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J. Blaas 《International Journal of Earth Sciences》1910,1(2):49-52
Ohne Zusammenfassung 相似文献
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G. Fischer G. Karakas M. Blaas V. Ratmeyer N. Nowald R. Schlitzer P. Helmke R. Davenport B. Donner S. Neuer G. Wefer 《International Journal of Earth Sciences》2009,98(2):281-298
The ocean off NW Africa is the second most important coastal upwelling system with a total annual primary production of 0.33 Gt
of carbon per year (Carr in Deep Sea Res II 49:59–80, 2002). Deep ocean organic carbon fluxes measured by sediment traps are also fairly high despite low biogenic opal fluxes. Due
to a low supply of dissolved silicate from subsurface waters, the ocean off NW Africa is characterized by predominantly carbonate-secreting
primary producers, i.e. coccolithophorids. These algae which are key primary producers since millions of years are found in
organic- and chlorophyll-rich zooplankton fecal pellets, which sink rapidly through the water column within a few days. Particle
flux studies in the Mauretanian upwelling area (Cape Blanc) confirm the hypothesis of Armstrong et al. (Deep Sea Res II 49:219–236,
2002) who proposed that ballast availability, e.g. of carbonate particles, is essential to predict deep ocean organic carbon fluxes.
The role of dust as ballast mineral for organic carbon, however, must be also taken into consideration in the coastal settings
off NW Africa. There, high settling rates of larger particles approach 400 m day−1, which may be due to a particular composition of mineral ballast. An assessment of particle settling rates from opal-production
systems in the Southern Ocean of the Atlantic Sector, in contrast, provides lower values, consistent with the assumptions
of Francois et al. (Global Biogeochem Cycles 16(4):1087, 2002). Satellite chlorophyll distributions, particle distributions and fluxes in the water column off NW Africa as well as modelling
studies suggest a significant lateral flux component and export of particles from coastal shelf waters into the open ocean.
These transport processes have implications for paleo-reconstructions from sediment cores retrieved at continental margin
settings. 相似文献
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The residual circulation over the continental slope, and in particular, its vertical structure, is analysed by means of an idealised hydrodynamic model. The model is based on the depth-dependent shallow-water equations under uniform along-isobath conditions and is forced by a prescribed meridional density gradient and tidal velocities. By means of expansion in the small Rossby number solutions are analysed for conditions representative for the continental slopes off the Hebrides and in the Bay of Biscay. The steady solution at zeroth order consists of a linear density-driven flow. At order a tidally rectified flow is found and a stationary flow due to self-interaction of the zeroth-order density-driven flow. At order 2 the leading-order effect of the interaction between the zeroth-order density-driven flow and the tides is found: the ‘interaction current’. The solutions up to and including order 2 constitute an along-isobath steady slope current which is comparable to field data. The slope current and the accompanying cross-shelf circulation depend strongly on the shelf and flow characteristics. For the Hebridean case the density forcing predominates, but for the Biscay case the tidal effects are of the same order of magnitude as the density effects. Under those conditions the interaction current is significant which implies that linear superposition of density and tidal effects differs from the non-linear combination of both. It is also shown that the depth-average of the interaction current differs essentially from the solution obtained from a depth-averaged model. 相似文献
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Meinte Blaas Changming Dong Patrick Marchesiello James C. McWilliams Keith D. Stolzenbach 《Continental Shelf Research》2007
Suspended sediment-transport processes in Santa Monica and San Pedro Bay are analyzed using the sediment-transport capabilities of the Regional Oceanic Modeling System (roms). A one-month simulation for December 2001 has been carried out with a set of nested domains. The model inputs include tides, winds, surface waves, and idealized initial sediment conditions for sand and non-cohesive silt. Apart from the control run, the sensitivity of the results to surface waves, ripple roughness and bed armoring has been analyzed. From the control experiment, the horizontal transport of sand turns out to be limited to within a few km of the nearshore erosion zones. During high wave events, silt is transported over further distances and also partly offshelf in distinct plumes. The effectiveness of horizontal silt transport depends strongly on vertical mixing due to both surface wind stress and wave-enhanced bottom stress. High wave events coincident with strong winds (hence strong vertical mixing) are the most optimal conditions for sediment-transport. Excluding wave effects in the simulation shows that surface waves are the dominant factor in resuspending bed material on the Southern Californian shelves. The sensitivity experiments also show that the direct influence of additional ripple roughness on erosion and deposition is relatively weak. Switching off bed armoring locally results in increases of near-bottom concentrations by a factor of 20 for silt and a factor of 5 for sand as well as stronger spatial gradients in grain size. 相似文献
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