First insights into the structure and environmental setting of cold-seep communities in the Marmara Sea     

Bénédicte Ritt  Jozée Sarrazin  Jean-Claude Caprais  Philippe Noël  Olivier Gauthier  Catherine Pierre  Pierre Henry  Daniel Desbruyères 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(9):1120-1136

A brackish-water cold seep on the North Anatolian Fault (NAF) in the Marmara Sea was investigated with the Nautile submersible during the MarNaut cruise in 2007. This active zone has already been surveyed and revealed evidence of active seeping on the seafloor, such as bubble emissions, patches of reduced sediments, microbial mats and authigenic carbonate crusts. MarNaut was the first opportunity to sample benthic communities in the three most common microhabitats (bioturbated and reduced sediments, carbonate crust) and to examine their relationships with environmental conditions. To do so, faunal communities were sampled and chemical measurements were taken close to the organisms. According to diversity indices, the bioturbated microhabitat exhibited the highest taxonomic diversity and evenness despite a lower number of samples. Conversely, the reduced sediment microhabitat exhibited the lowest taxonomic diversity and evenness. The carbonate crust microhabitat was intermediate although it had the highest biomass. Multivariate analyses showed that (1) fauna were relatively similar within a single microhabitat; (2) faunal community structure varied greatly between the different microhabitats; (3) there was a link between faunal distribution and the type of substratum; and (4) chemical gradients (i.e. methane, oxygen and probably sulphides) may influence faunal distribution. The estimated fluid flow velocity (0.4–0.8 m/yr) confirmed the presence of fluid emission and provided evidence of seawater convection in the two soft-sediment microhabitats. Our results suggest that the reduced sediments may represent a harsher environment with high upward fluid flow, which restrains seawater from penetrating the sediments and inhibits sulphide production, whereas bioturbated sediments can be viewed as a bio-irrigated system with sulphide production occurring at greater depths. Therefore, the environmental conditions in reduced sediments appear to prevent the colonization of symbiont-bearing fauna, such as vesicomyid bivalves, which are more often found in bioturbated sediments. Fluid flow appears to control sulphide availability, which in turn influences the horizontal and vertical distribution patterns of fauna at small spatial scales as observed at other seep sites.  相似文献   

Perceptions of risk and adaptation: Coffee producers,market shocks,and extreme weather in Central America and Mexico     

Catherine M. Tucker  Hallie Eakin  Edwin J. Castellanos 《Global Environmental Change》2010,20(1):23-32

This article explores the role of risk perception in adaptation to stress through comparative case studies of coffee farmers’ responses to climatic and non-climatic stressors. We hypothesized that farmers associating these changes with high risk would be more likely to make adaptations than those who saw the events as part of normal variation. Nevertheless, we found that farmers who associated events with high risk were not more likely to engage in specific adaptations. Adaptive responses were more clearly associated with access to land than perception of risk, suggesting that adaptation is more a function of exogenous constraints on decision making than perception.  相似文献   

Thermal Convection in Finite Prandtl Number Fluids at High Rayleigh Number     

Catherine A. Hier  Stephen Y. Bergeron  David A. Yuen  Erik O. Sevre  John M. Boggs 《Visual Geosciences》2001,6(3):1-4

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Mapping Coastal Aquifers by Joint Inversion of DC and TEM Soundings-Three Case Histories     

Yves Albouy  Pierre Andrieux  Gérard Rakotondrasoa  Michel Ritz  Marc Descloitres  Jean-Lambert Join  Eddy Rasolomanana 《Ground water》2001,39(1):87-97

Electrical and electromagnetic methods are well suited for coastal aquifer studies because of the large contrast in resistivity between fresh water-bearing and salt water-bearing formations. Interpretation models for these aquifers typically contain four layers: a highly resistive unsaturated zone; a surficial fresh water aquifer of intermediate resistivity; an underlying conductive, salt water saturated aquifer; and resistive substratum. Additional layers may be added to allow for variations in lithology within the fresh water and salt water layers. Two methods are evaluated: direct current resistivity and time domain electromagnetic soundings. Use of each method alone produces nonunique solutions for resistivities and/or thicknesses of the different layers. We show that joint inversion of vertical electric and time domain electromagnetic soundings produces a more tightly constrained interpretation model at three test sites than is produced by inversion methods applied to each data set independently.  相似文献   

