The Isotopic Evolution of Modern Water (δ18O and δD) Across the Andean Plateau Controlled by Easterly-Derived Precipitation Modified by Surface Water Recycling

Abstract:

The stable isotopic composition (δ¹⁸O and δD) of precipitation and derivative surface and ground water has been shown to be a function of elevation (precipitation-weighted hypsometric mean elevation in the case of surface or ground water), particularly for the windward side of mountain ranges. The elevation dependence of this relationship has been exploited for paleoaltimetry studies where δ-values of geological proxies for paleowaters have been used to estimate the paleoelevation and topographic growth (or decay) of many of the world’s mountain ranges and plateaux. However, these same water δ-elevation relationships have also been used to infer surface elevation and/or climate change on the leeward side of mountain ranges and interior of plateaux, where factors other than elevation have a significant impact.

Here, we present new surface water isotope data across the Andean plateau from the Eastern Cordillera to the Pacific Ocean. This dataset provides much needed constraints on modern water isotope evolution across the plateau and a context for the interpretation of elevation and climate proxies from the rock record in this continental environment. Our results show: 1) A progressive westward increase in δ¹⁸O that we attribute to a larger fraction of recycled water in precipitation, similar to what is observed on the Tibetan Plateau; 2) A southward increase in δ¹⁸O, likely due to the relatively arid climate in the south and; 3) That Easterly-derived precipitation appears to dominate the entire Andean plateau (including the Western Cordillera). These data suggest that paleoaltimetry studies based on samples collected on the Andean Plateau and Western Cordillera may overestimate increases in surface elevation.