STABLE ISOTOPES IN HYDRATED VOLCANIC GLASS FROM THE CORQUE SYNCLINE INDICATE LONG-LIVED HIGH ELEVATIONS IN THE CENTRAL ALTIPLANO, WESTERN BOLIVIA

A major, late Miocene decrease in δ¹⁸O values from carbonates in the Corque Syncline has been interpreted to indicate high-magnitude, rapid surface uplift between 10 and 6 Ma. However, carbonates are subject to several complications including temperature dependent fractionation of O isotopes during carbonate precipitation, diagenetic resetting of O isotopic compositions, and climatic influences on surface water composition. Climate models show that regional temperatures and the isotopic composition of precipitation are subject to threshold responses to surface uplift. Temperature changes would maximize the signal of threshold-related isotopic shifts preserved in carbonate, potentially obscuring the timing and/or pace of surface uplift.

Volcanic glass has become widely used in paleoaltimetry because it rapidly hydrates after eruption and the δD values of this water in hydrated volcanic glass can be retained over geologic timescales. Fractionation of D into volcanic glass is temperature independent, and thus may present a possibly simpler proxy for paleoelevation. Nevertheless, it is subject to the same elevation-threshold isotopic effects discussed above, so results must be interpreted in the context of uplift and global climate change.

We analyzed the D composition of volcanic glass samples from the Corque Syncline and Western Cordillera. In contrast to previous results, our preliminary data shows consistently low δD values since at least the middle Miocene, with a steady decrease from ~-135‰ to -162‰ between 10.8 and 4.8 Ma. We propose two possible explanations for the discrepancy between the H and O datasets. First, it is possible that either or both of the isotopic systems have been reset. If this is the case, additional research is needed to determine if either dataset can provide paleoelevation constraints. Alternatively, the volcanic glass may record primary changes in isotopic composition of precipitation and environmental water. In this second scenario the carbonates may record a convoluted signal of gradual isotopic change and threshold temperature changes at ~6 Ma, rather than directly recording rapid uplift. If the volcanic glass is pristine and tracking precipitation isotopic compositions, this suggests that the central Altiplano has been at near-modern elevations since at least 11 Ma.