The hypothesis, that at least certain marine microbial organisms respond to toxic stress by the development of resistance, was tested using the hypotric marine ciliate Euplotes vannus Muller as the test organism. Resistance to polychlorinated biphenyls (PCB, Aroclor 1254) was developed in E. vannus by exposing the animals to progressively higher PCB concentrations during a period of several months. The resistance to PCB persisted for at least 80 days (greater than 40 generations) after final exposure. This suggests either that genetic selection or persistent (lasting over many cell division cycles) phenotypic trait modification, possibly in the form of Dauermodification, had occurred. If resistance were widespread among marine microbial organisms in polluted environments it would be an important consideration in evaluating the long-term biological impact of both natural and man-induced chemical stress. 相似文献
Multiples contain valuable information about the subsurface, and if properly migrated can provide a wider illumination of the subsurface compared to imaging with VSP primary reflections. In this paper we review three different methods for migrating multiples. The first method is model-based, and it is more sensitive to velocity errors than primary migration; the second method uses a semi-natural Green's function for migrating multiples, where part of the traveltimes are computed from the velocity model, and part of the traveltimes (i.e., natural traveltimes) are picked from the data to construct the imaging condition for multiples; the third method uses cross-correlation of traces. The last two methods are preferred in the sense that they are significantly less sensitive to velocity errors and statics because they use “natural data” to construct part of the migration imaging conditions. Compared with the interferometric (i.e., crosscorrelation) imaging method the semi-natural Green's function method is more computationally efficient and is sometimes less prone to migration artifacts. Numerical tests with 2-D and 3-D VSP data show that a wider subsurface coverage, higher-fold and more balanced illumination of the subsurface can be achieved with multiple migration compared with migration of primary reflections only. However, there can be strong interference from multiples with different orders or primaries when multiples of high order are migrated. One possible solution is to filter primaries and different orders of multiples before migration, and another possible solution is least squares migration of all events. A limitation of multiple migration is encountered for subsalt imaging. Here, the multiples must pass through the salt body more than twice, which amplifies the distortion of the image. 相似文献
A method developed recently for constructing tectonic subsidence curves in early Paleozoic miogeoclines has produced new evidence for the breakup of a late Proterozoic supercontinent. Tectonic subsidence analyses in miogeoclines of eastern and western North America, northwestern Argentina, the Middle East and northwestern Australia limit the timing of the continental breakup to between 625 and 555 Ma. These results refine the implications of a much broader range of radiometric ages of rift-related igneous rocks and biostratigraphic ages of the transition from active extension to passive subsidence in miogeoclines.
The recognition of the timing and extent of rifting has led to testable hypotheses for latest Proterozoic and early Paleozoic continental histories. Breakup and onset of drift along an extensive system of continental fractures within a relatively short period of time would generate a large amount of young ocean floor, thereby reducing the volume of the global ocean basin and causing a sea level rise. Maximum reduction of ocean basin volume would postdate the time of breakup, probably by about 70 m.y., placing the transgressive peak at a time not older then about 510–520 Ma. That age agrees well with the time of maximum flooding on the continents close to the end of the Cambrian. Restriction of the breakup to between 625 and 555 Ma reduces the time gap between an essentially intact late Proterozoic supercontinent and the oldest reliable paleomagnetic reconstruction of the dispersed continents at about 560 Ma. A continental reconstruction produced by rotating Laurentia and Baltica into Gondwana a minimum distance from the 560 Ma position is consistent with limited geologic data. However, that reconstruction places Laurentia and Baltica in low latitudes which is difficult to reconcile with the absence of evaporites in syn-rift complexes in both continents. 相似文献
Summary An aerosol in charge equilibrium is theoretically and numerically investigated for plasma-like behaviour. It is shown that in sufficiently large containers the common type aerosols should manifest the properties of a plasma as regards their behaviour in an electric field. Such aerosols do not unambiguously possess a plasma frequency nor can they be termed a magnetoplasma. The results of this investigation may help to understand certain phenomena in atmospheric electricity. 相似文献
Glaciers respond to both long-term, persistent climate changes as well as the year-to-year variability that is inherent to a constant climate. Distinguishing between these two causes of length change is important for identifying the true climatic cause of past glacier fluctuations. A key step in addressing this is to determine the relative importance of year-to-year variability in climate relative to more persistent climate fluctuations. We address this question for European climate using several long-term observational records: a century-long, Europe-wide atmospheric gridded dataset; longer-term instrumental measurements of summertime temperature where available (up to 250 years); and seasonal and annual records of glacier mass balance (between 30 and 50 years). After linear detrending of the datasets, we find that throughout Europe persistence in both melt-season temperature and annual accumulation is generally indistinguishable from zero. The main exception is in Southern Europe where a degree of interannual persistence can be identified in summertime temperatures. On the basis of this analysis, we conclude that year-to-year variability dominates the natural climate forcing of glacier fluctuations on timescales up to a few centuries. 相似文献