Search

Triple oxygen isotope values of silica samples and formation water with tight temperature constraints from the Hellisheiði power plant in southwest Iceland and the natural Puchuldiza siliceous hot springs in northern Chile were used to evaluate potential fractionation effects of biogenic vs. abiogenic samples and silica samples of different crystallinity. Temperature estimates from the Hellisheiði power plant based on silica-water oxygen isotope thermometry are in excellent agreement with samples with measured temperatures, and lower for samples from within the heat exchanger where temperatures can only be estimated. The calculated θ values from this study are in close agreement with the calculated θ values from other studies and indicate a 0.00001 change in θ per degree Celsius at ~100°C. In a real-time silica precipitation experiment at the hot springs in Puchuldiza of northern Chile, silica only precipitated at the air-water interface. Amorphous silica and coexisting waters were collected in active hot springs ranging in temperatures from 63-84°C. In all cases, the calculated temperatures were less than the measured temperatures, ranging from 38-48°C. Recrystallized paleosinter record higher temperatures, 69.5°C and 89°C. All samples appear to be in equilibrium in the triple oxygen isotope system. We interpret all samples as recording the temperatures of their formation. The poorly crystalline modern samples likely precipitated at the air-water interface while the water was cooling, reaching saturation with respect to silica and preserving temperatures that are less than the measured geothermal water temperatures. In contrast, the more crystalline paleosinter samples record the temperature of hydrothermal recrystallization below the air-water interface. Silica re-precipitating from dissolved paleosinter in the presence of acidic fumarolic steam was not in equilibrium with the condensed steam water on the basis of the D'17O values, which was not indicated by the d

CO2 reduction is a pressing problem worldwide, and one obstacle in the electrochemical conversion of CO2 to more useful C1 products such as methanol and formic acid is the kinetic barrier known as overpotential. The overpotential of rearranging the nuclei in CO2 to accommodate electron transfer can be lowered by pre-bending the CO2 molecule in an adduct-mediated complex. The work presented in Part I utilizes a combination of hard and soft donors on pincer-like ligands to effect the bending of CO2 prior to electrochemical reduction. A large majority of the work consists of using Group 4 metallocenes, but Chapter 3 outlines Zn-based compounds that also show promise in the reduction of CO2. A refined methodology has been employed to increase the yield of the ligand 2-di-tert-butylphosphinophenol. Previous literature preparations could only accommodate approximately 1 g of product per batch, but by using the methodology found in this dissertation, the ligand can now be synthesized in large batches (>15 g). 2-diisopropylphosphinothiophenol and 2-diphenyl-phosphinothiophenol were synthesized for the first time by our group, and their utility was explored in relation to Group 4 metallocenes. The work presented in Part II of this dissertation illustrates the usefulness of homogenous electrochemical interactions in space-based applications. New ventures toward reversible electrochemical mirror (REM) technology were explored using silver and copper cations solvated between two transparent conductive films. Silver deposited upon these conductive thin films to produce seamless mirrors, and by controlling silver nanoparticle growth clusters with ramping voltages, thin colored films were also deposited.

The water cycle in the tropics and subtropics exerts a strong influence on Earth's climate. Isotopic ratios in modern water vapor can provide us with insights into low-latitude moisture-transport processes in the modern climate system, recorded in paleoclimate proxy records, and in general circulation models that we rely on to understand how our climate is changing. Advances in satellite and groundbased instruments in the past decade have improved measurements of isotopes in water vapor. In this study, I focus on satellite (chapters 2 and 3) and ground-based (chapters 2 and 4) measurements of isotopes in atmospheric water vapor to evaluate processes responsible for moisture transport in tropical and subtropical South America. Satellite-measured hydrogen isotope ratios (δD), mixing ratios (q), and outgoing longwave radiation (OLR) show that upwind convective intensity controls the seasonal variability in isotopic ratios in tropical Andean water vapor and leads to lower isotopic ratios than predicted by equilibrium isotope fractionation models (i.e. DδD = δDmeasured - δDRayleigh < 0°) (chapter 2). Deep convection in the South American Summer Monsoon domain in austral summer and in the Inter-tropical Convergence Zone in austral winter leads to zones where DδD is negative from the Lifted Condensation Level through the mid- to upper-troposphere and possibly above the Level of Neutral Buoyancy (chapter 3). In subtropical South America, nearly continuous measurements of isotopic ratios, mixing ratios, and deuterium-excess (i.e. d-excess = δD – (8*δ18O)) indicate that condensation under ice supersaturated conditions and mixing with moister air play important roles in controlling moisture transport to the hyperarid Chajnantor Plateau.

