STABLE ISOTOPES IN HYDRATED VOLCANIC GLASS INDICATE LONG-LIVED HIGH ELEVATIONS IN THE CENTRAL ALTIPLANO

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: (1) a temperature-dependent carbonate-water fractionation factor, (2) potential seasonality of carbonate precipitation, (3) diagenetic resetting of O isotopic compositions, and (4) 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 δD has also become widely used in paleoaltimetry and is subject to fewer complexities. We analyzed the D composition of 18 volcanic glass samples from the eastern limb of the Corque syncline and in the surrounding region in Bolivia. In contrast to previous δ¹⁸O results, our data shows consistently low δD values since at least the early Miocene, with consistent δD values of ~-175.7‰ to -129.0‰ between ~22.7 and 4.1 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 partially or fully 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. In this scenario both carbonate and volcanic glass isotopic compositions may be affected by threshold changes potentially including a decrease in seasonality or increase in mean annual precipitation which may accompany attainment of threshold elevations. Nevertheless, comparison of the δD data to δD values from a global dataset suggests that low values such as are observed in Bolivia are attainable only at elevations >3000 m. If accurate in the past, this suggests that the central Altiplano has been at near-modern elevations since at least ~23 Ma.