Browsing by Subject "Stable Isotopes"
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Item Halogen chemistry and stable chlorine isotope composition of thermal springs and arc lavas in the Cascade arc(2013-08) Cullen, Jeffery Todd; Barnes, Jaime DanielleThe stable isotope compositions (chlorine, oxygen, and hydrogen), major anion concentrations, and major/minor cation concentrations of 37 thermal (any spring water with temperature at least 6.5° C above mean ambient air temperature) and mineral springs from the Cascade volcanic arc system were measured in order to better determine chlorine sources within the Cascades hydrothermal systems, and thus place better constraints on halogen flux through the subduction zone. Typically, most subduction zone flux calculations have been limited to the study of the erupted magmas and gases from fumarole vents, yet magmatic discharge through thermal springs may be considerable, particularly those in the often ignored forearc. Additionally, 9 geochemically well characterized lavas from across the Mt. St. Helens/Mt. Adams region of the Cascade arc (Leeman et al. 2001, 2005) were analyzed for their halogen concentrations, as well as their Cl stable isotope composition. Cl concentrations in the thermal springs range from 6 to 13,850 ppm and have δ37Cl values that range from -0.1‰ to + 1.9‰ (average = +0.8 ± 0.4‰; error = ± 0.2‰), with no systematic variation along or across the arc. The slightly positive values (~0.0 to +0.9‰) may be explained by fluid-rock interaction with underlying lithologic units, such as 37Cl-enriched volcanic sequences, and/or serpentinites or oceanic crust of accreted oceanic terranes. Another process possibly contributing to these positive δ37Cl values, particularly those with δ37Cl > 1‰, is magmatic HCl fractionation during degassing generating an enriched 37Cl vapor which mixes with thermal waters. We cannot completely rule out slab-derived altered oceanic crustal chlorine that has degassed into the springs, although most slab Cl is believed to have already been devolatilized from the slab before reaching sub-arc depths corresponding to longitudes where these springs are located at the surface. Lavas from the Columbia transect across the arc exhibit highest Cl concentrations at the volcanic front compared to the forearc and backarc. Br, like Cl, exhibits highest concentrations along the volcanic front. F and I show a progressive decrease in concentration from forearc to backarc which may demonstrate the putative early surge of fluids/fluid mobile element loss early in subduction at relatively shallow depth. δ37Cl values range from -0.1 to +0.8‰ (error = ± 0.2‰) and may reflect a component of assimilation of crustal material, or is derived from an enriched mantle, although we cannot completely rule out some isotopic fractionation and/or slab-derived chlorine.Item Saltwater Incursion into Micro Tidal Wetlands: Case Studies from Matagorda, Texas and Humacao, Puerto Rico(2013-08-27) Colon, Ricardo J.Global climate change threatens the survival of microtidal wetlands by altering fundamental hydrological aspects such as precipitation patterns and tidal exchange. The combination of these stressors results in increased flooding period and soil salinity in coastal wetlands. In this study, we combined the use of detailed hydrological measurements (wetland water level and salinity), LIDAR elevation models, and water stable isotopes tracers (?D, ?18O) to study the balance between freshwater and saltwater inputs on two microtidal wetlands: a saltmarsh in Matagorda, Texas and a freshwater-forested wetland in Humacao, Puerto Rico. In Matagorda, Texas, we described the process of connectivity between different hydrologic units (isolated and connected ponds) within the saltmarsh. Pond connectivity only occurred when water levels in major water bodies adjacent to our study site reached a threshold elevation of 0.39 m. Connections events were correlated to rainfall and? to a lesser extent? wind speed and direction. We conclude that connectivity within the saltmarsh is driven by the combined effect of tidal influence and rainfall inputs, factors that will be altered by sea level rise and climate change-related changes in long term weather patterns. In Humacao, Puerto Rico, we gathered a detailed dataset of changes in salinity and water level in a freshwater forested wetland dominated by the endangered salt intolerant species Pterocarpus officinalis. In addition, we studied tree water use and identified important water sources to the wetland using stable isotope tracers. Firstly, we provide evidence that recent hydrological alterations have effectively transformed the system from mostly freshwater, to a saltwater wedge estuary. Salinity inputs travel via a tidal creek channel that allows the progression of a saltwater wedge to the inland parts of the forest. Our results suggest that inland progression of the saltwater wedge is influenced by amplitude of tidal exchange in the middle portions of the tidal creek and by extended dry periods in the headmost part of the tidal creek. Isotope data showed that surface standing water was influenced by tidal water sources during the dry season, although the spatial extent of this influence was constrained to areas of the forest that had been previously deforested. The isotopic content of groundwater samples taken at increasing distances from the tidal creek revealed that? although surface waters are dominated by freshwater inputs (rainfall and runoff) during the wet season? the influence of tidal water sources at soil depths greater than 60 cm persists throughout the year. Nonetheless, isotopic content of Pterocapus officinalis stem water samples suggest that tree water uptake is constrained to very shallow, unsaturated parts of the soil. We conclude from both case studies that the long term vulnerability of microtidal wetlands to climate change is determined by the interaction of increased annual variability of freshwater inputs along with a steady increase in mean sea levels, and aggravated by extreme climatic events.Item Stable Isotope Characterization and Proxy Records of Hypoxia-Susceptible Waters on the Texas-Louisiana Shelf(2012-02-14) Strauss, JosiahHypoxia, with dissolved oxygen levels < 1.4 ml L-1, is a recurring summer feature of Louisiana shelf bottom waters. Stable isotope characterization (delta^18O and delta D) of surface waters over the hypoxic zone shows a shift of dominant