Throughout the Holocene, a large sediment load coupled with river avulsions, tectonic activity and eustasy developed the complex stratigraphy of the Ganges-Brahmaputra-Meghna delta. Understanding this development is useful in predicting how the delta will respond to changes in the environmental setting such as climatic shifts and sea-level rise. The complex stratigraphy is also linked to high concentrations of naturally occurring arsenic in the shallow groundwater aquifers of Bangladesh through its controls on hydrogeology and aquifer biogeochemistry. This research investigates differences in the fluvio-deltaic deposits of the Ganges and Brahmaputra rivers, as well as differences in the tectonic setting across the lower delta plain. We investigate how these overarching controls influence stratigraphic architecture, the resulting aquifer systems, and ultimately arsenic distribution within the lower delta plain. A transect of 48 sediment cores spanning the lower delta plain of Bangladesh was drilled to a depth of 90m. Individual samples were analyzed for lithology, provenance and age to determine the stratigraphic architecture, the source of the sediments and the depositional history of the rivers. Delta stratigraphy was related to arsenic distribution by measuring groundwater arsenic near each borehole and combining this data with previously collected groundwater data. This study provides a better understanding of Holocene delta evolution and river behavior, as well as a more complete understanding of the geologic controls on arsenic and the characteristics of contaminant distribution in fluvial settings as they relate to stratigraphic features.

Riparian vegetation has been recognized as a controlling factor of stream channel morphology, but specific influences on bed topography and planform geometry are yet to be fully clarified. This project explores the influence of riparian vegetation on channel geometry in alluvial streams of different sizes. Field data suggest that the presence of hardwood vegetation modulates channel width, bed topography and planform geometry in low-order streams. In larger channels, rootwads have less influence on planform curvature, but create patchy variations in bed topography that amplify relief of curvature-dominated bedforms. Stream table experiments illustrate that greater values of relief and thalweg forcing are generated by larger riparian obstructions. Experiments regarding the downstream spacing of trees also reveal that specific spacing frequencies can amplify or disrupt bedform creation, suggesting that resonance between alternate-bar dynamics and forest density may generate increased bedform variation.

It is important to understand what triggers silicic eruptions because of the implications for modern-day systems. Here, we use phase equilibria modeling to determine to what extent magmas within the crust are induced to erupt due to external triggers (e.g. earthquakes; new magma injection) and to what extent they naturally evolve to a point where eruption is inevitable. Modelling the effect of fluid exsolution on the increase in system volume demonstrates that rhyolitic magmas can reach internal triggering. We find that internal triggering is effective in magmas that are stored at 10 km or shallower, revealing a window of eruptibility within the Earthâs shallow crust. Optical mineralogy is a skillset typically introduced in a microscope lab supporting lessons in Earth materials in the classroom. Challenges arise because students do not know what to look for when looking down the microscope. Eight videos were developed with accompanying pre-lab questions and hosted online as part of semester-long, undergraduate Earth materials courses at Vanderbilt University and Illinois State University. Vanderbilt students self-report increased confidence and their testing grades in 2015, when videos were implemented, were comparable or improved compared to the 2014 offering with no videos used.

A new phase equilibria âmagma-meterâ determines a number of magmatic storage and crystallization conditions, including pressure, oxygen fugacity (fO2), state of fluid saturation, and maximum temperature for glass-bearing rocks containing the assemblage plagioclase+orthopyroxene+clinopyroxene (plag+opx+cpx). This newly developed magma-meter can better constrain crystallization conditions of shallow, glass-bearing andesites to dacites. The magma-meter utilizes rhyolite-MELTS to determine crystallization conditions utilizing the glass compositions coexisting with the plag+opx+cpx assemblage in natural samples. The magma-meter retrieves crystallization conditions for experiments from the literature. We applied the magma-meter to the plag+opx+cpx-bearing systems: Mt. Ruapehu, in the southern Taupo Volcanic Zone, New Zealand, and the Palmas unit of the Paraná volcanic province, Brazil. The samples from Mt. Ruapehu and Paraná were tested from ~5 MPa to ~400 MPa and from super-liquidus to ~90% crystalline (~1200 ºC to ~700 ºC). Mt. Ruapehu is an active, structurally well-understood volcano; it serves as a methodological testing ground for the magma-meter. Results show a distribution of crystallization pressures for different eruptions, with modes of ~110 MPa and 130 MPa, consistent with field interpretations of different eruptive styles based on juvenile clast textures and previous knowledge of the magma plumbing system. The distribution indicates that the magma batches were stored over a range of pressures. Mt. Ruapehu magmas are water saturated, with fO2 of ÎQFM ~+1. Paraná includes extinct, super-eruption deposits, with preliminary results suggesting shallow (~90 MPa) storage conditions.

