High‐resolution modeling of sediment erosion and particle transport across the northwest African shelf

Abstract

[1] The region off Cape Blanc along the northwest African coast is dominated by persistent upwelling and strong activity of small-scale eddies, filaments, and jets. Vertical particle camera profiles obtained during recent cruises in this region show that there exist two well-marked maxima of particle abundance in the water column, one at the surface and the other in subsurface layers between 200 m and 400 m depths. Using a highresolution (2.7 km) terrain-following coordinate ocean model with built-in ecosystem and sediment transport modules, we show that the surface particle maximum can be explained by local productivity, while the deeper, subsurface particle cloud most likely originates from particulate material eroded from the shallow shelf and transported offshore by vigorous filament activity and dynamic features of the flow. In the numerical experiments, particles are produced either by primary production in the surface layer or from prescribed sediment sources to mimic suspension and erosion along the shelf areas. Good agreement of modeled particle distributions with the data is achieved with a typical settling velocity of 5 m day À1. Time-averaged effective transport patterns of particles reveal distinct maxima between 20.5°N and 23.5°N off Cape Blanc. In the south of Cape Bojador and off Cape Timiris, on the other hand, the effective transport distance patterns suggest energetic offshore activity.Citation: Karakaş, G., N. Nowald, M. Blaas, P. Marchesiello, S. Frickenhaus, and R. Schlitzer (2006), High-resolution modeling of sediment erosion and particle transport across the northwest African shelf,

Sinking rates and ballast composition of particles in the Atlantic Ocean: implications for the organic carbon fluxes to the deep ocean

“…These relatively low rates might indicate a significant lateral displacement of larger particles from their point of origin, most probably from the coast (e.g. Karakaş et al, 2006). Values between the surface and the deep traps were between 73 and 100 m d −1 for the deployments CB-1-3 and CB-13, respectively, again pointing to increasing sinking rates with depth.…”

“…Particle cameras apparently record more the normal distribution of particles in the water column (e.g. mid-water maxima off the shelves, Karakaş et al, 2006;, with larger particle volumes being suspended or sinking slowly. Nevertheless, in one particle profile off NW Africa , we were able to capture such an episodic sinking event occurring in spring.…”

“…fine-grained lithogenic components or tiny coccoliths (de La Rocha and Passow, 2007) resuspended from the seafloor but also from the shelf (e.g. Karakaş et al, 2006) are enhanced and might be incorporated into larger particles , thus increasing particle sinking rates.…”

“…Studies modelling particle transport paths currently operate with fixed sinking rates for the entire water column (e.g. Karakaş et al, 2006;Gruber et al, 2006). Lower sinking rates in the mesopelagic zone (e.g.…”

Deep ocean mass fluxes in the coastal upwelling off Mauritania from 1988 to 2012: variability on seasonal to decadal timescales

“…Where lower trap data (around 3500 m) were not available, the upper trap data (around 1000 m) were used, which mostly match the lower trap fluxes with respect to seasonality (Fischer et al, 2009b). When plotting all available lower and upper trap total mass fluxes for winter, a close correspondence is observed (r 2 = 0.84, N = 10), with slightly higher fluxes in the deeper trap due to lateral particle advection processes (Fischer et al, 2009b;Karakas et al, 2006Karakas et al, , 2009. However, considering the entire record presented here, it seems that the upper trap fluxes of the winter seasons 1998 and 2004 may be critical due to smaller filament areas.…”