PALEOCLIMATIC IMPLICATIONS OF THE d13C VALUES OF HUMIC SUBTANCES IN KARST TERRAINS IN THE SOUTHERN UNITED STATES

The d¹³C values of humic substances in karst terrains in the southern United States have values that serve as a proxy for determining the photosynthetic pathway of present day vegetation. Soil and cave drip water samples were taken from 3 active karst systems with variable types of vegetation: 1.) Cumberland Caverns, McMinnville, TN- a C3 dominated deciduous forest, 2.) Caverns of Sonora, Sonora TX- a mixed oak prairie with C4 bunch grasses, and 3.) Carlsbad Caverns, Carlsbad NM- a desert scrub land with C4 bunch grass, C3 scrub and CAM succulents. Soil organic matter and DOC in drip waters have d¹³C values that agree with the d¹³C values of present day vegetation. Additional soil samples were taken at 100 km intervals along a 1300km transect from the Louisiana/Texas state line, across The Great Plains of Texas, to El Paso, Texas to determine whether the d¹³C value of soil organic matter faithfully indicates C3 /C4 co-dominant landscapes. Soil organic matter from environments containing both C3 and C4 plants have d¹³C values that indicate the transitional, co-dominant landscape: d¹³C values of soil organic matter along the transect increased westward as C4 grasses became more abundant. The data also showed a slight depletion in ¹³C at the western terminus due to the abundance of CAM succulents. The C isotopic composition of stalagmites has been used as an indicator of shifts from C3 ("normal") to C4 (arid and/or hot) vegetation. However, the C isotope compositions of CaCO₃ can be misleading because of kinetic fractionation of C isotopes. Stalagmites also commonly contain humic substances, and this work shows that the C isotopic composition of such humic substances may be a much better record of past changes in vegetation than the C isotopic composition of CaCO₃ stalagmites. Isotopic analysis of humic substances within a stalagmite, coupled with dating of appropriate calcite layers, may give more reliable results that indicate changes in plant ecology resulting from major shifts of climatic conditions. Humic substances in stalagmites could also be helpful in the reconstruction of ancient ecosystems in geographic locations where calcite speleothems likely to have undergone kinetic fractionation could still be used.