Incorporation of Trace Elements into Soil Carbonate as a Novel Hydrological Proxy

Trace element compositions of soil carbonates are applied for the first time as possible paleo-proxy based on a case study on Chinese Loess Plateau. The Mg/Ca and Sr/Ca ratios of the soil carbonates picked from Chinese loess show large variation more than one order of magnitude and are positively correlated with each other. The observed Mg/Ca and Sr/Ca ratios of the soil carbonates are controlled by Rayleigh distillation rather than partition coefficient. Evolving soil solution as a result of progressive calcite precipitation due to transpiration and evaporation of soil water is responsible for the Rayleigh distillation. Theoretical and empirical examination demonstrates that the extent of Rayleigh distillation, and thus the Mg/Ca and Sr/Ca ratios of soil carbonates, largely depends on the fraction of soil water lost by runoff, i.e., the runoff coefficient. An application of the new proxy indicates dramatic change of hydrology on Chinese Loess Plateau following global ice volume during the past glacial cycles.