Abrupt Geographic Shift in Hydrogen Isotope Ratios of Meteoric Water Across the Western Andes, Peru

Quantitative isotopic paleoaltimetry has been applied in regions where Rayleigh distillation controls isotopic lapse rates. Air mass mixing and moisture recycling are viewed as complicating factors. We show here that, because of such effects, a cross-Andean transect of meteoric water delta D values precisely marks the geographic position of the Western Cordillera crest. This modern water signal is also recorded in Pliocene-Pleistocene hydrated volcanic glass delta D values. delta D values between the Pacific coast and Western Cordillera exhibit no trend up to 2.5 km elevation and 100 km inboard, consistent with an arid climate in which Amazonian moisture is topographically blocked and Pacific moisture is efficiently recycled. The result is a large delta D lapse rate (-98 parts per thousand/km) and an abrupt horizontal delta D shift (2 parts per thousand/km) at the Western Cordillera crest. Therefore, we conclude that cross-orogen delta D transects could locate the ancient Western Cordillera crest. Mountains have an outsized control on climate. Moist air masses rise and cool to cross high elevations, resulting in enhanced precipitation on the windward side and dry conditions downwind These processes influence the isotopic compositions of rainfall and of materials preserved in the geologic record that form from the interaction of rain with near-surface materials. Here we report data from transects across the Peruvian Central Andes and show that the isotopic compositions shift abruptly at the position of highest topography (the crest of the Western Cordillera). This suggest that isotopic compositions of materials preserved in the geologic record might help establish the geographic position of the crests of mountain belts in the past. There is a substantial shift in the hydrogen isotope ratios of meteoric water at the Andean Western Cordillera crest Volcanic glass younger than 5 million years old shows similar ratio distributions to modern soil and precipitation water values Volcanic arc migration over time can be identified with meteoric water stable isotope records