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991.
We document and characterize the climatology of the diurnal cycles encountered along a West African transect during the pre-monsoon and full-monsoon periods. The meridional gradient in low-level properties is fundamental for the monsoon dynamics and here, for the first time, it is studied based on a large set of observations from the African Monsoon Multidisciplinary Analysis (AMMA) campaign. A detailed analysis of surface energy budget, boundary-layer structures and cloud occurrence is carried out to investigate the diurnal cycles of the low levels. A relatively weak meridional gradient of net radiation is observed during the pre-monsoon period, and a large gradient in sensible heat flux is found over the transect with values increasing from south to north. This, as well as the boundary-layer structures, partly explains the large contrasts in the diurnal amplitude of potential temperature and specific humidity along the transect. During the monsoon period, the atmospheric regimes drastically change involving strong interactions between the surface, atmosphere and clouds. The maximum in net radiation is shifted northwards, towards the Sahel, which potentially has a significant impact on the monsoon circulation. The sensible heat flux is considerably reduced and the diurnal amplitude is strongly damped, while the daytime boundary-layer growth decreases significantly in the Sahel related to changes in the balance of boundary-layer processes. These results highlight the contrasted diurnal cycle regimes encountered over West Africa under dry, moist and wet conditions. They provide observationally-based diagnostics to investigate the ability of models to handle the representation of the diurnal cycle over land. 相似文献
992.
Wind-Direction Effects on Urban-Type Flows 总被引:2,自引:2,他引:0
Jean Claus O. Coceal T. Glyn Thomas S. Branford S. E. Belcher Ian P. Castro 《Boundary-Layer Meteorology》2012,142(2):265-287
Practically all extant work on flows over obstacle arrays, whether laboratory experiments or numerical modelling, is for cases
where the oncoming wind is normal to salient faces of the obstacles. In the field, however, this is rarely the case. Here,
simulations of flows at various directions over arrays of cubes representing typical urban canopy regions are presented and
discussed. The computations are of both direct numerical simulation and large-eddy simulation type. Attention is concentrated
on the differences in the mean flow within the canopy region arising from the different wind directions and the consequent
effects on global properties such as the total surface drag, which can change very significantly—by up to a factor of three
in some circumstances. It is shown that for a given Reynolds number the typical viscous forces are generally a rather larger
fraction of the pressure forces (principally the drag) for non-normal than for normal wind directions and that, dependent
on the surface morphology, the average flow direction deep within the canopy can be largely independent of the oncoming wind
direction. Even for regular arrays of regular obstacles, a wind direction not normal to the obstacle faces can in general
generate a lateral lift force (in the direction normal to the oncoming flow). The results demonstrate this and it is shown
how computations in a finite domain with the oncoming flow generated by an appropriate forcing term (e.g. a pressure gradient)
then lead inevitably to an oncoming wind direction aloft that is not aligned with the forcing term vector. 相似文献
993.
Alain Prinzhofer Jean Pierre Girard Stphane Buschaert Yvon Huiban Sonia Noirez 《Chemical Geology》2009,260(3-4):269-277
A methodology has been developed to determine chemical and carbon isotopic compositions of trace amounts of hydrocarbon gas compounds (methane, ethane, propane, iso- and normal-butane) present as dissolved compounds in the porewater of the low permeability Callovo-Oxfordian argillites in eastern Paris Basin, France. Results indicate that the studied hydrocarbons contain significant amounts of ethane, butane and propane, in addition to methane. Carbon isotopic compositions reflect primarily thermogenic origin (thermal cracking of organic matter), and lack of any significant biodegradation. Because temperature did not exceed 50 °C in the studied argillites, investigated hydrocarbons must have originated in hotter/deeper organic-bearing formations, possibly Stephanian coals. Data supports the predominance of high maturity thermogenic gas in the upper part of the Callovo-Oxfordian, and low maturity thermogenic gas mixed with minor bacterially produced methane in the lower part of the formation. A mixing between three end-member gases models quite well the data: one thermogenic gas with a low maturity (42% methane, with a δ13C of − 53‰), a gas with higher maturity (55% methane, with a δ13C of − 47‰) and a bacterial gas (99.45% methane, with a δ13C of − 80‰). This study illustrates that migration of hydrocarbon gases can take place in rocks with very low permeability and porosity, such as compacted mudrocks, given enough time. It further suggests that the studied fluid migration and transfer in aquitards would help characterization and understanding of fluid movements in sedimentary basins, as a complement to studies focused on water aquifers and hydrocarbon reservoirs. Chemical and isotopic composition of dissolved hydrocarbons in porewater can be used as natural tracers of fluid circulation in sedimentary basins, in addition to more conventional tracers. 相似文献
994.
