From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, November 06, 2009
2:00pm
The stable isotope composition of CO2 in soils and caves:
investigating processes in the critical zone.
Dan Breecker, Dept. of Geological Sciences, Jackson School of Geosciences, UT-Austin
The stable isotope composition of CO2 in soils and caves: investigating processes in the critical zone.
Carbon dioxide is of fundamental importance to the chemistry of groundwater, caves, soils and the vegetative canopy: Earth’s ‘Critical Zone.’ This presentation is a survey of several independent and ongoing studies that are all related by the central topic of CO2 in soils. The studies to be discussed are investigations of 1) the factors that control soil respiration, 2) the formation of soil carbonate, and 3) the transfer of CO2 from soils into caves.
CO2 is produced in soils by the decomposition of soil organic matter and by respiration associated with the roots of plants. Although many studies have been dedicated to quantifying the rates of soil respiration, few studies have investigated the stable isotope composition of soil respiration. A rapid incubation technique has been developed in which respired CO2 is automatically injected into a mass spectrometer. Initial results from soil incubations experiments will be discussed. Evidence for the dominance of root-associated respiration over decomposition respiration in soils from central New Mexico will also be discussed.
Monitoring the concentration and stable isotope composition of CO2 in calcic soils from the Arizona and New Mexico suggests that carbonate in these soils forms under hot, dry conditions and not mean growing season conditions as typically assumed. This conclusions is based on the simultaneous occurrence of carbon isotope equilibrium between soil CO2 and soil carbonate, oxygen isotope equilibrium between soil water and soil carbonate, low soil pCO2, high soil temperature and soil water loss by evapotranspiration. Formation of soil carbonate during hot, dry episodes rather than mean growing season has important implications for reconstructing ancient atmospheric CO2 concentrations, paleovegetation and paleoelevation.
Comparison of CO2 in the atmosphere of Cave of the Bells, southern Arizona, with CO2 in streambed and hillslope soils at several locations above the cave suggests that CO2 transfer into the cave may preferentially occur along the streambed. During the past year, the streambed and cave CO2 have had consistently similar carbon isotope compositions, once the effect of atmospheric mixing is removed. The similarity between streambed and cave CO2 suggests the two have the same source, which is likely organic matter in the streambed. CO2 in the streambed and in the cave has ?13C values that are lower than CO2 in the hillslope soils, which is consistent with a higher proportion of C4 vegetation on the hillslopes than in the streambed. Tracing CO2 from soils to caves will help to construct cave CO2 budgets, which are important for the formation of speleothems, and may help establish the controls on the carbon isotope composition of speleothem calcite.
|
