ASSESSING SPELEOTHEM C AND O ISOTOPIC EQUILIBRIUM IN A MODERN CALCITE-DRIPWATER SYSTEM

Carbon and oxygen isotopic variations in speleothem calcite have been used as records of changes in environmental parameters such as surface temperature, rainfall amount and sources, vegetation type and ecosystem productivity. These interpretations require an understanding of the extent to which calcite has precipitated in isotopic equilibrium with its associated dripwater.  We assessed controls on C and O isotopic variations in speleothems, including extent of isotopic equilibrium, by the analysis of the modern calcite-dripwater system. This analysis was conducted by placing glass plates under active drips on a monthly basis over seasonal cycles in a central Texas cave. We analyzed the C and O isotopic compositions of calcite precipitated on these plates and their corresponding dripwaters, and we monitored environmental conditions. At  the center of each plate, analogous to calcite sampled along a speleothem growth axis, the d ¹³C values of the calcite range from -10.6 to -4.5‰, and the d ¹⁸O values range from -5.5 to -4.2‰. These values vary from being in oxygen isotopic equilibrium with the associated dripwater to 0.5‰ higher than equilibrium. C and O isotopic compositions also vary within each individual plate. d ¹³C values of calcite sampled along the plate surface, analogous to sampling along a stalagmite growth layer, increase by 0.2 to 1.2‰ from the center to the plate edge.  Similarly, d ¹⁸O values increase by 0.2 to 1.2‰ from center to plate edge, indicating the maximum departure from equilibrium is at the plate edge. This systematic increase of both d ¹³C and d ¹⁸O values from center to edge and the co-variation between d¹³C and d ¹⁸O values are consistent with non-equilibrium calcite precipitation, per the Hendy test for speleothem isotope records.  These results imply that analyzing a speleothem along its growth axis may not ensure O isotopic equilibrium. These departures from equilibrium and spatial isotopic variations are consistent with kinetic isotope effects driven by CO₂ degassing and calcite precipitation.