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81.
Marco Patacci Mattia Marini Fabrizio Felletti Andrea Di Giulio Massimo Setti William McCaffrey 《Sedimentology》2020,67(5):2625-2644
The partitioning of different grain-size classes in gravity flow deposits is one of the key characteristics used to infer depositional processes. Turbidites have relatively clean sandstones with most of their clay deposited as part of a mudstone cap or as a distal mudstone layer, whereas sand-bearing debrites commonly comprise mixtures of sand grains and interstitial clay; hybrid event beds develop alternations of clean and dirty (clay-rich) sandstones in varying proportions. Analysis of co-genetic mudstone caps in terms of thickness and composition is a novel approach that can provide new insight into gravity flow depositional processes. Bed thickness data from the ponded Castagnola system show that turbidites contain more clay overall than do hybrid event beds. The Castagnola system is characterized by deposits of two very different petrographic types. Thanks to this duality, analyses of sandstone and mudstone composition allow inference of which proportion of the clay in each of the deposit types was acquired en route. In combination with standard sedimentological observations the new data allow insight into the likely characteristics of their parent flows. Clean turbidites were deposited by lower concentration, long duration, erosive, muddy turbidity currents which were more efficient at fractionating clay particles away from their basal layer. Hybrid event beds were deposited by shorter duration, higher-concentration, less-erosive sandier flows which were less efficient at clay fractionation. The results are consistent with data from other turbidite systems (for example, Marnoso-arenacea). The approach represents a new method to infer the controls on the degree of clay partitioning in gravity flow deposits. 相似文献
82.
Magnetic history of a dyke on Mount Etna (Sicily) 总被引:1,自引:0,他引:1
During the 1989 eruption of Mount Etna, two fracture systems, trending c. N45°E and N150°E, opened at the foot of its 3000 m high SE Crater and propagated quickly downslope to distances of ≈3 and 7 km, respectively. The northeastern fracture fed a flank eruption, whereas the southeastern fracture remained dry and offered contrasting volcanological and geophysical evidence of the presence of magma at a shallow depth. During the opening of this non-eruptive fracture system, a differential magnetic network was set up on a short profile across its distant extremity. Initially, the magnetic field did not display any change along the profile between frequent surveys. However, repeated measurements at intervals of about 3 months for two years revealed the slow build-up of a 130 nT anomaly. The anomaly vanishes laterally within 0.2 km of the surface expression of the fracture system. This exceptional set of observations constrains the location and time of cooling of a shallow dyke. The increase in magnetization of the dyke inferred by the rate of growth of the anomaly leads to the interpretation that the dyke was emplaced near the end of the eruption. 相似文献
83.
84.
Giorgio Socal Francesco Acri Mauro Bastianini Fabrizio Bernardi Aubry Franco Bianchi Daniele Cassin Joan Coppola Amelia De Lazzari Vinko Bandelj Gianpiero Cossarini Cosimo Solidoro 《Marine Ecology》2008,29(4):449-468
This paper reports on the main biogeochemical properties of the Northern Adriatic Sea in the period May 2003–November 2006 within the framework of the European program INTERREG III Italy‐Slovenia. Spatial and temporal distributions of water density, dissolved oxygen, nutrients (nitrogen, phosphorous and silicon) and chlorophyll a are presented. Multivariate methods such as fuzzy k‐means, self‐organising maps and cluster analysis were used to identify the different water masses and to characterise the temporal and spatial variability of the main biogeochemical features present in the area. The results confirm that the Po River outflows and the meteorological forcing factors are the main components triggering the alternation of stratification and mixing of the water column and that strongly affect the trophic state of the basin. In general, oligotrophic conditions dominate, and were more pronounced offshore, but mesotrophy occurred episodically in May 2004 and July 2005, when phytoplankton blooms were observed concomitant with vertical stability of the water column. A marked interannual variability was also observed, supporting the importance of maintaining long‐term observations of the basin. 相似文献
85.
Billy P. Glass Luigi Folco Matteo Masotta Fabrizio Campanale 《Meteoritics & planetary science》2020,55(2):253-273
We examined 16 white opaque inclusions exposed on two polished slices of a Muong Nong‐type Australasian tektite from Muong Phin, Laos. The inclusions usually consist of a core, surrounded by a froth layer, and a quartz neoblast layer. The cores are composed primarily of a mixture of silica glass, coesite, and quartz in varying proportions. A thin (up to ~4 μm) layer of SiO2‐poor glass enriched in FeO, MgO, CaO, Al2O3, and TiO2 is observed as a bright halo in backscattered electron images around the quartz neoblasts and in places contains μm‐sized crystals, which may be Fe,Mg‐rich spinel. The distribution and textural relationships between the coesite‐bearing inclusions and the tektite matrix point to an in situ formation of the coesite due to an impact, rather than to infall, from a nearby impact, into tektite melt produced by the aerial burst of a bolide. The quartz neoblasts probably formed by crystallization of silica melt squeezed out of the inclusion core during the development of the froth layer. The bright halo may be the result of silica diffusing from the adjacent tektite melt into the growing quartz neoblasts. We propose that the survival of coesite was possible due to the froth layer that acted as a heat sink during bubble expansion and then as a thermal insulator. 相似文献
86.
87.
