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1.
Davide Lazzati Rosalba Perna Gabriele Ghisellini 《Monthly notices of the Royal Astronomical Society》2001,325(4):L19-L23
The recent detection of a transient absorption feature in the X-ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma-ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor. 相似文献
2.
Gabriele Giuli Eleonora Paris Giovanni Pratesi Christian Koeberl Curzio Cipriani 《Meteoritics & planetary science》2003,38(8):1181-1186
Abstract— Libyan Desert Glass (LDG) is an enigmatic type of glass that occurs in western Egypt in the Libyan Desert. Fairly convincing evidence exists to show that it formed by impact, although the source crater is currently unknown. Some rare samples present dark‐colored streaks with variable amounts of Fe, and they are supposed to contain a meteoritic component. We have studied the iron local environment in an LDG sample by means of Fe K‐edge highresolution X‐ray absorption near edge structure (XANES) spectroscopy to obtain quantitative data on the Fe oxidation state and coordination number in both the Fe‐poor matrix and Fe‐rich layers. The pre‐edge peak of the high‐resolution XANES spectra of the sample studied displays small but reproducible variations between Fe‐poor matrix and Fe‐rich layers, which is indicative of significant changes in the Fe oxidation state and coordination number. Comparison with previously obtained data for a very low‐Fe sample shows that, while iron is virtually all trivalent and in tetrahedral coordination ([4]Fe3+) in the low‐Fe sample, the sample containing the Fe‐rich layers display a mixture of tetra‐coordinated trivalent iron ([4]Fe3+) and penta‐coordinated divalent iron ([5]Fe2+), with the Fe in the Fe‐rich layer being more reduced than the matrix. From these data, we conclude the following: a) the significant differences in the Fe oxidation state between LDG and tektites, together with the wide intra‐sample variations in the Fe‐oxidation state, confirm that LDG is an impact glass and not a tektite‐like glass; b) the higher Fe content, coupled with the more reduced state of the Fe, in the Fe‐rich layers suggests that some or most of the Fe in these layers may be directly derived from the meteoritic projectile and that it is not of terrestrial origin. 相似文献
3.
Geophysical evidence of mud diapirism on the Mediterranean Ridge accretionary complex 总被引:2,自引:0,他引:2
A. Camerlenghi M. B. Cita B. Della Vedova N. Fusi L. Mirabile G. Pellis 《Marine Geophysical Researches》1995,17(2):115-141
Mud volcanoes, mud cones, and mud ridges have been identified on the inner portion of the crestal area, and possibly on the inner escarpment, of the Mediterranean Ridge accretionary complex. Four areas containing one or more mud diapirs have been investigated through bathymetric profiling, single channel seismic reflection profiling, heat flow measurements, and coring. A sequence of events is identified in the evolution of the mud diapirs: initially the expulsion on the seafloor of gasrich mud produces a seafloor depression outlined in the seismic record by downward dip of the host sediment reflectors towards the mud conduit; subsequent eruptions of fluid mud may create a flat topped mud volcano with step-like profile; finally, the intrusion of viscous mud produces a mud cone.The origin of the diapirs is deep within the Mediterranean Ridge. Although a minimum depth of about 400 m below the seafloor has been computed from the hydrostatic balance between the diapiric sediments and the host sediments, a maximum depth, suggested by geometric considerations, ranges between 5.3 and 7 km. The presence of thermogenic gas in the diapiric sediments suggests a better constrained origin depth of at least 2.2 km.The heat flow measured within the Olimpi mud diapir field and along a transect orthogonal to the diapiric field is low, ranging between 16 ± 5 and 41 ± 6 mW m–2. Due to the presence of gas, the thermal conductivity of the diapiric sediments is lower than that of the host hemipelagic oozes (0.6–0.9 and 1.0–1.15 W m–1 K–1 respectively).We consider the distribution of mud diapirs to be controlled by the presence of tectonic features such as reverse faults or thrusts (inner escarpment) that develop where the thickness of the Late Miocene evaporites appears to be minimum. An upward migration through time of the position of the décollement within the stratigraphic column from the Upper Oligocene (diapiric sediments) to the Upper Miocene (present position) is identified. 相似文献
4.
The Agulhas Ridge is a prominent topographic feature that parallels the Agulhas-Falkland Fracture Zone (AFFZ). Seismic reflection
and wide angle/refraction data have led to the classification of this feature as a transverse ridge. Changes in spreading
rate and direction associated with ridge jumps, combined with asymmetric spreading within the Agulhas Basin, modified the
stress field across the fracture zone. Moreover, passing the Agulhas Ridge’s location between 80 and 69 Ma, the Bouvet and
Shona Hotspots may have supplied excess material to this part of the AFFZ thus altering the ridge’s structure. The low crustal
velocities and overthickened crust of the northern Agulhas Ridge segment indicate a possible continental affinity that suggests
it may be formed by a small continental sliver, which was severed off the Maurice Ewing Bank during the opening of the South
Atlantic. In early Oligocene times the Agulhas Ridge was tectono-magmatically reactivated, as documented by the presence of
basement highs disturbing and disrupting the sedimentary column in the Cape Basin. We consider the Discovery Hotspot, which
distributes plume material southwards across the AAFZ, as a source for the magmatic material. 相似文献
5.
