725 research outputs found
Low-temperature hydrogen production and consumption in partially-hydrated peridotites in Oman: implications for stimulated geological hydrogen production
No full textThe Samail Ophiolite in Oman, the largest exposed body of ultramafic rocks at the Earth’s surface, produces a continuous flux of hydrogen through low-temperature water/rock reactions. In turn, the scale of the subsurface microbial biosphere is sufficient to consume much of this hydrogen, except where H2 is delivered to surface seeps via faults. By integrating data from recent investigations into the alteration history of the peridotites, groundwater dynamics, and the serpentinite-hosted microbial communities, we identify feasible subsurface conditions for a pilot demonstration of stimulated geological hydrogen production. A simple technoeconomic analysis shows that the stimulation methods to be used must increase the rate of net hydrogen production at least 10,000-fold compared to the estimated natural rate to economically produce hydrogen from engineered water/rock reactions in the peridotite formations. It may be possible to meet this challenge within the upper 1–2 km, given the projected availability of reactive Fe(II)-bearing phases and the lower drilling costs associated with shallower operations. Achieving ≥10,000-fold increases in the H2 production rate will require a combination of stimuli. It will likely be necessary to increase the density of fracturing in the reaction volume by at least two orders of magnitude. Then, the H2-production rates must also be increased by another two orders of magnitude by increasing the water/rock ratio and modifying the chemistry of the injected fluids to optimize formation of Fe(III)-bearing secondary phases. These fluid modifications must be designed to simultaneously minimize microbial consumption of H2 within the stimulation volume. In contrast, preserving the high potentials for biological H2 consumption in the shallow groundwaters replete with oxidants such as nitrate, sulfate and dissolved inorganic carbon will reduce the potential for any inadvertent leaks of hydrogen to the atmosphere, where it acts as an indirect greenhouse gas
Molecular evidence for an active microbial methane cycle in subsurface serpentinite-hosted groundwaters in the Samail Ophiolite, Oman
No full textSerpentinization can generate highly reduced fluids replete with hydrogen (H
2) and methane (CH
4), potent reductants capable of driving microbial methanogenesis and methanotrophy, respectively. However, CH
4 in serpentinized waters is thought to be primarily abiogenic, raising key questions about the relative importance of methanogens and methanotrophs in the production and consumption of CH
4 in these systems. Herein, we apply molecular approaches to examine the functional capability and activity of microbial CH
4 cycling in serpentinization-impacted subsurface waters intersecting multiple rock and water types within the Samail Ophiolite of Oman. Abundant 16S rRNA genes and transcripts affiliated with the methanogenic genus Methanobacterium were recovered from the most alkaline (pH, >10), H
2- and CH
4-rich subsurface waters. Additionally, 16S rRNA genes and transcripts associated with the aerobic methanotrophic genus Methylococcus were detected in wells that spanned varied fluid geochemistry. Metagenomic sequencing yielded genes encoding homologs of proteins involved in the hydrogenotrophic pathway of microbial CH
4 production and in microbial CH
4 oxidation. Transcripts of several key genes encoding methanogenesis/methanotrophy enzymes were identified, predominantly in communities from the most hyperalkaline waters. These results indicate active methanogenic and methanotrophic populations in waters with hyperalkaline pH in the Samail Ophiolite, thereby supporting a role for biological CH
4 cycling in aquifers that undergo low-temperature serpentinization. IMPORTANCE Serpentinization of ultramafic rock can generate conditions favorable for microbial methane (CH
4) cycling, including the abiotic production of hydrogen (H
2) and possibly CH
4. Systems of low-temperature serpentinization are geobiological targets due to their potential to harbor microbial life and ubiquity throughout Earth's history. Biomass in fracture waters collected from the Samail Ophiolite of Oman, a system undergoing modern serpentinization, yielded DNA and RNA signatures indicative of active microbial methanogenesis and methanotrophy. Intriguingly, transcripts for proteins involved in methanogenesis were most abundant in the most highly reacted waters that have hyperalkaline pH and elevated concentrations of H
2 and CH
4. These findings suggest active biological methane cycling in serpentinite-hosted aquifers, even under extreme conditions of high pH and carbon limitation. These observations underscore the potential for microbial activity to influence the isotopic composition of CH
4 in these systems, which is information that could help in identifying biosignatures of microbial activity on other planets.