river influence from the Mississippi River during April to the Atchafalaya in July. Carbon isotopes of dissolved inorganic carbon (?13CDIC) in bottom waters reveal the respiration of terrestrial organic carbon (OC) at inshore localities of 10 m depth and the respiration of marine OC at depths equal to and greater than 20 m. delat^18O and delta^13C profiles of Louisiana shelf Conus shells collected in 1972 show no evidence for summer hypoxia. Comparison with modern Conus records reveal a delta^13CDIC reduction during the last four decades associated with intrusion of ^13C-depleted fossil fuel CO2. Summer delta^13C reductions in Texas shelf Pteria shells may imply dissolved oxygen (DO) was reduced by ?0.7 ml L-1, although this may be attributed to influence of Brazos River discharge on shell delta^18O and delta^13C. Foraminifera fauna measured in age-calibrated sediments from the Texas shelf reveal a low oxygen conditions on between 1960 and modern sediments. From 1950 to 1960, fauna indicate oxygenated bottom waters. Contemporaneous increases of foraminifera delta^13Cand delta^18O suggest this event is associated with severe drought (the Little Dust Bowl). The synchronicity of these data suggests a link between Brazos River discharge and shelf hypoxia.Item Tectonic fibrous veins: initiation and evolution. Ouachita Orogen, Arkansas(2009-05-15) Cervantes, PabloVeins are ubiquitous features in deformed rocks. Despite observations on syntectonic veins spanning two centuries, fundamental questions remain unanswered. Their origin as fractures is largely established but it is still not known why these fractures initiate where they do and how the vein evolves once started. We studied veins from the Lower Ordovician Mazarn Formation in the Arkansas? Ouachitas combining textural observations, stable isotopes, fluid inclusions, SEM-based cathodoluminescence and electron back-scattered diffraction to understand the initial stage of vein formation, its later evolution, the role of fluids and their environment of formation. The veins are located at boudin necks and are synchronous with cleavage formation. Texturally, veins are characterized by veinlets (thin veins between 5 and 25 ?m thick) that parallel the vein-host interface and fibers (columns of quartz or calcite) perpendicular to the vein-host interface between 30 and 350 ?m wide. Veinlets are localized fractures filled with quartz. The crystallographic orientation of the precipitated material in veinlets is inherited from host grains at the micron scale and replicated as fibers? lengths grow to centimeters. The vein-forming fluid was cyclically supersaturated yet never very far from saturation. ?18O values of vein quartz and host are within 2? of each other suggesting that the fluid was rock-buffered. Nevertheless, ?18O and ?13C define a ?J? shaped trend. Although it is not possible to date any portion of this curve, the simplest explanation is that the fluid evolved from rock-buffered in a closed system to fluid-dominated in an open system. The range of pressure-temperature conditions of vein formation is between 275 and 385 ?C and 1100 and 3400 bars, from fluid inclusions and quartz-calcite oxygen isotopes thermometry. By examining a vein from tip to middle, we have established a sequence of events from inception to maturity in vein growth. Vein formation starts with folding followed by flattening of resistant sandstone layers which in turn gives rise to boudinage. Boudinage formation allowed for fracture localization along boudin-necks. The vein grew by the repeated addition of veinlets in the neck region. Recrystallization later modified the fibers by obliterating some evidence of the veinlets and moving fiber walls.Item The Use of Stable and Radiocarbon Isotopes as a Method for Delineating Sources of Organic Matter in Anchialine Systems(2013-05-08) Neisch, Julie ASubmerged caves, locally referred to as cenotes, can be found throughout the Yucatan Peninsula of Mexico. These nutrient poor, aphotic ?underground estuaries? lack photosynthetic primary productivity, but are often found underlying high primary productivity areas such as mangroves and tropical forests. Adjacent ecosystems contribute organic carbon to the cave systems via percolation, where it is then utilized by the obligate, cave-dwelling fish and invertebrates. Another potential pathway through which organic carbon can enter the cave food web is through chemosynthesis. Chemoautotrophic sulfur-oxidizing or nitrifying bacteria have been found in the hydrogen sulfide layer or in the sediments of some anchialine caves. Our study utilizes 13C/12C and 15N/14N stable isotopes as well as 14C radiocarbon dating to determine and compare the sources of organic matter entering a coastal anchialine cave (Cenote Aak Kimin) versus an inland cave (Cenote Maya Blue) in the Yucatan Peninsula. Stable isotopes have long been employed in tropic investigations. This study, however, is the first to utilize radiocarbon isotopes in anchialine caves. The use of both stable and radiocarbon isotopes as source indicators provides greater discrimination in systems that contain numerous carbon sources or indistinct trophic levels, particularly to distinguish between chemoautotrophic versus photosynthetically derived carbon. Results indicate that chemosynthetically derived organic carbon contributes substantially to the diet of some crustaceans, such as the stygobitic shrimp Typhlatya, while other species remain dependent on detrital inputs. Depleted ?13C values and aged radiocarbon values (as low as -47.51? and 1840 yrs. for Typhlatya spp.) in comparison to particulate and sediment ?13C values (lowest -32.07? and -28.43?, respectively). A comparison of isotopic values between Cenote Aak Kimin and Cenote Maya Blue suggests that the trophic web of the coastal cave incorporates more photosynthetic or detrital carbon, while the inland cave, with more depleted 13C and 14C values, relies more heavily on chemoautotrophic carbon. Within both systems, however, distinct photosynthetic and chemoautotrophic levels were identified. Water quality parameters, especially dissolved oxygen and pH, support the hypothesis of bacterial activity at the halocline. Anchialine systems in the Yucatan Peninsula are threatened due to increases in tourism, development, and pollution. Quantifying and qualifying the inputs of organic carbon is vital for the management and conservation of the area?s freshwater resources.