2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM
Paper No. 205-12
Presentation Time: 4:30 PM-4:45 PM

Seasonal Biases In δ13C Values of Soil and Cave Carbonates

BREECKER, Dan O., Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712, breecker@jsg.utexas.edu, QUADE, Jay, Department of Geosciences, University of Arizona, Tucson, AZ 85721, FISCHER, T.B., Department of Geosciences, Penn State University, University Park, PA 16802, MCFADDEN, Leslie, Earth & Planetary Sciences, University of New Mexico, MSC03 2040, Albuquerque, NM 87131-0001, and SHARP, Zachary D., Earth and Planetary Sciences, Univ of New Mexico, Albuquerque, NM 87131

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.

2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM
General Information for this Meeting
Session No. 205
Critical Zone Studies of Soils and Weathering: Implications for Interpreting Climate and Landscapes of the Past
George R. Brown Convention Center: General Assembly Theater Hall A
1:30 PM-5:30 PM, Monday, 6 October 2008

Geological Society of America Abstracts with Programs, Vol. 40, No. 6, p. 276

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