This two-part study is focused on the evolution of silicic magma in two very different settings. The first part is a study of zircon to understand the generation, storage, and evolution of felsic magmas erupted in historical times in Iceland. Icelandic zircon compositions, which are correlated with proximity to the main axial rift, are distinct from those of continental zircon. Icelandic zircon exhibit generally low U (<200 ppm), low U/Th (<1), low Hf (<10,000 ppm) and uniformly high Ti (>10 ppm). Ti concentrations imply that zircon grew at relatively high temperatures (800-950 °C). U-Th ages demonstrate the range of ages is far less than is common in continental settings, but zircon growth predates eruptions by thousands to tens of thousands of years. These magmas did not evolve by monotonic fractionation, but experienced zircon entrainment and open-system magma recharge. The second part of this study is a modeling project focused on better understanding the conditions that lead to the evolution and destabilization of the Peach Spring Tuff (AZ, CA, NV) in particular, and large (super-eruption) silicic magmas in general. Analyses of pumice and fiamme from intracaldera and outflow exposures reveal systematic heterogeneity in the underlying chamber at the time of eruption. Whole-rock compositions and modeling results are compatible with differentiation in a shallow magma reservoir at ~200-300 MPa. Destabilization of this large chamber appears to be strongly related to the timing of water saturation relative to other phases and to the pseudo-invariant, both of which are correlated with pressure and water content. Changes in total magma volume and density are modest for systems with high initial water contents (7 wt. %) and significant for systems with low initial water (2-3 wt. %). In the latter case, crystallization and bubble exsolution likely promote magma destabilization and eruption.

The Peach Spring Tuff (PST), a large Miocene ignimbrite located in the southwestern USA, is unique in its abundance of U, Th, and REE concentrating accessory minerals (zircon, sphene, allanite, chevkinite). Textures and compositions of these accessory minerals and magnetite were studied by a variety of methods. Samples of pumice clasts and fiamme were collected from locations in the outflow sheet and intracaldera deposits. Samples range in composition and crystal content, from relatively crystal-poor rhyolites to crystal-rich trachytes, with intracaldera fiamme on the more mafic end of this spectrum. REE trends in zircon and sphene grains reveal a simple fractionation history in rhyolites, but MREE enrichment in sphene edges in trachytes suggest final growth from a less evolved melt. Ti-in-zircon and Zr-in-sphene thermometry reveals lower temperature growth at edges of grains from rhyolites, while edges from trachytes record warmer temperatures. Trace element variations and temperatures also suggest that zircon has a more protracted history of growth than other accessory phases. Textures corroborate the geochemical results. Phenocrysts in rhyolites tend to be euhedral, while those from intracaldera trachytes display resorption features. Sphene size distributions of accessory minerals are kinked, suggestive of growth during decompression. These results suggest that the PST was a zoned system affected by a late-stage heating event, which may have triggered eruption, followed by eruptive decompression. One outflow pumice clast of trachyte composition follows compositional trends of intracaldera trachytes and textural trends of outflow rhyolites, suggesting that these events affected different regions of the chamber to different extents.

A multiple-proxy study for borehole sediments from the Bengal Basin reveals stratigraphic, paleoenvironmental, and provenance data which suggest repeated event deposition, river avulsion and abandonment, and shifting provenance sources to the Ganges-Brahmaputra (G-B) margin during Late Quaternary delta evolution. Results indicate that the G-B delta system is not simply an amalgam of homogenous sediments, but, rather, is comprised of stacked units with distinct lithologies deposited from at least three major provenance sources. For the first time, trends in down-core strontium concentration are used to distinguish Brahmaputra, Ganges, and Eastern Himalayan provenance signatures within delta sediments. Major controls on delta development are reflected within two sediment cores, one dominated by fluvial processes and one by coastal marine processes. Results from this study introduce new, non-trivial mechanisms of G-B delta evolution including: 1) the significance of Eastern Himalayan tributaries to delta development, 2) direct borehole evidence for the apparent shut-off of Brahmaputra sediment flux to the margin, and 3) the deposition of discrete, anomalously coarse-grained sand and gravel pulses interbedded within typical delta sediments. This study suggests the filling, and subsequent draining, of previously documented glacially dammed lakes in the Himalaya may be the principle mechanism for temporary shut-off Brahmaputra sediment input to the margin and the deposition of anomalously coarse grained deposits within fluvial and coastal-dominated environments.

