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- All Subjects: Geochemistry
- All Subjects: Biogeochemistry
- Genre: Doctoral Dissertation
- Creators: Anbar, Ariel D
Variable and positive shifts of 238U/235U were observed during U uptake by primary abiotic and biotic calcium carbonates, carbonate diagenesis, and anoxic deposition of carbonates. Previous CaCO3 coprecipitation experiments demonstrated a small but measurable U isotope fractionation of ~0.10 ‰ during U(VI) incorporation into abiotic calcium carbonates, with 238U preferentially incorporated into the precipitates (Chen et al., 2016). The magnitude of U isotope fractionation depended on aqueous U speciation, which is controlled by water chemistry, including pH, ionic strength, carbonate, and Ca2+ and Mg2+ concentrations. Based on this speciation-dependent isotope fractionation model, the estimated U isotope fractionation in abiotic calcium carbonates induced by secular changes in seawater chemistry through the Phanerozoic was predicted to be 0.11–0.23 ‰. A smaller and variable U isotope fractionation (0–0.09 ‰) was observed in primary biogenic calcium carbonates, which fractionated U isotopes in the same direction as abiotic calcium carbonates. Early diagenesis of modern shallow-water carbonate sediments from the Bahamas shifted δ238U values to be 0.270.14 ‰ (1 SD) higher than contemporaneous seawater. Also, carbonate sediments deposited under anoxic conditions in a redox-stratified lake—Fayetteville Green Lake, New York, USA— exhibited elevated δ238U values by 0.160.12 ‰ (1 SD) relative to surface water carbonates with significant enrichments in U.
The significant U isotope fractionation observed in these studies suggests the need to correct for the U isotopic offset between carbonate sediments and coeval seawater when using δ238U variations in ancient carbonate rocks to reconstruct changes in ocean anoxia. The U isotope fractionation in abiotic and biogenic primary carbonate precipitates, during carbonate diagenesis, and under anoxic depositional environments provide a preliminary guideline to calibrate 238U/235U in sedimentary carbonate rocks as a paleoredox proxy.
system missions seek environments providing liquid water, energy, and nutrients, the three ingredients necessary to sustain life.
Such environments include hydrothermal systems, spatially-confined systems where hot aqueous fluid circulates through rock by convection. I sought to characterize hydrothermal microbial communities, collected in hot spring sediments and mats at Yellowstone National Park, USA, by measuring their bulk elemental composition. To do so, one must minimize the contribution of non-biological material to the samples analyzed. I demonstrate that this can be achieved using a separation method that takes advantage of the density contrast between cells and sediment and preserves cellular elemental contents. Using this method, I show that in spite of the tremendous physical, chemical, and taxonomic diversity of Yellowstone hot springs, the composition of microorganisms there is surprisingly ordinary. This suggests the existence of a stoichiometric envelope common to all life as we know it. Thus, future planetary investigations could use elemental fingerprints to assess the astrobiological potential of hydrothermal settings beyond Earth.
Indeed, hydrothermal activity may be widespread in the solar system. Most solar system worlds larger than 200 km in radius are dwarf planets, likely composed of an icy, cometary mantle surrounding a rocky, chondritic core. I enhance a dwarf planet evolution code, including the effects of core fracturing and hydrothermal circulation, to demonstrate that dwarf planets likely have undergone extensive water-rock interaction. This supports observations of aqueous products on their surfaces. I simulate the alteration of chondritic rock by pure water or cometary fluid to show that aqueous alteration feeds back on geophysical evolution: it modifies the fluid antifreeze content, affecting its persistence over geological timescales; and the distribution of radionuclides, whose decay is a chief heat source on dwarf planets. Interaction products can be observed if transported to the surface. I simulate numerically how cryovolcanic transport is enabled by primordial and hydrothermal volatile exsolution. Cryovolcanism seems plausible on dwarf planets in light of images recently returned by spacecrafts. Thus, these coupled geophysical-geochemical models provide a comprehensive picture of dwarf planet evolution, processes, and habitability.
Humans have dramatically increased phosphorus (P) availability in terrestrial and aquatic ecosystems. As P is often a limiting nutrient of primary production, changes in its availability can have dramatic effects on ecosystem processes. I examined the effects of calcium carbonate (CaCO3) deposition, which can lower P concentrations via coprecipitation of phosphate, on P availability in two systems: streams in the Huachuca Mountains, Arizona, and a stream, Río Mesquites, in Cuatro Ciénegas, México. Calcium carbonate forms as travertine in the former and within the microbialites of the latter. Despite these differences, CaCO3 deposition led to lowered P availability in both systems. By analyzing a three-year dataset of water chemistry from the Huachuca Mountain streams, I determined that P concentrations were negatively related to CaCO3 deposition rates. I also discovered that CaCO3 was positively correlated with nitrogen concentrations, suggesting that the stoichiometric effect of CaCO3 deposition on nutrient availability is due not only to coprecipitation of phosphate, but also to P-related constraints on biotic nitrogen uptake. Building from these observations, bioassays of nutrient limitation of periphyton growth suggest that P limitation is more prevalent in streams with active CaCO3 deposition than those without. Furthermore, when I experimentally reduced rates of CaCO3 deposition within one of the streams by partial light-exclusion, areal P uptake lengths decreased, periphyton P content and growth increased, and periphyton nutrient limitation by P decreased. In Río Mesquites, CaCO3 deposition was also associated with P limitation of microbial growth. There, I investigated the consequences of reductions in CaCO3 deposition with several methods. Calcium removal led to increased concentrations of P in the microbial biomass while light reductions decreased microbial biomass and chemical inhibition had no effect. These results suggest that CaCO3 deposition in microbialites does limit biological uptake of P, that photoautotrophs play an important role in nutrient acquisition, and, combined with other experimental observations, that sulfate reduction may support CaCO3 deposition in the microbialite communities of Río Mesquites. Overall, my results suggest that the effects of CaCO3 deposition on P availability are general and this process should be considered when managing nutrient flows across aquatic ecosystems.
