Are two elements better than one? Dual isotope-ratio detrending of evaporative effects on lake carbonate paleoelevation proxies

Abstract

Stable isotope-based proxy methods enhance our ability to interpret geodynamical histories for tectonic provinces via paleoelevational reconstructions. These methods require that the unmodified isotopic composition of meteoric water is recorded by authigenic minerals, a critical assumption that has not been tested across wide-ranging environmental and topographical contexts. Here, we show that Quaternary lake carbonate δ 18 O values are not strongly, nor significantly, correlated with regional elevation due to the isotopic modification of in-flow waters following entry into the lake environment. These modifications are largely caused by surface water evaporation, and can result in >3 km errors in paleoelevation estimates if not accounted for. However, our analysis suggests that positive shifts in surface water δ 18 O are accompanied by similar magnitude shifts in δ 13 C-DIC. This positive co-variation in δ 18 O and δ 13 C may be used to detrend lake carbonate compositions for the effects of surface water evaporation using a parameter we define here as the 13 C-excess. When Tibetan lakes are excluded from the data set, 13 C-excess values are significantly correlated with mean up-slope hypsometric altitude with an error of 500 m. Application of the 13 C-excess approach to Cenozoic lake carbonate records from the western U.S. Cordillera both challenges and reinforces previous paleoelevational interpretations based on δ 18 O alone. ©2012. American Geophysical Union. All Rights Reserved.