2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 11
Presentation Time: 10:50 AM


BREECKER, Dan O., SHARP, Zachary and MCFADDEN, Leslie, Earth & Planetary Sciences, University of New Mexico, MSC03 2040, Albuquerque, NM 87131-0001, breecker@unm.edu

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.