STRONTIUM AND MAGNESIUM VARIATIONS IN A SPELEOTHEM FROM CREVICE CAVE, MISSOURI: POTENTIAL FOR SEASONAL SCALE PALEOCLIMATE RECONSTRUCTION

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. However, analysis by X-ray diffraction reveals a calcite mineralogy, with no measurable aragonite or strontianite.

Our results demonstrate that strontium and magnesium variations can be easily resolved on a seasonal scale, assuming the optical bands represent semiannual changes in environmental conditions. This knowledge can provide insights into the variety of changes in climate and groundwater conditions that occurred in and above Crevice Cave in the past.