The Ganges and Brahmaputra River, which drain the crystalline rocks of the High Himalayas, Lesser Himalayas and Tibetan Sedimentary Series, have transported the major portion of late Quaternary sediments to the Ganges - Brahmaputra (G-B) delta, Bangladesh. These major lithologic groups also have distinctive Sr concentrations and isotopic ratios that are used in this study to understand the late Quaternary river avulsion and abandonment, and shifting provenance sources for the G-B delta. Sr concentrations and isotopic ratios of the Pleistocene sediment collected from 12 boreholes across the G-B delta indicate transport by the Brahmaputra River alone mixing sediment from the Tibetan Plateau and Higher Himalaya. In contrast, the Holocene sediment from the same boreholes show that the western delta contains Ganges input alone from the Higher Himalaya; the central delta is a mixing zone between the Ganges and Brahmaputra sediment and in the eastern delta, Holocene sediments were deposited only by the Brahmaputra. The Sr concentrations and isotopic ratios of these borehole sediments at different depths from all over the G-B delta supports a model where the Brahmaputra was the principal source of sediment deposited in the delta during the Late Pleistocene. Decreasing Sr concentrations in the Early Holocene sediment in the western delta suggest that the Ganges River avulsed from the West Bengal of India into the western G-B delta in Bangladesh. The avulsion of the Ganges is associated with a similar shift of the Brahmaputra to the east, possibly indicating a basin wide reorganization of the river channels.

The spatial and temporal variability of water chemistry complicates the process of observing and identifying changes to water quality, particularly in catchments where data are spare. In this study, the effect of coal mining on water quality was assessed for the New River and upland, sub-catchment Indian Fork, in eastern Tennessee. Through the use of fragmented water quality and discharge records, hydrochemical trends were identified and compared between catchments and across time. Concentration-discharge relationships were characterized and recursive time-series modeling was completed to quantify long and short term hydrochemical trends. The inability of the New River 2007 time-series model to accurately perform under 1980 low-flow conditions suggests the basin wide shift in reclamation practices during the 1980s affected hydrologic pathways. Little temporal change was anticipated for the Indian Fork as mining ceased prior to available data, however hydrochemistry shifted from chemostatic to dilution response, likely due to spatial heterogeneity of the stream. The hydrochemical spatial disparity between the catchments is likely attributable to the percent of mining disturbed area within the New River (~7%) and Indian Fork (23%). Alternating c-Q loop rotation in the New River was found to be dependent on peak flows above (CW) or below (CCW) 25 m3/s discharge. Due to the magnitude of disturbance, the Indian Fork is conceptualized as a two component system: (1) impacted (i.e. spoil water) and (2) non-impacted water. The New River is conceptualized as a three component system: (1) impacted water, (2) non-impacted water, and (3) event water.

This study aims at understanding the processes by which silicic magmas are generated, how silicic magmatic systems evolve and how they are terminated, and the link between magmatic processes at depth and eruptive centers on the Earths surface. We present evidence for volcano-pluton connections from two perspectives: the evolution of a plutonic silicic system, as recorded in the mineral zircon, at the Austurhorn Intrusive Complex (AIC), SE Iceland, and the death throes of a silicic magmatic system, as indicated by field relationships and geochemistry, in the Highland Range Silicic Volcanic Sequence (HRSVS), S Nevada. The two systems preserve a striking record of the complex interactions that take place between mafic and felsic magmas both in an intrusive (AIC) and an extrusive (HRSVS) setting, and we emphasize that both are part of greater systems that preserve intrusive as well as extrusive components. From our results, we conclude that zircon at Austurhorn records a long and complex magmatic history characterized by multiple episodes of mafic magma recharge into a dominantly felsic magma reservoir. Their elemental compositions form a coherent low-Hf and low-U/Yb array, consistent with fractional crystallization, that falls within the field established by Carley et al (2010) for silicic rocks from other tectonic settings. We establish a U-Pb intrusion age for the AIC at 6.45±0.04 Ma, and a major mafic recharge event at 5.99±0.6 Ma. At the Highland Range, we interpret the capping mingled lava (CML), rhyolite porphyries, and andesite dike and lavas to be derived from nearby Searchlight Pluton. Our results suggest an origin of the CML by mingling of porphyry-like rhyolite and andesite magmas, and the CML is the last silicic unit exposed in the HRSVS, thus marking the death of the HR-SLP silicic magmatic system.

