Up to what temperature is petroleum stable? New insights from a 5200 free radical reactions model     

Florent Domin  Roda Bounaceur  Gerard Scacchi  Paul-Marie Marquaire  Daniel Dessort  Bernard Pradier  Olivier Brevart 《Organic Geochemistry》2002,33(12)

The discovery of crude oils and condensates at ever higher temperatures casts doubt on the validity of the usual geochemical modelling approach, that uses empirical reactions and rate constants. The solution used to account for such a high thermal stability is presently to adjust the rate parameters, but the physical meaning and scientific value of such a strategy can be questioned. We have developed a mechanistic model consisting of 5200 lumped free radical reactions to describe the thermal evolution of a mixture of 52 organic species meant to represent light petroleum. Rate constants used are those available in the literature or estimated using well established thermochemistry-reactivity correlations. Chemical structures included in the model are linear, branched and cyclic hydrocarbons, hydro- and alkyl-aromatics, PAHs, and three heteroatomic compounds. Reactions include cracking and alkylation chains and inhibiting and accelerating reactions from the various reactants. This model has been applied to several mixtures with various proportions of reaction inhibitors and accelerators, and to a composition representing a light mature oil. From the results obtained, we conclude that mature oils will be stable up to 240–260 °C, depending on their composition, and that the thermal cracking of oil to gas is not possible under reasonable basin conditions. The kinetics of petroleum cracking are thus much slower than generally recognized.  相似文献   

Anhydrite-bearing rocks from the Rožná district (Moldanubian zone,Czech Republic): high-grade metamorphosed exhalites?     

Bohdan Kříbek  Jana Hladíková  Daniel Holeczy 《Mineralium Deposita》2002,37(5):465-479

Several types of anhydrite-bearing rocks have been found in the amphibolite-facies metamorphosed rocks at the north-eastern margin of the Moldanubian Zone. Anhydrite either forms monomineralic bands up to 40 cm thick, or occurs in the form of disseminated grains in surrounding calc-silicate gneiss together with feldspar, scapolite, amphibole, pyroxene, epidote and pyrite. The isotopic composition of sulphur ('³⁴S=30.6 to 32.3‰) and strontium (⁸⁷Sr/⁸⁶Sr=0.70797 to 0.70781) in anhydrite may indicate a marine source of sulphate. The isotopic ratio of strontium is in the same range as that of metamorphosed strata-bound barite-sulphide ores, which have been previously described in the same area. The '³⁴S values of coexisting pyrite range from 21.4 to 22.5‰, the (³⁴S_{anhydrite-pyrite} corresponding to the metamorphic temperature of 600 to 660 °C. In contrast to many submarine-exhalative deposits, the oxygen isotopic compositions of anhydrite ('¹⁸O=9.3 to 10.2‰) are lighter than that of barite ('¹⁸O=10.4 to 13.8‰). This indicates that the both minerals are not in isotopic equilibrium. Therefore, it is probable that anhydrite and barite from the Ro—ná district were deposited from fluids that contained different proportions of seawater and hydrothermal fluids or from hydrothermal fluids that underwent variable extent of oxygen isotope exchange with seafloor rocks. The '¹³C values in calcite ('¹³C=-17.2 to -18.7‰) from anhydrite-bearing rock are lower than those in distant marbles. As graphite is absent in anhydrite- and calcite-bearing rocks, impoverishment in the ¹³C isotope cannot be attributed to the graphite-carbonate isotopic exchange during metamorphism. It is proposed that low '¹³C values in carbonates are caused by pre-metamorphic oxidation of organic matter in course of hydrothermal processes. Anhydrite and anhydrite-bearing calc-silicate gneiss from the north-eastern part of the Moldanubian Zone are interpreted to be the high-grade metamorphosed analogue of anhydrite-rich exhalites commonly found in submarine-exhalative hydrothermal deposits.  相似文献   

Obituary: Vassil T. Vuchev (1935–2001)     

