1,721,004 research outputs found
The paradox of the end Permian global oceanic anoxia
No full textGeochemical results argue against global anoxia as a or the cause for the end Permian mass extinction. Instead, they suggest that the water column was oxic and sometimes the sediment pore water was suboxic, which is the norm. Anoxia, if present, may reflect local conditions developed in deep water settings, but that have to be supported by concrete and direct evidence
METHANE HYDRATE: KILLER CAUSE OF EARTH’S GREATEST MASS EXTINCTION
No full textThe cause for the end Permian mass extinction, the greatest challenge life on Earth faced in its geologic history, although still hotly debated it bears an important message for humanity. The most significant marker of this event is the negative δ13C shift and rebound recorded in marine carbonates with a duration ranging from 2000 to 19,000 years. Leading cause for the event are Siberian trap volcanism and its emission of greenhouse gases with consequent global warming, but other leading contenders are oceanic anoxia and acidification. We present measurements of gases vaulted in shell calcite of end Permian brachiopods and whole rock, documenting significant differences in normal atmospheric equilibrium concentration (NAEC) of gases between modern and end Permian seawaters. The gas composition recorded in end Permian brachiopod gas-inclusions reflects dramatically higher seawater methane contents leading up to the event. Initial global warming of 8 to 11°C sourced by isotopically light carbon dioxide from volcanic emissions triggered the sudden release of methane from permafrost and shelf sediment hydrate. Consequently, the huge quantities of methane emitted into the atmosphere and the oceans accelerated global warming and marked the negative δ13C spike observed in marine carbonates marking the onset of the biggest mass extinction in Earth history. The rapidity of the methane emission lasting from several years to thousands of years was tempered by the equally rapid oxidation of the atmospheric and oceanic methane that gradually reduced its warming potential but not before global warming had reached levels lethal to most life on land and in the oceans. Based on measurements of gases trapped in biogenic and abiogenic calcite, the release of methane (of ~ 8 -28 % of total C stored) from permafrost and shelf sediment hydrate was the ultimate source and cause for the dramatic life-changing global warming (GMAT > 34°C) and oceanic (negative carbon isotope excursion) changes observed during the end Permian. Global warming triggered by the exorbitant release of carbon dioxide may be catastrophic, but the subsequent release of methane hydrate may be apocalyptic. The end Permian may have an important lesson for humanity regarding the issue of global warming and climate change it faces today
High resolution Carbon-isotope stratigraphy of the Cambrian-Ordovician GSSP: An Enhanced International Correlation Tool.
No full textIsotope curves delineating d13Ccarb and d16O variations across the
Cambrian–Ordovician boundary at Green Point, western Newfoundland (Canada)
are presented. The d13Ccarb profile of the section reveals a composite (three peaks)
negative shift of *6.0 % and starts immediately at the base of bed 23 of the
Broom Point Member. The 13Ccarb profile ties to conodont and graptolite zones and
indicates that the lowest occurrence of planktic graptolites in Bed 25 largely
coincides with the C. lindst
Diagenetic constraints in carbon-isotope stratigraphy of the Upper Jurassic-Lower Cretaceous Vaca Muerta-Quintuco system, Neuquén Basin, Argentina: Implications for a global correlation
