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11.
Gabriel Godin 《大气与海洋》2013,51(3):263-284
Abstract Differences in the speed or direction of a harmonic component of the current between ebb and flood, cause a transfer of spectral energy from one tidal band to the others. Cyclic changes in the speed or direction of a component between large and small tides, induce a leakage to neighbouring frequencies. Transient fluctuations, even in a perfectly isotropic flow, have the same effects. As a consequence, the spectral estimates of point measurements of current are not independent. 相似文献
12.
Secondary magnetic remanences residing in pyrrhotite and anisotropy of magnetic susceptibility (AMS) were studied in low-grade metamorphic carbonates of the Tethyan Himalaya in Nar/Phu valley (central Nepal) and used for interpretation of tectonic deformations. The characteristic remanence (ChRM) is likely of thermomagnetic origin related to post-peak metamorphic cooling occurring after the Eohimalayan phase (35–32 Ma). The ChRM postdates small-scale folding (main Himalayan folding F1 and F2) as shown by a negative fold test of site mean directions at 99% confidence level, and has been probably acquired between 32 and 25 Ma. Late-orogenic long-wavelength folding associated with the Chako antiform (CA) is recorded by the spatial dispersion of ChRM directions and the distribution of the main axes of the AMS tensor. The mean tilting of the ChRM direction since remanence acquisition (≈20–30°) approximately coincides with the tilting of the CA (31°) at the study area indicating that the pyrrhotite remanence predates the CA (CA formed at <18 Ma according to preliminary U/Pb dating). However, comparison of tilt angles of remanence directions and AMS tensor axes suggests that remanence acquisition was not completed before the onset of the CA formation. This could imply a younger age (Early Miocene or even younger) of the ChRM. Using the distribution of remanence directions along a small-circle as well as the distribution of AMS tensor axes, a clockwise mean rotation of 16° is obtained for a remanence age of ≈30 Ma. An Early Miocene remanence age would not change this result substantially. Compilation of rotations in the Tethyan Himalaya deduced from secondary pyrrhotite remanences reveals an increasing clockwise rotation from the Hidden valley in the W to the Shiar valley in the E (≈150 km distance), incompatible with an oroclinal bending model. 相似文献
13.
Abstract We evaluate a set of current measurements done in a section of the Strait of Juan de Fuca. The flow is of estuarine character, the upper layer flow usually being directed seaward. The RMS value of steady current exceeds its mean value appreciably in the upper layer; it also exceeds the mean near the bottom. The near‐surface currents do change their direction on occasions and can run landward for over five consecutive days, especially in the southern part of the channel. The lower layer flow (landward) and the upper layer flow (seaward) varied in magnitude from 90 to 160 thousand m3/sec, and their fluctuations were in phase; their difference, the net flow, is of the order of 5% to 10% of these flows and could not be calculated with any confidence. Tidal motion is barotropic in the section, with some deformation in the vertical caused by bottom friction and internal stresses. The M2 tide in the system Juan de Fuca‐Georgia Strait can be represented by a standing Kelvin wave influenced by friction. The K1 tide can also be represented by a standing Kelvin wave if some leakage is allowed in the northern end of Georgia Strait. Topographic effects mask the Coriolis influence on the intensity of tidal currents. 相似文献
14.
Renaud Soucy La Roche Laurent Godin John M. Cottle Dawn A. Kellett 《Journal of Metamorphic Geology》2019,37(2):239-269
New phase equilibrium modelling, combined with U–Th/Pb petrochronology on monazite and xenotime, and 40Ar/39Ar geochronology on white mica, reveal the style of deformation and metamorphism near the southern tip of the extruded Himalayan metamorphic core (HMC). In the Jajarkot klippe, west Nepal foreland, greenschist to lower amphibolite facies metamorphism is entirely constrained to the Cenozoic Himalayan orogeny, in contrast with findings from other foreland klippen in the central Himalaya. HMC rocks exposed in the Jajarkot klippe yield short‐lived, hairpin pressure–temperature–time–deformation paths that peaked at 550–600°C and 750–1,200 MPa at 25 Ma. The Main Central thrust (MCT) and the South Tibetan detachment (STD) bound the base and the top of the HMC, respectively, and were active simultaneously for at least part of their deformation history. The STD was active at c. 27–26 Ma and possibly as late as c. 19 Ma, while the MCT may have been active as early as 27 Ma and was still active at c. 22 Ma. The tectonometamorphic conditions in the Jajarkot klippe are characteristic of crustal thickening and footwall accretion of new material at the tip of the extruding metamorphic orogenic core. Our new results reveal that collisional processes active in the middle to late Miocene at the base of the HMC now exposed in the hinterland were also active earlier, during the Oligocene, at the tip of the southward‐extruding middle crust. 相似文献
15.
Abstract A series of extensive current measurements was carried out in sections of the Straits of Juan de Fuca and Johnstone during 1973, whose primary objective was the definition of the net circulation around Vancouver Island. Careful use of the data collected during the survey failed to yield reliable values for such net flows; the reasons for this lack of success are investigated. 相似文献
16.
Internationally, shark conservation is now being recognized as a major environmental challenge, but management efforts to halt the overexploitation of sharks have lagged behind. This review examines the state of knowledge for shark species in Canadian waters and analyzes the role of existing management and legislation in ensuring shark conservation. Despite Canada's early leadership, the present management framework reveals major shortcomings with regard to legal protection, bycatch and finning regulations. These problems are not unique to Canada but illustrate broader issues pertaining to the global management of endangered fish species. To strengthen the conservation and management of sharks, this paper recommends a set of key policies and management priorities, which exemplify proper precautionary management of endangered shark species in Canada and could serve as a blueprint for improving conservation efforts internationally. A structured approach for grading progress in shark conservation efforts against best practices is also presented and could be used as a goalpost elsewhere. 相似文献
17.