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Reply to “Methane origin in the Samail ophiolite: Comment on ‘Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability’” [Geochim. Cosmochim. Acta 179 (2016) 217–241]
No full textModern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability
No full textThe Samail ophiolite in Oman is undergoing modern hydration and carbonation of peridotite and may host a deep subsurface biosphere. Previous investigations of hyperalkaline fluids in Oman have focused on fluids released at surface seeps, which quickly lose their reducing character and precipitate carbonates upon contact with the O2/CO2-rich atmosphere. In this work, geochemical analysis of rocks and fluids from the subsurface provides new insights into the operative reactions in serpentinizing aquifers. Serpentinite rock and hyperalkaline fluids (pH > 10), which exhibit millimolar concentrations of Ca2+, H2 and CH4, as well as variable sulfate and nitrate, were accessed from wells situated in mantle peridotite near Ibra and studied to investigate their aqueous geochemistry, gas concentrations, isotopic signatures, mineralogy, Fe speciation and microbial community composition.The bulk mineralogy of drill cuttings is dominated by olivine, pyroxene, brucite, serpentine and magnetite. At depth, Fe-bearing brucite is commonly intermixed with serpentine, whereas near the surface, olivine and brucite are lost and increased magnetite and serpentine is detected. Micro-Raman spectroscopy reveals at least two distinct generations of serpentine present in drill cuttings recovered from several depths from two wells. Fe K-edge X-ray absorption near-edge spectroscopy (XANES) analysis of the lizardite shows a strong tetrahedral Fe coordination, suggesting a mixture of both Fe(II) and Fe(III) in the serpentine. Magnetite veins are also closely associated with this second generation serpentine, and 2–10 ?m magnetite grains overprint all minerals in the drill cuttings. Thus we propose that the dissolved H2 that accumulates in the subsurface hyperalkaline fluids was evolved through low temperature oxidation and hydration of relict olivine, as well as destabilization of pre-existing brucite present in the partially serpentinized dunites and harzburgites. In particular, we hypothesize that Fe-bearing brucite is currently reacting with dissolved silica in the aquifer fluids to generate late-stage magnetite, additional serpentine and dissolved H2.Dissolved CH4 in the fluids exhibits the most isotopically heavy carbon in CH4 reported in the literature thus far. The CH4 may have formed through abiotic reduction of dissolved CO2 or through biogenic pathways under extreme carbon limitation. The methane isotopic composition may have also been modified by significant methane oxidation. 16S rRNA sequencing of DNA recovered from filtered hyperalkaline well fluids reveals an abundance of Meiothermus, Thermodesulfovibrionaceae (sulfate-reducers) and Clostridia (fermenters). The fluids also contain candidate phyla OP1 and OD1, as well as Methanobacterium (methanogen) and Methylococcus sp. (methanotroph). The composition of these microbial communities suggests that low-temperature hydrogen and methane generation, coupled with the presence of electron acceptors such as nitrate and sulfate, sustains subsurface microbial life within the Oman ophiolite
The Universal Constraint Set: Convention not Fact
No full textThis paper looks at two different views of the universality of the constraint set in Optimality Theory. The first view is that the universality is a statement of mental reality of speakers of human languages, and so is a real, objective fact of the world. The second view is that the constraint set is a convention, like the phonetic alphabet, which reflects some functionally motivated categories, but is at heart arbitrary. I examine seven arguments for one view or the other, and conclude that the arguments for the objectivist position are weak, while it is hard deny the usefulness of a conventional constraint set.This is a pre-publication version of the of a chapter in the the book 'Optimality theory: phonology, syntax, and acquisition'.ISBN: 0-19-823844-4 (published book
Citizenship
No full textCitizenship as a status concerns who gets what from the terms of membership within a given community. Citizenship as a socio-cultural practice shines light on how and why some are recognized as (worthy) members whilst others are not. Reflecting on this distinction, this chapter starts by briefly outlining T. H. Marshall’s seminal account that has proven influential in shaping, and in many ways constraining, contemporary understandings of citizenship within society and social policy. The chapter considers the contested functions of social citizenship when it comes to capitalism, democracy and inequality. It then problematizes some of the claims underpinning normative and ideological accounts of citizenship. The chapter concludes by discussing the emergence of multiple, shifting citizenships that currently reflect and condition welfare politics. The author argues that the terms of citizenship are being reformulated not just through – but also in revolt against – de-territorialized memberships and ‘flexible’ forms of belonging and entitlement
