EVALUATION OF ISOTOPIC EQUILIBRIUM IN STABLE ISOTOPES OF SPELEOTHMES AS A PROXY FOR CURRENT CLIMATE CONDITIONS

The study of paleoclimates and climate change depends on a variety of sources to measure and analyze past climates. One potentially promising source is the stable isotopes analysis of speleothems, as these formations grow over long periods of time at a relatively constant temperature. However, there are many factors that can lead to faulty data. One of the original assumptions of when this type of data analysis began was that the calcite precipitated out of the water would be in isotopic equilibrium. The goal of the project is to evaluate the validity of this assumption, and to determine the spatial point in the speleothems of where isotopic equilibrium is reached. Ideally, water precipitating on these speleothems would have been sampled, but this was not possible because of the difficulty collecting samples.

Speleothem isotopic studies use carbon and oxygen isotopic compositions as proxies for climate change. Stalagmites form as dripping water strikes the cave floor and precipitates out calcite. The largest amount of calcite is deposited where the drop strikes first, so repeated drops will build up the conical structure that forms a stalagmite. This creates a structure with an axis that can be used to measure time and growth over long periods. The typical method for taking samples in stalagmites is to take them along the axis, but if they are not at isotopic equilibrium at this point, this method would be invalid.

For this study, samples were collected from multiple caves in the southern Ozark region in Arkansas and Missouri, with the greatest care taken to minimize aesthetic damage. Most of the collected materials were small stalagmites (~2-4 cm high) that showed current growth. Small samples (4-5 mg) were ground from the surface of the stalagmites, descending from the top of the formation to the distal end, to if there had been any changes in the stable isotope ratio as calcite was deposited. To determine this, the samples were analyzed with a dual-inlet mass spectrometer. Comparison of the results showed if the samples for each speleothem were at equilibrium relative to each other. This study was sponsored as a Research Experience for Undergraduates (REU) at the University of Arkansas at Fayetteville during the summer of 2006 and special thanks to the National Science Foundation (NSF) for their generous support of this program.