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151.
Glass samples from alkali-trachytic pumice of the Lower Member (LM, level LM1) and Upper Member (UM) of the Neapolitan Yellow Tuff at Campi Flegrei Caldera were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy. The EPR data show that the relative occupancy of the different iron sites varies in samples from the Lower Member LM1 and Upper Member of the eruptive sequence. The LM1 glasses are characterized by a more homogeneous distribution of the cationic site population of iron compared with UM and by a different Fe3+/Al3+ substitution in the C2v symmetrical sites of the silicate framework. 29Si MAS NMR spectra on glasses indicate that LM1 glasses are more depolymerized than UM glasses, indicating the existence of structurally distinct magma batches. 27Al MAS NMR spectra indicate the occurrence in UM glasses of aluminum in both tetrahedral and octahedral coordination; the observed AlVI is believed to be a primary feature of the glass, since 27Al CP-MAS NMR experiments do not reveal detectable esa-hydrated Al. The Al in sixfold coordination could be ascribed either to the more abundant alkali–OH terminations in UM, in which alkalies would be extracted from their charge-balance role for Al, thus altering its fourfold structural position, or to permanent compaction during fast cooling and decompression of high-pressure melts. 23Na CP-MAS NMR experiments indicate the existence of Na–OH groups in both LM1 and UM glasses, relatively more abundant in the latter. Although preliminary, the results indicate the possible application of atomic-scale studies to variables that determine the regimes of explosive volcanism. Received: 1 July 1998 / Accepted: 1 October 1998  相似文献   
152.
We present an idealized network model for storm surges in the Wadden Sea, specifically including a time-dependent wind forcing (wind speed and direction). This extends the classical work by H.A. Lorentz who only considered the equilibrium response to a steady wind forcing. The solutions obtained in the frequency domain for the linearized shallow-water equations in a channel are combined in an algebraic system for the network. The velocity scale that is used for the linearized friction coefficient is determined iteratively. The hindcast of the storm surge of 5 December 2013 produces credible time-varying results. The effects of storm and basin parameters on the peak surge elevation are the subject of a sensitivity analysis. The formulation in the frequency domain reveals which modes in the external forcing lead to the largest surge response at coastal stations. There appears to be a minimum storm duration, of about 3–4 h, that is required for a surge to attain its maximum elevation. The influence of the water levels at the North Sea inlets on the Wadden Sea surges decreases towards the shore. In contrast, the wind shearing generates its largest response near the shore, where the fetch length is at its maximum.  相似文献   
153.
Origin of sedimentary organic matter in the north-western Adriatic Sea   总被引:1,自引:0,他引:1  
In order to evaluate the origin and the transformation of organic matter on the shallow shelf of the NW Adriatic Sea, organic carbon, total nitrogen and stable isotope ratios of organic carbon were analysed in riverine suspended matter and sediments as well as in marine suspended and sedimentary organic matter, in marine phytoplankton and zooplankton.The deposition of organic matter is influenced by fine sediment concentration. Surface sediments were characterised by highly variable biogeochemical conditions on the sea floor, whereas sub-surface sediments showed a more homogeneous hypoxic/anoxic environment.Low Corg/N ratio and high organic carbon and nitrogen concentrations in riverine suspended organic matter indicate an important contribution of freshwater phytoplankton within rivers, particularly during low flow regimes, which adds to the marine phyto- and zooplankton at shelf locations.In order to evaluate the importance of terrestrial, riverine and marine sources of OM in shelf sediments, a three end-member mixing model was applied to shelf surface sediments using 13C/12C values for organic matter and N/C ratios. The model showed an elevated contribution of terrestrial organic substances at intermediate depths (10–15 m), mostly corresponding to an area of coarser grain-size, whereas the riverine and marine organic fractions were mainly accumulating near the coast and offshore, respectively.  相似文献   
154.
Statistical analysis of laboratory-measured compressional wave (sound) velocity, porosity, wet bulk density, and selected textural parameters of surface sediments from the Barents Sea reveals that clay content and mean grain size are the best indices to sound velocity. These parameters are followed closely by porosity and wet bulk density, while sand content provides the weakest index. Although Barents Sea surface sediments are characterized by fairly high variability, the results of the present study are in general agreement with studies of similar sediment types in other continental shelf environments.  相似文献   
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