2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 8
Presentation Time: 1:30 PM-5:30 PM


GUILFOYLE, Amber L.1, BANNER, Jay L.1, STERN, Libby1, MICKLER, Patrick2, JAMES, Eric1, CHRISTIAN, Lance1 and MACK, Larry1, (1)Department of Geological Sciences, University of Texas at Austin, 1 University Station C1100, Austin, TX 78712, (2)Chapel Hill, NC 27516, amberguilfoyle@yahoo.com

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.