Mineral ballast and particle settling rates in the coastal upwelling system off NW Africa and the South Atlantic |
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Authors: | G Fischer G Karakas M Blaas V Ratmeyer N Nowald R Schlitzer P Helmke R Davenport B Donner S Neuer G Wefer |
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Institution: | (1) Geosciences Department and Research Center Ocean Margins (RCOM), Klagenfurter Strasse, University of Bremen, 28359 Bremen, Germany;(2) School of Life Sciences, Arizona State University, Tempe, AZ, USA;(3) Alfred-Wegener-Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany;(4) Delft Hydraulics, P.O. Box 177, Delft, The Netherlands |
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Abstract: | 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. |
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Keywords: | Ballast Particle settling rates Particle fluxes Lateral advection Coccolithophorids Diatoms Modelling Dust Mauretania Upwelling |
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