Refractory inclusions, calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs), of variable textures and mineralogies in CO3 chondrites have been investigated in detail using the focused ion beam (FIB) sample preparation technique combined with transmission electron microscopy (TEM) in order (1) to characterize the microstructures, compositions and textural relationships of their various constituent phases at the micrometer to nanometer scales and (2) to provide additional constraints on their formational and subsequent thermal histories both in the solar nebula and on their asteroidal parent bodies. In particular, refractory inclusions in CO3 chondrites are very fine-grained so that they are sensitive recorders of both primary formational and secondary alteration processes, which can only be adequately characterized by TEM. This work represents the first microstructural study of AOAs and CAIs from two CO3 chondrites, Allan Hills A77307 and Kainsaz. The microstructural and compositional characteristics of AOAs and CAIs in the pristine CO3.0 chondrite, Allan Hills A77307, provide evidence that various high-temperature condensation and gas-solid reactions occurred in the solar nebula under both equilibrium and disequilibrium conditions. Diverse gas-solid reactions reflect localized differences in the primary mineral assemblages in individual AOAs and CAIs that controlled reaction pathways. Despite large variations in their texture and mineralogy, mineralogically- and compositionally-zoned refractory components in AOAs, spinel-pyroxene CAIs and melilite-rich CAIs may have formed by a similar sequence of high-temperature condensation and gas-solid reactions, as follows; (1) the formation of the cores of the inclusions from primary equilibrium condensates (i.e., perovskite, spinel and melilite), (2) disequilibrium, probably short-lived reactions of the primary core minerals with a nebular gas under highly dynamic conditions, forming diopside with r

I carried out a petrologic and Mn-Cr isotopic study of carbonates in the paired CM1 chondrites, ALH 84049 and ALH 84051, in an effort to understand the origin and chronology of formation of carbonates in the most heavily altered CM chondrites. Dolomite is strongly compositionally zoned (Ca, Mg, Fe, Mn), indicating very heterogeneous formation conditions, yet all carbonate Mn-Cr analyses form individual isochrons. In this study, I also analyzed the Mn-Cr isotope systematics of the CR1 chondrite GRO 95577 and determined that siderite is the youngest secondary mineral yet observed in carbonaceous chondrites. This has implications for the CR parent body as it either was large enough to retain heat for long periods of time or was heated by impact after most aqueous alteration in carbonaceous chondrites had ceased. This study also presents analyses of carbonates in the same samples (ALH 84049 and GRO 95577) in-situ for their oxygen, and, in ALH 84049, carbon isotope composition to constrain aqueous alteration. The results show that multiple generations of carbonates must have occurred in ALH 84049 from a carbon source with either heterogeneous carbon isotopes or with changing carbon isotope compositions from ongoing methane formation. Furthermore, in GRO 95577, the oxygen isotope values suggest that calcite precipitated before siderite if CR chondrite fluids followed a closed system oxygen isotope evolution path similar to CM chondrites.

