OXYGEN ISOTOPE VALUES OF MODERN SPELEAN CARBONATES AND SOURCE WATERS: NON-EQUILIBRIUM DEPOSITION IS STATUS QUO

Oxygen isotopic values of 35 modern calcite deposits and contemporaneous water samples from three Central Texas caves provide evidence that non-equilibrium deposition is common. All spelean sample pairs of calcite and immediately adjacent water are from streams and pools. Calcite precipitated on glass substrates, at the edge of pools, or on the water surface as floating rafts. Oxygen isotopic values range from –5.0 to –2.2 permil (PDB) and from -5.0 to 0.0 permil (SMOW) for calcite and water, respectively. Differences between theoretical and measured d¹⁸O values of these calcite samples range from –1.1 to +2.8 permil (mean=0.5; s=1.01; n=35).

The temperature of the water from which a carbonate precipitated can be calculated assuming: 1) oxygen isotopic composition of the water, and 2) equilibrium fractionation between mineral and water. To assess the effect of observed disequilibrium fractionation on temperature calculations, the actual d¹⁸O values for carbonate and water sample pairs were used to calculate water temperature, treating temperature as an unknown. The resulting discrepancies ranged from –3.7 to +12.3°C relative to field-measured temperatures (mean=3.2; s=4.4; n=31).

Evidence for non-equilibrium calcite deposition in three Central Texas caves is in agreement with data from a travertine-depositing stream within the same karst region as well as 6 carbonate-depositing surface streams from other areas (both ambient- and thermal-temperature) from which sample pairs of carbonate (aragonite and calcite) and water were analyzed. Without exception, non-equilibrium deposition is most common in these systems in California, Colorado, Oklahoma, Texas, Wyoming, and Italy. Collectively, these data provide strong evidence that equilibrium fractionation of oxygen isotopes is rare in these terrestrial environments. Thus, poor results from temperature calculations, using detailed isotope data (no assumptions) from natural systems, emphasize the potential for errors in interpretation of ancient depositional or climatic conditions.