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881.
A new set of parameters for the total plate tectonic reconstruction of South America and south central Africa is presented: euler pole 46.75°N, 32.65°W; rotation angle 56.40°. This fit is constrained by at least three pre-drift tectonic features crossing from one continent to the other: (1) the geophysically defined eastern and western boundaries of the submarine Jurassic Outeniqua Basin (South Africa) and the Falkland Plateau Basin; (2) the Late Precambrian transcurrent fault and mylonite belts of Pernambuco (Brazil) and Foumban (West Africa); and (3) the Triassic northern tectonic front of the Cape Fold Belt and the major morphological feature on the Falkland Plateau with which it is closely lined up. Isotopic ages of Falkland Plateau gneisses correspond to Cape Pluton and Cape Fold Belt ages, suggesting their palaeoposition was within the realm of the Cape Fold Belt.In addition, the bathymetrically and geophysically defined northeastern apex of the Falkland Plateau fits into the re-entrant angle defined on the South African margin by the steep southeast-facing sheared Agulhas margin and the southern face of the Tugela Cone. Simultaneously known Precambrian outcrops in northeastern Brazil and in the Gulf of Benin area of West Africa are juxtaposed rather than overlapped. Reconstructions producing a closer fit of these cratonic areas are considered untenable.  相似文献   
882.
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884.
To better characterize human health risks associated with potentially contaminated seafood, 56 composite samples of edible tissue of several finfish and shellfish species were analyzed for residues of toxaphene using gas chromatography with electron capture and negative ion mass spectrometric detection (GC-ECD and GC-ECNI-MS). Toxaphene in these samples, collected in 1997 near a former toxaphene plant in Brunswick, Georgia, were previously reported as non-detectable using non-selective techniques. Estimated total toxaphene concentrations (ΣTOX) ranged from less than 0.01 to 26 μ g−1 on a wet tissue basis. Smaller, bottom dwelling finfish such as croaker, mullet, and spot exhibited the highest ΣTOX (0.76–26 μg g−1), larger predatory fish including seatrout contained intermediate levels (0.08–4.4 μg g−1), and shellfish (blue crab and shrimp) contained the lowest levels (<0.01 to 0.27 μg g−1). For a given species, samples from the site furthest from the toxaphene plant had lower ΣTOX than samples from the other 3 sites. On a congener specific basis, levels ranged from <0.0025 to 3.5 μg g−1. Congener distributions were, in general, dominated by 2-exo, 3-endo, 6-exo,8,9,10-hexachlorobornane (Hx-Sed) and 2-endo,3-exo,5-endo,6-exo,8,9,10-heptachlorobornane (Hp-Sed), breakdown products of Cl8−Cl10 toxaphene homologs. Other prominent congeners confirmed by GC-ECNI-MS included Parlar numbers 26, 40/41, 42, 44, 50, 62, and 63, as well as several unidentified Cl6−Cl9 homologs. Minor differences in congener distribution among species and sampling locations suggested that exposure regimes and/or intrinsic biotransformation capabilities were not uniform. These results indicate that toxaphene residues were detectable in all species surveyed and at concentrations higher than estimated previously.  相似文献   
885.
Lost circulation, the inadvertent injection of drilling fluids into a highly permeable and/or fractured aquifer during rotary drilling, may result in collection of spurious information if the lost drilling fluids are not adequately purged before sampling the ground water. The purpose of this study was to determine whether removal of the volume of water lost during coring of a monitoring well in the carbonate Scotch Grove Formation (Silurian, east central Iowa) necessarily ensures collection of representative ground water samples. To monitor dilution of the ground water due to lost circulation, rhodamine dye was added to the drilling water and dye recovery was measured in samples collected during purging of five separate 5- to 10-foot intervals.
Circulation loss occurred in all five intervals, ranging from nearly 200 gallons in the upper permeable portion of the Scotch Grove to 25 gallons in the less permeable Buck Creek Member below. When the volume of water purged from the upper three intervals corresponded to the volume of water lost during coring, the purge water still contained 11 to 20 percent dyed drilling water. As purging continued, the proportion of drilling water in the samples decreased slowly. After purging more than 200 gallons of water, 86 to 98 percent of the dyed drilling water was recovered from the five test intervals. Four traditionally measured water quality parameters-pH, temperature, specific conductance, and dissolved oxygen — were less useful than the dye recovery for distinguishing drilling water from formation water in those zones in which the ground water quality was similar to the drilling water. These results indicate that the determination of the quantity of water to be purged prior to sampling must be based, at least in part, on aquifer lithology and hydraulic characteristics.  相似文献   
886.
In eastern England the Chalk aquifer is covered by extensive Pleistocene deposits which influence the hydraulic conditions and hydrochemical nature of the underlying aquifer. In this study, the results of geophysical borehole logging of groundwater temperature and electrical conductivity and depth sampling for major ion concentrations and stable isotope compositions (δ18O and δ2H) are interpreted to reveal the extent and nature of the effective Chalk aquifer of north Norfolk. It is found that the Chalk aquifer can be divided into an upper region of fresh groundwater, with a Cl concentration of typically less than 100 mg l−1, and a lower region of increasingly saline water. The transition between the two regions is approximately 50 m below sea-level, and results in an effective aquifer thickness of 50–60 m in the west of the area, but less than 25 m where the Eocene London Clay boundary is met in the east of the area. Hydrochemical variations in the effective aquifer are related to different hydraulic conditions developed in the Chalk. Where the Chalk is confined by low-permeability Chalky Boulder Clay, isotopically depleted groundwater (δ18O less than −7.5‰) is present, in contrast to those areas of unconfined Chalk where glacial deposits are thin or absent (δ18O about −7.0‰). The isotopically depleted groundwater is evidence for groundwater recharge during the late Pleistocene under conditions when mean surface air temperatures are estimated to have been 4.5°C cooler than at the present day, and suggests long groundwater residence times in the confined aquifer. Elevated molar Mg:Ca ratios of more than 0.2 resulting from progressive rock-water interaction in the confined aquifer also indicate long residence times. A conceptual hydrochemical model for the present situation proposes that isotopically depleted groundwater, occupying areas where confined groundwater dates from the late Pleistocene, is being slowly modified by both diffusion and downward infiltration of modem meteoric water and diffusive mixing from below with an old saline water body.  相似文献   
887.
