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

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.