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991.
Using the diving submersible survey NAUTICA we investigated the central part of the Caribbean large igneous province (CLIP) to observe and sample internal portions of this proposed oceanic plateau. Most of the samples are gabbroic and doleritic rocks; basalts are scarce. Radiometric dating by 40Ar/39Ar incremental heating experiments indicate that the intrusive rocks are Campanian in age (81–75 Ma). In some places these intrusive rocks underlie older Santonian (85–83 Ma) extrusive basaltic rocks, suggesting that the Campanian rocks represent a sill injection and an underplating episode. Results of the diving program supplemented by information from ODP and DSDP drilling sites document a 20 m.y. period (94–75 Ma) of igneous activity in the submerged portion of the Caribbean large igneous province (CLIP). In the northern part of the Beata Ridge late Campanian and/or post Campanian uplift is documented by prominent Maastrichtian (71–65 Ma) erosion and the establishment of a Paleocene-middle Eocene (65–49 Ma) carbonate platform. During and after the uplift an extensional period is indicated by seismic images and the subsidence (3 km depth) of the carbonate platform. Paleocene ages (55–56 Ma) determined on some volcanic samples are attributed to localised decompression mantle melting that accompanied the extension. We document a prolonged period of magmatic and tectonic events that do not fit with the current models of short-lived plateau formation during mantle plume initiation but shares many similarities with the constructional histories of other oceanic large igneous provinces.  相似文献   
992.
993.
The far-UV spectrum of the T Tauri stars (TTSs) provides important clues about the structure of the stellar atmospheres, winds and accretion shocks. The IUE ( International Ultraviolet Explorer ) Final Archive contains the most complete data base for such studies. A new extraction system, the IUE Newly Extracted Spectra ( ines ), has been developed to overcome the disadvantages of the extraction system used in the IUE Final Archive, the Signal Weighted Extraction Technique ( swet ). We have compared the ines spectra of the whole sample of TTSs in the far-UV range (1200–2000 Å) with the swet low-resolution spectra available in the IUE Final Archive. Although in most of the cases there is a good agreement between both samples, an important enhancement of the ines line fluxes with respect to the swet line fluxes is reported for particular spectra. The line fluxes are enhanced by as much as a factor of ∼2.5 in some objects, which is significant for variability studies of TTSs because the variations of the UV lines are typically of this order. The emission-measure distributions built to study the atmospheres of these stars are based on the UV emission line fluxes, so the new system is susceptible to introduce changes in these models. Moreover, the non-linear enhancement of the ines line fluxes produces variations in diagnostic line ratios usually taken as temperature and density tracers in late-type stars. These line ratios can vary by as much as a factor of 3 when the ines data are compared with the swet , with the subsequent variation of the physical parameters derived from them.  相似文献   
994.
995.
Petrographic and geochemical data obtained on the Araguainha impact crater (Goiás/Mato Grosso States, Brazil) indicate the existence of several molten products that originated during impact‐induced congruent melting of an alkali‐granite exposed in the inner part of the central uplift of the structure. Although previous studies have described these melts to some extent, there is no detailed discussion on the petrographic and geochemical variability in the granite and its impactogenic derivatives, and therefore, little is known about the geochemical behavior and mobility of trace elements during its fusion in the central part of the Araguainha crater. This paper demonstrates that the preserved granitoid exposed in the core of the structure is a magnesium‐rich granite, similar to postcollisional, A‐type granites, also found in terrains outside the Araguainha crater, in the Brasília orogenic belt. The molten products are texturally distinct and different from the original rock, but have very similar geochemical composition, making it difficult to separate these lithotypes based on concentrations of major and minor elements. This also applies for trace and rare earth elements (REE), thus indicating a high degree of homogenization during impact‐induced congruent melting under high pressure and postshock temperature conditions. Petrographic observations, along with geochemical data, indicate that melting occurs selectively, where some of the elements are transported with the melt. Simultaneously, there is an effective dissolution of the rock (granite), which leads to entrainment of the most resistant solid phases (intact or partially molten minerals) into the melt. Minerals more resistant to melting, such as quartz and oxides, contribute substantially to a chemical balance between the preserved granite and the fusion products generated during the meteoritic impact.  相似文献   
996.
