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Testing stable isotope paleoaltimetry with Quaternary volcanic glasses from the Ecuadorian Andes

Abstract:

Paleoaltimetry provides critical constraints on orogenic processes. Validation of paleoaltimeters enhances confidence in their application to geologic problems and requires investigations of proxy materials formed at known elevations over known time frames. We evaluated isotope-elevation relationships for late Pleistocene to Holocene (220-0 ka) hydrated volcanic glasses deposited over a 4 km elevation range spanning the Pacific coastal forearc and Andean magmatic arc of Ecuador (0°-1.5°S). Reconstructed δD values of paleometeoric water (δD<inf>pw</inf>) in the glasses decrease systematically with elevation. Holocene δD<inf>pw</inf> values are similar to δD values of modern meteoric water (δD<inf>mw</inf>), whereas late Pleistocene δD<inf>pw</inf> values are 10‰-30‰ lower than both Holocene δD<inf>pw</inf> values and δD<inf>mw</inf> values at high (> 2 km) elevations. An elevation reconstruction based on δD differences from a modern or late Pleistocene low-elevation datum (ΔδD<inf>mw</inf> or ΔδD<inf>pw</inf>, respectively) using a one-dimensional thermodynamic Rayleigh distillation model parameterized with modern temperature accurately predicts Holocene sample elevations but overpredicts elevations of volcanic glass samples that were deposited during glacial marine isotope stage (MIS) 6 by 1-1.5 km. Late Pleistocene sample elevations are accurately predicted by applying a realistic correction for estimated lower air temperature (by ~5 °C) during glacial MIS 6. This study validates the applicability of volcanic glass δD values in paleoaltimetry studies, underscores the importance of accounting for climate-induced changes in isotope lapse rates when calculating paleoelevations, and suggests that ΔδD<inf>pw</inf> might be a sensitive proxy for climate change when applied on time scales over which elevation change was minimal.