Browsing by Subject "Isotope geochemistry"
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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 Constraints from mantle xenoliths on the geodynamic evolution of Earth’s upper mantle(2014-05) Byerly, Benjamin Lee; Lassiter, John C.; Gardner, James E; Grand, Stephen P; Rowe, Michael C; Zhao, DonggaoGeophysical studies identify a region of slow seismic velocity mantle beneath the central Rio Grande rift that potentially represents a region of hot asthenospheric mantle that has replaced destabilized lithosphere. We determine that the majority of mantle xenoliths from Elephant Butte on the central Rio Grande rift axis are, based on their geochemical affinity to depleted mantle, derived from asthenospheric mantle that has accreted to the base of the Proterozoic lithosphere. Using mantle heat flow models, we estimate the boundary between residual lithosphere and accreted asthenosphere to be at ~45km depth. The amount of lithosphere thinning that has occurred cannot be accounted for by rift-related extension and we therefore suggest that convective removal of a large portion of Proterozoic lithosphere has occurred. Convecting upper mantle-derived peridotites display extreme isotopic depletions that are not observed in mid-ocean ridge basalts (MORB). Previous studies suggest that these isotopically ultradepleted domains represent rare refractory mantle domains that do not participate in MORB petrogenesis. We demonstrate the isotopically ultradepleted domains are not only a ubiquitous feature of convecting upper mantle, but are also capable of melting if advected beneath mid-ocean ridges. To explain the lack of MORB with ultradepleted isotopes, we suggest that MORB compositions are biased towards fertile enriched source components. Estimates of upper mantle composition based on MORB therefore overestimate the fertility of the upper mantle. The Lu-Hf system is commonly used to estimate the timing of melt depletion events in the lithospheric mantle. This is typically done with pseudoisochrons from genetically related mantle xenoliths. Most studies, however, misinterpret Lu/Hf – Hf isotopes correlations by using Cpx Lu/Hf ratios when whole rock Lu/Hf ratios are appropriate due to equilibration above the Lu-Hf closure temperature. Hafnium isotopes do not typically correlate with indicators of melt depletion, which suggests that Hf isotopes do not record ancient melting events. This is likely due to overprinting of Hf isotopes by later metasomatic events.Item Evaluating stable isotope proxies (δ¹³C, δ¹⁸O, and δ¹⁵N) for detecting photosymbiosis in fossil corals(2016-05) Tornabene, Chiara; Martindale, Rowan C.; Bell, Christopher J.; Breeker, Daniel O.; Shanahan, Timothy M.Photosymbiosis is a mutually beneficial relationship that many scleractinian corals have developed with dinoflagellates called zooxanthellae. Photosymbiosis is often considered the evolutionary innovation that allowed corals to become major reef-builders; nevertheless it is difficult to determine whether ancient reef-building corals harbored symbionts because zooxanthellae are not preserved in the fossil record. Two stable isotope measurements were previously proposed as proxies for ancient photosymbiosis: δ18O/δ13C and δ15N. Modern zooxanthellate (Z) and azooxanthellate (AZ) corals can be differentiated by skeletal δ18O and δ13C due to fractionation caused by zooxanthellae photosynthesis (Stanley and Swart, 1995) and by the δ15N of their skeletal organic matrix because nitrogen is influenced by trophic lifestyle (Muscatine et al., 2005). In this study, Modern, Holocene, Oligocene, and Triassic coral skeletons with varying morphologies (Z-like and AZ-like morphologies) were analyzed for δ18O/δ13C and δ15N to test whether these geochemical techniques are applicable to the fossil record. To avoid altered signals due to recrystallization, samples were visually tested for diagenetic alteration through petrography and scanning electron microscopy (SEM). The δ18O and δ13C data displayed enriched isotopic values for both C and O in fossil corals and higher variability in Triassic corals than previously reported suggesting that the proxy is not dependable when used on fossil corals. The δ15N analyses of fossil material failed to reproduce the significant Z vs. AZ offset originally identified by Muscatine et al. (2005). Although AZ-like specimens were specifically selected, all of the samples were collected from shallow-water platforms and it is possible that they were zooxanthellate despite their morphology. It is also possible that all corals shared the same nitrogen sources by living in the same environment thus leading to similar organic matrix δ15N values. Future research on this proxy should focus on understanding the influence of the nitrogen cycle on organic matrix δ15N and analyzing fossil deep-water corals to establish whether the δ15N of fossil coral organic matrix can serve as a reliable proxy for ancient photosymbiosis. The establishment of a successful proxy for photosymbiosis will allow scientists to define the evolutionary relationship between coral-zooxanthellae symbiosis and reef-building.