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951.
Volker?St?hleEmail author Rainer?Altherr Mario?Koch Lutz?Nasdala 《Contributions to Mineralogy and Petrology》2004,148(2):150-159
A dense (~3.34 g cm–3) garnet–sillimanite-rich metamorphic rock from the suevite breccia of the Ries impact crater was studied by scanning-electron microscopy and Raman microprobe spectroscopy. In the strongly shocked rock clast kyanite was formed from sillimanite under momentary high pressures of natural shock waves. Kyanite aggregates were found as thin (~0.3–2.0 m) seams along grain boundaries between, and fractures within, sillimanite grains. Within these seams kyanite c-axes are oriented perpendicular to original grain boundaries and fractures. In addition, larger (up to 10 m) isolated kyanite grains were rarely found within host sillimanite. Filamentary kyanite aggregates and isolated crystals typically show shrinkage cracks due to volume decrease (~10%). Locally, broad interstices between sillimanite crystals are filled with aluminosilicate glass containing a high volume fraction of sub-micrometer-sized euhedral crystals. The silica-rich glass suggests incongruent melting of sillimanite at local post-shock temperatures significantly higher than 1,300°C. The edges of adjacent sillimanite grains are thermally and chemically altered. The local generation of temperature spikes is attributed to strong shock wave interactions due to very high shock impedance contrasts. 相似文献
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St 《International Journal of Earth Sciences》1910,1(4):169-169
Ohne Zusammenfassung 相似文献
958.
H.A. Karl W.C. Schwab A.St.C. Wright D.E. Drake J.L. Chin W.W. Danforth E. Ueber 《Ocean & Coastal Management》1994,22(3)
The oceans have been and will continue to be disposal sites for a wide variety of waste products. Often these wastes are not dumped at the designated sites or transport occurs during or after dumping, and, subsequent attempts to monitor the effects the waste products have on the environment are inadequate because the actual location of the waste is not known. Acoustic mapping of the seafloor with sidescan sonar is a very effective technique for locating and monitoring dredge-spoil material and other debris. Sidescan sonar provides an acoustic image or sonograph of the sea floor that is similar to a satellite image of the Earth's land surface. In effect sidescan sonar allows the water column to be stripped from the sea floor, thereby providing a clear, unobstructed view of the sea bed.An example of the potential of this technique is summarized herein for the Gulf of the Farallones region. More than 47 800 drums (55 gallon) and other containers of low-level radioactive waste were dumped on the continental margin offshore the San Francisco Bay between 1946 and 1970. These drums now litter a large area (1200 km2) of the sea floor within the Gulf of the Farallones National Marine Sanctuary (GFNMS). The exact location of the drums and the potential hazard the drums pose to the environment are unknown. To evaluate the risk, samples of the sediment, biota and water must be collected near and distant from the concentrations of barrels. To do this the exact location of the barrels must be known prior to sampling. The USGS, through a cooperative research agreement with GFNMS, used sidescan sonar to map two areas within the sanctuary. Total sea-floor coverage was obtained and computer-processed sonographic mosaics were constructed on board ship. Many small nongeologic targets were distributed throughout the survey areas that covered about 70 km2 on the shelf and 120 km2 on the slope. Analysis of the sidescan data suggests that the targets are 55-gallon drums. This interpretation was confirmed at one site with an underwater video and 35-mm camera system. Data were collected with both a 30-kHz and a 120-kHz sidescan system within a 15-km2 area on the shelf. We found that the barrels were more easily detected with the mid-range 30-kHz system than with the higher resolution 120-kHz system. Maps of barrel distribution derived from the sonographs are being used to design sampling schemes to evaluate the risk that the radioactivity may have on the biota and environment. 相似文献
959.
Coccolithophores are one of the major, living phytoplankton groups and play important roles in geochemical cycles and climate. They are a particularly dominant group in oligotrophic waters, yet a lot more needs to be learned about their horizontal and vertical distributions. Coccolithophores were collected at different photic depths from 15 stations across the Caribbean Sea during cruise 35/1 of the RV Meteor from April to May 1996. A total of 67 species was identified, with an average of 19 species per station. Coccolithophores were encountered at nearly all stations but abundances were fairly low (102-103 cells 1-1). Coccolithophore assemblages in the four oceanographical provinces identified (Granada Basin waters, Atlantic surface waters, northern Caribbean waters, and Pedro Bank waters) differed in their species composition and abundances. Abundance peaks occurred near the surface and in the deeper photic zone (140-150 m) just below the deep chlorophyll-a maximum at the top of the nitrate nutricline. 相似文献
960.
Estimating nitrogen transformation rates in aquatic ecosystems by isotope dilution techniques is simplified by directly measuring nitrogen isotopic ratios for NH4+ in the water using high performance cation exchange liquid chromatography (HPLC). Modifications of HPLC conditions and implementation of a median-area method for retention time determination improved and linearized a previously reported sigmoid relationship between the retention time shift (RTshift) of the NH4+ peak and the ratio of [15NH4+]: [Total NH4+] in seawater fortified with 15NH4+. Increasing the temperature of the HPLC column from 47 to 85 °C increased mobile phase buffer flow rate relative to column back pressure, decreased the retention time for NH4+, and allowed the buffer pH to be optimized relative to the pK of NH4+. The use of median-area rather than maximum-height to define the retention time of NH4+ further improved the linearity (r > 0.995) of the relationship between the ratio [15NH4+]: [Total NH4+] and RTshift over the range of isotope ratios. Reduction of NO3− to NH4+ by adding zinc dust to acidified (pH 2) seawater or lakewater samples, followed by pH neutralization, and subsequent analysis of NH4+ isotope ratios by HPLC, extended application of the method to isotope dilution experiments with NO3−. Advantages of this direct-injection method over mass-measurement approaches traditionally used for isotope dilution experiments include small sample size and minimal sample preparation. 相似文献