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; mean annual precipitation is ~3000 mm on the windward side W of the mountains, contrasting with only ~300 mm to their E. On the W side of the mountains, the δ¹⁸O values of precipitation and surface waters show a steady eastward decrease. This is presumably the result of progressive condensation and rainout during the orographic uplift of moisture-bearing air masses; the δ¹⁸O values of precipitation and surface waters systematically decrease with elevation by ~0.3 ‰/100 m. On the leeward E side of the mountains, in contrast, the δ¹⁸O values of precipitation and surface waters are consistently low, and show no systematic trend with local elevation. Geologic data imply that the present-day rain shadow has been established or significantly enhanced during Miocene surface uplift of this Andean segment, in good agreement with evidence for climatic and ecologic changes in the eastern foreland of the Patagonian Andes during this time. To provide constraints on changes in Miocene plant ecosystems and the isotopic composition of 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 ~22-14 Ma. From ~17-14 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.