排序方式： 共有18条查询结果，搜索用时 15 毫秒
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Ramón A. López‐Pérez Luis E. Calderón‐Aguilera Héctor Reyes‐Bonilla José D. Carriquiry Pedro Medina‐Rosas Amilcar L. Cupul‐Magaña María D. Herrero‐Pérezrul Héctor A. Hernández‐Ramírez Miguel Á. Ahumada‐Sempoal Betsabe M. Luna‐Salguero 《Marine Ecology》2012,33(4):407-416
Corals in the Eastern Pacific extend south from the Gulf of California to Ecuador and oceanic Chile, and west from Colombia to Clipperton Atoll. Nevertheless, large stretches of the Mexican Pacific remain fundamentally unstudied. Therefore, to assess the current conditions of coral communities, a coastal fringe ～300 km long (17°40′ N, 101°39′ W to 16°46′ N, 99°49′ W) was surveyed within the Southern Mexican Pacific, between 2005 and 2009. Fifteen stony coral species were identified at 13 coral communities and six Pocillopora‐dominated fringing reefs, with Pocillopora verrucosa and Pocillopora damicornis the primary contributing taxa. Reef development was identified in embayments or behind rocks or islands that offered shelter from northern and northwestern winds. Observations of Pocillopora effusus, Pocillopora inflata, Porites lobata, Pavona clavus, and Pavona varians expanded the species known geographic ranges by several degrees of latitude, suggesting reef building fauna comprised a mixture of widespread and relatively rare Eastern Pacific corals. Results indicated greater live coral cover in the Ixtapa‐Zihuatanejo area (15–73%) than in the Acapulco localities, which had high algal dominance; the reefs in the latter region exhibited high erosion. Regional differences are likely the result of long‐standing anthropogenic pressures around Acapulco since 1950, when it became an important tourist destination. This paper is the first detailed report of ecologically stressed corals and coral reefs from the state of Guerrero on the Mexican Southern Pacific coast. 相似文献
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János Kovács Szabolcs Á. Fábián Gábor Varga Karoly Németh Corina Risso Francisco Nullo Gabor Kereszturi Titusz Bugya Szabolcs Á. Fábián Noémi L. Görcs István P. Kovács Bertalan Radvánszky Gabriella Barta Rudolf Musil Alice Ghiselli Marzio Merazzi Andrea Strini Roberto Margutti Michele Mercuriali Rauf Gardashov Daria Gushchina Boris Dewitte Martin Michálek Marián Putiš Christoph A. Hauzenberger Jindřich Šancer Martin Štrejbar Aneta Maleňáková George Migiros George D. Bathrellos Hariklia D. Skilodimou Theodoros Karamousalis 《Central European Journal of Geosciences》2011,3(2):229-229
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Á.S. Dias R.A. Mills R.N. Taylor P. Ferreira F.J.A.S. Barriga 《Chemical Geology》2008,247(3-4):339-351
Hydrothermal sediment mineralogy and geochemistry can provide insights into seafloor mineralization processes and changes through time. We report a geochemical investigation of a short (22 cm) near-vent hydrothermal metalliferous sediment core from the Lucky Strike site (LS), on the Mid-Atlantic Ridge (MAR). The sediment was collected from the base of an active white smoker vent and comprises pure hydrothermal precipitates, mainly chalcopyrite, sphalerite, pyrite and barite, with negligible detrital and biogenic inputs. Geochemically, the core is enriched in elements derived from high-temperature hydrothermalism (Fe, Cu, Zn and Ba) and depleted in elements derived from low-temperature hydrothermalism (Mn), and metasomatism (Mg). The U/Fe content ratio is elevated, particularly in the deeper parts of the core, consistent with uptake from seawater associated with sulphide alteration. Rare earth elements (REE) concentrations are low and chondrite-normalized patterns are characteristic of high-temperature vent fluids with an enrichment in light REE and a pronounced positive Eu anomaly. A stronger positive Eu anomaly associated with higher Lan/Smn at the core top is controlled by barite precipitation. The hydrothermal influence on the REE decreases downcore with some evidence for a stronger seawater influence at depth. Nd isotopes also exhibit an increased detrital/seawater influence downcore. Pb isotope ratios are uniform and plot on the Northern Hemisphere Reference Line in a small domain defined by LS basalts and exhibit no detrital or seawater influence. Lucky Strike sediments are derived from high-temperature mineralization and are overprinted by a weak seawater–sediment interaction when compared with other Atlantic hydrothermal sites such as TAG. The larger seawater input and/or a larger detrital contribution in deeper layers can be explained by variable hydrothermal activity during sediment formation, suggesting different pulses in activity of the LS hydrothermal system. 相似文献
