Seasonal moisture sources and the isotopic composition of precipitation, rivers, and carbonates across the Andes at 32.5–35.5°S

collected on both sides of the range in 2002 and 2007 were analyzed. We employ a Rayleigh isotope fractionation modeling approach to explore spatial and temporal variations in precipitation and river water compositions. The results indicate that precipitation on the eastern slopes of the Andes at ~33S, at elevations above 2km, is largely derived from a westerly, Pacific-source component and a mixture of easterly and westerly sources below 2km. Further south at ~35S, river water compositions exhibit a strong winter influence. At 33S, rivers have an isotopic minimum of ~18% across the core of the range, which has an average elevation of 4000m, and are topographically offset from similar isotopic values of precipitation by +1000m. Comparison of precipitation and river water data with temperature-corrected d 18 O estimates from pedogenic carbonates illustrates that carbonates capture the range of variability observed in modern precipitation and Rayleigh fractionation models.