Massimo Chiaradia Jean Vallance Lluis Fontboté Holly Stein Urs Schaltegger Joshua Coder Jeremy Richards Mike Villeneuve Ian Gendall 《Mineralium Deposita》2009,44(4):371-387
New U–Pb, Re–Os, and 40Ar/39Ar dates are presented for magmatic and hydrothermal mineral phases in skarn- and porphyry-related ores from the Nambija and
Pangui districts of the Subandean zone, southeastern Ecuador. Nambija has been one of the main gold-producing centers of Ecuador
since the 1980s due to exceptionally high-grade ores (average 15 g/t, but frequently up to 300 g/t Au). Pangui is a recently
discovered porphyry Cu–Mo district. The geology of the Subandean zone in southeastern Ecuador is dominated by the I-type,
subduction-related, Jurassic Zamora batholith, which intrudes Triassic volcanosedimentary rocks. The Zamora batholith is in
turn cut by porphyritic stocks, which are commonly associated with skarn formation and/or porphyry-style mineralization. High
precision U–Pb and Re–Os ages for porphyritic stocks (U–Pb, zircon), associated prograde skarn (U–Pb, hydrothermal titanite),
and retrograde stage skarn (Re–Os, molybdenite from veins postdating gold deposition) of the Nambija district are all indistinguishable
from each other within error (145 Ma) and indicate a Late Jurassic age for the gold mineralization. Previously, gold mineralization
at Nambija was considered to be Early Tertiary based on K–Ar ages obtained on various hydrothermal minerals. The new Jurassic
age for the Nambija district is slightly younger than the 40Ar/39Ar and Re–Os ages for magmatic–hydrothermal minerals from the Pangui district, which range between 157 and 152 Ma. Mineralization
at Nambija and Pangui is associated with porphyritic stocks that represent the last known episodes of a long-lived Jurassic
arc magmatism (∼190 to 145 Ma). A Jurassic age for mineralization at Nambija and Pangui suggests that the Northern Andean
Jurassic metallogenic belt, which starts in Colombia at 3° N, extends down to 5° S in Ecuador. It also adds a new mineralization
style (Au-skarn) to the metal endowment of this belt.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
995.
Field, microstructural, and anisotropy of magnetic susceptibility (AMS) or magnetic fabric studies were applied to identify the sequence and character of the Pan-African structures in the basement of Eastern Cameroon at both sides of the regional scale Bétaré-Oya Shear Zone (BOSZ). The NE-SW trending BOSZ separates older gneisses and migmatites towards SE (domain I) from the younger rocks of the Lom meta-volcano-sedimentary basin towards NW (domain II). In domain I, early, ductile compressional deformation occurred in two events, D1 and D2, under relatively high T conditions. During subsequent cooling, strain partitioned between the competent basement gneisses with only mild compression and the bordering shear zone (BOSZ) with intense simple shear-wrenching (D3). Strain in the less competent rocks of domain II is dominated by simple shear, strike-slip wrenching (D3), with an earlier stage of compressional deformation preserved only in some low strain pods.Magnetic fabrics (AMS) document a progressive change from oblate ellipsoids towards prolate ellipsoids in domain I, when proceeding from the south towards the BOSZ. Foliations are mostly steep but define a girdle with a pole plunging gently towards WSW. The magnetic lineations also plunge mostly towards WSW at shallow angles. These fabrics indicate a compression approximately normal to the BOSZ, which is also the SE margin of the Lom Basin. In the Lom metasediments (domain II), AMS ellipsoids are typically oblate. Foliations trend NE-SW with mostly steep dips. Magnetic lineations plunge gently NE or SW. This fabric with foliations mostly steep and subparallel with the major BOSZ, combined with generally subhorizontal lineations implies the BOSZ as a Pan-African strike–slip shear zone with a subordinate component of compression.At a larger scale, the area is part of a continent-scale shear zone, separating external Pan-African domains of compression along the northern margin of the Congo craton from internal domains dominated by high-angle strike–slip and transpressional deformation. Together with published data, the present study thus demonstrates that transpression is a regional phenomenon in the Pan-African orogen of central and eastern Cameroon. 相似文献
996.
