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

• Seasonal timing of pedogenic carbonate formation is explored across four different precipitation regimes.
• At multiple sites, clumped isotope data suggest carbonate formation during cool points of the year.
• Seasonal fluctuations in soil water content can explain the carbonate formation temperatures observed at different sites.
• Factors affecting soil water storage, such as texture, should be considered when using soil carbonate in paleoapplications.

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 or mean 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 °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 agree with the annual timing of soil water depletion, suggesting soil moisture 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.