In order to better understand organic transformations in natural systems, the reactivities of oxygen- and nitrogen-bearing organic functional groups were investigated under experimental hydrothermal conditions, at 250°C and 40 bar. The model compounds benzylamine and α-methylbenzylamine were used as analogs to environmentally relevant amines, ultimately elucidating two dominant deamination mechanisms for benzylamine, SN1 and SN2, and a single SN1 mechanism for deamination of α-methylbenzylamine. The presence of unimolecular and bimolecular mechanisms has implications for temperature dependent kinetics, indicating that Arrhenius rate extrapolation is currently unreliable for deamination.
Hydrothermal experiments with benzyl alcohol, benzylamine, dibenzylamine, or tribenzylamine as the starting material indicate that substitution reactions between these compounds (and others) are reversible and approach metastable equilibrium after 72 hours. These findings suggest that relative ratios of organic compounds capable of substitution reactions could be targeted as tracers of inaccessible geochemical conditions.
Metastable equilibria for organic reactions were investigated in a natural low-temperature serpentinizing continental system. Serpentinization is a water-rock reaction which generates hyperalkaline, reducing conditions. Thermodynamic calculations were performed for reactions between dissolved inorganic carbon and hydrogen to produce methane, formate, and acetate. Quantifying conditions that satisfy equilibrium for these reactions allows subsurface conditions to be predicted. These calculations also lead to hypotheses regarding active microbial processes during serpentinization.
Increasingly heavier 98Mo are found in shales deposited during the Neoarchean (2,800 to 2,500 million years ago, or Ma), which would be a predicted consequence of progressive ocean oxygenation across this timeframe. Increasingly heavier 205Tl across a well-documented Mesozoic Oceanic Anoxic Event (~94 Ma), on the other hand, would be a predicted consequence of progressive ocean de-oxygenation. An anti-correlation in the first combined application of Mo and Tl isotopes in ancient shales provides a strong fingerprint for previously unrecognized levels of ocean oxygenation at ~2,500 Ma. Lastly, neither 98Mo or 205Tl behave as predicted in shales deposited during three Ediacaran Ocean Oxygenation Events (~635 Ma, ~580 Ma, and ~560 Ma). These unexpected trends are due, at least in part, to local-scale overprints that must be taken into consideration when pairing together Mo and Tl isotopes in shales.
The ability of the Mo and Tl isotope systems to track changes in past ocean oxygenation is confirmed in this dissertation. Both isotope systems have the potential to track these changes independently, but their combined utility is particularly powerful. Under ideal conditions, their combined application can provide an even more robust fingerprint for changes in past ocean oxygenation. Even under non-ideal conditions, their combined application makes it possible to decipher local-scale overprints from signals of past ocean oxygenation. It is therefore ideal, whenever possible, to measure both 98Mo and 205Tl in the same shale samples to assess past changes in ocean oxygenation.
Modelling Geochemical and Geobiological Consequences of Low-Temperature Continental Serpentinization
Around 140 fluids were sampled from the Oman ophiolite and analyzed for their compositions. Fluid compositions can be accounted for by thermodynamic simulations of reactions accompanying incipient to advanced stages of serpentinization, as well as by simulations of mass transport processes such as fluid mixing and mineral leaching. Thermodynamic calculations were also used to predict compositions of end-member fluids representative of the shallow and deep peridotite aquifers that were ultimately used to quantify energy available to various subsurface chemolithotrophs. Calculations showed that sufficient energy and power supply can be available to support deep-seated methanogens. An additional and a more diverse energy supply can be available when surfacing deep-seated fluids mix with shallow groundwater in discharge zones of the subsurface fluid pathway. Finally, the consequence of the evolving continental composition during the Archean for the global supply of H2 generated through komatiite serpentinization was quantified. Results show that the flux of serpentinization-generated H2 could have been a significant sink for O2 during most of the Archean. This O2 sink diminished greatly towards the end of the Archean as komatiites became less common and helped set the stage for the Great Oxidation Event. Overall, this study provides a framework for exploring the origins of fluid and volatile compositions, including their redox state, that can result from various low-temperature serpentinizing environments in the present and past Earth and in other rocky bodies in the solar system.