A three-dimensional model for correlation functions and spectra in theatmospheric, convective boundary layer (CBL) is presented. The modelincludes vertical inhomogeneities introduced by eddy-blocking at the ground.By assuming the disturbance to the turbulent flow resulting from the groundblocking is irrotational, an equation is developed which allows one to writethe inhomogeneous, two-dimensional (2D) cross spectra for the blocked flowin terms of the 2D cross spectra for a homogeneous flow. VonKármán's energy spectrum then is used to determine thehomogeneous, 2D cross spectra. Although there are only two adjustableparameters in the model, the variance and a length scale, the model is shownto agree quite well with a diversity of previous results for the CBL.  相似文献   
888.
The phenomenon of post-flare coronal arches, initially discovered with the Hard X-Ray Imaging Spectrometer (HXIS), was investigated using observations made with the SMM Flat Crystal Spectrometer (FCS) on 20 through 23 January, 1985. Since these observations were made with a different type of instrument from HXIS, they provide independent information on the physical characteristics of the arch phenomenon and extend our knowledge to lower coronal temperatures.Conspicuous arch activity was observed after three flares and after a disturbance which could not be identified. (1) A dynamic flare starting on 20 January at 20: 39 UT was responsible for the formation of the primary arch structure. (2) An arch revival, showing characteristics very similar to those of the arch revivals observed with HXIS, took place after the dynamic flare starting on 21 January at 23: 50 UT. The most conspicious difference relates to the moving thermal disturbance observed very shortly after the onset of the parent flare, in particular to its propagation velocity. This difference in the arch revival is probably related to the different range of plasma temperatures covered by the FCS observations (3 × 106 K through 6 × 106 K) and the HXIS observations (>107 K) and the consequently more important effects of radiative cooling in the FCS arch revival. (3) More arch activity was observed after a (possibly dynamic) flare starting at 03: 40 UT on 21 January and (4) after an unidentified event with estimated time of occurrence near 23: 00 UT on 22 January. Similar to the arch revival, this activity was primarily characterized by the energization of (i.e., input of energy to) a pre-existing arch structure. The activity after the unidentified event suggests the existence of a mode of arch activation which is different from the typical flare-associated revival and is characterized by the absence of significant activity at chromospheric levels.  相似文献   
889.
Validation of a decadal OGCM simulation for the tropical Pacific   总被引:2,自引:0,他引:2  
An ocean general circulation model is forced with NCEP reanalysis over the 1948–1999 period. The simulated dynamic height and sea level are compared respectively to the dynamic height computed from hydrological data and to the sea level measured by tide gauges in the tropical Pacific. The model is shown to capture important features of the temporal structure of variability in the tide gauge data over the last several decades. However, the comparison reveals a largely artificial trend in the simulation, which consists of a decreasing dynamic height and sea level in the southwest and northwest of the tropical Pacific. Model sensitivity experiments show this trend is controlled by the NCEP surface wind stresses and more precisely by a weakening in the trade winds and a trend in the off-equatorial wind curl, with this trend existing mainly before the mid 1970s. For studies of decadal variability, the simple removal of a linear trend is an inadequate way to solve the problem, due to the inhomogeneities in the data used in reanalysis products and the non-linearity of models.  相似文献   
890.
In the marine environment, stability of the glacier terminus and the location of subglacial streams are the dominant controls on the distribution of grounding-line deposits within morainal banks. A morainal bank complex in Muir Inlet, Glacier Bay, SE Alaska, is used to develop a model of terminus stability and location of subglacial streams along the grounding line of temperate marine glaciers. This model can be used to interpret former grounding-line conditions in other glacimarine settings from the facies architecture within morainal bank deposits.The Muir Inlet morainal bank complex was deposited between 1860 A.D. and 1899 A.D., and historical observations provide a record of terminus positions, glacial retreat rates and sedimentary sources. These data are used to reconstruct the depositional environment and to develop a correlation between sedimentary facies and conditions along the grounding line.Four seismic facies identified on the high-resolution seismic-reflection profiles are used to interpret sedimentary facies within the morainal bank complex. Terminus stability is interpreted from the distribution of sedimentary facies within three distinct submarine geomorphic features, a grounding-line fan, stratified ridges, and a field of push ridges. The grounding-line fan was deposited along a stable terminus and is represented on seismic-reflection profiles by two distinct seismic facies, a proximal and a distal fan facies. The proximal fan facies was deposited at the efflux of subglacial streams and indicates the location of former glacifluvial discharges into the sea. Stratified ridges formed as a result of the influence of a quasi-stable terminus on the distribution of sedimentary facies along the grounding line. A field of push ridges formed along the grounding line of an unstable terminus that completely reworked the grounding-line deposits through glacitectonic deformation.Between 1860 A.D. and 1899 A.D. (39 years),

m3 of sediment were deposited within the Muir Inlet morainal bank complex at an average annual sediment accumulation rate of

m3/a. This rate represents the annual sediment production capacity of the glacier when the Muir Inlet drainage basin is filled with glacial ice.  相似文献   
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