997.
998.
Regional variations in water quality and relationships to soil and bedrock weathering in the southern Sacramento Valley, California, USA 总被引:2,自引:2,他引:0
Richard B. Wanty Martin B. Goldhaber Jean M. Morrison Lopaka Lee 《Applied Geochemistry》2009,24(8):1512
Regional patterns in ground- and surface-water chemistry of the southern Sacramento Valley in California were evaluated using publicly available geochemical data from the US Geological Survey’s National Water Information System (NWIS). Within the boundaries of the study area, more than 2300 ground-water analyses and more than 20,000 surface-water analyses were available. Ground-waters from the west side of the Sacramento Valley contain greater concentrations of Na, Ca, Mg, B, Cl and SO4, while the east-side ground-waters contain greater concentrations of silica and K. These differences result from variations in surface-water chemistry as well as from chemical reactions between water and aquifer materials. Sediments that fill the Sacramento Valley were derived from highlands to the west (the Coast Ranges) and east (the Sierra Nevada Mountains), the former having an oceanic provenance and the latter continental. These geologic differences are at least in part responsible for the observed patterns in ground-water chemistry. Thermal springs that are common along the west side of the Sacramento Valley appear to have an effect on surface-water chemistry, which in turn may affect the ground-water chemistry. 相似文献
999.
Stable isotope data, a dissolved gas tracer study, groundwater age dating, and geochemical modeling were used to identify and characterize the effects of introducing low-TDS recharge water in a shallow aerobic aquifer affected by a managed aquifer recharge project in California’s San Joaquin Valley. The data all consistently point to a substantial degree of mixing of recharge water from surface ponds with ambient groundwater in a number of nearby wells screened at depths above 60 m below ground surface. Groundwater age data indicate that the wells near the recharge ponds sample recently recharged water, as delineated by stable O and C isotope data as well as total dissolved solids, in addition to much older groundwater in various mixing proportions. Where the recharge water signature is present, the specific geochemical interactions between the recharge water and the aquifer material appear to include ion exchange reactions (comparative enrichment of affected groundwater with Na and K at the expense of Ca and Mg) and the desorption of oxyanion-forming trace elements (As, V, and Mo), possibly in response to the elevated pH of the recharge water. 相似文献
1000.
A major environmental and societal event struck the Mediterranean basin during the 9th millennium cal BP. A sudden and major climatic crisis occurred in the Northern Hemisphere around 8200 cal BP leading to hyper arid conditions along a tropical zone between 15° and 40° North (Near and Middle East), cooler and wetter conditions in western and central Europe, and marked climatic irregularity in the northern Mediterranean basin. At the same time, frequent cultural gaps are observed in cave infillings from Greece to the Spanish peninsula between 8500 and 8000 cal BP, making the vision of the European Mesolithic–Neolithic transition more complex. Furthermore, a stratigraphic and socio-economic rupture associated with a spatial redistribution of sites characterizes the PPNB-NC/Yarmoukian transition in the Near East. The impact of these climatic and environmental changes in the first centuries of the neolithisation of Mediterranean Europe is discussed, using the socio-cultural, economic, stratigraphic and chronological evidence for the first farmers and last hunter-gatherers. This evidence is compared to recent paleoclimatic and geo-archaeological data obtained from prehistoric contexts, in order to measure the hydro-morphological impact on activities in valleys and karstic rockshelters. 相似文献