1,721,022 research outputs found
Imbalance in the oceanic strontium budget
No full textPalmer and Edmond [Earth Planet. Sci. Lett. 92 (1989) 11-26] indicated that thermally plausible oceanic hydrothermal inputs of strontium to the oceans are not sufficient to balance the riverine input. It has recently been suggested that off-axis low-temperature hydrothermal circulation may reconcile this discrepancy [e.g. Butterfield et al., Geochim. Cosmochim. Acta 65 (2001) 4141-4153]. Strontium isotope alteration profiles are compiled for sampled in situ ocean and ophiolite crust to calculate a sustainable cumulative hydrothermal flux to the oceanic strontium budget. High-temperature circulation contributes ~1.8 x 109 mol yr-1 of basaltic strontium to the oceans. Enhanced hydrothermal systems in arc-related spreading environments (10% of the crust) may increase this to ~2.3 x 109 mol yr-1. It is shown that low-temperature flow cannot supply the remaining flux required to reconcile the oceanic strontium budget (~8.7 x 109 mol yr-1) because this would require 100% exchange of seawater strontium for basaltic strontium over an 820 m section of MORB-like crust. Currently sampled in situ ocean crust is not altered to this extent. The isotopic alteration intensity of 120 Myr crust sampled in DSDP Holes 417D and 418A indicates that off-axis low-temperature flow may contribute up to ~8 x 108 mol yr-1 of basaltic strontium (9% of that required). The ocean crust can sustain a total basaltic strontium flux of ~3.1+/-0.8 x 109 mol yr-1 (87Sr/86Sr ~0.7025) to the oceans. This is consistent with hydrothermal flux estimates, but remains less than a third of the flux required to balance the oceanic strontium budget. The ocean crust cannot support a higher hydrothermal contribution unless the average ocean crust is significantly more altered than current observation
Secrets of the sea floor
No full textOcean drilling is the most successful long-standing international collaboration in the geosciences. The invaluable archive of samples and data that has been built underpins our understanding of the Earth, its surface environment and climate. Planning the next phase is at an advanced stage
Partitioning riverine sulfate sources using oxygen and sulfur isotopes: Implications for carbon budgets of large rivers
Full text linkThe weathering of carbonate rocks with sulfuric acid releases carbon dioxide (CO2) to the atmosphere, offsetting the CO2 drawdown from carbonic acid weathering of silicates thought to regulate global climate. Quantifying CO2 release from sulfuric acid weathering requires the partitioning of riverine sulfate between its two main sources: sedimentary sulfate and sulfide. Although the sulfur ( δ 34 S S O 4 ) and oxygen ( δ 18 O S O 4 ) isotope ratios of sedimentary sulfates (gypsum and anhydrite) of different ages are well constrained, the δ 34 S of sulfide minerals is highly variable, restricting the utility of δ 34 S for partitioning sulfur sources. Here, we use oxygen isotope ratios in the river water ( δ 18 O H 2 O ) and sulfate molecules ( δ 18 O S O 4 ) to partition the fraction of sulfate and associated uncertainty delivered by the oxidative weathering of pyrite ( f pyr ). The partitioning is illustrated using the Mekong River, one of the world's largest river basins, presenting new δ 18 O S O 4 , δ 18 O H 2 O and δ 34 S S O 4 data collected on 18 tributaries and 6 mainstem sites over two field seasons at peak flux. The geological, geomorphological and climatic diversity of the Mekong River basin make it an ideal field site to quantify the role of sulfuric acid weathering and its implications for the carbon cycle. There is a 12‰ range in both the difference between δ 18 O S O 4 and δ 18 O H 2 O ( Δ 18 O S O 4 − H 2 O ) and δ 34 S in the river waters of the basin. In the Mekong tributaries, sources of sulfate are highly variable with the fraction of sulfate derived from pyrite oxidation ( f pyr ) ranging from 0.19 to 0.84. In the mainstem, f pyr reflects the flux-weighted mean of these tributary inputs, with 56 ± 7 % (1σ) of the sulfate delivered to the ocean at the Mekong mouth being derived from the oxidative weathering of pyrite. As a result, we estimate that ∼70% of CO2 consumed through silicate weathering in the Mekong basin is offset by the release of CO2 via the dissolution of carbonates by sulfuric acid
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Fluid-rock interactions in a carbon storage site analogue, Green River, Utah
Full text linkReactions between CO2-charged brines and reservoir minerals might either enhance the long-term storage of CO2 in geological reservoirs or facilitate leakage by corroding cap rocks and fault seals. Modelling the progress of such reactions is frustrated by uncertainties in the absolute
mineral surface reaction rates and the significance of other rate limiting steps in natural systems.
