Nepheloid layers

Bacon MP, Rutgers van der Loeff MM (1989) Removal of Thorium-234 by scavenging in the bottom nepheloid layer of the ocean. Earth Planet Sci Lett 92 157-164 Bacon MP, Cochran JK, Hirschberg DJ, Hammar TR, Fleer AP (1996) Export flux of carbon at the equator during the EqPac time-series cruises estimated from " Th measurements. Deep-Sea Res II 43 1133-1153... 

In the case of the turbidity currents, this redistribution usually occurs along the foot of the continental slope and is largely responsible for the accumulation of sediments in the continental rise. The resuspension of particles by contour currents can also maintain permanent nepheloid layers as shown in Figure 13.10. 

Nepheloid layer Deep and bottom waters that have high concentrations of resuspended sediment. 

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. 

Rhone River Nepheloid layer 720 192 528 144 Inhibitor Bianchi et al., 1999 ... 

Intermediate nepheloid layer and quantitative significance of denitrification in remineralization... 

Ransom B., Shea K. E., Burkett P. J., Bennett R. H., and Baerwald R. (1998b) Comparison of pelagic and nepheloid layer marine snow implications for carbon cycling. Mar. Geol. 150, 39-50. 

Naqvi, S.W.A., Kumar, M.D., Narvekar, P.V., de Sousa, S.N., George, M.D. and D Silva, C. (1993) An intermediate nepheloid layer associated with high microbial metabolic rates and denitrification in the Northwest Indian ocean. Journal of Geophysical Research, 98, 16469-16479. 

Fig. 12.9 Occurrence of nepheloid layers (based on beam attenuation and light scattering profiles) across the continental margin off Namibia at 25.5°S. As a result of the lateral particle transport, a TOC-rich depot center has develloped on the mid-slope (compiled from Inthorn et al. subm. a and b).

Inthorn, M., Mohrholz, V. and Zabel, M., subm. a. Nepheloid layer distribution in the Benguela upwelling area offshore Namibia.- subm. to Deep Sea Research... 

Sediment-trap measurements have sometimes allowed determination of the source as well as the rate of particulate organic transport. Spencer et al. (1978) deployed sediment traps 200 m off the Sargasso Sea floor within the bottom nepheloid layer. Resuspended bottom material, especially clays, made a major contribution to the total trap material collected. Interpretation of elemental and isotope data led them to postulate that 95% of the clay and 10% of the organic matter, but Eilmost none of the calcium carbonate in the traps was contributed by resuspended bottom material instead of... 

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. 

Biscaye, P.E., and Eittreim, S.L. 1977. Suspended particulate loads and transport in the nepheloid layer of the abyssal Atlantic Ocean. Marine Geology, 23 155-172. 

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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