Low‐Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite
No full textSerpentinized peridotite is reacting with groundwater in the subsurface of the Samail ophiolite in Oman. Although these rocks are partially to completely serpentinized, they interact with a groundwater aquifer containing hyperalkaline fluids rich in H2 and CH4. Since the mechanisms by which H2 production may continue at low temperatures (<50°C) are not fully understood, core recovered during the Oman Drilling Project provides an excellent opportunity to study the mineralogy and Fe speciation in highly serpentinized harzburgite recording multiple stages of water/rock interaction. In Hole BA3A, early hydration of olivine and pyroxene, which likely occurred at temperatures of ∼100°C–200°C, produced serpentine, Fe-rich brucite, awaruite, and little magnetite. Notably, Fe-rich brucite is only preserved at >∼100 m depth in the core. Fe-rich brucite is nearly absent within two near-surface reaction zones where later stages of reaction are recorded, which include replacement of Fe-rich brucite by Fe(III)-bearing serpentine, and increases in the proportion of other Fe(III)-bearing phases such as magnetite and hydroandradite. Thus, Fe-rich brucite at depth represents substantial stored capacity for H2 production that can continue at low temperature, even after primary olivine and pyroxene are exhausted, thereby sustaining habitable conditions for microbial life
Partial ordering and topologies on finite sets, 1971
No full textA topological space is a I4 -space if and only if for each pair x and y of distinct points, there is an open set containing one of the points and not the other. There is a connection between T -topologies and partial orderings on finite sets. The relation matrix corresponds to a topology on S, a finite set
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Climate change adaptations of Vietnamese robusta coffee (Coffea canephora) farmers : analysis of potential strategies
No full textVietnam is considered one of the most climate change-vulnerable countries due to its high dependence on agriculture and its coastal location. Increased atmospheric temperatures caused by climate change also significantly threaten one of Vietnam's most important cash crops, robusta coffee (Coffea canephora). Suitable cultivation sites of robusta, which comprises 97% of Vietnam's coffee industry, will be eliminated by 2050 (Bunn et al., 2015) under RCP 6.0 if status quo management is maintained. The typical current management of Vietnamese coffee farms has also led to significant land degradation, reducing yields and the industry's long-term sustainability. To ensure that coffee remains a viable crop in Vietnam in the future, farmers and, indeed, all industry stakeholders will need to increase efforts to adopt adaptation techniques to counter anticipated climate change and existing environmental degradation.
Although multiple adaptation strategies exist for farmers, it is proposed that they maintain their existing sites and adopt agroforestry methods that can moderate farm microclimates to reduce loss of cultivation suitability (option 4, Table 8). Coffee agroforestry is an accessible, biotic, and low technological adaptation system that reduces air temperatures and increases available water resources by introducing woody perennial support plants. The system also provides additional ecosystem services that benefit the environment and farmers while diversifying farmer crops, livelihoods, and their capacity to respond to future environmental and biological shocks.
Vietnamese coffee farmers' knowledge and understanding of agroforestry systems is low, representing one barrier to entry. Additionally, farmers are hesitant to adopt agroforestry systems because of low financial resources and delayed profit returns. Therefore, more support and a multi-pronged approach from the Government of Vietnam (GOV), academia, NGOs, and the private sector will be needed to increase farmer adoption of agroforestry. If the hurdles to farmer adoption can be lowered and overcome, agroforestry represents a feasible and appealing potential adaptation strategy that can offset the anticipated losses to robusta cultivation suitability from anticipated future climate change and existing mismanagement of farms
Concentration in Knowledge Output: A case of Economics Journals
No full textThis paper assesses the degree of author concentration in seven economics journals, which were published in India during 1990-2002. To measure the degree of author concentration, Lotka's Law was used. Moreover, we also make an exploratory analysis of the geographic, economics subfield and institutional concentration in 704 economics journals. An important finding of this paper is that specialized journals in the sample report the highest degree of author concentration. This result is quite similar to the findings by Cox and Chung (1991). Furthermore, there are several instances showing that the journals lean towards certain norms; this may affect the flow of innovative ideas into economics. We conclude that a knowledge activity, involving the high degree of concentration and a biased publication process, may affect the flow of new ideas into the discipline.Concentration, Lotka's Law
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