Daniel F. Merriam  Elaine Kiroff 《Mathematical Geology》2002,34(3):361-364

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Climate change impacts on U.S. Coastal and Marine Ecosystems   总被引：1，自引：0，他引：1  

Donald Scavia  John C. Field  Donald F. Boesch  Robert W. Buddemeier  Virginia Burkett  Daniel R. Cayan  Michael Fogarty  Mark A. Harwell  Robert W. Howarth  Curt Mason  Denise J. Reed  Thomas C. Royer  Asbury H. Sallenger  James G. Titus 《Estuaries and Coasts》2002,25(2):149-164

Increases in concentrations of greenhouse gases projected for the 21st century are expected to lead to increased mean global air and ocean temperatures. The National Assessment of Potential Consequences of Climate Variability and Change (NAST 2001) was based on a series of regional and sector assessments. This paper is a summary of the coastal and marine resources sector review of potential impacts on shorelines, estuaries, coastal wetlands, coral reefs, and ocean margin ecosystems. The assessment considered the impacts of several key drivers of climate change: sea level change; alterations in precipitation patterns and subsequent delivery of freshwater, nutrients, and sediment; increased ocean temperature; alterations in circulation patterns; changes in frequency and intensity of coastal storms; and increased levels of atmospheric CO₂. Increasing rates of sea-level rise and intensity and frequency of coastal storms and hurricanes over the next decades will increase threats to shorelines, wetlands, and coastal development. Estuarine productivity will change in response to alteration in the timing and amount of freshwater, nutrients, and sediment delivery. Higher water temperatures and changes in freshwater delivery will alter estuarine stratification, residence time, and eutrophication. Increased ocean temperatures are expected to increase coral bleaching and higher CO₂ levels may reduce coral calcification, making it more difficult for corals to recover from other disturbances, and inhibiting poleward shifts. Ocean warming is expected to cause poleward shifts in the ranges of many other organisms, including commercial species, and these shifts may have secondary effects on their predators and prey. Although these potential impacts of climate change and variability will vary from system to system, it is important to recognize that they will be superimposed upon, and in many cases intensify, other ecosystem stresses (pollution, harvesting, habitat destruction, invasive species, land and resource use, extreme natural events), which may lead to more significant consequences.  相似文献   

Determining the trophic guilds of fishes and macroinvertebrates in a seagrass food web     

Joseph J. Luczkovich  Garcy P. Ward  Jeffrey C. Johnson  Robert R. Christian  Daniel Baird  Hilary Neckles  William M. Rizzo 《Estuaries and Coasts》2002,25(6):1143-1163

We established trophic guilds of macroinvertebrate and fish taxa using correspondence analysis and a hierarchical clustering strategy for a seagrass food web in winter in the northeastern Gulf of Mexico. To create the diet matrix, we characterized the trophic linkages of macroinvertebrate and fish taxa present inHalodule wrightii seagrass habitat areas within the St. Marks National Wildlife Refuge (Florida) using binary data, combining dietary links obtained from relevant literature for macroinvertebrates with stomach analysis of common fishes collected during January and February of 1994. Heirarchical average-linkage cluster analysis of the 73 taxa of fishes and macroinvertebrates in the diet matrix yielded 14 clusters with diet similarity ≥ 0.60. We then used correspondence analysis with three factors to jointly plot the coordinates of the consumers (identified by cluster membership) and of the 33 food sources. Correspondence analysis served as a visualization tool for assigning each taxon to one of eight trophic guilds: herbivores, detritivores, suspension feeders, omnivores, molluscivores, meiobenthos consumers, macrobenthos consumers and piscivores. These trophic groups, corss-classified with major taxonomic groups, were further used to develop consumer compartments in a network analysis model of carbon flow in this seagrass ecosystem. The method presented here should greatly improve the development of future network models of food webs by providing an objective procedure for aggregating trophic groups.  相似文献   

An estuarine benthic index of biotic integrity for the Mid-Atlantic region of the United States. II. Index development     