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The Saldanha hydrothermal field is hosted atop a mafic–ultramafic seamount, located at a non-transform offset on the Mid-Atlantic Ridge. Previous observations revealed a field where transparent low-temperature fluids discharge through centimeter-sized vents without the formation of chimney structures. We present geochemical and stable isotope (O and C) analyses from sediment samples collected at this field, both at and far from the vent area. Most sediments, including some directly adjacent to orifice vents, are pelagic oozes with only a weak hydrothermal overprinting. Hydrothermal precipitates are characterized by Fe–Mn oxyhydroxides and a minor amount of Cu–Zn sulphide minerals. However, one of the cores (SCD7) collected at the vent area shows a much stronger hydrothermal signature. This core is composed of a matrix of serpentine + talc ± chlorite with high porosity, where calcite + chalcopyrite + sphalerite/wurtzite ± pyrite–pyrrhotite were precipitated. In this core, metal enrichments, REE patterns, and the oxygen and carbon isotope composition of calcites indicate that mineralization must have occurred in the subsurface by high-temperature fluids, with minor mixing with seawater and with a significant magmatic contribution. Thus, while most samples confirm previous findings indicating that Saldanha hydrothermal fluid discharge is mainly diffuse and of low temperature, data from core SCD7 suggest that areas of high-temperature hydrothermal activity also occur, where temperatures of the fluids could reach > 260 °C and maximum temperatures of 330 °C. We suggest that fluids can flow through faults at the top of the mount and discharge in a more focused way through vent orifices, producing intense hydrothermal alteration of the sediments. At these locations complex hydrothermal processes occur, including reactions of the hydrothermal fluids with mafic and ultramafic rocks and magma degassing, as suggested by the carbon isotope composition of hydrothermal calcites. The high temperature of the fluid inferred from the geochemistry of the hydrothermal minerals requires a significant heat input to the system, suggesting an additional magmatic heat source to the already proposed exothermic serpentinization reactions. 相似文献
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International Journal of Earth Sciences - One of the key requirements for a Global Stratotype Section and Point (GSSP) is the absence of tectonic disturbance. The GSSP for the... 相似文献
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This study demonstrates discernible biosilicification of natural microbial mats through batch laboratory experiments. Identification of the geochemical requirements for this process to occur includes thermodynamically favorable, but sluggish silica reaction kinetics associated with acidic conditions, and the necessity for colloidal silica rather than dissolved silicic acid species. This study provides the first results to bridge the apparent literature discrepancy between widespread, in-situ observations of microbial silicification, and the inability to demonstrate a detectable microbial impact in this process under well-constrained laboratory conditions. We compared the silica scavenging abilities of three natural microbial mats collected from Yellowstone National Park (YNP) hotsprings, relative to those of both abiotic particle (TiO2) and solution controls at constant, near-saturated aqueous silica concentrations, while experimental pH and temperature conditions were varied, using both dissolved and colloidal SiO2 forms. We specifically evaluated three microbial mats sampled from YNP sites all exhibiting saturation with respect to amorphous SiO2, but possessing variable pH and temperature conditions that should reflect differential kinetics (and therefore biological opportunity) relative to silica polymerization: (1) most biologically favorable, acidic-mesophile (AM: pH 3, T = 35 °C); (2) biologically possible, but less opportune, alkaline, mesophile (ALK-M: pH 8, T = 35 °C) and (3) unlikely to be biologically favorable, alkaline-thermophile (ALK-T: pH 8, T = 80 °C). Comparison of field and laboratory results substantiates the requirements for thermodynamically favorable, but kinetically slower SiO2 polymerization conditions. Results show that acidic moderate temperature conditions were required for an observable biosilicification impact. Moreover, they also identified for the first time, the necessity specifically for colloidal silica forms which are surface bound under acidic pH conditions, to distinguish discernible biosilicification compared to mineral particle controls. Results also highlight the important influence of mat surface characteristics in this process, specifically the extent of live, non-mineralized, exposed biological mat surface. Greater colloidal SiO2 scavenging abilities are associated with non-mineralized microbial mat surfaces than with mineral particle surfaces or microbial mat surfaces encrusted with authigenic silica. These results are the first to demonstrate that biosilicification can be a microbially mediated, discernible geobiological process, shedding new light on the longstanding argument in the literature, and opening the door for more sensitive evaluation of this phenomenon in natural systems. 相似文献