Methane (CH&#8324) in the atmosphere accounts for 18% of the climate warming attributed to greenhouse gases. The majority of CH&#8324 emitted is due to natural mechanisms (biogenic CH&#8324), but even the smaller contribution of anthropogenically sourced CH&#8324 (thermogenic CH&#8324) will have a deleterious effect on global temperatures. A portion of this human-derived methane stems from the rapid growth in high volume hydraulic fracturing (HVHF) technologies used to procure natural gas from the subsurface. In order to address growing concerns, we measured the flux and carbon isotope composition of CH&#8324 emitted from the soil into the atmosphere in eastern Tennessee (Morgan Co.). These measurements were made with cavity ring down spectroscopy (CRDS); CRDS permitted the collection of in situ and rapid (1 Hz) measurements of CH&#8324 emissions. We designed our study by making identical sets of measurements at three HVHF wells and four geologically similar corresponding comparison sites, where HVHF activity was absent. The primary objective was to better understand the strength of the connection between HVHF activity and the increasing concentration greenhouse gases in the atmosphere. Through measurements made during two different field sessions (10 Oct 2015 and 29 Feb-01 Mar 2016), we found elevated background concentrations (> 2.0 ppm) of CH&#8324. Through measurements made during two different field sessions (10 Oct 2015 and 29 Feb-01 Mar 2016), we found elevated background concentrations (> 2.0 ppm) of CH&#8324 at the test sites relative to the comparison sites. Furthermore, our data, though not significantly different, showed positive CH&#8324 fluxes (from soil into the atmosphere) at the test sites, whereas CH&#8324 fluxes were generally negative at comparison sites. These results suggest excess CH&#8324 in the soil that may be connected to leakage contributed by HVHF activity. Evidence from stable carbon isotope analysis of the emitted CH&#8324 supports the conclusion of the presence of thermogenic gas at two of the three wells measured. at the test sites relative to the comparison sites. Furthermore, our data, though not significantly different, showed positive CH&#8324 fluxes (from soil into the atmosphere) at the test sites, whereas CHâ´ fluxes were generally negative at comparison sites. These results suggest excess CH&#8324 in the soil that may be connected to leakage contributed by HVHF activity. Evidence from stable carbon isotope analysis of the emitted CH&#8324 supports the conclusion of the presence of thermogenic gas at two of the three wells measured.

Supereruptions are rare but giant and violent volcanic eruptions that have the potential to wreak havoc on life and infrastructure. Two key questions surrounding supereruptions are investigated in this work: (1) What are the timescales over which giant magma bodies accumulate and erupt? Three-dimensional x-ray tomography of quartz-hosted melt inclusions and cathodoluminescence imaging of compositional zoning in quartz crystals are used to assess timescales from melt inclusion faceting and diffusion chronometry, respectively. Results from three eruptions (240 ka Ohakuri-Mamaku, 26.5 ka Oruanui Taupo Volcanic Zone, New Zealand; 760 ka Bishop Tuff California, USA) suggest that large to giant systems accumulate over extremely short timescales (10¹-10³ a) and that quartz growth rates are ~10^-12-10^-12.5 m/s. (2) What is the geometry of supereruptive systems in the Earths crust? Phase-equilibria and amphibole geobarometry are used to investigate the residence depth of the Peach Spring (southwest USA) magma body. Results indicate that this magma body resided at a pressure of ~200-250 MPa. The analysis also shows that rhyolite-MELTS phase-equilibria geobarometry is an excellent method for obtaining pressure information and weeding out altered glass analyses. Finally, these questions and methods are linked together in an effort to assess the longevity and geometry of a single supereruptive system the Oruanui, the most recent supereruption in Earths history. Melt inclusion faceting indicates this crystal-poor high-silica rhyolite magma was short lived in the crust (10¹-10² a). Such short timescales are notably different from those derived from some other methods and likely reflect the unstable condition of crystal-poor, buoyant magma parcels residing in the shallow crust, rather than the development of the broad magmatic system and pre-accumulation priming of the crust. Phase-equilibria geobarometry suggests that the typical model of a single magma body is not appropriate to describe the Oruanui magmatic system. Instead, results suggest the Oruanui was comprised of multiple magma batches that resided at different depths in the crust but were erupted contemporaneously. This result fits well with an increasingly common model for supereruptive systems in which several vertically and/or laterally juxtaposed magma bodies are erupted simultaneously.

Dramatic environmental changes associated with a global cooling trend beginning in the late Miocene, and the onset of glacial-interglacial cycles in the Pleistocene served as a backdrop to the evolutionary radiation of modern bears (family Ursidae). These environmental changes likely prompted changes in food availability, and triggered dietary adaptations that served as motive forces in ursid evolution. Here, I assess correspondence of dental microwear textures of first and second lower molars with diet in extant ursids. I use the resulting baseline data to evaluate the hypothesis that the giant short-faced bear, <i>Arctodus simus</i>, was a bone consumer and hyper-scavenger at Rancho La Brea, California. Significant variation along the tooth row is consistent with functional differentiation, with the second molar serving as a better dietary recorder than the first. Results evince significant variation among species: carnivorous and omnivorous ursids (<i>Ursus maritimus, U. americanus</i>) have significantly higher and more variable complexity (<i>Asfc</i>) than more herbivorous bears (<i>Ailuropoda melanoleuca, Tremarctos ornatus, U. malayanus</i>), and <i>A. melanolueca</i> is differentiated from <i>U. maritimus</i> and <i>U. americanus</i> by significantly higher and more variable anisotropy (<i>epLsar</i>) values. <i>Arctodus simus</i> exhibits wear attributes most comparable to its closest modern relative (<i>T. ornatus</i>) and inconsistent with hard object (e.g., bone) consumption, and the hypothesis that short-faced bears were bone consuming hyper-scavengers across their range. Rather, plant matter was likely an important component of the diet of <i>Ar. simus</i> at Rancho La Brea.