The mineral zircon (ZrSiO4) is a particularly robust geochronometer and arguably one of the most important minerals available to study the evolution of the Earths crust. Despite a high preservation potential, zircon grains extracted from high-grade metamorphic rocks associated with mountain building display evidence of alteration, recrystallization, or new-growth, during which new geochronological information is generated. Geochemical and textural evidence suggests these zircon reactions are catalyzed by hydrous fluids generated during the metamorphic evolution of their host rock. Because these metamorphic rocks are witnesses to the most extreme tectonic and metamorphic processes, linking the geochemistry and geochronology of zircon grains to their metamorphic environment remains one of the most vital yet unresolved barriers to understanding crustal geodynamics. This study focuses on unraveling the response of zircon to mountain building using geological samples from the uplifted and exposed roots of the Dabie Mountains in Central East China, and experimentally measuring zircon/fluid element partition coefficients using quartz-saturated hydrothermal fluids. Results suggest that when fluids are present, zircon is susceptible to recrystallization and alteration involving loss of radiogenic Pb and rejection of a non-ideal thorite (ThSiO4) solid solution component. Detailed exploration of zircon geochemical and isotopic characteristics can distinguish specific mechanisms of zircon recrystallization and growth, which can be used to date events during the evolution of mountain belts. The results of our zircon/fluid element partitioning study show zircon to incorporate the Rare Earth Elements (i.e. lanthanides) with an increasing preference with decreasing ionic radii (i.e. Dzircon/fluid La << Dzircon/fluid Lu), similar to zircon/melt element partitioning. Overall, partition coefficients are low, not exceeding ~10 for the most compatible elements (i.e. Yb, Lu, Y, Sc, Th, and U). Ratios of Dzircon/fluid Th/U range from 0.3 to 2.7, suggesting that zircon Th/U ratios of 10-2 to 10-3 for grains from metamorphic rocks result from loss of a thorite component during metamorphism.

Dietary studies of xenarthrans are challenging as geochemical or microscopic examinations utilize enamel, which xenarthrans lack. The relative softness of dentin makes it potentially more prone to taphonomic alteration; thus, we aim to determine if dental microwear texture analysis (DMTA, which quantifies microwear in 3-D) of dentin can yield biologically meaningful results. Here we ask the following questions: i) are there consistent differences between dentin and enamel from teeth that have been subjected to the same food and bite forces (e.g., carnassial teeth); ii) can DMTA differentiate between extant and extinct xenarthrans with different diets (observed and morphologically inferred, respectively); and iii) is dentin microwear texture a reliable and comparable indicator of dietary ecology? DMTA characters of different dental tissues in the carnassials of Florida panthers (Puma concolor coryi) were not significantly different from one another (except textural fill volume and heterogeneity); however, differences between tissues are highly variable. Dentin DMTA characteristic in sloths (Bradypus variegatus and Choloepus hoffmanni) and nine-banded armadillos (Dasypus novemcinctus) lack significant differences, except for greater textural fill volume and complexity in D. novemcinctus as compared to B. variegatus and Pilosa, respectively. All extinct taxa showed significant overlap in complexity and anisotropy, with values often below or above expected values from similarly feeding mammals with enamel teeth. Further, no differences were observed between like taxa over time. The high degree of overlap in most DMTA characters, despite divergent diets and differential grit consumption, questions the use of DMTA of dentin in extant and extinct xenarthrans.

Climate change and sea level rise will continue to stress the human and natural landscapes over the next century with low-lying deltaic regions characterized by extensive aquifer systems being highly vulnerable to these perturbations. Specifically, groundwater and surface water interactions may be adversely affected by the estimated rise (56-200 cm) in sea level through aquifer salinization. The Bengal Basin is one of the most densely populated of these regions with brackish groundwater already an issue in the tidally-dominated reaches near the Bay of Bengal. Previous studies suggest the groundwater system is isolated from the surface water system with the brackish groundwater being of Pleistocene age. Here, we employed a one-dimensional finite-difference model of a conceptualized aquifer system between two tidal channels in an attempt to discern groundwater and surface water connectivity. The model varied aquifer transmissivity (TA), channel transmissivity (TC), aquifer storativity (S), and tidal range (TR) over a realistic range of values. The dominant control on connectivity was found to be hydraulic diffusivity (TA/S and TC/S). The results suggest some degree of hydraulic connectivity between the aquifer and tidal systems is possible under present conditions. These results call for a reevaluation of our understanding of groundwater-surface water dynamics in the region and may influence our mitigation strategies in the face of climate change.

The future of deltas worldwide will be determined by a balance of sedimentation, subsidence and sea-level rise, in addition to human-caused perturbations. The Ganges-Brahmaputra delta is perceived to be at great risk to degradation and submergence under multiple sea-level rise (SLR) scenarios. The lower delta plain in southwest Bangladesh is isolated from fluvial processes, yet still receives up to 1 cm of sediment accretion annually via the tidal channel network. In the pristine Sundarbans mangrove forest sedimentation has kept pace with relative SLR. Adjacent landscapes have experienced locally inhibited sediment delivery caused by embankment construction in the 1960s. In the ensuing five decades the land has continued to subside and compact, and in the absence of sediment accretion has experienced a net elevation loss of more than 1 m. The effects of this elevation offset were felt in 2009 when the embankments of several large islands breached during Cyclone Aila and land remained tidally inundated for two years. Despite sustained human suffering during this time, the newly reconnected landscape rebounded with tens of centimeters of tidally deposited sediment, accounting for decades-worth of normal sedimentation, but only partly restoring the elevation lost over the previous five decades. This study demonstrates that the evolutions of these two adjacent landscapes lie on very different trajectories, a finding that has immediate and profound implications for the human population on Polder 32 and other embanked landscapes in the tidal delta plain.