Mantle degassing is a fundamental process that modifies the chemical composition of the atmosphere, crust, and deep Earth. Elements are returned to the mantle from surface reservoirs through subduction at convergent margins. Melts generated in the mantle carry dissolved volatiles from depth and upon decompression exsolve a fluid or vapour phase. These volatiles drive explosive eruptions, change climate, and form economic metal deposits. Thus, the sources and processes involved in degassing of the mantle and melts are a crucial topic in geochemistry. This dissertation is composed of four chapters, each contributing original and new insights into sources and processes involved in mantle degassing. Chapters 1 and 2 focus on volatiles and the origin of carbon-rich magmatic emissions in the East African Rift, which is perhaps the least understood tectonic system on Earth in terms of mantle degassing. East Africa hosts a diverse array of enigmatic magma compositions, best exemplified by Oldoinyo Lengai volcano in Tanzania, which is the only active carbonatite volcano on Earth and an important alkaline endmember magmatic system. In Chapter 1 (published in Earth and Planetary Sciences in January, 2013) we show that nephelinite melts at Oldoinyo Lengai are the most carbon-rich natural silicate melts known to science. However, we argue that the mantle source for these melts is not unusually rich in carbon. Rather, extreme degrees of fractional crystallization and the high carbon solubility in these alkali-rich melts are responsible for the high observed carbon contents. We also show that the Oldoinyo Lengai magmatic system is rich in water, contrary to prior assumption, and that water degassing plays a fundamental role in the eruptive behaviour and magmatic evolution at Oldoinyo Lengai. The appendix to Chapter 1 contains additional data (including carbon, oxygen ,and sulfur isotope compositions and assessment of sulfur behaviour in the magma system) from Oldoinyo Lengai that

The chlorine isotope composition of chondrites provides information about isotopic reservoirs that were present in the solar nebula and the effect of secondary processes on the isotopic composition. We analyzed twenty chondrites that included both unaltered (type 3) and altered (types 1-2, 4-6) chondrites from the carbonaceous, ordinary, and enstatite classes. Petrologic type 3 chondrites are the least equilibrated and are those most likely to retain primary nebular signatures. By analyzing unaltered, pristine, chondrites from each class, we can determine if chlorine isotope variations existed among the classes and thus if multiple isotopic reservoirs existed. Since chlorine behaves as a hydrophile and volatile element, analyses of aqueously altered (types 1-2) and metamorphosed (types 4-6) chondrites provide insight into effects of secondary processes on the chlorine isotope composition. Type 3 chondrites have similar δ37Cl values that average -0.3± 0.4°. This value is indistinguishable from the δ37Cl values of the bulk Earth, lowest measured value of the Moon, and the average of martian meteorite samples from the literature. From this similarity, we conclude that the inner regions of the solar nebula had a homogeneous chlorine isotope reservoir. For samples that have undergone secondary processing, we observe slight variations from -2.1 to 1.0° for metamorphosed samples (average -0.3°) and -2.0 to 2.7° for aqueously altered samples (average 0.7°). We conclude that these processes were not significant in fractionating the chlorine isotopes.

Soils in the arid to semi-arid piedmont landscapes of central New Mexico were examined in order to evaluate surficial processes and landscape evolution in the central Rio Grande Rift. These soils were found to contain key information regarding pedologic and geomorphic responses to climate change, rifting and base level drop. A rare association of sepiolite and palygorskite occurs in old soils formed in eolian fines. Pedologic and mineralogical data indicate that palygorskite formed by transformation of dust-borne, aluminum-rich minerals, and that sepiolite subsequently formed by transformation of palygorskite. The association of these two minerals, and geomorphic observations, confirm the existence of a 1 – 3.5 Ma abandoned geomorphic surface of the ancestral Rio Grande. In contrast to Mormon Mesa, Nevada, and Ogallala Formation paleosols, massive cementation by pedogenic carbonate, and chaotic pedogenic carbonate pendants are notably absent in soils formed on the 1 – 3.5 Ma surface. It is hypothesized that abandonment of geomorphic surfaces, largely due to down-cutting by the ancestral Rio Grande, and slow accumulation of fine eolian sediments resulted in the absence of well-developed petrocalcic soil properties usually observed in soils of this age. Other geomorphic observations confirm the hypothesis of an ancestral Palo Duro drainage in the McKensie Flats Basin. In contrast to monogenetic soils, only one polygenetic soil demonstrates stable carbon isotope depth trends similar to the diffusion model of Cerling et al. (1989). Low delta 13C carbonate values in the near-surface reflect the introduction of exogenous material. Low delta 13C carbonate values at >50 cm of depth reflect buried rooting zones of plant assemblages that occurred in the past. An association between buried soil boundaries and shifts in delta 13C carbonate depth trends indicates that changes in C3 and C4 plant assemblages have occurred in the past. These shifts may reflect paleo-environmenta

