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51.
The subtidal circulation of the southeast Greenland shelf is described using a set of high-resolution hydrographic and velocity transects occupied in summer 2004. The main feature is the East Greenland Coastal Current (EGCC), a low-salinity, high-velocity jet with a wedge-shaped hydrographic structure characteristic of other surface buoyancy-driven currents. The EGCC was observed along the entire Greenland shelf south of Denmark Strait, while the transect north of the strait showed only a weak shelf flow. This observation, in conjunction with water mass considerations and other supporting evidence, suggests that the EGCC is an inner branch of the East Greenland Current (EGC) that forms south of Denmark Strait. It is argued that bathymetric steering is the most likely reason why the EGC apparently bifurcates at this location. Repeat sections occupied at Cape Farewell between 1997 and 2004 show that the alongshelf wind stress can have an influence on the structure and strength of the EGCC and EGC on timescales of 2-3 days. Accounting for the wind-induced effects, the volume transport of the combined EGCC/EGC system is roughly constant (∼2 Sv) over the study domain, from 68°N to Cape Farewell near 60°N. The corresponding freshwater transport increases by roughly 60% over this distance (59-96 mSv, referenced to a salinity of 34.8). This trend is consistent with a simple freshwater budget of the EGCC/EGC system that accounts for meltwater runoff, melting sea-ice and icebergs, and net precipitation minus evaporation. 相似文献
52.
Grant Saff 《The Professional geographer》1998,50(4):449-464
During the late 1980s some informal settlements were established within or near affluent Cape Town suburbs. Despite objections by suburban residents that these settlements would lead to a rapid escalation in crime and a dramatic fall in the value of properties, the authorities allowed the settlements to remain. Using data on property transfers, which are then mapped using GIS, the paper examines the effects that the informal settlement of Marconi Beam had on house prices and housing turnover within the adjacent suburb of Milnerton. The results show that most of the objections regarding Marconi Beam's effect on property values were groundless as the settlement had only a negligible effect on property values in the adjacent suburb. 相似文献
53.
M.?L.?Cox G.?A.?Sturrock P.?J.?FraserEmail author S.?T.?Siems P.?B.?Krummel 《Journal of Atmospheric Chemistry》2005,50(1):59-77
There are large uncertainties in identifying and quantifying the globally significant sources and sinks of methyl bromide (CH3Br) and methyl iodide (CH3I). Long-term, quasi-continuous observations can provide valuable information about their regional sources, which may be significant in the global context. We report 3 years of in situobservations of these trace gases from the AGAGE (Advanced Global Atmospheric Gas Experiment) program at Cape Grim, Tasmania (41 °S, 145 °E). The average background levels of CH3Br and CH3I during March 1998–March 2001 were 8.05 and 1.39 ppt (dry air mole fractions expressed in parts per 1012), respectively. The CH3Br background data showed little seasonal variability. Trajectory analyses reveal that air masses showing elevated CH3Br levels at Cape Grim have had significant contact with coastal-terrestrial and/or coastal-seawater and/or urban source regions. The CH3I background data showed a seasonal cycle with a 3-year average amplitude of 0.47 ppt and maximum concentrations in summer, suggesting that the Southern Ocean is a significant source.Trajectory analyses reveal that air masses showing highly elevated CH3I levels at Cape Grim have had significant contact with coastal-terrestrial and/or coastal-seawater regions and/or the open-ocean regions of Bass Strait and the Tasman Sea. 相似文献
54.
Multichannel seismic reflection profile data have been used to determine the internal structure of Mesozoic oceanic crust in the vicinity of the Cape Verde islands. The data show the oceanic crust to be characterized by both dipping and sub-horizontal reflectors. Several lines of evidence argue against the reflectors being scattering artifacts arising, for example, from rough basement topography. Instead, the reflectors are attributed to tectonic and magmatic processes associated with the accretion of oceanic crust at the Mid-Atlantic Ridge. The upper crust shows variable reflectivity due to both dipping and sub-horizontal events. We interpret the dipping reflectors, which have been identified on both ridge-normal and ridge-parallel profiles, as sub-surface expressions of normal faults that formed at or near the Mid-Atlantic Ridge. There is no evidence that the faults are caused by loading of the oceanic crust by either the Cape Verde islands or their associated topographic swell. Some faults, however, can be traced into the overlying sediments suggesting they may have been re-activated since their formation at the ridge. The origin of the sub-horizontal reflectors is not as clear. We believe them to be boundaries of different igneous lithologies, such as that between basalts and gabbros. The lower crust is highly reflective in some areas, whereas in others only a few dipping and sub-horizontal reflectors are observed. Some of the dipping reflectors can be traced into the upper crust, suggesting they are also normal faults. Others, however, appear to be confined to the lower crust. The sub-horizontal, discontinuous, reflectors about 2.0–2.5 seconds two-way travel time below the top of oceanic basement are attributed to the Moho. 相似文献
55.