Roberto J. Llansó  Lisa C. Scott  Jeffrey L. Hyland  Daniel M. Dauer  David E. Russell  Frederick W. Kutz 《Estuaries and Coasts》2002,25(6):1231-1242

A benthic index of biotic integrity was developed for use in estuaries of the mid-Atlantic region of the United States (Delaware Bay estuary through Albemarle-Pamlico Sound). The index was developed for the Mid-Atlantic Integrated Assessment Program (MAIA) of the U.S. Environmental Protection Agency using procedures similar to those applied previously in Chesapeake Bay and southeastern estuaries, and was based on sampling in July through early October. Data from seven federal and state sampling programs were used to categorize sites as degraded or non-degraded based on dissolved oxygen, sediment contaminant, and sediment toxicity criteria. Various metrics of benthic community structure and function that distinguished between degraded and reference (non-degraded) sites were selected for each of five major habitat types defined by classification analysis of assemblages. Each metric was scored according to thresholds established on the distribution of values at reference sites, so that sites with low scoring metrics would be expected to show signs of degradation. For each habitat, metrics that correctly classified at least 50% of the degraded sites in the calibration data set were selected whenever possible to derive the index. The final index integrated the average score of the combination of metrics that performed best according to several criteria. Selected metrics included measures of productivity (abundance), diversity (number of taxa, Shannon-Wiener diversity, percent dominance), species composition and life history (percent abundance of pollution-indicative taxa, percent abundance of pollution-sensitive taxa, percent abundance of Bivalvia, Tanypodinae-Chironomidae abundance ratio), and trophic composition (percent abundance of deep-deposit feeders). The index correctly classified 82% of all sites in an independent data set. Classification efficiencies of sites were higher in the mesohaline and polyhaline habitats (81–92%) than in the oligohaline (71%) and the tidal freshwater (61%). Although application of the index to low salinity habitats should be done with caution, the MAIA index appeared to be quite reliable with a high likelihood of correctly identifying both degraded and non-degraded conditions. The index is expected to be of great utility in regional assessments as a tool for evaluating the integrity of benthic assemblages and tracking their condition over time.  相似文献   

Une méthode de quantification de la dégradation d'un escarpement de faille au cours des cycles climatiques du Quaternaire : la faille de Jobourg (Nord Cotentin,France)Quantification of a fault-scarp degradation through Quaternary: the Jobourg fault (North Cotentin,France)     

Marianne Font  Jean-Louis Lagarde  Daniel Amorese  Jean-Pierre Coutard  Jean-Claude Ozouf 《Comptes Rendus Geoscience》2002,334(3):171-178

The degradation of the Jobourg fault-scarp occurred by cryoclastic processes in a periglacial environment during a part of Quaternary time. An attempt of quantification indicates a bulk scarp erosion of about 39 m³ m^?2, while the head accumulated at the bottom of the fault scarp only represents 4.6 m³ m^?2. To cite this article: M. Font et al., C. R. Geoscience 334 (2002) 171–178.  相似文献   

Distribution and effectiveness of nivation in Mediterranean mountains: Peñalara (Spain)     

David Palacios  Nuria de Andrs  Enrique Luengo 《Geomorphology》2003,54(3-4):157-178

Snow accumulation is responsible for geomorphic and biogeographic processes taking place in the southern sector of the Peñalara massif in central Spain (40°51′N, 3°57′W; max. altitude 2428 m at Pico de Peñalara). This work compares the intensity of nivation on the eastern slope, leeward of the prevailing westerly winds and heavily eroded by glacial activity during the Pleistocene, to that of the western slope on the windward side, unaffected by glacial erosion and completely covered by a thick weathering mantle. On the eastern slope, nivation is effective only where the weathering mantle is exposed or on morainic formations. It does not occur on the landforms derived from glacial erosion. In contrast, the western side shows almost no evidence of snow action except where catastrophic mass movements have altered the regularity of the slope. During the post-glacial epoch, nivation cirques formed in the scars left by mass wasting. In the last 30 years, spring temperatures have increased and this activity has diminished. The pattern of evolution observed at Peñalara can be extrapolated to other Mediterranean mountains with similar characteristics such as marginal glacial activity during the Pleistocene, unconsolidated formations on the summits caused by chemical weathering, and dry, hot summers that can increase the effectiveness of nivation.  相似文献   