Metasomatic hydrous fluids in amphibole peridotites from Zabargad Island (Red Sea)     

Pierre Agrinier  Catherine Mvel  Delphine Bosch  Marc Javoy 《Earth and Planetary Science Letters》1993,120(3-4):187-205

Combining textural, petrological, chemical and isotopic (Sr, H and O) data for amphiboles and whole rocks from the Zabargad peridotite diapir allows three different events to be distinguished. During each event, which can be related to a specific tectonic process of the rifting of the Red Sea, hydrous fluids produced amphiboles.

The first and the second generations of amphiboles have characteristics consistent with the involvement of mantle-derived hydrous fluids. The first generation consists of scarce Ti-pargasites which crystallized from small amounts of fluid at temperatures of around 900–1000°C. Their growth was linked to magma percolation in the peridotites before their deformation during diapiric uplift. The second generation consists of Cr-pargasites which crystallized locally (and abundantly) during reaction between the peridotites and a sodium/potassium-bearing hydrous fluid at temperatures of around 700–800°C. These amphiboles grew synchronously with the diapiric uplift. The hydrous fluids probably originated in the sub-continental mantle and were released during the diapiric uplift of the peridotites.

The third generation consists of amphiboles (pargasitic hornblende, hornblende sensu lato and tremolite) which are localized in shear zones and veins. They crystallized at temperatures estimated between 700°C and 450°C, again from a sodium/potassium-bearing hydrous fluid. However, this fluid is extraneous to the peridotites, as shown by the Sr, H and O isotope compositions which suggest seawater penetration either during or after the final emplacement of the peridotite diapir.

Although the peridotite diapir was emplaced in granulitic gneisses of the pan-African deep continental crust, no evidence was found for a contribution of hydrous continental fluids in the production of the amphiboles present in the peridotite bodies of Zabargad Island.  相似文献   

Les eaux thermales des granites de Galice (Espagne): des eaux carbogazeuses aux eaux alcalines (Thermal waters from granites of Galicia (Spain): from CO₂-rich to high-pH waters)     

Gil Michard  Catherine Beaucaire 《Chemical Geology》1993,110(4):345-360

Thermal waters from Galicia, Spain, range from CO₂-rich, low-pH to high-pH, sulfide-rich waters, going through intermediate waters. In order to explain the variations of their chemical composition, we present a discussion of the major factors governing the water composition. We eiscuss especially the isochemical recrystallization of a granite. It is shown that the equilibrium mineral association of the water-alkali granite system is unique. For chalco-alkaline granites, two different associations are possible, depending on the chemical composition of the granite. The effect of an addition of CO₂ is also discussed.

Some criteria allowing a distinction between fully equilibrated and non-equilibrated waters are presented.

Finally, the composition of Galician waters is explained by different initial CO₂ contents and by different values of the degree of the granite-water reaction progress (ξ).

Résumé

Les eaux thermales de Galice (Espagne) vont des eaux riches en CO₂ aux eaux typiquement alcalines en passant par différents intermédiaires. Pour comprendre l'évolution de la composition chimique de ces eaux, nous avons été conduits à discuter, de façon plus générale, des facteurs majeurs qui fixent la composition minéralogique résultant d'une recristallisation isochimique du granite. Nous avons pu montrer que les granites alcalins se transformaient toujours en la même association minérale, tandis que les granites calcoalcalins pouvaient donner naissance à deux associations d'équilibre différentes. L'apport de CO₂ au système eau-roche peut faire passer d'une association minérale à l'autre, et, si le CO₂ est en grande quantité, à une association contenant un minéral de moins et du CO₂ en excès. On propose également, pour les solutions géothermales, des critères d'équilibre ou de non équilibre, basés sur la composition en éléments majeurs. La variation de composition des eaux de Galice peut alors s'interpréter par des teneurs variables en CO₂ et des degrés d'avancement différents de la réaction d'interaction eau-granite.  相似文献   