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Á. Bazsó R. Dvorak E. Pilat-Lohinger V. Eybl Ch. Lhotka 《Celestial Mechanics and Dynamical Astronomy》2010,107(1-2):63-76
It is known since the seminal study of Laskar (1989) that the inner planetary system is chaotic with respect to its orbits and even escapes are not impossible, although in time scales of billions of years. The aim of this investigation is to locate the orbits of Venus and Earth in phase space, respectively, to see how close their orbits are to chaotic motion which would lead to unstable orbits for the inner planets on much shorter time scales. Therefore, we did numerical experiments in different dynamical models with different initial conditions—on one hand the couple Venus–Earth was set close to different mean motion resonances (MMR), and on the other hand Venus’ orbital eccentricity (or inclination) was set to values as large as e = 0.36 (i = 40°). The couple Venus–Earth is almost exactly in the 13:8 mean motion resonance. The stronger acting 8:5 MMR inside, and the 5:3 MMR outside the 13:8 resonance are within a small shift in the Earth’s semimajor axis (only 1.5 percent). Especially Mercury is strongly affected by relatively small changes in initial eccentricity and/or inclination of Venus, and even escapes for the innermost planet are possible which may happen quite rapidly. 相似文献
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Griselda V. Garaffo María L. Jaubet María de los Á. Sánchez María S. Rivero Eduardo A. Vallarino Rodolfo Elías 《Marine Ecology》2012,33(3):272-279
The intertidal zone around Mar del Plata’s sewage discharge (38° S, 57° W) is characterized by the presence of the non-indigenous spionid polychaete Boccardia proboscidea. This species has been classified as tolerant to moderate and high levels of organic contamination. During early stages of colonization this species can reach very high densities without suffering from interspecific competition, building biogenic structures such as reef. The aim of this work was evaluate the recovery time of the reef to a small-scale experimental physical disturbance. Five independent rocks with B. proboscidea reefs on them were selected randomly and a small-scale disturbance was generated by corers (16 cm2). At the beginning of the experiment, six corers were collected in each reef to produce the disturbance. The original density of B. proboscidea in each reef was determined in these corers. The reefs with disturbed corer were sampled on successive days to assess the reef recovery time. The number of larvae, juveniles and adults was quantified. Polychaete reefs had very high densities before the disturbance (mean density: 1,021,250 ind m−2). Boccardia proboscidea reefs had a fast recovery rate after small-scale disturbance. Four days after disturbance the density reached about 50% of the original density and after 5 days the disturbed reefs could not be differentiated from the undisturbed reef. The initial recolonization of disturbed patches occurs as a result of migration which appears to be driven by larvae and juveniles. These reefs can not be seen as biodiversity hotspots and the presence of the species demonstrates great environmental deterioration. 相似文献
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Titusz Bugya Szabolcs Á. Fábián Noémi L. Görcs István P. Kovács Bertalan Radvánszky 《Central European Journal of Geosciences》2011,3(2):119-128
This article presents results from the survey conducted on Dunaszekcs? loess bluff after the last major rotational sliding event in 2008. The study area is a region of 25×30 m located on loess bluff close to the recent scarp. The relative elevation change of the surface was surveyed in 2.5×5 m grid network in relation to a marked base point. The survey was conducted using simple equipment such as analogue theodolite and leveller with regular time interval during a year and control measurements were taken after six months. It was assumed that measurements to the nearest cm are sufficient to recognize vertical displacements of the surface. The study focused on identifying the pattern of general vertical movements for the study area by the relative movements of individual points. Our results show significant cm scale vertical displacements. Most of the grid points have a slow decreasing tendency, but close to the scarp a more significant displacement was found. The main character of the spatial pattern is subsidence, which is more definitive on southern part of the study area than the northern part. Our observations correlate with the broader geomorphological characteristics of loess bluffs along the Danube. 相似文献