RESPONSE OF METEORIC D18O TO SURFACE UPLIFT - IMPLICATIONS FOR CENOZOIC ANDEAN PLATEAU GROWTH

The timing and magnitude of surface uplift provides important constraints on geodynamic models of orogen formation. Oxygen isotope (δ18O) and mass-47 isotopologue (Δ47) compositions from terrestrial carbonate sediments have been used with modern isotope and temperature lapse rates to infer past surface elevations of the Andes. However, these paleoaltimetry interpretations are contentious because variations in the oxygen isotope composition in meteoric water (δ18Op) are caused by changes in elevation (orographic) and regional climate. Here, we use a limited-domain isotope-tracking general circulation model to simulate changes in δ18Op and isotopic lapse rates in response to Andean surface uplift, and to re-evaluate δ18O and Δ47 changes in late Miocene carbonates previously associated with rapid Andean growth. Results indicate that Andean surface uplift leads to changes in low-level atmospheric circulation and an increase in precipitation along the eastern Andean flank which influences isotopic source and amount effects. Simulated changes in Andean δ18Op are not systematic with an increase in surface elevation, but are instead a function of orographic thresholds that abruptly change regional climate. A δ18Op decrease of >5‰ over the central Andes and an increase in isotopic lapse rates (up to 0.8‰ km-1) coincide with Andean surface uplift from 75 to 100% of modern elevation. These changes in the isotopic signature could account for the entire 3-4‰ δ18O depletion in late Miocene carbonate nodules, and suggest an Andean paleoelevation of ~3000 m (75% of modern elevations) before 10 Ma.