Paola Del Negro Mauro Celussi Erica Crevatin Alessandro Paoli Fabrizio Bernardi Aubry Alessandra Pugnetti 《Marine Ecology》2008,29(3):375-386
The prokaryotic community, both in terms of abundance and activity (exoenzymatic hydrolysis of proteins, polysaccharides and phosphorylated molecules and leucine uptake), was investigated seasonally for a 3‐year period (2004–2006) in the Gulf of Venice (northern Adriatic Sea). By focusing on spatial and temporal variability, the prokaryote dynamics showed significant variations on a horizontal and seasonal scale, but no substantial differences were observed among years. The basin‐scale variability was mainly influenced by allochthonous inputs from the Po river and the Venice Lagoon, which were the main source of nutrients, acting as a ‘bottom up’ control on prokaryotes. On a seasonal scale, all the microbial parameters (except the polysaccharide degradation) showed significant variations following the temperature fluctuations. The annual rate of change was very low for all the chemical, physical and biological parameters and only the abundance and phosphatase activity of the prokaryotes differed significantly among years. 相似文献
88.
Matteo Lelli Sergio Grassi Michele Amadori Fabrizio Franceschini 《Environmental Earth Sciences》2014,71(9):3907-3919
A detailed hydrogeochemical study of groundwater in the Cecina coastal plain (Livorno province, Italy) and its inner sectors was undertaken in 2008, as chemical analyses carried out on groundwater since 2006 have revealed Cr(VI) concentrations of up to 49 μg/L (well above the permissible limit of 5 μg/L). Ophiolite outcrops are present throughout the study area, and their fragments likely represent a significant portion of the existing multilayered aquifer skeleton. Waters delivered by the serpentinite outcrops have a typically Mg–HCO3 composition, whereas those of the coastal plain are prevailingly of the Ca/Mg–HCO3 type with significant Mg contents. Significant NO3 contamination characterises the studied coastal plain, and an interesting negative correlation exists between Cr(VI) and both NO3 and SO4 deriving from the widespread use of (NH4)2SO4 as a farm fertilizer. Chromium speciation calculations carried out using the EQ3NR code reveal that the prevailing Cr(VI) species in solution is CrO4 2?; however, CaCrO4° and MgCrO4° neutral complexes represent significant percentages (up to 42 %). These findings suggest that the mobility and consequently the bioavailability of Cr(VI) can be significantly enhanced by these neutral complexes, which are not considered to be affected by adsorption/desorption processes. The Cr(VI) source, investigated by means of the Mg/SiO2 molar ratio, seems to be represented mainly by Mg-bearing minerals of the chlorite group. Petrographic observations confirm the occurrence of this mineral group. The interaction between rainwater and the local serpentinite rock was simulated at different $P_{{{\text{CO}}_{ 2} }}$ and $P_{{{\text{O}}_{ 2} }}$ conditions by reaction path modelling using the EQ3/6 software package. $P_{{{\text{O}}_{ 2} }}$ was varied in accordance with the assumption that redox conditions are determined in part by NO3. Results are in good agreement with experimental data on spring waters and subordinately with data on some coastal plain groundwater, which plot in a rather wide $P_{{{\text{CO}}_{ 2} }}$ and $P_{{{\text{O}}_{ 2} }}$ field. Although the dissolved Cr content is mostly of natural origin, fertilization may affect its fate. 相似文献
89.
Geoffrey D. Bromiley Fabrizio Nestola Ming Zhang 《Geochimica et cosmochimica acta》2010,74(2):705-445
The solubility and incorporation mechanisms of water in synthetic and natural MgAl2O4 spinel have been investigated in a series of high-pressure/temperature annealing experiments. In contrast to most other nominally anhydrous minerals, natural spinel appears to be completely anhydrous. On the other hand, non-stoichiometric Al-rich synthetic (defect) spinel can accommodate several hundred ppm water in the form of structurally-incorporated hydrogen. Infrared (IR) spectra of hydrated defect spinel contain one main O-H stretching band at 3343-3352 cm−1 and a doublet consisting of two distinct O-H bands at 3505-3517 cm−1 and 3557-3566 cm−1. IR spectra and structural refinements based on single-crystal X-ray data are consistent with hydrogen incorporation in defect spinel onto both octahedral and tetrahedral O-O edges. Fine structure of O-H bands in IR spectra can be explained by partial coupling of interstitial hydrogen with cation vacancies, or by the effects of Mg-Al disorder on the tetrahedral site. The concentration of cation vacancies in defect spinel is a major control on hydrogen affinity. The commercial availability of large single crystals of defect spinel coupled with high water solubility and similarities in water incorporation mechanisms between hydrous defect spinel and hydrous ringwoodite (Mg2SiO4) suggests that synthetic defect spinel may be a useful low-pressure analogue material for investigating the causes and consequences of water incorporation in the lower part of Earth’s mantle transition zone. 相似文献
90.
Antigorite equation of state and anomalous softening at 6 GPa: an in situ single-crystal X-ray diffraction study 总被引:1,自引:0,他引:1
Fabrizio Nestola Ross J. Angel Jing Zhao Carlos J. Garrido Vicente López Sánchez-Vizcaíno Giancarlo Capitani Marcello Mellini 《Contributions to Mineralogy and Petrology》2010,160(1):33-43
The compressibility of antigorite has been determined up to 8.826(8) GPa, for the first time by single crystal X-ray diffraction in a diamond anvil cell, on a specimen from Cerro del Almirez. Fifteen pressure–volume data, up to 5.910(6) GPa, have been fit by a third-order Birch–Murnaghan equation of state, yielding V 0 = 2,914.07(23) Å3, K T0 = 62.9(4) GPa, with K′ = 6.1(2). The compression of antigorite is very anisotropic with axial compressibilities in the ratio 1.11:1.00:3.22 along a, b and c, respectively. The new equation of state leads to an estimation of the upper stability limit of antigorite that is intermediate with respect to existing values, and in better agreement with experiments. At pressures in excess of 6 GPa antigorite displays a significant volume softening that may be relevant for very cold subducting slabs. 相似文献