The Mediterranean coralligenous substratum is a hard bottom of biogenic origin, mainly composed of calcareous algae, growing in dim light conditions. Sponges are among of the most representative taxa of the coralligenous assemblages, with more than 300 recorded species of different habits: massive, erect, boring and insinuating. When sponges die, their siliceous spicules remain trapped in the biogenic concretion, offering the opportunity to describe the coralligenous spongofauna over a very long span of time, virtually dating back to a large part of the Holocene period. The data reported here were obtained from core samples collected from four coralligenous concretions. Each block was collected in a different locality of the Ligurian Sea: Santo Stefano Shoals, Bogliasco, Punta del Faro (Portofino Promontory) and Punta Manara. Radiocarbon age determinations indicate for these conglomerates a maximal age between 1600 and 3100 years. The spicules trapped in the cores show deep dissolution marks in the form of circular holes on their surface or present an enlargement of the axial canal. However, their original shape, generally intact, suggests the absence of mechanical injuries and allows a tentative identification at the species level. The analysis of these old spicules reveals an ancient sponge assemblage composed of 30 recognisable species. This indicates that almost one half of the sponge community today settled on coralligenous substrata has been present in the conglomerates for their entire existence. 相似文献
Mass movements varying in type and size, some of which are periodically reactivated, affect the urban area of Avigliano. The disturbed and remoulded masses consist of sandy–silty or silty–clayey plastic material interbedded with stone fragments and conglomerate blocks. Five landslides that were markedly liable to rainfall-associated instability phenomena were selected.
The relationships between landslides and rainfall were investigated using a hydrological and statistical model based on long-term series of daily rainfall data. The model was used to determine the return period of cumulative daily rainfall over 1–180 days. The resulting hydrological and statistical findings are discussed with the aim of identifying the rainfall duration most critical to landslides.
The concept of a precipitation threshold was generalized by defining some probability classes of cumulative rainfall. These classes indicate the thresholds beyond which reactivation is likely to occur. The probability classes are defined according to the return period of the cumulative rainfall concomitant with landslide reactivation. 相似文献
The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive
presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite
(websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted
major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (LaN/YbN = 8–17) and depleted in Nb. It has high radiogenic Sr (87Sr/86Sr(270) = 0.7055–0.7093), low radiogenic Nd (ɛNd(270) = −1 to −3) and EMII-like Pb isotopes. Two pyroxenite – peridotite sections examined in detail show the virtual absence of
major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile
major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes
exhibit LREE-enriched patterns (LaN/YbN ∼16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative
Sr anomaly, but incompatible element concentration ∼1.9 (Sr) to ∼7.9 (Ti) times higher than that of coexisting clinopyroxene.
Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the
sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg## and higher in Na2O, and spinels and phlogopite are richer in Cr2O3. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical
signatures of “carbonatite” metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and
Sr concentrations than amphibole (amph/cpxDREE,Sr = 0.7–0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at
Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained
by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because
of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting
temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with
the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling,
the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics.
In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways
through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process,
which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition
is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence
of some CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting
in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and
with the occurrence of modal metasomatism before peridotite incorporation in the crust.
Received: 20 July 1998 / Accepted: 28 October 1998 相似文献
An analysis of Ca ii spectroheliograms obtained at Catania Astrophysical Observatory throughout the years 1967–1977 has been carried out, to throw light on the complex relationship linking (Ternullo, 1986) the angular rotation rate of Ca plages with their age, as well as with solar cycle phase, and with latitude. Given the rotation law w =, a + b sin2I, solar-cycle-related oscillatory properties both of a and b coefficients are described, both for young and old Ca plages.The aging-dependent rate-increments vary, for each epoch, both in module and sign with latitude; that results in deep distortions of the differential rotation latitudinal profile, which exhibits, when old plages are taken into account, a fine structure (Ternullo, 1987). Such a fine structure is absent in the young plage differential rotation profile.Throughout the time interval under examination, the torsional waves observed by Howard and LaBonte (1980) have appeared to be in close spatial relationship with the latitude bands where aging-dependent rate increments occur.相似文献
The transfer equations for the Stokes parameters in the presence of magnetic field and under the hypothesis of LTE are derived in an original way by the use of density matrix techniques.The results are substantially the same as those previously obtained by other authors. We finally compare our results to the previous ones in order to clarify some discrepancies still present in the literature. 相似文献
We study the general relativistic orbital equation and using a straightforward perturbation method and a mathematical device
first introduced by d’Alembert, we work out approximate expressions of a bound planetary orbit in the form of trigonometrical
polynomials and the first three terms of the power series development of the perihelion advance. The results are applied to
a more precise determination of the total mass of the double pulsar J0737-3039. 相似文献
This paper presents closed form expressions linking the ultimate bearing capacity to the ultimate curvature of rectangular RC sections subjected to axial load and bending moment acting in one of the two symmetry planes of the section. With respect to possible simplified formulations the following effects are also considered: confinement of the concrete, hardening of the longitudinal reinforcement, and presence of reinforcing bars distributed orthogonally to the neutral axis. The formulation is proposed in dimensional terms after a preliminary definition of the geometrical and mechanical parameters governing the structural response of the class of sections considered. The analytical expressions derived using the proposed approach also allow one to determine the compression level that makes the ultimate bending moment maximum as well as to evaluate the curvature corresponding to the first yielding of the principal reinforcement in tension. Comparing this value of curvature with the ultimate one, an approximate estimation of the available ductility of curvature of the section can be made. The analytical procedure is validated by comparing the results with those obtained using a typical numerical approach. Some experimental results are also considered. 相似文献