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. 8
Presentation Time: 3:20 PM

Oxygen and Carbon Isotope Values of Modern Spelean Carbonates and Source Waters: Is Non-Equilibrium Deposition Status Quo?


TAYLOR, Penny M., Dept. of Geology and Geography, Mount Holyoke College, 50 College Street, South Hadley, MA 01075 and CHAFETZ, Henry S., Dept. of Earth and Atmospheric Sciences, Univ of Houston, Houston, TX 77204-5007, pmtaylor@mtholyoke.edu

Oxygen and carbon isotope values of 36 modern calcite deposits and adjacent contemporaneous water samples from three central Texas caves provide evidence for non-equilibrium deposition.  Spelean sample pairs (calcite and water) are predominantly from cave pools.  Calcite precipitated on glass substrates, at the edge of pools, and on the water surface (floating rafts).  d18O value of water samples range from -34.6 to -29.9‰ VPDB (-5.2 to 0.0‰ VSMOW).  d18O values for calcite range from -5.6 to -2.2‰ VPDB.  The oxygen isotopic value of calcite differs from theoretical (equilibrium) values by an average of 0.2‰ (s = 1.0; n = 36), but the differences (measured – equilibrium) between individual pairs range from -3.2 to +0.9‰.  d13C for water ranges from -15.2 to -4.3‰ VPDB.  d13C value for calcite precipitates range from -10.1 to -4.6‰ VPDB.  d13C of calcite differs from theoretical (equilibrium) values by an average 1.6‰ VPDB (s = 2.3; n = 36).  Differences between measured and theoretical values for individual pairs range from -3.3 to +5.4‰.

Water temperature calculated using the d18O values for calcite and water sample pairs compared to field-measured temperature demonstrate that agreement is seldom found.  Temperature discrepancies average -0.9°C (s = 4.6; n=36) however, the range is from -14.7 to +4.1°C (relative to field-measured temperatures).

The data suggest non-equilibrium calcite deposition in three caves, a surface stream in the region, and 6 additional travertine-depositing surface streams  (both ambient- and thermal-temperature systems). Collectively, these data bring to question if equilibrium fractionation of oxygen and carbon isotopes exists in specific modern terrestrial environments (California, Colorado, Oklahoma, Texas, Wyoming, and Italy).  Therefore, these results emphasize a potential for error in interpretation of ancient, terrestrial, depositional, and climatic conditions, for which equilibrium precipitation is requisite.