The first interpretation of the stratigraphic record of Holocene river-channel switching between the Brahmaputra-Jamuna and Old Brahmaputra paleovalleys is presented here in the context of Last Glacial Maximum (LGM) river incision and postglacial sea-level rise. Motivated by the historic diversion of the Old Brahmaputra channel into the present-day Brahmaputra-Jamuna course, we have obtained sediment and radiocarbon samples from 41 boreholes along a 120-km transect crossing these two braided-river valleys. The stratigraphy of this transect reveals two adjacent sand-dominated Holocene channel systems, each bounded by remnant, mud-capped Pleistocene stratigraphy. Using lithology and bulk strontium concentration as provenance indicators, we define the geometry and channel-occupation history of each paleovalley. The relatively steep valley margins and low width-to-thickness ratio (W/T: ~240) of the sub-valleys of the Old Brahmaputra indicate that it was filled primarily through vertical channel sand aggradation. Conversely, the gently-sloped valley margins and high width-to-thickness ratio (W/T: ~890) of the Brahmaputra-Jamuna valley suggest that it was filled primarily through lateral accretion and reworking of braidbelt sands. We attribute this disparity in valley geometry and fill processes to the previous occupation history of each valley. The much larger Brahmaputra-Jamuna was the principal lowstand river course, whereas the smaller Old Brahmaputra valley was first occupied during the early post-glacial transgression. We also demonstrate that the river has experienced 3-4 major avulsions since the LGM and occupies the Brahmaputra-Jamuna valley for longer time periods. Together these observations indicate that occupation history and antecedent morphology are important controls on river course mobility and avulsion behavior.

The 18.78 Ma Peach Spring Tuff (PST), the lone supereruption from the northern Colorado River extensional corridor (NCREC), exhibits diverse elemental characteristics, from crystal-rich intracaldera trachyte to crystal-poor high-silica rhyolite. By contrast, it is remarkably uniform isotopically, which suggests unusually thorough mixing during magma chamber assembly. Its isotopic composition requires that the PST magma was composed of ancient Proterozoic crustal and enriched mantle-derived materials, a hybrid origin typical of felsic NCREC magmas. Closed and open system trace element modeling cannot distinguish between derivation by open-system processes involving juvenile mafic magma and Proterozoic crust or by anatexis of igneous crust formed by similar processes during the Mesozoic, but the elemental data provide constrains on the petrogenetic conditions leading to the PST magma. Concentrations of Ba and Sr much lower and concentrations of Zr much higher than other NCREC magmas indicate that the PST underwent extensive feldspar fractionation prior to accessory mineral fractionation, potentially a consequence of relatively high temperatures and low water content. Most NCREC felsic magmas exhibit elemental characteristics indicative of cooler temperatures and higher water contents, similar to extension-related magmas in the Basin and Range province to the north, while PST magma is more akin to the hot, dry magmas erupted on the Snake River Plain. It is likely that the relatively hot and dry conditions inferred for the PST allowed the magma body to convect vigorously, homogenizing itself isotopically, while smaller batches of cooler, wetter magmas were unable to do so, leading to the NCRECs regional isotopic heterogeneity.

Desert vegetation in the Southwestern United States influences sediment transport. Specifically, rainsplash processes create mounds beneath shrubs which regulate the sediment flux on hillslopes. In turn, complex plant community dynamics make it difficult to predict changes in transport rates over climate-change timescales. Southwestern climate predictions suggest changes in precipitation, which affect desert shrub populations and sediment transport rates. We use a coupled, physically based model of shrub population dynamics and sediment transport to understand the implications of arid-region climate change and compare normal precipitation to precipitation scenarios influenced by climate change. Using this model, we track changes in shrub populations and resulting fluctuations in transport rates. In particular, we track changes in the vegetation, measured in terms of biomass, using the validated WBE scaling model, and relate it to sediment transport rates. This allows for biomass calculations from structural measurements defined in the coupled model. Furthermore, simulated values from the model confirm the idea that increasing amounts of biomass, for two analyzed shrub species, indeed lead to decreasing sediment flux values. This correlation opens an opportunity to use biomass as a common currency to describe sediment flux. Variations in shrub morphology between species leads to differences in shrub response to climate change, particularly aridification. This work provides an opportunity for large scale monitoring, and possibly prediction, of changes in shrub populations and associated sediment transport on Southwestern desert hillslopes when considering shrub sensitivity to fluctuations in precipitation.