Title

Strontium Variations in a Speleothem from Crevice Cave, Missouri: Potential for High Resolution Paleoclimate Reconstruction

Degree Year

2003

Document Type

Thesis

Degree Name

Bachelor of Arts

Department

Geology

Abstract

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 I present an investigation of magnesium and strontium variations in a calcite stalagmite from Crevice Cave, Missouri. Using an electron microprobe, researchers at the University of Minnesota and I were able to produce quantitative, high resolution line transects and qualitative map analyses across optical banding in the stalagmite. The alternating light and dark optical bands are visible using transmitted light on a petrographic microscope. My measurement of these bands and U/Th dating of the stalagmite completed by Jeff Dorale indicate that the bands are either or seasonal or annual scale. Our quantitative line transect shows that the variations in strontium concentration are of the same scale as the optical banding. The strontium 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 also varies regularly, yet on a different scale. Our microprobe analyses have uncovered extraordinarily high strontium concentrations (~20,OOO ppm) in this particular speleothem from Crevice Cave. However, analysis by X-ray diffraction reveals a calcite mineralogy, with no measurable aragonite or strontianite. We have also found large amounts of strontium in other Crevice Cave stalagmites through ICP-MS analyses. The extraordinary amounts of strontium present in Crevice Cave speleothems most likely originate from celestite and strontianite deposits located in the lower portion of the Joachim Formation in which Crevice Cave has formed. Variations in dissolution of these minerals likely led to variations in strontium concentration of Crevice Cave groundwater, and subsequently to the stronitum variations measured in the stalagmite. The solubility product (Ksp) of celestite can be affected by several environmental factors. Using the equation for Ksp as a function of temperature presented by Reardon and Armstrong (1987), I was able to rule out variations in temperature above Crevice Cave as a cause for the strontium variations in the stalagmite. Celestite dissolution is also controlled by the reaction occurring at the mineral-water interface. Therefore, as more celestite will dissolve with longer contact with groundwater, changes in groundwater residence time above the cave are likely responsible for the measured strontium variations. Another possible cause for the strontium variations is changes in groundwater flow routes due to events of large groundwater infiltration. These events could force groundwater down less traveled paths, thus exposing it to larger or less dissolved populations of strontium-rich minerals.

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