The generation and propagation mechanisms of a Kyucho and a bottom intrusion in the Bungo Channel, Japan, have been studied
numerically using the hydrostatic primitive equations by assuming density stratification during summer. The experiments are
designed to generate a Kuroshio small meander in Hyuga-Nada, which acts as a trigger for these disturbances. After the current
speed of the Kuroshio is changed, a small meander is generated. At the head of the small meander, warm Kuroshio water is engulfed,
and encounters the southwest coast of Shikoku. However, convergence of heat flux on the bump off Cape Ashizuri suppresses
the generation of a warm disturbance, if the current speed is large. As the cold eddy associated with the small meander approaches
Cape Ashizuri, the heat flux diverges on the bump. This heat source forces a warm disturbance, which intrudes along the east
coast of the Bungo Channel as a baroclinic Kelvin wave (a Kyucho). After the cold eddy passes off Cape Ashizuri, the Kuroshio
approaches the bump again. Strong convergence of heat flux then occurs on the bump, which forces a cold disturbance. This
disturbance propagates as a topographic Rossby wave along the shelf break at the mouth of the channel. After the topographic
wave reaches the west end of the shelf break, it intrudes along the bottom layer of the channel as a density current (a bottom
intrusion). These results suggest that a Kyucho and a bottom intrusion are successive events associated with the propagation
of the small meander. 相似文献
56.
Javier Arístegui Eric D. Barton Paul Tett María F. Montero Mercedes García-Muoz Gotzon Basterretxea Anne-Sophie Cussatlegras Alicia Ojeda Demetrio de Armas 《Progress in Oceanography》2004,62(2-4):95
The variability in dissolved and particulate organic matter, plankton biomass, community structure and metabolism, and vertical carbon fluxes were studied at four stations (D1–D4), placed along a coastal-offshore gradient of an upwelling filament developed near Cape Juby (NW Africa). The filament was revealed as a complex and variable system in terms of its hydrological structure and distribution of biological properties. An offshore shift from large to small phytoplankton cells, as well as from higher to lower autotrophic biomass, was not paralleled by a similar gradient in particulate (POC) or dissolved (DOC) organic carbon. Rather, stations in the central part of the filament (D2 and D3) presented the highest organic matter concentrations. Autotrophic carbon (POCChl) accounted for 53% (onshore station, D1) to 27% (offshore station, D4) of total POC (assuming a carbon to chlorophyll ratio of 50), from which nano- and pico-phytoplankton biomasses (POCA < 10 μm) represented 14% (D1) to 79% (D4) of POCChl. The biomass of small hetrotrophs (POCH < 10 μm) was equivalent to POCA < 10 μm, except at D1, where small autotrophs were less abundant. Dark community respiration (Rd) in the euphotic zone was in general high, almost equivalent to gross production (Pg), but decreasing offshore (D1–D4, from 108 to 41 mmol C m−2 d−1). POC sedimentation rates (POCsed) below the euphotic zone ranged from 17 to 6 mmol C m−2 d−1. Only at D4 was a positive carbon balance observed: Pg−(Rd + POCsed) = 42 mmol C m−2 d−1. Compared to other filament studies from the NE Atlantic coast, the Cape Juby filament presented lower sedimentation rates and higher respiration rates with respect to gross production. We suggest that this is caused by the recirculation of the filament water, induced by the presence of an associated cyclonic eddy, acting as a trapping mechanism for organic matter. The export capacity of the Cape Juby filament therefore would be constrained to the frequency of the interactions of the filament with island-induced eddies. 相似文献
57.
