The circulation of cold, deep water is one of the controlling factors of the Earth's climate. Forty percent of this water
enters the world ocean through the Southwest Pacific as a deep western boundary current (DWBC) flowing northwards at bathyal
to abyssal depths, east of the New Zealand microcontinent. South of latitude 50°S, the DWBC is intimately linked with the
Antarctic circumpolar current (ACC), which is the prominent force for the shallow-water circulation. The Pacific DWBC is presently
the largest single contributor of deep ocean water, and deciphering its evolution is of fundamental importance to understanding
ocean and climate history, and global ocean hydrography. The evolution of the DWBC system, and of related circum-Antarctic
currents, has taken place since 30–25 Ma when plate movements created the first oceanic gaps south of Australia and South
America. The stratigraphic record preserved in sediment drifts of the Southwest Pacific, in eastern New Zealand, is the best
available for deciphering the Neogene history of Southern Ocean water masses, and of the circulation of the ACC, DWBC and
their precursor systems. Major current activity commenced on the New Zealand margin in the late Eocene or early Oligocene
(Hoiho Drift; early ACC) and was widespread by the mid-late Oligocene (Marshall Paraconformity and Weka Pass Limestone drift;
ACC). During the Neogene the eastern South Island continental shelf built seawards by accretion at its outer edge of large
Miocene current drifts up to tens of kilometres long and hundreds of metres thick (Canterbury drifts). Also commencing in
the mid-Cenozoic, but in depths >2000 m, the DWBC emplaced large deep-water sediment drifts. Rates of drift deposition accelerated
considerably in the late Neogene, when climatic change (and particularly glacial sea-level falls) caused the delivery of large
volumes of turbiditic sediment into the path of the DWBC via the Bounty and Hikurangi channels.
Received: 9 August 1995 / Accepted: 15 January 1996 相似文献
The 117.38 m of gabbroic core drilled during the Ocean Drilling Program (ODP) Leg 153 at Sites 921 to 924 in the Mid-Atlantic Ridge (MAR) between 23 °N and the Kane Fracture Zone, exhibits a remarkable primary compositional heterogeneity, such as magmatic layering, intrusive contacts and late magmatic veining, which express a succession of magmatic events. Textural indicators suggest that the cooling of the crystal mush occurred in a dynamic environment, with infiltration of progressively evolved liquids. Magmatic features include random shape fabric and magmatic lamination; the subsequent deformational overprint occurred in subsolidus conditions. The ductile deformation, generally concentrated in discrete domains of the gabbro, is associated with continuous re-equilibration of the metamorphic assemblages of (1) olivine + clinopyroxene + orthopyroxene + plagioclase + ilmenite + Ti-magnetite, (2) olivine + clinopyroxene + plagioclase + ilmenite + Ti-magnetite + red hornblende. At lower temperatures brittle deformation prevails and subsequent fractures control the development of metamorphic assemblages: (3) clinopyroxene + plagioclase + red brown hornblende + Ti-magnetite + magnetite (?) + ilmenite, (4) plagioclase + brown hornblende + Ti-magnetite + magnetite + hematite + titanite ± Ti-oxide, (5) plagioclase + green hornblende + magnetite + titanite, (6) plagioclase + actinolite + chlorite + titanite + magnetite, (7) albite + actinolite + chlorite + prehnite ± epidote ± titanite and (8) albite + prehnite + chlorite ± smectite. Assemblages 1 to 8 express increasing water/rock ratios and decreasing degrees of recrystallization.
During the ductile phase, red hornblende is stable and its abundance increases with deformation intensity, possibly as an effect of the introduction of hydrous fluids. During the brittle phase, water diffusion controls the development of the fracture-filling mineral assemblages and re-equilibration of the adjacent rock; temperatures decrease further, as demonstrated by mineral zoning and incompletely re-equilibrated assemblages. The lowest temperatures correspond to the development of hydrothermal assemblages.
Compared with oceanic gabbros from fast-spreading transform environments, high-temperature ductile phases (granulite and amphibolite) are well developed, whereas brittle phases are widespread, as microcracks, prevalent on fracturing associated with discrete veins. 相似文献
Acidification of groundwater lags behind acid deposition due to the relatively long water residence time in conjunction with
various buffering processes in the soil zone and deeper aquifer (chemical weathering, cation exchange, sulfate sorption, and
N uptake by the biomass). Extensive field data from eight forested catchments in the Bunter Sandstone of the Black Forest,
including results from water budget studies and hydrochemical analysis of stream and spring waters, were used to simulate
the future evolution of ground-water acidification with the MAGIC model. The present acid deposition exceeds the “critical
load” (here meaning buffering due to chemical weathering and protonation of organic acids) in six of eight catchments. Two
catchments are well buffered because they contain carbonate-bearing layers in the Upper Bunter sandstone. Transient buffering
(i.e., cation exchange, N uptake, the sulfate sorption) thus far prevents worse acidification, but this effect will decline
in the future. For one of the poorly buffered catchments (Seebach), a two-layer simulation was carried out, based on extensive
data from 10 years of measurements. Validation of the long-term simulations by hydrochemical and soil data was hampered by
strong annual variations but generally supported by paleolimnological studies. In the future, reductions in the S deposition
by 20% and the N deposition by 10% up to the year 2030 are assumed as the most probable scenario. N uptake through soil and
vegetation will come to an end as suggested by decreasing C/N ratios of the organic matter. This process is arbitrarily included
in the simulations. In the periglacial soil layer, acidification will decrease until the year 2030 and then approach a steady-state
condition. In the fractured aquifer, acidification will also proceed at a decreasing rate; however, sulfate desorption up
to the year 2130, the end of simulated period, will prevent earlier remediation. Despite a significant reduction in S deposition
since the mid-1980s, further efforts are necessary to reduce the emission of acidifying substances. Liming in the recharge
area is partially effective to ameliorate “shallow” groundwater but largely fails to ameliorate “deeper” groundwater in the
sandstone aquifer.
