Monday, 6 October 2008: 4:30 PM
George R. Brown Convention Center, General Assembly Theater Hall A
The carbon isotope compositions of modern soil and speleothem carbonate were studied in the southwestern USA in order to investigate their relationship to local plant cover. Our results suggest that δ13C values of both soil and speleothem carbonate tend to be biased toward seasonal extremes rather than “average” plant distributions. Evidence from modern soils in central New Mexico indicates that soil carbonate forms during the warmest and driest times of year. Reduced respiration rates under these conditions lower soil CO2 concentrations and help drive the precipitation of calcite. Soil carbonates are biased toward C4 plants because they record the annual maximum δ13C values of soil CO2 that occur during dry episodes. In addition, if pedogenic carbonate records minimum rather than mean growing season soil pCO2 values, then paleoatmopsheric CO2 concentrations calculated using the paleosol barometer may be significantly overestimated. By contrast, the δ13C values of modern speleothem calcite in Cave of the Bells near Tucson, Arizona are 3 to 6‰ lower than soil carbonate formed above the cave. While the δ13C value of CO2 in the soil above the cave fluctuates seasonally, the δ13C value of cave CO2 is nearly constant and closest in value to winter soil CO2. Hence, the fraction of C3 biomass implied by the δ13C values of cave CO2 and speleothem calcite are much higher than actually observed at the surface, where C4 plants dominate. The exact opposite (i.e. a strong C4 bias) would be expected if limestone dissolution contributed 50% of the carbon in speleothem calcite, as typically assumed. We conclude that carbon contribution from dissolution of limestone is minimal and that δ13C values in this cave are biased toward C3 plants, either because they are deeper rooted, or because they dominate during the winter and spring months when deep recharge events reach the cave.