Seasonal Biases In δ13C Values of Soil and Cave Carbonates

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.