Seasonal bias in pedogenic carbonate formation: implications for interpreting paleosol temperatures from clumped isotopes

Clumped isotope thermometry of paleosol carbonates is potentially a powerful technique for reconstructing terrestrial paleoenvironments and paleotopography from the geologic record. Clumped isotope measurements independently determine the temperature of carbonate formation and the δ18O value of the carbonate, from which the isotopic composition of the soil water can be calculated. Temperature and δ18O values of precipitation and surface waters are strongly influenced by climate and elevation. However, interpretation of paleosol temperatures may be problematic without rigorous evaluation of how the timing and temperature of carbonate formation are influenced by environmental factors such as seasonal variations in temperature, precipitation, vegetation, and snow cover. Our work investigates the relationship between environmental conditions and the formation temperatures of modern pedogenic carbonates along a transect spanning 2 km of relief in the southern Central Andes of Argentina (33°S). The transect exhibits high seasonal variability in precipitation and temperature, and clumped-isotope thermometry offers a direct way to quantify their influence on the seasonality of carbonate formation. As carbonate formation is likely favored under warm, dry conditions, temperatures from clumped isotope thermometry of pedogenic carbonate are expected to exceed mean annual air temperature. Our preliminary results support this, with clumped isotope temperatures exceeding mean annual air temperature by at least 4°C, regardless of sample depth. This excess increases with elevation (particularly over 2000 m), suggesting that simply interpreting carbonate formation temperatures as seasonal summer temperatures is not valid in all cases. In addition, preliminary results suggest the depth dependence of carbonate formation temperature also may vary with environment. At lower elevations (<2000 m), average temperatures for shallow (0-50 cm depth) samples are 1-2°C warmer than deep (50-100 cm) samples, but at higher elevations, average temperatures for shallow samples are 2-4°C cooler than deep samples. These results are important not only for how temperature estimates are interpreted, but also for sampling strategies from soil profiles. Simply sampling deeper in the soil profile to avoid larger temperature variations at the surface (e.g. below 50 cm) could potentially introduce greater biases in soil temperature estimates. Developing an understanding of how specific environmental factors influence clumped isotope estimates of pedogenic carbonate formation temperature is crucial for accurate reconstructions of paleoclimate and paleoelevation using ancient carbonates.