This study uses the chemical evolution of groundwater from the Jurassic Navajo Sandstone, part of a leaking natural accumulation of CO2 at Green River, Utah, in the Colorado Plateau, USA, to place constraints on the rates and potential controlling mechanisms of the mineral-fluid reactions,under elevated CO2 pressures, in a natural system. The progress of individual reactions, inferred from changes in groundwater chemistry is modelled using mass balance techniques. The mineral reactions are close to stoichiometric with plagioclase and K-feldspar dissolution largely balanced by precipitation of clay minerals and carbonate. Mineral modes, in conjunction with published surface area measurements and flow rates estimated from hydraulic head measurements, are then used to quantify the kinetics of feldspar dissolution. Maximum estimated dissolution rates for plagioclase and K-feldspar are 2x10-14 and 4x10-16 mol·m-2·s-1, respectively. Fluid ion-activity products are close to equilibrium (e.g. DGr for plagioclase between -2 and -10 kJ/mol) and lie in the region in which mineral
surface reaction rates show a strong dependence on DGr. Local variation in DGr is attributed to the injection and disassociation of CO2 which initially depresses silicate mineral saturation in the
fluid, promoting feldspar dissolution. With progressive flow through the aquifer, feldspar hydrolysis reactions consume H+ and liberate solutes to solution which increase mineral saturation in the fluid and rates slow as a consequence. The measured plagioclase dissolution rates at low DGr would be compatible with far-from-quilibrium rates of ~1x10-13 mol·m-2·s-1 as observed in some experimental studies. This suggests that the discrepancy between field and
laboratory reaction rates may in part be explained by the differences in the thermodynamic state of natural and experimental fluids, with field-scale reactions occurring close to equilibrium
whereas most laboratory experiments are run far-from-equilibrium.
Surface carbonate deposits and cementation within the footwall of the local fault systems record multiple injections of CO2 into the Navajo Aquifer and leakage of CO2 from the site over ca. 400,000 years. The d18O, d13C and 87Sr/86Sr of these deposits record rapid rates of CO2 leakage
(up to ~1000 tonnes/a) following injection of CO2, but rates differ by an order of magnitude between each fault, due to differences in the fault architecture. Elevated pCO2 enhances rates of feldspar dissolution in the host aquifer and carbonate precipitation in fracture conduits. Silicate mineral dissolution rates decline and carbonate precipitation rates increase as pH and the CO2 charge dissipate. The Sr/Ca of calcite cements record average precipitation rates of ~2x10-6 mol/m2/s, comparable to laboratory derived calcite precipitation rates in fluids with elevated
Mn/Ca and Fe/Ca, at cc of ~1 to 3. This suggests that far-from-equilibrium carbonate
precipitation, which blocks fracture conduits and causes the leaking system to self-seal, driven by CO2 degassing in the shallow subsurface, can be accurately modeled with laboratory derived rates. Sandstones altered in CO2 leakage conduits exhibit extensive dissolution of hematite grain coatings and are chemically bleached as a result. Measurements of Eh-pH conditions in the modern fluid, and modeling of paleo-Eh-pH conditions using calcite Fe and Mn concentrations, suggests that the CO2-charged groundwaters are reducing, due to their low dissolved O2 content and that pH suppression due to high pCO2 is capable of dissolving and transporting large concentrations of metals. Exhumed paleo-CO2 reservoirs along the crest of the Green River
anticline have been identified using volatile hosting fluid inclusions. Paleo-CO2-charged fluids mobilized hydrocarbons and CH4 from deeper formations, enhancing the reductive dissolution of hematite, which produced spectacular km-scale bleached patterns in these sediment
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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