Volcanic Glass as a Proxy for Cenozoic Elevation and Climate in the Cascade Mountains, Oregon, USA

After deposition, volcanic glass hydrates with ambient water, recording the average hydrogen isotope ratio (δD or δ2H) of local meteoric water during the hydration period. Previous researchers have used ancient glass δD values to reconstruct paleotopography and paleoclimate, while others have questioned the long-term reliability of the proxy as a recorder of ancient meteoric water. In this study, we sampled volcanic glasses ranging in age ~33 Ma to(east) side of the Oregon Cascade Mountains. Our results strongly suggest that volcanic glass acquires and preserves δD values that are proportional to the stable isotopic composition of environmental water at the time of ash deposition based on 1) a 20‰ difference in δD values between samples of different ages (~8 Ma apart) from the same locality, 2) preservation of stable isotopic compositions consistent with lacustrine and non-lacustrine depositional environments in coeval samples, and 3) substantial differences between δD values of ancient volcanic glass (\u3e1 Ma) and local meteoric water (converted to glass δD values) throughout the study area. We propose a paleoenvironmental interpretation of volcanic glass results that resolves previously published isotopic data and agrees well with the petrologic, structural, and stratigraphic record. Namely, the Oregon Cascades have been a significant topographic barrier since at least the mid-Miocene, and likely as far back as the Oligocene. Since reaching a topographic maximum during the eruption of Columbia River flood basalts in the mid-Miocene, surface elevations in Oregon have decreased, while the northern Cascades in Washington continue to rise