Stable Isotopic And Geochemical Studies Of Late Quaternary Stalagmites, South-Central Appalachian Mountains, Eastern North America
Buckles, Jessica Ann
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A tripartite investigation was conducted utilizing speleothem geochemistry and modern cave environments: reconstruction of paleoclimate variability in southeastern West Virginia that spans the late Pleistocene to mid-Holocene, the development and optimization of Microbeam X-Ray Fluorescence analysis of speleothems, and high-frequency cave drip monitoring and cave environment study and implications for stalagmite paleoclimate studies. Paleoclimate interpretations of stable isotopic and geochemical results from three coeval 230Th-dated stalagmites provide insight into the timing, magnitude, and forcing mechanisms of past climate variability. Stable isotopic δ13C and δ18O and trace element (Sr) time series demonstrate variability between the coeval records, indicating different hydrologic histories of speleothem calcite. Many of the large-scale variations in the stalagmite record are shown to coincide with global climate events, especially the Younger Dryas, which is well-constrained as abrupt shifts in the proxy record. The stalagmite record displays clear linkages with SST and meltwater records from the GOM.Trace metal abundances and ratios in speleothems are extensively used as indicators of the hydrogeochemical processes active in overlying epikarst and serve as valuable proxies for precipitation amount and source, water residence time, and vegetation cover. Conventional methods of trace metal acquisition can be expensive, time-consuming and destructive. The use of microbeam XRF in measuring trace elements in speleothems has several advantages, as it is non-destructive, rapid, and is capable of providing high-quality data. A suite of test scans was performed using a Rh microfocus beam and a W collimator beam in order to define the appropriate methodology for the application of µ-XRF. Statistical simulations confirm that the data are governed by a Poisson distribution, through which statistical parameters may be calculated. While these data are typically reported as ratios to Ca, it is more appropriate for µ-XRF results to be reported as Sr counts, eliminating complications of differential surface and near-surface effects due to the more shallow penetration depth of the X-ray beam into the sample material for Ca. Calibration curves were created that convert the Sr counts to concentrations (ppm), allowing for the direct conversion to quantitative data.Over a 2.5 year period, three caves in southeastern West Virginia were continuously monitored for temperature and relative humidity. Surface precipitation drip frequency loggers were deployed near each of the cave entrances and compared to six cave frequency loggers installed under active cave drips. The behavior of the cave drip frequency responses to meteoric precipitation events reveals insight into the flowpaths of the water through the epikarst. Hydrochemical analyses reveal a wide variety of behaviors, and the dilution of dripwaters during a period of increased precipitation is evidenced by the abrupt change in multiple parameters. Only half of the active stalagmites studied with the cave drip loggers were determined to be suitable for paleoclimate studies, which highlights the importance of cave monitoring prior to speleothem collection.