EXTREME CONDITIONS OF PEDOGENIC CARBONATE FORMATION AND IMPLICATIONS FOR PALEOENVIRONMENTAL RECONSTRUCTION AND PALEOSOL PCO2 BAROMETRY

We report results from a year-long study of the stable isotope composition and concentration of soil CO₂ in young, pedogenic carbonate-bearing soils from a site in central New Mexico. δ¹³C values and concentrations were measured as a function of depth and season in four distinct biomes. Below 50 cm, δ¹³C values were lowest in winter and highest in spring (2-3‰ variation) while soil CO₂ was lowest in winter and highest in summer (2000-3000 ppm variation). Respiration rates estimated with a steady state model were lowest in winter and highest in summer while δ¹³C values of soil-respired CO₂ were lowest in winter and highest in spring. Measured δ¹³C values of recently formed pedogenic carbonate were always heavier than predicted from measured δ¹³C values of soil CO₂. Modeling soil CO₂ suggests that equilibrium with pedogenic carbonate occurs only when respiration rates decrease to their lowest values and the isotopic composition of soil-respired CO₂ increases to its highest value. These conditions likely occur during the driest, hottest season of dry years when soil moisture and respiration rates are low and reduced discrimination against ¹³C during photosynthesis combined with an increase in the relative contribution from C₄ plants maximizes δ¹³C soil-respired CO₂. We conclude that carbonate forms only during the driest times when soil CO₂ is anomalously low and δ¹³C values are anomalously high. This model is supported by the fact that evaporation of soil water, low soil CO₂ and high temperature all favor calcium carbonate precipitation. If we are correct, then pedogenic carbonate is biased toward high δ¹³C values (C₄ signal) and typically does not record average compositions of vegetation growing in the soil. If carbonate indeed forms under dry, hot conditions, with low soil CO₂ and reduced discrimination, then previously calculated values for atmospheric pCO₂ using carbonate paleosol barometry are probably overestimated by up to 1000 ppm.