1,720,979 research outputs found
Mineral-bound lipid formation in soils and sediments: the importance of microbial pathways
No full textDecline in oceanic sulfate levels during the early Mesoproterozoic
No full textMultiple-sulfur isotope compositions ([superscript 32]S, [superscript 33]S, [superscript 34]S and [superscript 36]S) were analyzed for paired carbonate-associated sulfate (CAS) and disseminated pyrite (PY) from the ∼1.6-Ga Gaoyuzhuang Formation of the North China Craton to reconstruct the history of sulfate levels in Proterozoic oceans. The 200-m-thick study interval yielded relatively constant values for δ[superscript 34]S[subscript CAS] (13.0 ± 1.8‰), δ[superscript 34]S[subscript PY] (8.0 ± 2.3‰), and Δ[superscript 34]S[subscript CAS-PY] (∼5‰), as well as relatively constant Δ[superscript 33]S (0 ± 0.05‰) and Δ[superscript 36]S (0.35 ± 0.15‰) for both CAS and pyrite. Limited variation in δ[superscript 34]S[subscript PY] and slightly lower Δ[superscript 33]S of pyrite relative to CAS suggest water-column precipitation of pyrite. Limited fractionation of sulfur during microbial sulfate reduction (as documented by Δ[superscript 34]S[subscript CAS-PY]) implies low seawater sulfate concentrations in the early Mesoproterozoic ocean. We quantitatively constrained paleo-seawater [SO[subscript 4][superscript 2−]] using a novel modeling approach based on measured values of Δ[superscript 34]S[subscript CAS-PY] and ∂δ[superscript 34]S[subscript CAS]/∂t(max). For the study unit, Δ[superscript 34]S[subscript CAS-PY] is 5.4 ± 1.4‰ (n = 17), and ∂δ[superscript 34]S[subscript CAS]/∂t(max) is 6.8–34‰ Myr[superscript −1] based on sedimentation rates of 30–150 m Myr[superscript −1]. These data indicate early Mesoproterozoic seawater [SO[subscript 4][superscript 2−]] of ∼<0.1 to 0.35 mM (with a maximum possible concentration of 1.8 mM), a range that is lower and more tightly constrained than earlier estimates for the Mesoproterozoic. Compilation of published data suggests that low seawater sulfate concentrations began about ∼1.7 Ga and persisted until at least the mid-Mesoproterozoic (∼1.4 Ga), documenting a distinct early Mesoproterozoic perturbation in ocean chemistry that may have been related to a decline in atmospheric pO[subscript 2] after Great Oxidation Event I.National Basic Research Program of China (973 Program) (Grants 2011CB808800 and 2013CB955704)National Natural Science Foundation (China) (Grant 41472170)National 111 Project of China (Grant B08030)China University of Geosciences (CUG 130406 and CUG120117)National Science Foundation (U.S.) (Grant 1338810)NASA Astrobiology InstituteSimons Foundatio
Rapid oxygenation of Earths atmosphere 2.33 billion years ago
Get PDFMolecular oxygen (O[subscript 2]) is, and has been, a primary driver of biological evolution and shapes the contemporary landscape of Earth’s biogeochemical cycles. Although “whiffs” of oxygen have been documented in the Archean atmosphere, substantial O2 did not accumulate irreversibly until the Early Paleoproterozoic, during what has been termed the Great Oxygenation Event (GOE). The timing of the GOE and the rate at which this oxygenation took place have been poorly constrained until now. We report the transition (that is, from being mass-independent to becoming mass-dependent) in multiple sulfur isotope signals of diagenetic pyrite in a continuous sedimentary sequence in three coeval drill cores in the Transvaal Supergroup, South Africa. These data precisely constrain the GOE to 2.33 billion years ago. The new data suggest that the oxygenation occurred rapidly—within 1 to 10 million years—and was followed by a slower rise in the ocean sulfate inventory. Our data indicate that a climate perturbation predated the GOE, whereas the relationships among GOE, “Snowball Earth” glaciation, and biogeochemical cycling will require further stratigraphic correlation supported with precise chronologies and paleolatitude reconstructions.National Science Foundation (U.S.) (EAR-1338810)National Natural Science Foundation (China) ((grant no. 41472170)Wellcome Trust Sanger Institute ( 111 Project grant no. B08030)National Basic Research Program of China (973 Program)United States. National Aeronautics and Space Administration (NASA Astrobiology Institute award NNA13AA90A
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
No evidence for high atmospheric oxygen levels 1,400 million years ago
No full textZhang et al. (1) recently proposed atmospheric oxygen levels of ∼4% present atmospheric levels (PAL) based on modeling a paleoenvironment reconstructed from trace metal and biomarker data from the 1,400 Ma Xiamaling Formation in China. Intriguingly, this pO2 level is above the threshold oxygen requirements of basal animals and clashes with evidence for atmospheric oxygen levels <<1% PAL in the mid-Proterozoic (2). However, there are fundamental problems with the inorganic and organic geochemical work presented by Zhang et al. (1)
The Jenkyns Event (early Toarcian OAE) in the Ordos Basin, North China
No full textThe early Toarcian (Early Jurassic) Oceanic Anoxic Event (T-OAE) or Jenkyns Event (~183 Ma) was marked by a globally synchronous negative carbon-isotope excursion, large-scale organic carbon burial and widespread oxygen depletion in the oceans. These features have been associated with volcanic activity of the Karoo and Ferrar large igneous provinces. The Jenkyns Event is well investigated in the Tethyan Ocean, but its expression in continental settings outside of Europe are still poorly understood. Here, the lacustrine Anya (AY) section in the NE Ordos Basin, North China, is studied for carbon-isotope geochemistry and palynology. Palynostratigraphy constrains the age of the section to the Pliensbachian-Toarcian transition, and a positive trend in δ13C, interrupted by a pronounced negative carbon-isotope excursion, has been identified in bulk organic matter and long-chain n-alkanes. The negative excursion is the expression of the Jenkyns Event, and coincides with variable total organic carbon (TOC) content in lake sediments. The maximum TOC (~21.5%) values precede the most negative carbon isotopes. A turnover from spore and seed fern-dominated palynological assemblages to Classopollis superabundance is coincident with the isotopic excursion and indicates clearly environmental perturbation. Biomarker data suggest that the organic matter was mainly sourced from higher plants. The magnitude of the Toarcian excursion in the bulk organic matter of this section (~ − 12.5‰) is much larger compared to that typically observed in marine and lake strata. Humidity-related fractionation effects in land plants and changes in plant groups might have contributed to the recorded magnitude. This magnitude is also larger than that of other terrestrial records, and casts doubt on the use of terrestrial C-isotope data to constrain deep time pCO2. Nevertheless, our findings show that the Jenkyns Event is well-recorded in lacustrine facies of the Ordos Basin
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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