The solubility of fluorite in hydrothermal solutions,an experimental study     

Catherine K. Richardson  H.D. Holland 《Geochimica et cosmochimica acta》1979,43(8):1313-1325

The solubility of fluorite in NaCl solutions increases with increasing temperature at all ionic strengths up to about 100°C. Above this temperature, the solubility passes through a maximum and possibly a minimum with increasing temperature at NaCl concentrations of 1.0M or less, and increases continuously with increasing temperature at NaCl concentrations above 1.0M. At any given temperature, the solubility of fluorite increases with increasing salt concentration in NaCl, KCl and CaCl₂ solutions. The solubility follows Debye-Hückel theory for KCl solutions. In NaCl and CaCl₂ solutions, the solubility of fluorite increases more rapidly than predicted by Debye-Hückel theory: the excess solubility is due to the presence of NaF^c, CaF⁺, and possibly of Na₂F⁺. The solubility of fluorite in NaCl-CaCl₂ and in NaCl-CaCl₂-MgCl₂ solutions is controlled by the common ion effect and by the presence of NaF^c, CaF⁺, and MgF⁺. The solubility of fluorite in NaCl-HCl solutions increases rapidly with increasing initial HCl concentration; the large solubility increase is due to the presence of HF^c. It seems likely that complexes other than those identified in this study rarely play a major role in fluoride transport and fluorite deposition at temperatures below 300°C.  相似文献   

Fluorite deposition in hydrothermal systems   总被引：1，自引：0，他引：1  

Catherine K. Richardson  H.D. Holland 《Geochimica et cosmochimica acta》1979,43(8):1327-1335

During the formation of fluorite deposits fluorite is precipitated either as a consequence of changes in temperature and pressure along the flow path of hydrothermal solutions or due to fluid mixing, or as the result of the interaction of hydrothermal solutions with wall rocks.A decrease in temperature in the flow direction is the most appealing, though still unproven, mechanism of fluorite deposition in Mississippi Valley fluorite deposits.Mixing can produce solutions which are either undersaturated or supersaturated with respect to fluorite. The most important parameters are the temperature, the salinity, and the calcium and fluoride concentration of the fluids prior to mixing.A variety of wall rock reactions can lead to fluorite precipitation. Among these reactions which increase the pH of initially rather acid (pH ≤ 3) hydrothermal solutions are apt to be particularly important.  相似文献   

Mineral-chemistry and stable-isotope constraints on the magmatism, hydrothermal alteration, and related PGE–(base-metal sulphide) mineralisation of the Mesoarchaean Baula-Nuasahi Complex, India     

