CARBON AND CLUMPED ISOTOPES IN SOILS AND PALEOSOLS

Carbon and clumped isotopes in soil carbonate are very useful archives of vegetation and temperature change in the past. As paleorecords, however, they tend to be narrowly biased to the warmest and driest part of the year, when soil dewatering is most intense and hence soil carbonate preferentially forms. The d¹³C value of soil carbonate reflects the contribution of C₃, C₄, and atmospheric CO₂ in soil atmosphere. It will be biased toward the C₄ plants and perhaps atmospheric CO₂ that dominate soil CO₂ in the warmest, driest month (s). This may account for discrepancies with soil organic archives of paleovegetation, which are probably not so summer biased. Temperature reconstructed from clumped isotopes is well above (10-15°C) but strongly correlated with (r² = 0.9) local mean annual air temperature, making it a potentially powerful continental geothermometer.

Carbon isotopes in paleosol carbonate are the most resistant of the isotope archives to diagenetic alteration, much as they are in marine carbonates, withstanding burial temperatures >250°C. Oxygen isotopes of soil carbonate are less resistant, probably altering in the 125-250°C range depending on local hydrologic conditions. Clumped isotopes reset at the lowest burial temperatures of all, starting in the 100-125°C range. Any temperature >40°C reconstructed from clumped isotopes of paleosol carbonate should be viewed with caution, a reflection of burial, not primary surficial conditions.