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91.
Ho KT Burgess RM Pelletier MC Serbst JR Ryba SA Cantwell MG Kuhn A Raczelowski P 《Marine pollution bulletin》2002,44(4):286-293
The identification of toxicants affecting aquatic benthic systems is critical to sound assessment and management of our nation's waterways. Identification of toxicants can be useful in designing effective sediment remediation plans and reasonable options for sediment disposal. Knowledge of which contaminants affect benthic systems allows managers to link pollution to specific dischargers and prevent further release of toxicant(s). In addition, identification of major causes of toxicity in sediments may guide programs such as those developing environmental sediment guidelines and registering pesticides, while knowledge of the causes of toxicity which drive ecological changes such as shifts in benthic community structure would be useful in performing ecological risk assessments. To this end, the US Environmental Protection Agency has developed tools (toxicity identification and evaluation (TIE) methods) that allow investigators to characterize and identify chemicals causing acute toxicity in sediments and dredged materials. To date, most sediment TIEs have been performed on interstitial waters. Preliminary evidence from the use of interstitial water TIEs reveals certain patterns in causes of sediment toxicity. First, among all sediments tested, there is no one predominant cause of toxicity; metals, organics, and ammonia play approximately equal roles in causing toxicity. Second, within a single sediment there are multiple causes of toxicity detected; not just one chemical class is active. Third, the role of ammonia is very prominent in these interstitial waters. Finally, if sediments are divided into marine or freshwater, TIEs perforMed on interstitial waters from freshwater sediments indicate a variety of toxicants in fairly equal proportions, while TIEs performed on interstitial waters from marine sediments have identified only ammonia and organics as toxicants, with metals playing a minor role. Preliminary evidence from whole sediment TIEs indicates that organic compounds play a major role in the toxicity of marine sediments, with almost no evidence for either metal or ammonia toxicity. However, interpretation of these results may be skewed because only a small number of interstitial water (n = 13) and whole sediment (n = 5) TIEs have been completed. These trends may change as more data are collected. 相似文献
92.
93.
Low-frequency current fluctuations in the deep central equatorial Atlantic are analyzed using current meter measurements recorded
from November 1992 to November 1994. Current meters were located at about 14°W of longitude and 1° of latitude on both sides
of the equator between 1,700 m depth and the ocean bottom. At all sampling depths, the velocity fluctuations are dominantly
zonal and symmetrical with respect to the equator. At 1,700 and 2,000 m, the flow is dominated by annual period fluctuations,
at 3,000 m, the velocity field amplitude presents a minimum, and at 3,750 and 3,950 m, the flow is modulated by annual and
semiannual period variability. The annual signal exhibits an apparent upward phase propagation. When considering the phase
and the amplitude of the seasonal fluctuations, the data compare well with the outputs of a realistic numerical simulation
of the Atlantic Ocean. Together with a previous analysis of the model simulations, this supports the idea that the observed
annual fluctuations are due to wind-forced vertically propagating Kelvin and Rossby waves. Data and model do not provide deciding
evidences of the presence of semiannual equatorial waves deeper than 3,500 m depth in the central equatorial Atlantic Ocean. 相似文献
94.
95.
Herwig Dejonghe Anne Mathieu Veronique De Bruyne Sven De Rijcke 《Celestial Mechanics and Dynamical Astronomy》1998,72(3):219-233
We argue that the temperature of the stellar phase is likely a monotically decreasing function of radius in the central regions
of elliptical galaxies with cusps. We show that it requires the presence of a central mass concentration. An estimate of its
minimum mass based on few parameters is given. This lower bound is consistent with current mass values of the central objects
in galaxies derived from detailed models.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
96.
The ML 5.3 Épagny (French Alps) earthquake of 1996 July 15: a long-awaited event on the Vuache Fault
97.
Stphane Gaffet Christophe Larroque Anne Deschamps Florence Tressols 《Soil Dynamics and Earthquake Engineering》1998,17(7-8):475-484
We analyze observed three-component seismograms recorded by a dense array of 21 short-period receivers (f>2 Hz) installed on a homogeneous Quaternary plain with 1.5×3.5 km2 horizontal dimensions. The experiment was conducted in the district of Caille, in SE France for 3 months. The observed waveform data are related to diffraction and refraction of the direct wavefield by different geological structures and topographic morphologies surrounding the array. The origin of the coda energy following the energetic Pg or Sg arrivals is either influenced by heterogeneities close to and inside of the valley, for instance thrust of Jurassic limestones over Cretaceous marls in the northern and southern limits of the valley, or by heterogeneities located outside of the valley such as the interface unconformity between crystalline basement and sedimentary cover in the northern border of the Maures–Tanneron ranges. The main energetic arrivals of the seismograms arrive at the array with a definite range of azimuths that is not correlated with the event back-azimuth. Moreover, these arrivals do not appear with azimuths related to the characteristic NS and EW valley orientations, and thus may be related to heterogeneities located outside of the valley. 相似文献
98.
Anne Martine Tréhu 《Earth and Planetary Science Letters》1975,27(2):287-304
The straight-line relationship between depth and the square root of age predicted by recent variations of the thermal contraction model for ocean rise elevation is confirmed to an age of 80 m.y.We then examine this relationship in the immediate vicinity of the rise crest in an attempt to determine the sensitivity of the slopes thus obtained. Depth versus t1/2 profiles from a variety of rise types ranging from the topographically smooth, fast-spreading Pacific-Antarctic rise to the rough, slow-spreading Mid-Atlantic rise are discussed, ages having been assigned using a finite rotation pole. Because of the variety of superimposed anomalous features concentrated within a limited and well-surveyed region, the Galapagos Spreading Center has provided a suitable arena for determining the precision with which the method can decompose such an agglomeration into distinctly recognizable components. Although topographic “noise” precludes precise quantification of the slopes, it is concluded that, by removing the first-order effect of thermal contraction, the method can be quite revealing when topography is examined in relation to other data. Slopes for several profiles across the Pacific-Antarctic and Pacific-Nazca rises reveal the pattern expected in the case of asymmetric spreading, a conclusion which has independently been derived from the magnetic anomalies. In the Galapagos region “jumps” of the spreading center, a basic compositional difference, and uplift from below are exposed by their predictable effect on the slopes obtained from the depth versus t1/2 plots. 相似文献
99.
100.
Anne Lähteenmäki Merja Tornikoski Talvikki Hovatta Esko Valtaoja 《Astrophysics and Space Science》2007,311(1-3):347-351
A wealth of information on the properties of jets in Active Galactic Nuclei (AGNs) can be derived from total flux density observations at high radio frequencies. This includes, for example, the Doppler factor, the Lorentz factor, and the viewing angle of the jet. We have earlier calculated these parameters for a sample of ~80 sources of different AGN types using almost 20 years of 22 and 37 GHz data from Metsähovi Radio Observatory. We have now gathered data for an additional ten years, and studied the long term characteristic variability time scales of a large sample of AGNs using the first order structure function, the discrete autocorrelation function and the Lomb-Scargle periodogram. Some of the results will be presented in this paper. We also stress the importance of long term observations of AGNs, the main reason for this being misinterpretations of source properties due to insufficient time coverage. Only a few observing epochs will too easily lead to incorrect conclusions about variability, continuum spectra, and the general detectability of the source, not to mention the exclusion of interesting objects from further studies. This is particularly important when considering, for example, the Planck satellite for which the quality of the main mission product, the accurate cosmic microwave background anisotropy maps, depends heavily on the elimination of foreground sources such as AGNs. 相似文献