the oxygen isotope data increases significantly. We interpret these changes as the consequence of >1 km surface uplift in this Andean segment (from the δ18O values), and increased aridity to its east (from the δ13C values and the increased scatter in the δ18O values). Sediments overlying the Santa Cruz Formation are very limited in extent and volume, and dominated by coarse conglomerates related to Pleistocene and older glaciations. It thus seems that, by ∼14 Ma, deposition in the eastern foreland of the Southern Patagonian Andes had essentially ceased as the result of rain shadow formation.