Guy Libourel Aurelie Verney-Carron Andreas Morlok Stéphane Gin Jérôme Sterpenich Anne Michelin Delphine Neff Philippe Dillmann 《Comptes Rendus Geoscience》2011,343(2-3):237-245
The knowledge of the long-term behavior of nuclear waste in anticipation of ultimate disposal in a deep geological formation is of prime importance in a waste management strategy. If phenomenological models have been developed to predict the long-term behavior of these materials, validating these models remains a challenge, when considering the time scale of radioactive decay of radionuclides of environmental concern, typically 104–105 yrs. Here we show how natural or archaeological analogues provide critical constraints not only on the phenomenology of glass alteration and the mechanisms involved, but also on the ability of experimental short-term data to predict long-term alteration in complex environments. 相似文献
18.
Developing conceptual hydrogeological model for Potsdam sandstones in southwestern Quebec,Canada 总被引：1，自引：1，他引：0
A hydrogeological study was conducted in Potsdam sandstones on the international border between Canada (Quebec) and the USA (New York). Two sandstone formations, arkose and conglomerate (base) and well-cemented quartz arenite (upper), underlie the study area and form the major regional aquifer unit. Glacial till, littoral sand and gravel, and marine silt and clay discontinuously overlie the aquifer. In both sandstone formations, sub-horizontal bedding planes are ubiquitous and display significant hydraulic conductivities that are orders of magnitude more permeable than the intact rock matrix. Aquifer tests demonstrate that the two formations have similar bulk hydrologic properties, with average hydraulic conductivities ranging from 2?×?10?5 to 4?×?10?5 m/s. However, due to their different lithologic and structural characteristics, these two sandstones impose rather different controls on groundwater flow patterns in the study area. Flow is sustained through two types of fracture networks: sub-horizontal, laterally extensive fractures in the basal sandstone, where hydraulic connectivity is very good horizontally but very poor vertically and each of the water-bearing bedding planes can be considered as a separate planar two-dimensional aquifer unit; and the more fractured and vertically jointed system found in the upper sandstone that promotes a more dispersed, three-dimensional movement of groundwater. 相似文献
19.
Gabriel Godin 《Estuarine, Coastal and Shelf Science》1984,19(4):451-461
A comparison is made between a set of current measurements in a section across the Strait of Juan de Fuca and those recorded further upstream at Race Rocks at a single depth. No outstanding common qualitative features are noted. Cross spectra reveal a low correlation for the subtidal current and suggest an intensification of the diurnal modulation of the flow at Race Rocks. The high correlation in the tidal bands allows to calculate this portion of the tidal current at Race Rocks which is coherent with the one sensed across the section. 相似文献
20.
R. D. Bojkov E. Kosmidis J. J. DeLuisi I. Petropavlovskikh V. E. Fioletov S. Godin C. Zerefos 《Meteorology and Atmospheric Physics》2002,79(3-4):127-158
Summary Umkehr observations taken during the 1957–2000 period at 15 stations located between 19 and 52° N have been reanalyzed using
a significantly improved algorithm-99, developed by DeLuisi and Petropavlovskikh et al. (2000a,b). The alg-99 utilizes new
latitudinal and seasonally dependent first guess ozone and temperature profiles, new vector radiative transfer code, complete
aerosol corrections, gravimetric corrections, and others. Before reprocessing, all total ozone values as well as the N-values
(radiance) readings were thoroughly re-evaluated. For the first time, shifts in the N-values were detected and provisionally
corrected. The re-evaluated Umkehr data set was validated against satellite and ground based measurements. The retrievals
with alg-99 show much closer agreement with the lidar and SAGE than with the alg-92. Although the latitudinal coverage is
limited, this Umkehr data set contains ∼ 44,000 profiles and represent the longest (∼ 40 years) coherent information on the
ozone behavior in the stratosphere of the Northern Hemisphere. The 14-months periods following the El-Chichon and the Mt.
Pinatubo eruptions were excluded from the analysis. Then the basic climatological characteristics of the vertical ozone distribution
in the 44–52° N and more southern locations are described. Some of these characteristics are not well known or impossible
to be determined from satellites or single stations. The absolute and relative variability reach their maximum during winter–spring
at altitudes below 24 km; the lower stratospheric layers in the middle latitudes contain ∼ 62% of the total ozone and contribute
∼ 57% to its total variability. The layer-5 (between ∼ 24 and 29 km) although containing 20% of the total ozone shows the
least fluctuations, no trend and contributes only ∼ 11% to the total ozone variability. Meridional cross-sections from 19
to 52° N of the vertical ozone distribution and its variability illustrate the changes, and show poleward-decreasing altitude
of the ozone maximum. The deduced trends above 33 km confirm a strong ozone decline since the mid-1970s of over 5% per decade
without significant seasonal differences. In the mid-latitude stations, the decline in the 15–24 km layer is nearly twice
as strong in the winter-spring season but much smaller in the summer and fall. The effect of including 1998 and 1999 years
with relatively high total ozone data reduces the overall-declining trend. The trends estimated from alg-99 retrievals are
statistically not significantly different from those in WMO 1998a; however, they are stronger by about 1% per decade in the lower stratosphere and thus closer to the estimates by sondes.
Comparisons of the integrated ozone loss from the Umkehr measurements with the total ozone changes for the same periods at
stations with good records show complete concurrence. The altitude and latitude appearances of the long-term geophysical signals
like solar (1–2%) and QBO (2–7%) are investigated.
Received April 12, 2001 Revised September 19, 2001 相似文献