Understanding the constituents of fine-grained nebular dust prior to asteroidal accretion is a significant challenge in meteoritics. The key to understanding nebular dust lies in chondritic meteorites, which are composed of undifferentiated solar system materials. Chondritic matrices are a subnanometer mixture of amorphous material, silicate grains, sulfides, and other phases, that act as the "glue" for larger components. In the pristine chondrites (petrologic type 3.00) the matrix preserves some record of the primary characteristics of nebular dust. Two CR carbonaceous chondrites, MET 00426 (CR3.00) and QUE 99177 (CR3.00), are likely the most pristine meteorites ever described (Abreu and Brearley, 2010). The mineralogy of crystalline silicate grains in the matrices of these two chondrites has been studied here using focused ion beam (FIB) and transmission electron microscopy (TEM) techniques. Dark-field scanning transmission electron microscopy (STEM) imaging, along with bright-field TEM and energy dispersive X-Ray spectroscopy (EDS) techniques were used to understand mineralogical and chemical characteristics. FIB sections consist of extremely fine-grained, amorphous material (<50 nm in size). Additionally, each section contains crystalline or nanocrystalline silicate grains, varying in size from <100 to 1000 nm in diameter. These grains are MgO-rich or FeO-bearing olivines and pyroxenes. These crystalline silicates exhibit unequilibrated major and minor element compositions, but distinct compositional groupings of olivines and pyroxenes can be identified. We infer that the crystalline silicate materials in the matrices of these chondrites formed by different mechanisms that include nebular condensation under equilibrium and disequilibrium conditions, followed in some cases by annealing prior to accretion. Our data discount chondrule fragmentation as a significant source of the fine-grained matrix olivines. Although these chondrites have similar matrix mineralogy

Several techniques have been introduced in the last decades for the dehydration and release of O2 from biogenic silica (opal-A) for oxygen-isotope analysis. However, only one silica standard is universally available: a quartz standard (NBS28) distributed by the IAEA, Vienna. Hence, there is a need for biogenic silica working standards. This paper compares the existing methods of oxygen-isotope analyses of opal-A and aims to characterize additional possible working standards to calibrate the δ18O values of biogenic silica. For this purpose, an inter-laboratory comparison was organized. Six potential working standard materials were analysed repeatedly against NBS28 by eight participating laboratories using their specific analytical methods. The materials cover a wide range of δ18O values (+23 to +43‰) and include diatoms (marine, lacustrine), phytoliths and synthetically-produced hydrous silica. To characterize the proposed standards, chemical analyses and imaging by scanning electron microscopy (SEM) were also performed. Despite procedural differences at each laboratory, all methods are in reasonable agreement with a standard deviation (SD) for δ18O values between 0.3‰ and 0.9‰ (1σ). Based on the results, we propose four additional biogenic silica working standards (PS1772-8: 42.8‰; BFC: 29.0‰; MSG60: 37.0‰; G95-25-CL leaves: 36.6‰) for δ18O analyses, available on request through the relevant laboratories.

I completed atmospheric transport modeling studies into distributions of subtropical water vapor, to identify the mechanisms by which humidity varies over distinct temporal scales: inter-annual time-scales associated with the El Nino – Southern Oscillation, over the next century in response to global warming, and over periods of about 2 to 7 days. The latter study on 2 to 7 day variability is a modeling component of a field campaign from October – November, 2008, involving continuous in-situ measurement of water vapor stable isotope compositions at the Mauna Loa Observatory, Hawaii. These studies were conducted from a last saturation perspective, considering humidity and water vapor isotope composition of sub-saturated air to have effectively been set by the conditions at which the air parcels last encountered saturation. During El Nino northern winter, the free troposphere over the subtropical north Pacific is both drier and warmer than during La Nina. For instance, during El Nino (La Nina) northern winter, 57% (49%) of the air at 20˚N and 633 hPa over the north Pacific was last saturated poleward of 20˚N and above 500 hPa. ENSO humidity variability can be explained in terms of changes in the location of last saturation, and not by changes in the temperature field. Modeled specific humidity at the subtropical relative humidity minimum is projected to increase by about 0.2-0.3 g/kg, by the end of this century, in the simulations completed for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Most of this projected increase can be attributed to last saturation within what will be a warmer free troposphere. The increase in specific humidity is partially offset by a poleward and upward shift of baroclinic instability and saturation patterns that effectively yield drier air to the RH minimum via isentropic transport. Two to 7 day variability of nighttime water vapor δD values at the National Oceanic and Atmospheric Administration's Mau