No full textThe δ13Ccarb profiles of the Vaca Muerta Formation (Neuquén Basin, Argentina), span the Tithonian to lower Valanginian (Jurassic-Cretaceous transition). The investigated interval was sampled from two sections: the Pampa de Tril (PDT, ∼120 m-thick covering the lower Vaca Muerta Formation) and Puerta Curaco (PC, ∼670 m-thick covering the entire Vaca Muerta and Quintuco formations). The succession consists of organic-rich marine carbonates alternating with mudrock beds. A multitechnique screening protocol, including petrography and geochemistry, has been applied to evaluate the degree of preservation of the studied carbonates. The Pampa de Tril and Puerta Curaco sections have δ13Ccarb values that vary from −12.2 ‰VPDB (at the bottom) to 2.9 ‰ VPDB (at the top) and δ18Ocarb values from −13.2 ‰VPDB to −5.3 ‰VPDB. The δ13Ccarb values of the analysed samples, exhibit insignificant correlations with their Mn/Sr (rs = −0.13) and ∑REE (rs = −0.19) counterparts. This supports the preservation of at least near-primary δ13Ccarb compositions that can be implemented to construct a reliable high-resolution carbon-isotope profile for global correlations. The δ13Ccarb profiles of the Vaca Muerta Formation display a negative excursion of ∼8–10 ‰ at the bottom of the formation (Tithonian) which can be correlated with the VOICE (Volgian Isotope Carbon Excursion) event.Fil: Lanz, Maria del Rosario. Universidad Nacional del Sur. Departamento de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Azmy, Karem. Memorial University Of Newfoundland; CanadáFil: Fortunatti, Natalia Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Sur. Departamento de Geología; ArgentinaFil: Rainoldi, Ana Laura. Universidad Nacional del Sur. Departamento de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Cesaretti, Nora Noemi. Universidad Nacional del Sur. Departamento de Geología; ArgentinaFil: Irastorza, Ainara. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; Argentina. Universidad Nacional del Sur. Departamento de Geología; Argentin
Diagenesis of the Vaca Muerta Formation, Neuquén Basin: Evidence from petrography, microthermometry and geochemistry
No full textThe Tithonian Vaca Muerta Formation (500 m-thick marine carbonates) in the Pampa de Tril area is a potential unconventional hydrocarbon reservoir but earlier studies rarely focused on its diagenetic history. It constitutes the upper part of the lower Mendoza Group in the Neuqu´en Basin, western Argentina. Petrographic examinations reveal four generations of calcite (C1, C2, Cb and C3). The microbial lime mudstone (C1, <4 μm), which exhibits dull cathodoluminescence (CL), is the most abundant with mean δ18O value of − 7.2 ± 1.3‰ (VPDB) and δ13C of − 1 ± 3.3‰ (VPDB) and deemed to have deposited in tropical warm shallow-water settings. The equant calcite cement (C2, 25 μm–140 μm) exhibits red CL and is associated with recrystallization of bioclasts and micritic matrix. The fibrous calcite (Cb, 35 μm–125 μm) fills parallel-to-bedding antitaxial veins (beefs) and shows bright orange CL. Its δ18O and δ13C values are more depleted (− 10.7 ± 2.2‰ VPDB and − 4.8 ± 3.9‰ VPDB, respectively) relative to those of C1, which is consistent with the occurrence of hydrocarbon fluid inclusions and may imply precipitation during a mid-to late stage of diagenesis within the oil window. The fracture-filling C3 consists of coarse subhedral crystals (40 μm–5 mm) showing bright orange CL, which has been interpreted as precipitated during deep burial conditions at temperatures around 118 ± 26.8 ◦C based on fluid-inclusion microthermometry. The relative depletion of Fe and Mn from C1 to C3 and the low concentration of U, may imply a relatively closed system of low water/rock interaction and restricted meteoric alteration. The shale normalized (REESN) patterns of C1, Cb and C3 generally exhibit parallel patterns with a distinct positive Eu anomaly and progressive enrichment towards the MREE and HREE. The positive Eu (Eu/Eu*)SN anomalies reflect the impact of dissolution of plagioclase feldspars by circulation of the diagenetic fluids through the surrounding siliciclastics in the basin. The Ce (Ce/Ce*)SN and La (La/La*)SN anomalies of Cb and C3 are consistent with precipitation under reducing conditions of deep burial settings. The Vaca Muerta carbonates are enriched in organic matter (up to 12%) that fills the pores but permeability is insignificant.Fil: Lanz, Maria del Rosario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Geología; ArgentinaFil: Azmy, Karem. Memorial University Of Newfoundland; CanadáFil: Cesaretti, Nora Noemi. Universidad Nacional del Sur. Departamento de Geología; ArgentinaFil: Fortunatti, Natalia Beatriiz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; Argentin
Subsurface diagenetic evolution and porosity evaluation of Middle-Upper Devonian Kee Scarp reef, Norman Wells, Northwest Territories, Canada.