Received: 30 July 1996/Accepted: 23 January 1997 相似文献
In situ seismic attenuationQ−1logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate- and slow-spreading oceanic crust.Q−1logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in Ocean Drilling Program Holes 504B and 896A and in Hole 395A, located in 5.9–7.3 Ma crust on the Pacific and Atlantic plates, respectively. Our results strongly tie crustal properties to seismic measurables and observed geological structures: we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity. We interpret a distinct decrease in the Q−1log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2A/2B boundary. The boundary is located at 465 m depth below the sea floor in both Hole 395A and 504B, although this is likely to be a coincidence of the sediment thickness at these sites. Layer 2A is estimated to be approximately 150 m thick in Hole 504B and > 300 m thick in Hole 395A. Cyclic sequences of high-porosity pillows and low-porosity massive units in the uppermost 100 m of volcanics in Hole 395A result in large velocity heterogeneities which cause > 5 times more attenuation in this layer than in Hole 504B. In Hole 896A, by contrast, fewer pillows, more massive flows, and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from Hole 504B. We conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration. The observed differences inQ−1profiles and layer 2A thickness at these sites may be attributed to variations in the volume and duration of volcanic activity at mid-ocean spreading centers for these Pacific and Atlantic ridge segments. 相似文献
Abstract A series of paleogeographic maps of the Japanese Islands, from their birth at ca 750–700 Ma to the present, is newly compiled from the viewpoint of plate tectonics. This series consists of 20 maps that cover all of the major events in the geotectonic evolution of Japan. These include the birth of Japan at the rifted continental margin of the Yangtze craton ( ca 750-700 Ma), the tectonic inversion of the continental margin from passive to active ( ca 500 Ma), the Paleozoic accretionary growth incorporating fragments from seamounts and oceanic plateaux ( ca 480-250 Ma), the collision between Sino-Korea and Yangtze (250–210 Ma), the Mesozoic to Cenozoic accretionary growth (210 Ma-present) including the formation of the Cretaceous paired metamorphic belts (90 Ma), and the Miocene back-arc opening of the Japan Sea that separated Japan as an island arc (25-15 Ma). 相似文献
Using the data of ECMWF (European Center for Medium-range Weather Forecasts) to undertake composite diagnoses of 16 explosive cyclones occurring at the Atlantic and the Pacific Oceans,it is found that there are a lot of obvious discrepancies on the basic fields between these strong and weak explosive cyclones.The major reasons why the explosive cyclones over the Atlantic are stronger than those over the Pacific Ocean are that the non-zonal upper jet and the low-level warm moist flow over the Atlantic are stronger.The non-zonal upper jet offers stronger divergence,baroclinicity and baroclinic instability fields for explosive cyclones.Anticyclonic curvature at the high level of strong explosive cyclones is easy to make the inertia-gravitational wave developing at the moment of northward transfer of energy and stimulate the cyclones deepening quickly.Warm advection and diabatic heating can cause the upper isobaric surface lifting,as a result,the anticyclone curvature of cyclones enlarges,and wave energy develops easily as well.The most powerful period of the development of explosive cyclones is just the time when the positive vorticity advection center is located over the low vortex.At the upper level,when the distribution of potential vorticity contours changes suddenly from rareness to denseness,and the large values of the potential vorticity both in the west and north sides of cyclones extend downwards together,then cyclones are easy to explosively develop.The formation of strong explosive cyclones is closely related with the non-zonality of upper jet and the anticyclonic curvature. 相似文献
Sewage sludge ash (SSA), the waste generated in sewage sludge incineration, was obtained from Wuhan Sewage Treatment Plant and used as a low-cost sorbent for removing Cu(Ⅱ) from wastewaters. The sorbent was first modified with 5 % sulfuric acid to increase its sorption capacity. The specific surface area, porosity, cation-exchange capacity (CEC) and pHZPC of the sorbent were measured. Batch experiments were made to study the effect of contact time, solution pH value and temperature on sorption. Both Langmuir and Freundlich models well described the Cu(Ⅱ) sorption process, with correlation coefficient (R2) values of 0.993 4 and 0.989 9 respectively. And the sorption process follows the Lagergren first order kinetic model. The equilibrium sorption capacity of acidified SSA to Cu(Ⅱ) is estimated to be 7.78 mg/g under optimal conditions. 相似文献