Nicolas Nowald

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

[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 � 1 . Time-averaged effective transport patterns of particles reveal distinct maxima between 20.5N and 23.5N 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.

In-situ sinking speed measurements of marine snow aggregates acquired with a settling chamber mounted to the Cherokee ROV

Marine snow plays a key role in the global carbon cycle because it transfers huge amounts of carbon dioxide (around 1–2 Gt per year) from the ocean surface to the deep-sea, thus removing it from the global system. It is of major field of study for several decades to quantify the amount of particulate matter settling through the water column. A central parameter for ocean mass flux estimates is the settling velocity of larger particles. Most of the few available datasets have been acquired by Scuba divers but are they are limited to a diving depth of a few tenth of meters. Particle settling speeds for the deeper water column may be estimated with the help of sediment trap recordings, having the disadvantage to integrate settling speeds over a long period of time and for the entire particle population settling through the water column. In situ sinking speed measurements of individual aggregates however, are rare and difficult to obtain. We present results from a settling chamber constructed for in situ sinking speed measurements of marine snow. The settling chamber was mounted to the MARUM Cherokee ROV during RV Poseidon Cruise 365 in 2008 off Cape Blanc, Mauritania. It was constructed in consideration of a similar device used by the MBARI ROV Ventana. It is a simple plexiglas box which can be opened and closed to allow an infinite number of measurements with little disturbance inside. A collimated light source illuminates a defined sample volume in which aggregates can be observed after the box has been closed. We sampled a total of 51 aggregates at four depth levels, between 50m and 400m water depth. The depths were chosen after collecting a vertical particle profile acquired by a deep-sea still image camera system before the deployment of the ROV. Sinking speeds ranged from 10m d−1to 287 m d−1 with a mean value of 57 m d−1. No clear relationsship between the size of the particles and their sinking speed was found. Furthermore we could not observe increasing particle sinking speeds with increasing water depth as found by other authors. This underlines the complexity of such studies and implies more deployments during upcoming cruises and comparison of in situ measurements with additional methods in the future.