Browsing by Subject "Speleothem"
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Item A 20,000 year flowstone record of Gulf of Mexico sourced moisture in Texas(2016-05) Charlton, Timothy Callison; Banner, Jay L.; Breecker, Dan O.; Musgrove, MaryLynnUnderstanding how future atmospheric warming may affect moisture distribution in the American Southwest is becoming increasingly important. To this end, various paleoclimatic proxies have been used to investigate the range of climatic fluctuations forced by natural processes during past periods of warming. As part of this effort, speleothem-based oxygen isotope records have been used to hypothesize the contribution of either Gulf of Mexico (GoM) or Pacific Ocean sourced moisture since the Last Glacial Maximum (LGM) at cave sites across the Southwest. In this study, a new oxygen isotope time series is developed from a flowstone in central Texas as a means to test this hypothesis. Due to significant age reversals in the U-series chronology, multiple age models were created that use a series of assumptions to exclude questionable age determinations. By comparing these age models, portions of the flowstone that are most likely to produce robust chronologies were identified. The rate, timing, and amplitude of oxygen isotope shifts in all of the age models are consistent with the only other Texas speleothem and one of the few GoM sediment cores that cover the same time period and have an oxygen isotope record. These results support the hypothesis that central Texas had a dominant GoM moisture source since the LGM and that speleothems in the region might record this signal. Furthermore, this study developed a novel workflow that could be applied to other flowstone samples, which would allow for greater spatial coverage of speleothem-based paleoclimate reconstructions when high-quality stalagmite samples are not available.Item New insights into the carbon isotope composition of speleothem calcite : an assessment from surface to subsurface(2012-05) Meyer, Kyle William; Banner, Jay L.; Breecker, Dan O.; Musgrove, MaryLynnThe purpose of this study was to provide new insights into the interpretation of speleothem (cave calcite deposit) δ13C values. We studied two caves in central Texas, which have been actively monitored for over 12 years. We compared δ13C values of soil CO2 (δ13Cs), cave drip water (δ13CDIC), and modern cave calcite (δ13Ccc). Measured average δ13C values of soil CO2 were -13.9 ± 1.4‰ under mixed, shallowly-rooted C3-C4 grasses and were -18.3 ± 0.7‰ under deeply-rooted ashe juniper trees (C3). The δ13CDIC value of minimally-degassed drip water in Natural Bridge Caverns was -10.7 ± 0.3‰. The carbon isotope composition of CO2 in equilibrium with this measured drip water is -18.1 ± 0.3‰. The agreement between juniper soil CO2 and drip water (within ~0.2‰) suggests that the δ13C value of drip water (δ13CDIC) that initially enters the cave is controlled by deeply-rooted plants and may be minimally influenced by host-rock dissolution and/or prior calcite precipitation (PCP). At Inner Space Caverns, δ13CDIC values varied with vegetation above the drip site, distance from the cave entrance, and distance along in-cave flow paths. Whereas CO2 derived from deeply-rooted plants defines the baseline for drip water δ13CDIC entering the caves, kinetic effects associated with the degassing of CO2 and simultaneous precipitation of calcite account for seasonal variability in δ13CDIC and δ13Ccc. We documented increases in δ13CDIC at a rate of up to 0.47‰/hour during the season of peak degassing (winter), suggesting that δ13CDIC variations may be controlled by total elapsed time of CO2 degassing from drip water (Ttotal). We also observed seasonal shifts in the δ13C values of modern calcite grown on glass substrates that are correlated with shifts in drip water δ13CDIC values and drip-rate. Therefore, we suggest that increased aridity at the surface above a given cave results in, slower drip-rates, higher Ttotal, and therefore higher δ13CDIC values. We propose that large variability (>2‰) in speleothem δ13Ccc values dominantly reflect major vegetation changes, and/or increasing Ttotal by slowing drip-rates. Based on these findings, variability in speleothem carbon isotope records may serve as a proxy for paleoaridity and/or paleovegetation change.Item Physical and geochemical response in cave drip waters to recent drought, central Texas, USA : implications for drought reconstruction using speleothems(2015-08) Hulewicz, Michelle; Banner, Jay L.; Breecker, Dan; Musgrove, MaryLynnA five-year study (2009-2014) of eight drip sites in Inner Space Cavern (IS), a cave on the Edwards Plateau in central Texas, was undertaken to assess the physical and geochemical response of cave drip waters to extreme drought. Drip rate, calcite growth rate, and dripwater geochemistry were monitored before, during, and after the peak of a record-breaking drought in central Texas that began and peaked in 2011, and which continued