Nepheloid layers benthic

Baker, J.E., Eisenreich, S.J., Johnson, T.C., Halfman, B.M. (1985) Chlorinated hydrocarbon cycling in the benthic nepheloid layer of Lake Superior. Environ. Sci. Technol. 19, 854-861. 

Guo, L., and Santschi, P.H. (2000) Sedimentary sources of old high molecular weight dissolved organic carbon from the ocean margin benthic nepheloid layer. Geochim. Cosmochim. Acta 64, 651-660. 

A very similar situation exists near the seafloor, where resuspended sediment particles scavenge " Th from the bottom water. The resulting depletion of " Th in the benthic nepheloid layer (BNL) is a measure of the intensity of the resuspension-sedi-mentation cycle on a timescale of weeks. The tracer thus shows whether a nepheloid layer is advected over large distances or sustained by local resuspension. 

Modes of resuspension interactively depend on the concentration profile. The vertical structure of concentration is conveniently subdivided into four zones (Fig. 27.11). In the upper zone the suspension layer (DSL) is dilute, and is characterized by Newtonian flow behavior. The lower zone is occupied by the benthic nepheloid layer (BNL), which contains fluid mud. In the benthic suspension layer (BSL), the concentration is intermediate between DSL and BNL. The suspension in BSL is non-Newtonian but the concentration is not high enough for setthng to be hindered. Finally, at the bottom a consolidating bed (CB) occurs. It possesses an effective stress but is soft enough (i.e., not fully consolidated) for the sediment to be susceptible to resuspension when wave forcing is sufficiently strong. 

At the deep site (Fig. 27.1(a)), wave-induced bed shear stresses were too small to erode the sediment. We will conveniently assume that neither the consolidating bed (CB) nor the benthic nepheloid layer (BNL) contributed to resuspension, and that sediment exchange mainly occurred between the benthic suspension layer (BSL) and the dilute suspension layer (DSL). As seen from a typical concentration profile in Fig. 27.16, BSL does not have a well-defined thickness. However, its mean height ifg can be taken as 0.80 m based on the equal area assumption, which idealizes the water column as composed of a distinct BSL beneath sediment-free water (i.e., DSL with zero suspended matter). The mean concentration in BSL is 0.07kgm . Over the several months of measurement of suspended sediment concentration profile at this site, it was found that 0.80 m was a representative mean value within the range of about 0.2 to 1 m for He- ... 

Instrumented tripods with flowmeters, transmissiometers, optical backscatter sensors (OBS), in situ settling cylinders, and programmable camera systems have often been used in marine environments, for example, oceanographic studies of flow conditions and suspended particle movements in the bottom nepheloid layer [37,38]. These instruments were deployed to study suspended-sediment dynamics in the benthic boundary layer and were able to collect small water samples (1-2 L) at given distances from the seafloor. An instrumented tripod system (Bioprobe), which collects water samples and time-series data on physical and geological parameters within the benthic layer in the deep sea at a maximum depth of 4000 m, has been described [39]. For biogeochemical studies, four water samples of 15 L each can be collected between 5 and 60 cm above the seafloor. Bioprobe contains three thermistor flowmeters, three temperature sensors, a transmissiometer, a compass with current direction indicator, and a bottom camera system. 

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