No full textThe Middle-Upper Devonian Kee Scarp Formation at Norman Wells consists of a carbonate platform and an overlying reefal buildup. Episodic increases in the rate of sea-level rise produced multiple cycles of reef growth, with a backstepping character. Reservoir porosity of these reefs is mostly represented by micropores of various types developed during diagenesis by the action of aggrading neomorphism and dissolution. The microporosity was developed as intercrystalline microvoids within the present low-magnesium calcite (LMC) stromatoporoid, algal and matrix components. SEM studies show that microporosity development responded to neomorphism at various rates. Reef margins, grain shoals and lower, middle and upper foreslope facies have the highest and best developed microporosity compared to lagoon, reef flat and tidal flat facies. The micropores in Kee Scarp limestone can be classified into four categories based on their shapes: (1) stepwise rhombic, about 1m to 2m in diameter; mainly developed in stromatoporoids; (2) intercrystalline rhombic, about 1m in diameter; mainly in algal aggregates; (3) microvugs, 4m to 10 m in diameter; mainly in algal aggregates; (4) microchannels, 12m length and 0.5m width; in algal aggregates and stromatoporoids. (Abstract shortened by UMI.)Dept. of Geology and Geological Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1992 .A958. Source: Masters Abstracts International, Volume: 31-04, page: 1695. Thesis (M.Sc.)--University of Windsor (Canada), 1992
Geochemistry of Late Ordovician dalmanelloid brachiopods from Laurentia: Testing the effects of paleolatitudinal gradient
Full text linkDalmanelloid brachiopod shells were collected from the Upper Ordovician Lexington Formation (lower Katian) of Kentucky, Sheguindah Shale (middle Katian) on Manitoulin Island, Ontario, and the Stony Mountain Formation (upper Katian) in the Winnipeg area, Manitoba. They were investigated to test the hypothesis of paleo-latitudinal zonation of the shelly benthos. A multi-technique approach was applied to evaluate the petrographic and geochemical (isotopic and elemental) preservation of the secondary layer of shells. Preliminary conventional microscopy, cathodoluminescence (CL), and scanning electron microscopy (SEM) confirmed the retention of primary shell ultrastructure (prismatic low-Mg calcite). The geochemical diagenesis proxies (e.g., Sr, Mn, Fe, and ΣREE) show insignificant correlations with the δ18O and δ13C values, thus supporting the preservation of at least near-primary geochemical compositions. Among the three lots of shells, the mean δ18O value is the highest in those from the Lexington Formation (–4.5‰ ± 0.3‰ VPDB), lowest from the Stony Mountain Formation (–6.8‰ ± 0.4‰ VPDB), and intermediate from the Sheguindah Shale (–6.0‰ ± 0.8‰ VPDB). The relative gradient in δ18O increase is in agreement with the paleo-latitudinal gradient, with Kentucky in subtropical, southern Ontario in mid-tropical, and southern Manitoba in subequatorial latitudes. The Lexington Formation shells also have the highest mean δ13C value (0.8‰ ± 0.2‰ VPDB) and relatively high P contents (170 ± 27 ppm), suggesting higher organic productivity, which is consistent with previous interpretation of frequent upwelling of nutrient-rich cool waters along the southeastern margin of Laurentia during the Katian. The Lexington shells also have a lower mean Th/U (0.6 ± 0.6), which is consistent with blooming organic productivity that likely led to more consumption of oxygen in the water column.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author
Isotopic composition of Silurian brachiopods: Implications for coeval seawater.
No full textTwo hundred and thirty six calcitic brachiopod shells, covering the entire Silurian Period (30 Ma), were collected at high temporal resolution from stratotype sections on Anticosti Island (Canada), in Wales (UK), in the Oslo region (Norway), on Gotland (Sweden), and in Podolia (Ukraine), Estonia, Latvia and Lithuania. Data from petrography, scanning electron microscopy (SEM), cathodoluminescence, isotopes and trace elements all confirmed excellent preservation in most shells, thus ensuring retention of primary isotope signals. The only exceptions were samples from the Oslo region. The \delta\sp O and \delta\sp C values for the well preserved samples range from 3 to {-}6.5\perthous and from -1 to 7.5\perthous (PDB), respectively. In terms of temporal trends, oxygen- and carbon-isotopes vary in parallel, with an overall decrease with decreasing age of {\sim}1\perthous, through the Silurian. Superimposed on these general trends are short-term variations that are negatively correlated with sea-level changes. Three successive positive \delta\sp O shifts in early Aeronian, latest Aeronian and early Wenlock correlate with sea-level lowstands and with glacial diamictite deposits in the Amazon Basin and in Africa. The \sp‡Sr/\sp†Sr values range from 0.707930 0.000011 to 0.708792 0.000017, increasing with decreasing age. This may indicate increasing riverine flux of radiogenic Sr into the ocean from weathering of continental sialic rocks due to progressive global warming. The Sr-isotope curve shows distinct inflexion points in the earliest Wenlock and in mid-Pridoli These can be used to correlate the Llandovery/Wenlock boundary in the U.K., on Gotland and in Lithuania and for the Kaugatuma/Ohesaare boundary in the Baltic states and Podolia. Partial linear regressions for Sr-isotope data form a pattern that climbs stepwise with decreasing age, with local drops around the Llandovery/Wenlock boundary and in the latest Ludlow. The slopes of the partial regression lines can be used for reliable age estimates with a resolution of about 2 biozones (1.5 to 2 Ma)
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
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