I present oxygen isotope data from modern diatom frustules that were collected from freshwater systems in the Jemez Mountains of northern New Mexico. The frustules display a constant silica-water fractionation over the measured temperature range (5.1 to 37.8°C) regardless of species. This relationship is in close agreement with other published silica-water fractionation factors for laboratory cultured diatom samples; however, it is as much as 8° lower than equilibrium quartz-water fractionations and 3 to 4° lower than observed silica water fractionations in diatomaceous silica collected from sediment traps and sediment cores. Post mortem loss of organic material results in an alteration or 'maturation' of diatom silica in which silica reequilibrates with a silica-water fractionation closer to the equilibrium quartz-water fractionation. Post-mortem alteration of silica d18O values occurs within 0.5 years in diatom frustules from a lacustrine setting in the Valles Caldera, New Mexico resulting in an increase of ~7.4° in the d18O values. The rapid post-mortem alteration of diatom d18O values explains much of the disparate data regarding silica-water fractionation for diatom silica and has profound implications for the use of diatom silica d18O values as a paleoclimate proxy. In the setting of open basin lacustrine environments where bottom temperatures are nearly constant at 4ºC, the maturation effect may facilitate reconstruction of variations in paleowater d18O values without the conflating influence of fluctuating water temperature and variable d18O water values that occur at the surface. Lastly, I present d18O values of diatom silica from a middle-Pleistocene lacustrine core (VC-3) from the Valles Caldera, northern New Mexico, which record considerable (>25°) variability in lake water d18O values during interglacial Marine Isotope Stage 11 (MIS 11) and glacial MIS 12. There is a strong correspondence between the d18O values of diatoms throughout the VC-3 core and m

A detailed electron microprobe (EMP) study was performed on chondrules of various textural types in four petrologic type 3 chondrites: MET 00526 (L3.05), MET 00426 (CR3.0), Kainsaz (CO3.2) and Kakangari (K3). Bulk compositions of twenty chondrules in each meteorite were determined with modal recombination analysis. This study provides a self-consistent dataset that combines chondrule textures with mineralogy and bulk chemical compositions. It allows us to make comparisons between different chondrite groups. In order to interpret the compositional relationship between chondrules and matrix, bulk matrix compositions were obtained as well, using EMP defocused beam analyses. In Chapter 1, we compare the mineralogy and bulk chemistry of chondrules and matrix in MET 00526 (L), MET 00426 (CR) and Kainsaz (CO). These three chondrites represent some of the most pristine material that formed in the solar nebula. Chondrule characteristics and the complementary relationship between the compositions of chondrules and matrix suggest open system behavior during chondrule formation, in the form of evaporation and recondensation of volatile and siderophile elements. While chondrules of the same textural types (e.g., FeO-poor (type I) and FeO-rich (type II) porphyritic chondrules) are present in all three chondrites and show similar characteristic features, there are also significant differences between the chondrite groups. This indicates that they probably formed in different regions of the solar nebula. One significant difference can be found in the Fe-Mn systematics of FeO-rich porphyritic olivine (type IIA) chondrules (Chapter 2). We also recognized that Fe-Mn systematics can be used to identify relict grains in type IIA chondrules. Chapter 3 deals with the chondrite Kakangari, which has been thought of as a very pristine chondrite in previous studies. Our study reveals that it records a complex series of events including reduction, thermal metamorphism, sulfidization and low-te