Statistical Analyses of d18O in Meteoric Waters From the Western US and East Asia: Implications for Paleoaltimetry

Questions on the timing of Tibetan Plateau uplift and its associated influence on the development of the Indian and Asian monsoons are best addressed through accurate determinations of regional paleoelevation. Previous determinations of paleoaltimetry utilized the stable isotopic composition of paleo-meteoric waters as recorded in various proxies (authigenic minerals, fossils, etc.), in combination with empirically and model determined elevation isotopic lapse rates. However, the applicability of these lapse rates, derived principally from orogenic settings, to high continental plateaus remains uncertain. Our research aims to gain a better understanding of the potential controls on the δ18O composition of meteoric waters over continental plateaus through a principal component analysis (PCA) of modern waters from eastern Asia and the western US. In particular, we investigate how various environmental parameters (elevation, latitude, longitude, MAP, and MAT) influence the δ18O composition of these waters. First, these analyses reveal that elevation and latitude are the primary controls on isotopic composition in all regions investigated, as expected. Second, PCA results yield elevation lapse rates from orogenic settings (i.e. Sierra Nevada, Himalaya) of ~ -3‰/km, in strong agreement with both empirical and Rayleigh distillation model derived lapse rates. The Great Plains of the US, although not an orogenic setting, represents a monotonic topographic rise, and is also characterized by a ~ -3‰/km lapse rate. In high, arid plateau regions (Basin and Range, Tibet), however, elevation lapse rates are ~ -1.5‰/km, half that of orogenic settings. An empirically derived lapse rate from small source area springs collected over a 2 km elevation change from a single mountain range in the Basin and Range yields an identical rate. One clue as to the source of this lowered lapse rate is eastern China, which also displays an elevation lapse rate of ~ -1.5‰/km, despite being a relatively low elevation, humid region. All three regions of lowered lapse rates are dominated by convective storms, which violate basic assumptions of simple Rayleigh distillation. The similarity of lapse rates between these regions suggests that convective storm systems may result in a predictable change in elevation lapse rates. Third, the effect of latitude changes on isotopic composition should be considered in major orogenic systems. In the western US, best-fit linear models reveal latitude lapse rates of ~ -0.5‰/°N, thus significant northward or southward tectonic translations may be misinterpreted as elevation changes. The mixing of multiple moisture sources over eastern Asia appears to result in a polynomial function for latitude lapse rate. The determination of the effects of this latitude lapse rate on paleoelevation histories is ongoing. Finally, comparison of PCA models of modern isotopic composition with actual meteoric water values offers an opportunity to assess the accuracy of paleoelevation estimates. Predictive capabilites of our derived models are significantly better in orogenic settings (± ~950m 2σ) than over continental plateaus (± ~1950m 2σ). These statistical models enhance our understanding, and the predictive capability, of stable isotopes over high, arid plateaus. In particular, they point to the controlling effect of convective storms on elevation lapse rates, and thus the potential effect of the growth of the Tibetan Plateau, and onset of monsoonal climate conditions, in driving time-dependent elevation isotopic lapse rates.