South American Climate Dynamics and Evolution of the Andes.

Abstract

Reconstructing the uplift history of the Andean Plateau (AP) provides important constraints for evaluating tectonic and geodynamic models of orogenesis and plateau formation. Recent elevation reconstructions of the AP, mainly based on stable isotope paleoaltimetry, suggest a rapid and recent rise of ~2.5±1 km of elevation of the central Andes during the Miocene (10-6 Ma) in response to lithospheric delamination. However, this uplift scenario is based on the assumption that climate did not change during deposition of paleoaltimetry proxies, and is inconsistent with geological evidence that implies Andean elevations of >2 km at 19-13 Ma. To address these issues, this thesis utilizes regional general circulation models (RegCM3.0, REMOiso5.0) to quantify changes in South American climate as a function of AP uplift and to evaluate the effects of surface uplift and climate change on the processes that control stable oxygen isotopic compositions in meteoric waters (d18Op). General results show that the Andes have a significant impact on moisture transport, deep convective processes, and precipitation over much of South America through mechanical forcing of the South American low-level jet and topographic blocking of westerly flow from the Pacific Ocean. In particular, Andean surface uplift intensifies convergence, orographic lifting, moisture transport, and latent heat release, which fuels convection and leads to an increase in precipitation along the eastern flank. Precipitation amount, water vapor source, wind pattern, and temperature are the factors that control d18Op along the Andes. Changes in the climatic conditions in response to Andean surface uplift influence isotopic source and amount effects and consequently d18Op. Changes in simulated d18Op are not systematic with an increase in Andean surface elevations, but differ in magnitude and timing across the AP. Low d18Op compositions of Miocene carbonates are consistent with simulated fractionation processes at elevations above 3000 m, and the 3-4‰ decrease in carbonate d18Op is smaller than the >5‰ decrease in d18Op simulated for Andean surface uplift from ~3000 to 4000 m. Model result suggest that the isotopic signal preserved in Miocene Andean carbonates reflects regional changes in low-level atmospheric circulation and precipitation associated with relatively minor (~1 km) surface uplift.