We present a case study of low-level wind jets induced in sequence by orographic effects off the Pacific coast of northern
Japan during 7–11 June 2003, and demonstrate that the transition of the wind jets causes areal differences of wave height
variations along the coast. First, we describe evolution and structure of the wind jet by analyzing SeaWinds scatterometer
wind measurements. Under the easterly wind, a strong wind jet formed after passing by Cape Erimo. As the wind shifted to the
southeast, the wind jet started to decay. In turn, the southerly wind along the coast led to another wind jet in the lee of
the easternmost tip of the Sanriku coast. We then identify onsets and decays of the wind jets from time series of wind speed
at meteorological stations. Finally, we demonstrate that the transition of the wind jets has local impacts on wave height
variations. Significant wave heights measured by altimeters were correlated positively with local wind energy, i.e., squares
of wind speeds. Accompanying the wind jet formation/decline, significant differences of wave height variations became marked
among wave observation stations located along the coast at intervals of up to 50 km. 相似文献
58.
59.
《International Geology Review》2012,54(10):1213-1225
P- and S-wave tomography of the upper mantle beneath the Cape Verde hotspot is determined using arrival-time data measured precisely from three-component seismograms of 106 distant earthquakes recorded by a local seismic network. Our results show a prominent low-velocity anomaly imaged as a continuous column <100 km wide from the uppermost mantle down to about 500 km beneath Cape Verde, especially below the Fogo active volcano, which erupted in 1995. The low-velocity anomaly may reflect a hot mantle plume feeding the Cape Verde hotspot. 相似文献
60.
Sampling the Cape Verde Mantle Plume: Evolution of Melt Compositions on Santo Antao, Cape Verde Islands 总被引:1,自引:0,他引:1
HOLM P. M.; WILSON J. R.; CHRISTENSEN B. P.; HANSEN L.; HANSEN S. L.; HEIN K. M.; MORTENSEN A. K.; PEDERSEN R.; PLESNER S.; RUNGE M. K. 《Journal of Petrology》2006,47(1):145-189
The volcanic history of Santo Antão, NW Cape Verde Islands,includes the eruption of basanitephonolite series magmasbetween 7·5 and 0·3 Ma and (melilite) nephelinitephonoliteseries magmas from 0·7 to 0·1 Ma. The most primitivevolcanic rocks are olivine ± clinopyroxene-phyric, whereasthe more evolved rocks have phenocrysts of clinopyroxene ±FeTioxide ± kaersutite ± haüyne ± titanite± sanidine; plagioclase occurs in some intermediate rocks.The analysed samples span a range of 190·03% MgO;the most primitive have 3746% SiO2, 2·57%TiO2 and are enriched 50200 x primitive mantle in highlyincompatible elements; the basanitic series is less enrichedthan the nephelinitic series. Geochemical trends in each seriescan be modelled by fractional crystallization of phenocrystassemblages from basanitic and nephelinitic parental magmas.There is little evidence for mineralmelt disequilibrium,and thus magma mixing is not of major importance in controllingbulk-rock compositions. Mantle melting processes are modelledusing fractionation-corrected magma compositions; the modelssuggest 14% partial melting of a heterogeneous mantleperidotite source at depths of 90125 km. Incompatibleelement enrichment among the most primitive magma types is typicalof HIMU OIB. The Sr, Nd and Pb isotopic compositions of theSanto Antão volcanic sequence and geochemical characterchange systematically with time. The older volcanic rocks (7·52Ma) vary between two main mantle source components, one of whichis a young HIMU type with 206Pb/204Pb = 19·88, 7/4 =5, 8/4 0, 87Sr/86Sr = 0·7033 and 143Nd/144Nd= 0·51288, whereas the other has somewhat less radiogenicSr and Pb and more radiogenic Nd. The intermediate age volcanicrocks (20·3 Ma) show a change of sources to two-componentmixing between a carbonatite-related young HIMU-type source(206Pb/204Pb = 19·93, 7/4 = 5, 8/4 = 38,87Sr/86Sr = 0·70304) and a DM-like source. A more incompatibleelement-enriched component with 7/4 > 0 (old HIMU type) isprominent in the young volcanic rocks (0·30·1Ma). The EM1 component that is important in the southern CapeVerde Islands appears to have played no role in the petrogenesisof the Santo Antão magmas. The primary magmas are arguedto be derived by partial melting in the Cape Verde mantle plume;temporal changes in composition are suggested to reflect layeringin the plume conduit. KEY WORDS: radiogenic isotopes; geochemistry; mantle melting; Cape Verde 相似文献