Combining soil water balance and clumped isotopes to understand the nature and timing of pedogenic carbonate formation

Pedogenic carbonate is an important archive for paleoclimate, paleoecology, and paleoelevation studies. However, it can form under seasonal environmental conditions that differ significantly from the mean growing season environment ormean annual conditions, potentially complicating its use for proxy reconstructions. The observed seasonal temperature is typically, but not always, biased high relative to mean annual air temperature (MAT). To evaluate the annual timing of pedogenic carbonate formation, ten different soils were sampled across the western United States. Sites were selected to span a variety of precipitation regimes and soil orders. Precipitation regimes ranged from arid sites (mean annual precipitation (MAP) < 20 cm) that receive the majority of precipitation during the winter to wetter sites (MAP > 50 cm) dominated by summer precipitation. Pedogenic carbonate formation temperatures derived from clumped isotope measurements ranged between 6 and 22 degrees C, with most samples falling at or below MAT. Clumped isotope temperatures were compared to monthly precipitation normals and modeled monthly values of evapotranspiration and soil water content. Results show that carbonate formation temperatures agreewith the annual timing of soilwater depletion, suggesting soilmoisture content is a primary control on the timing of pedogenic carbonate formation. Although the seasonal bias is a function of environmental factors that are difficult to reconstruct in paleo- studies, the use of other paleosol proxies can help to assess if changes in clumped isotope temperatures are a function of changes in air temperature or hydrology. These results have important implications for the production of accurate paleoclimate and paleoelevation estimates. (C) 2016 Elsevier B.V. All rights reserved.