The dynamics of two counter-streaming electron–positron–ion unmagnetized plasma shells with zero net charge is analysed in the context of magnetic field generation in gamma-ray burst internal shocks due to the Weibel instability. The effects of large thermal motion of plasma particles, arbitrary mixture of plasma species and space charge effects are taken into account. We show that, although thermal effects slow down the instability, baryon loading leads to a non-negligible growth rate even for large temperatures and different shell velocities, thus guaranteeing the robustness and the occurrence of the Weibel instability for a wide range of scenarios.  相似文献   
997.
998.
With the development of high-resolution multibeam echosounder systems (MBES) for surveying shallow-water areas a new tool is available to monitor rapid changes in seabed morphology as, e.g., caused by the dumping of dredge spoil in coastal waters. In this study, four data sets of repeated bathymetric surveys with a MBES were processed and analyzed. The data were collected in a 1.94-km2 dumping site in the outer Weser Estuary (German Bight). Between June and December 1998, 2.6 million m3 of dredged sediment were deposited there. The bathymetric maps generated in the course of this study reveal features such as subaqueous dunes, scour holes, and mounds of dumped dredge spoil. The mean water depth decreased by about 1 m during the dumping period. Furthermore, difference grids showing changes in sediment volume allowed a calculation of the sediment budget for the monitored area. After a time period of only 5 months, 0.5 million m3 of the originally dumped 2.6 million m3 of dredge spoil had already been removed from the dumping site.  相似文献   
999.
In the estuary of the river Scheldt, where an oxygen gradient exists in addition to the salinity gradient, redox processes will be of major importance for trace metal mobilisation. In this study, the influence of salinity and pH on the redox processes of dissolved Zn and Cd sulphides is investigated together with the effects on the ratio of the dissolved Zn and Cd concentrations. The speciation of these metals is calculated with the chemical equilibrium programme +. Zn sulphides are oxidised at lower oxygen concentrations than Cd sulphides, due to lower stability constants, causing a sudden increase or peak in the dissolved Zn/Cd ratio. The formation of dissolved Cd chloride complexes when oxidation occurs at high salinities (S=15) increases the mobility of Cd, causing a decrease in the Zn/Cd peak of the total dissolved concentrations. The peak is three to four times smaller at S=15 than when oxidation occurs at S=2. The simple model calculations compare very well with field data. The Scheldt estuary is suitable to illustrate these calculations. In the 1970s, the anoxic part of the estuary reached S=15–20, but since the early 1980s it has dropped to S=2–10. Historic data on metals in the estuary from 1978, 1987 and the 1990s were used to compare with the equilibrium calculations. The increase of the dissolved Zn/Cd peak at low salinity as a consequence, of the decreasing anoxic region is confirmed well by the data. The good agreement between model calculations and field data is a proof of the extreme importance of redox processes for the solubility of Zn and Cd sulphides in the estuary.  相似文献   
1000.
Eighteen sandy beaches were sampled along the 1659 km of the Galician coast (NW Spain) six months after the Prestige oil-spill to study the impact of the fuel and the clean-up activities on the macroinfauna community. A transect was extended at each beach, from above the drift line to below the swash line at five sampled levels; at each level six 0.05 m2 replicates were taken to a depth of 30 cm and sieved through a 1mm mesh, and the organisms collected and preserved. Results were compared with previous data obtained using the same procedures. The macroinfauna was numerically dominated by the amphipod Pontocrates arenarius, the isopod genus Eurydice, the polychaete Scolelepis squamata, and the amphipod Talitrus saltator. As a result of the Prestige oil-spill and the clean-up activities, beach populations were reduced, with Eurydice and S. squamata as the most affected taxa.  相似文献   
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