The integration of seismic data with core data should provide ground-truth to a structural interpretation of seismic data. The main difficulty in such an integration effort is the correct translation of physical property measurements on cores to a form which can be used in seismostratigraphic interpretation. In the absence of down-hole well data and check-shots, required knowledge of the velocity structure at the drilling locations can be obtained directly from measurements of the physical properties of core samples. This involves upscaling of the data from physical properties of cores to the sample interval used in the seismic data. In the present study, three of the seven drill-sites of ODP (Ocean Drilling Program) Leg 177 in 1997/1998, located on the Agulhas Ridge in the south-eastern Atlantic (sites 1088–1090), were connected with eight seismic profiles. Physical properties data measured on the cores from the various holes at each site were combined to create a single continuous log and used to construct synthetic seismograms. The synthetics generally show a good agreement with real seismic data in terms of amplitude and waveform. Some reflections in these generated traces may have a time-shift due to sections with incomplete or spurious P-wave velocity measurements in the ODP datasets. The main reflectors identified in the real seismic data correspond to hiatuses or periods of reduced sedimentation rates, and correlate well with density variations. One particular hiatus, clearly observable in the real seismic data, was not unequivocally identifiable in the various types of core data, and tying core data to seismic data can confirm its existence in the core data, showing the benefit of including seismic data in an interpretation of core log data. On the other hand, core data provide a calibration tool for the geological timescale of seismic data and information about the lithology, needed in the interpretation of seismic data. 相似文献
6.
Roberto Danovaro Mauro Fabiano Giancarlo Albertelli Norberto Della Croce 《Marine Ecology》1995,16(2):103-116
Abstract. Quantitative information on the vertical distribution of meiofaunal abundances and biomass were obtained from samples collected at 3 bathyal stations in the Eastern Mediterranean Sea located at the same depth but characterized by different food supply. Vertical distribution patterns of nieiofauna were investigated in relation to the biochemical composition of the sediment organic matter (proteins, carbohydrates, and lipids) and compared to benthic bacterial standing stocks. No significant relationship between bacteria and meiofauna was found, whereas a significant relationship between protein and lipid concentrations and total meiofauna density was observed. These data suggest that labile organic matter. considered as material readily aVdihbk to benthic consumers, may be an important factor regulating meiofaunal abundance and vertical distribution in deep-sea sediments. 相似文献
7.
To study the time-varying influence of the Congo River and the Benguela Current on the deposition at the Angola Continental Margin, a high-resolution reflection seismic survey was carried out on the northern Congo Fan. Four seismostratigraphic units have been defined for the upper 800 m (1000 ms TWT) of the data. The units record different depositional environments, ranging from pre-establishment of the Congo River drainage system to the influence of the Benguela Current. An indication of a general change in the turbidite system is provided by a shift in channel distribution and a relocation of the depocentre of coarse material. The ascent of salt is recorded up to the Pliocene. Gas that has migrated out of Lower Cretaceous shales and that was produced from large quantities of organic matter in the younger sediments can be found on the flanks and on top of the salt domes. In a few places, this gas even ascends to the ocean floor along structural pathways through the topmost unit. 相似文献
8.
In recent years many fuller ship hull forms have been designed and constructed in various shipbuilding countries, but the data available for the development of the fuller forms are inadequate from the point of view of preliminary ship design. In this paper the authors describe how they have systematically tested vessel forms of block coefficients ranging from 0.80 to 0.90. The analysis and presentation of the test results have been made in such a manner that designers can produce hull forms like those of tankers and other bulk carriers quickly and reliably. 相似文献
9.
R. Danovaro N. Della Croce A. DellAnno A. Pusceddu 《Deep Sea Research Part I: Oceanographic Research Papers》2003,50(12):1411-1420
The Atacama trench, the deepest ecosystem of the southern Pacific Ocean (ca. 8000 m depth) was investigated during the Atacama Trench International Expedition. Sediments, collected at three bathyal stations (1040–1355 m depth) and at a hadal site (7800 m) were analyzed for organic matter quantity and biochemical composition (in terms of phytopigments, proteins, carbohydrates and lipids), bacterial abundance, biomass and carbon production and extracellular enzymatic activities. Functional chlorophyll-a (18.0±0.10 mg m−2), phytodetritus (322.2 mg m−2) and labile organic carbon (16.9±4.3 g C m−2) deposited on surface sediments at hadal depth (7800 m) reached concentrations similar to those encountered in highly productive shallow coastal areas. High values of bacterial C production and aminopeptidase activity were also measured (at in situ temperature and 1 atm). The chemical analyses of the Atacama hadal sediments indicate that this trench behaves as a deep oceanic trap for organic material. We hypothesize that, despite the extreme physical conditions, benthic microbial processes might be accelerated as a result of the organic enrichment. 相似文献
10.
M. Cristina Fossi Silvia Casini Letizia Marsili Stefania Ancora Gabriele Mori Giovanni Neri Antonella Ausili Teresa Romeo Alessandra Moscatelli Giuseppe Notarbartolo Di Sciara 《Marine Ecology》2002,23(S1):184-189
Abstract. Endocrine Disrupting Chemicals (EDCs) have the potential to alter hormone pathways that regulate reproductive processes in wildlife and fisheries. In this research the unexplored hypothesis that Mediterranean top predator species (such as large pelagic fish and marine mammals) are potentially at risk due to EDCs is investigated. In the Mediterranean environment, top predators accumulate high concentrations of polyhalogenated aromatic hydrocarbons (PHAHs) and toxic metals, incurring high toxicological risk. Here we illustrate the need to develop sensitive biomarkers for evaluation of toxicological risk in top marine predators (Xiphias gladius, Thunnus thynnus thynnus) and non-lethal techniques, such as non-destructive biomarkers, for the hazard assessment of threatened species exposed to EDCs, such as marine mammals ( Stenella coeruleoalba, Tursiops truncatus, Delphinus delphis and Balaenoptera physalus ). 相似文献