Seismic imaging of mid-Pleistocene tunnel-valleys in the North Sea Basin—high resolution from low frequencies     

Daniel Praeg   《Journal of Applied Geophysics》2003,53(4):273-298

Seismic reflection methods provide continuous access both to stratigraphy (vertical) and to subsurface morphology (horizontal), for which the scales of interest may differ by orders of magnitude. Seismic surveys of Quaternary successions have generally sought to optimise vertical resolution, through the use of higher source frequency content. Here, I show that low-frequency bandwidth is not necessarily a limiting factor for the seismic resolution of glacigenic morpho-sedimentary features. Observations are presented from a buried network of large mid-Pleistocene (Elsterian) tunnel-valleys in the southern North Sea Basin, across a 100×130 km study area with water depths less than 30 m. Low-frequency 2D and 3D seismic multi-channel data, acquired for deeper hydrocarbon exploration, are compared with previously available high-frequency single- and multi-channel profiles (5–15 km grid spacing). The low-frequency data contribute to a new understanding of the basal morphology and fill stratigraphy of the tunnel-valleys, in part due to higher data densities (≥1 km grid spacing), but also to improved imaging of reflectors at depth. The tunnel-valleys are seen to be overdeepened troughs, shallow (≤0.5 km) relative to their widths (≤6 km). The basal unconformity defines a series of arborescent elements, convergent to the south; erosional overlap by younger elements to the north has resulted in anastomosing patterns in places. The fill is dominated by axially downlapping clinoforms, descending to the north, onlapped and overlain by subhorizontal reflectors. Well data show that sand-dominated glaciofluvial sediments are overlain by glaciolacustrine to marine muds. Better definition of the clinoforms on low- versus high-frequency multi-channel data is suggested to reflect the coarse spatial scale of the backset glaciofluvial strata. The results support a simple interpretation of time-transgressive tunnel-valley formation by coeval glaciofluvial erosion and backfill beneath the outer tens of kilometres of the northward receding Elsterian ice sheet margin. Comparable submarginal interpretations have been proposed for drainage features (tunnel-valleys and eskers) of the last deglaciation of both northern Europe and North America using integrated geomorphologic and stratigraphic methods. Commercial 2D and 3D seismic data are widely available from exploration areas such as the North Sea and are argued to constitute an underexploited resource for Quaternary research.  相似文献   

Using DC resistivity tomography to detect and characterize mountain permafrost   总被引：4，自引：0，他引：4  

Christian Hauck  Daniel Vonder Mühll  Hansruedi Maurer 《Geophysical Prospecting》2003,51(4):273-284

Direct-current (DC) resistivity tomography has been applied to different mountain permafrost regions. Despite problems with the very high resistivities of the frozen material, plausible results were obtained. Inversions with synthetic data revealed that an appropriate choice of regularization constraints was important, and that a joint analysis of several tomograms computed with different constraints was required to judge the reliability of individual features. The theoretical results were verified with three field experiments conducted in the Swiss and the Italian Alps. At the first site, near Zermatt, Switzerland, the location and the approximate lateral and vertical extent of an ice core within a moraine could be delineated. On the Murtel rock glacier, eastern Swiss Alps, a steeply dipping boundary at its frontal part was observed, and extremely high resistivities of several MΩ indicated a high ice content. The base of the rock glacier remained unresolved by the DC resistivity measurements, but it could be constrained with transient EM soundings. On another rock glacier near the Stelvio Pass, eastern Italian Alps, DC resistivity tomography allowed delineation of the rock glacier base, and the only moderately high resistivities within the rock glacier body indicated that the ice content must be lower compared with the Murtel rock glacier.  相似文献