OSTER, Jessica L^l, DORALE, Jeffrey A², RAY, Sushmita³, WANG, Xianfeng³, EDWARDS, R. Lawrence³, (1) Geology, Oberlin College, Carnegie Building, 52 West Lorain St, Oberlin, OH 44074, jessica.oster@oberlin.edu, (2) Geology, Univ. of Missouri, 101 Geology Building, Columbia, MO 65211, (3) Geology & Geophysics, Univ. of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455

Speleothems are capable of providing high-resolution terrestrial paleoclimate records, through proxies that include carbon and oxygen isotopes, luminescence banding, and variations in elemental concentrations. Here we present preliminary work on magnesium and strontium variations in a calcite stalagmite from Crevice Cave, Missouri.

Using an electron microprobe, we are able to produce quantitative, high resolution line transects across optical banding in the stalagmite. The alternating light and dark optical bands are visible using transmitted light on a petrographic microscope, and are likely seasonal. Quantitative line transects measuring strontium and magnesium concentrations show that their variations are positively correlated to each other and are of the same scale as the optical banding. The elemental record is in phase with the optical banding, with peaks generally associated with the light portion of the band and troughs with the darker portion. Magnesium concentration varies on scales that are likely seasonal, yet temperature in the cave has negligible seasonal amplitude. Thus, this data does not support the viability of Mg/Ca thermometry in this cave. Microprobe analysis has also revealed extraordinarily high strontium concentrations (~20,000 ppm) in this particular speleothem from Crevice Cave. TIMS measurements indicate a strontium concentration of ~ 17,000 ppm, confirming the microprobe measurement. However, analysis by X-ray diffraction reveals a calcite mineralogy, with no measurable aragonite or strantianite.

Several hypotheses can be advanced to explain the seasonal strontium variations identified in CC-94-D5. For example, the distribution coefficient of Sr has been found to increase with increasing calcite precipitation rate, thus incorporating more Sr into the calcite at higher precipitation rates (Lorens, 1981). Previous studies have discovered both anticorrelated and correlated strontium and magnesium variations in speleothems and attributed them to variable groundwater residence time or changes in the groundwater dissolution-precipitation process in the bedrock above the cave respectively (Roberts et al. 1998, Verheyden et al. 2000). However, our results indicate that, while Sr and Mg variations appear to be positively correlated on one scale (400 micron line), there is decoupling of the two elemental records on a smaller scale (100 micron transect). Magnesium variations in CC-94-D5 are less clear and regular than strontium variations in the sample. Furthermore, the extremely high strontium concentration verified in CC-94-D5, and the absence of orthorhombic carbonate phases in the sample which can incorporate more strontium into their structure, raises even more questions about this speleothem and the nature of its precipitation. Thus, further research is needed to discern the exact relationship between the strontium and magnesium variations and prevailing climatic conditions.

In the future, our research will focus on determining the timing of precipitation of the optical bands seen in CC-94-D5 and the mechanism behind the incorporation of strontium and magnesium into the calcite structure. We will also investigate possible explanations for the anomalous strontium content of CC-94-D5. This will include further examination of the Roxanna and Peoria loess deposits located above Crevice Cave as a possible strontium source. Additional speleothems from Crevice Cave will be analyzed for high strontium concentrations, optical bandmg, and strontium and magnesium variations. We will also examine speleothems from Crevice Cave for luminescence banding caused by organic acids. Finally, the optical banding, elemental variations, and possible luminescence banding resolved in the speleothem will be analyzed in the context of seasonal scale paleoclimate variations. The use of elemental variations in speleothems can then be evaluated as a tool for investigating high-resolution paleoclimate variations in terrestrial cave localities elsewhere on Earth.