through to early 2015. Three groups of drip sites are identified based on average drip rate (slow sites, 0.2 -- 0.4 mL/min; intermediate sites, 1.2 -- 4.4 mL/min; fast sites, 6.7 -- 18 mL/min) and similarities in geochemical variation. Drip rates of slow sites have the lowest rate and magnitude of response to changes in hydrological conditions, while fast sites have the largest rate and magnitude of response. In contrast, the geochemical response to drought of the three groups does not correspond to the drip rate response. Slow and fast sites exhibit limited geochemical responses to changes in hydrologic conditions, including dripwater Mg/Ca, Sr/Ca, and Ba/Ca ratios, Sr isotope values, Ca concentrations, and oxygen isotope values. This lack of response indicates limited water-rock interaction (WRI) and/or prior calcite precipitation (PCP) affects these dripwaters. Intermediate drip sites exhibit the greatest geochemical response to changes in hydrologic conditions, including extreme drought, expressed by a decrease in Sr isotope values and an increase in Mg/Ca ratios during drier periods. Quantitative modeling indicates that both WRI and PCP can account for trace-element and Sr isotope variations at intermediate sites. The peak of the drought in 2011 coincides with high cave-air CO₂ and slow calcite growth rates, yet PCP may be an important process at two intermediate drip sites during drought. Geochemistry of intermediate drip waters at IS is likely controlled by water supplied by conduit and matrix flow and may provide the preferred speleothem record for reconstructing past droughts in central Texas using trace-elements ratios. Flow-route characteristics of drip sites at other caves that may be expected to show drought response in terms of trace elements include drip rate response to changes in moisture conditions but relatively low drip rate coefficient of variation and sub-equal matrix- and conduit-flow contributions. The monitoring of key geochemical and physical parameters at a range of sites in a given cave may allow for the identification of speleothems that are most likely to be geochemically responsive to changes in climate, making the speleothem sampling process more informed and less destructive.Item Trace element incorporation in modern speleothem calcite and implications for paleoclimate reconstruction(2014-12) Hatch, Rosemary; Banner, Jay L.Trace element compositions, expressed as ratios relative to Ca (Mg/Ca, Sr/Ca and Ba/Ca), in drip water and modern speleothem calcite were measured at multiple sites within a single cave system (Natural Bridge Caverns) in central Texas. These measurements are used to investigate how water and calcite compositions respond to changes in climate. Drip water trace element ratios respond to changes in climate and in soil, vadose zone and/or in-cave processes, which are in turn influenced by climate. It is commonly assumed that speleothem calcite directly reflects these changes in the drip water composition. To test this assumption, this study quantifies the partitioning of trace elements into speleothem calcite in a natural cave setting. To determine the controls on calcite trace element ratios, empirical partition coefficients (K [subscript D]) for Mg, Sr, and Ba are measured using a unique time series of water and modern calcite geochemistry at two drip sites. One drip site, dominantly supplied by conduit flow, has relatively invariant calcite trace element compositions that reflect correspondingly small variations in drip water chemistry. A second drip site, supplied by a combination of conduit and diffuse flow, exhibits a seasonal change in drip water composition due to changes in cave-air CO₂ concentrations. The drip water seasonality at this site is recorded in the calcite trace element compositions; however the partitioning of Mg/Ca from drip water to calcite is not controlled by the same mechanism(s) that control Sr/Ca and Ba/Ca partitioning. Results of this study indicate that the partitioning of Mg changes with drip water Mg concentration, temperature and location of the calcite relative to the point of drip impact. Calcite Sr/Ca and Ba/Ca ratios are more strongly influenced by changes in cave-air CO₂ that cause changes in CO₂ degassing, affecting calcite precipitation. This element specific partitioning complicates the interpretation of speleothem trace element records, since calcite Mg/Ca and Sr/Ca may not always covary even at drip sites that experience PCP. Although there is a strong correlation between Mg/Ca and Sr/Ca ratios in the drip water at these sites, there is considerable scatter between these two values in calcite. Average K [subscript D] values at both sites fall within the range of previous theoretical and empirical studies and are 0.025, 0.12 and 0.15 for Mg, Sr and Ba respectively. It is important to note that not all trace element partitioning is controlled by the same mechanism, since this has implications for interpretations of hydrologic processes from speleothems.