Thierry Augé  Catherine Lerouge 《Mineralium Deposita》2004,39(5-6):583-607

The Baula-Nuasahi Complex, on the southern flank of the Singhbhum Archaean nucleus in north-eastern India, exposes a series of Mesoarchaean igneous suites. These are (1) a gabbro–anorthosite unit, which is petrographically homogeneous, although mineral-chemistry data hint at a subtle eastward differentiation; (2) a peridotite unit (with three chromitite layers) together with (3) a pyroxenite unit which display cumulate textures, modal layering, and (for the peridotite unit) differentiation trends in both mineralogy and mineral chemistry; and (4) the Bangur gabbro (~3.1 Ga), which defines an oblong intrusion, crosscutting the older igneous suites in the southern part of the complex, with a curvilinear NW-trending apophysis, 2 km long and up to 40 m wide. Magmatic breccia comprising ultramafic and chromitite wall-rock clasts in a gabbro matrix is exposed at the contact of the main Bangur gabbro body and also forms the entire Bangur gabbro apophysis. Concentrations of platinum-group minerals (PGMs) are found where the breccia contains abundant chromitite clasts, and two types of platinum-group-element (PGE) mineralisation are recognised. Type 1 (Pt 1.1–14.2, Pd 0.1–2.1 ppm, with an average Pt/Pd=8–9) is a contact-type mineralisation which occurs in the breccia at the contact between the Bangur intrusion and its ultramafic host. The PGMs—Pt alloys (isoferroplatinum) and sulphides (braggite, malanite)—are enclosed by pyroxene and plagioclase, reflecting a magmatic origin. Significant wall-rock assimilation by the magma (giving rise to the Bangur gabbro) is indicated by changes in pyroxene composition and by the presence of relicts of chromite (from the host) now altered to secondary ferritchromite in the contact zone. Type 2 PGE mineralisation (Pt 0.3–1.6, Pd 1.8–6.0 ppm, with Pt/Pd~0.5–3.0) is restricted to the breccia apophysis of the Bangur gabbro where it occurs in the breccia matrix, associated with an intense hydrothermal alteration which does not exist in the contact zone. PGMs (PGE arsenides, tellurides, bismuthides and antimonides) and, where present, base-metal sulphides (BMSs) form intergrowths with hydrous silicates, reflecting a hydrothermal origin. Oxygen isotope geothermometry documents the main stages of hydrothermal alteration within a decreasing temperature range between 700–1,000 and 500–600 °C, and oxygen, hydrogen and sulphur isotopes show that the hydrothermal fluids were derived from the magma rather than an external source. Pervasive hydrothermal alteration in the breccia apophysis likely represents upward channelling of late-magmatic fluids along a narrow, near-vertical, subplanar conduit which led away from the main magma chamber. We suggest that Type 2 mineralisation was produced by late-magmatic hydrothermal remobilisation and reconcentration of Type 1 PGE mineralisation, and that the composition of the hydrothermal fluids controlled whether BMSs were enriched along with the PGMs.Editorial handling: P. Lightfoot  相似文献   

Mössbauer spectroscopic determination of Fe<Superscript>3+</Superscript>/Fe<Superscript>2+</Superscript> in synthetic basaltic glass: a test of empirical<Emphasis Type="Italic"> f</Emphasis>O<Subscript>2</Subscript> equations under superliquidus and subliquidus conditions     

Georg?M.?PartzschEmail author  Dominique?Lattard  Catherine?McCammon 《Contributions to Mineralogy and Petrology》2004,147(5):565-580

One of the most widely used methods to estimate magmatic oxygen fugacity involves the use of empirical equations relating fO₂ to the iron redox state in quenched silicate liquids; however none of the equations have been calibrated experimentally under subliquidus conditions at temperatures and oxygen fugacities that are relevant to natural magmas. To address this problem, we tested two empirical relationships [Eq. (1) in Kress and Carmichael 1991; Eq. (6) in Nikolaev et al. 1996] on synthetic glasses synthesized from a ferrobasaltic and a transitional alkali-basaltic composition at sub- to superliquidus temperatures (1,132–1,222°C) and controlled oxygen fugacities (FMQ=–2 to +1.4). Fe³⁺/Fe was determined using conventional and milliprobe Mössbauer spectroscopy, and verified using wet chemical analysis on selected samples. For the ferrobasaltic bulk composition SC1-P, both empirical models reproduce the Fe³⁺/Fe ratio of the quenched liquids generally within 0.03 for sub- as well as superliquidus temperatures, although agreement is worse at higher oxygen fugacities (FMQ>+1) at subliquidus temperatures. For the transitional alkali-basaltic composition 7159V-P, both models reproduce the Fe³⁺/Fe ratio of the quenched liquids generally within 0.04, although agreement is worse for both models at high oxygen fugacities (FMQ>+1). Such behaviour may be related to a change in melt structure, where a progressive change in Fe³⁺ coordination is inferred to occur as a function of Fe³⁺/Fe based on Mössbauer center shifts. Recasting the data in terms of oxygen fugacity shows that calculated oxygen fugacities deviate from those actually maintained during the equilibration of the sample material by generally no more than 0.5 log-bar unit, with maximum deviations that only rarely exceed one log-bar unit.Editorial responsibility: J. Hoefs  相似文献