Benthic boundary layer sediments

The structure of turbulence in the transition zone from a fully turbulent fluid to a nonfluid medium (often called the Prandtl layer) has been studied intensively (see, for instance, Williams and Elder, 1989). Well-known examples are the structure of the turbulent wind field above the land surface (known as the planetary boundary layer) or the mixing regime above the sediments of lakes and oceans (benthic boundary layer). The vertical variation of D(x) is schematically shown in Fig. 19.8b. Yet, in most cases it is sufficient to treat the boundary as if D(x) had the shape shown in Fig. 19.8a. 

Figure 6.8 Conceptual diagram of the different scales of the components of the benthic boundary layer (BBL). In bottom water above the sediment-water interface where the Eckman layer occurs as flow is affected by the rotation of the Earth and bottom friction, where w = friction velocity and / = Coriolis parameter the logarithmic layer predominates when the velocity profile is well described using a logarithmic function a viscous sublayer is formed by molecular viscosity a diffusive boundary layer forms, whereby solute transport is controlled by molecular diffusion. (Modified from Boudreau and Jprgensen, 2001.)...

The high vertical flux of particulates in river/estuarine plume regions commonly results in the accumulation of particles in the formation of a benthic boundary layer (BBL) and/or mobile and fluid muds (see chapter 6 for more details). The BBL is defined by Boudreau and Iprgensen (2001, p. 1) as those portions of sediment and water columns that are affected directly in the distribution of their properties and processes by the presence of... 

Benthic boundary layer those portions of sediment and water columns that are affected directly in the distribution of their properties and processes by the presence of the sediment-water interface. 

Jprgensen, B.B., and Boudreau, B.P. (2001) Diagenesis and sediment-water exchange. In The Benthic Boundary Layer (Boudreau, B.P., and Jprgensen, B.B., eds.), pp. 211-244, Oxford University Press, New York. 

Chemical and biological processes in the sediments and benthic boundary layer (BBL) are important contributors to oceanic biogeochemical cycles, especially in the Arabian Sea due to its uncommon geographical setting. The oceanographic conditions experienced by various margins (e.g. Somalia/Oman versus India/Pakistan) are widely different, which in conjunction with the extensive mid-depth 02 deficiency produce a variety of BBL and sedimentary environments with respect to, among other factors, food supply, redox status and the nature and activity of benthic communities (Cowie, 2002). 

Stirring within a flux-chamber controls the thickness of the benthic boundary layer and thus the diffusive flux of oxygen into the sediment. A decrease or even an intermption of stirring results in a significant increase of benthic efflux of redox-sensitive constituents such as iron and manganese. 

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. 

An instrument that collects water from the benthic boundary layer in more shallow waters with a maximum depth of 50 m is described in Ref. [40]. Four water samples of 7 L each can be collected between 5 and 40 cm above the sediment. Handling is easy and the sampling operation is brief enough to allow repeated employment even on time-limited, routine investigations. 

De Beer D, Kiihl M (1998) Interfacial processes, gradients and metabolic activity in microbial mats and biofilms. In Boudreau B, Jorgensen BB (eds) The benthic boundary layer. Oxford University Press, Oxford De Beer D, Schramm A, Santegoeds CM, Nielsen HK (1998) Anaerobic proasses in activated sludge. Wat Sci Technol 37(4—5) 605-608 De Beer D (1999a) Use of microelectrodes to measure in situ microbial activities in biofilms, sediments and microbial mats. In Akkermans ADL, van Elsas JD, de Bruin FJ (eds) Mol nilar microbial ecology manual. Kluwer, Dordrecht... 

Scenario 1. Chemical solubilization across the WS interface. Fate models consisting of one or more differential equation require appropriate boundary conditions that will account for the mass-transfer resistance occurring on both sides of the interface. This MT situation was covered in the previous section under diffusive-type individual process where the flux equations appear see Equations 4.1 and 4.2. Figure 4.2 depicts the interface situation for this mass-transfer scenario. The positive z-direction is from the sediment into the water column. S, I, and W denote positions of defined water concentrations in the sediment bed (S), the interface (I), and water (W), respectively. A tortuous dashed line tipped with an arrowhead traces the chemical flux during its molecular diffusion process pathway from within the porous bed, to interface and through the benthic boundary layer finally arriving in the turbulent water column. 

A benthic boundary layer forms on the water side of the sediment-water interface with the same fluid dynamic and ftansport characteristics described in Section 12.2 for the marine BBL. 

The water-sediment interface. It is not readily accessible and may be occupied by macro and micro fauna as well as macro flora. Except for sites where the presence of subaquatic vegetation dominates the transport behavior at the benthic boundary layer, the grade for the water-side is good to average, though this depends to a certain extent on the type of aquatic system (fresh. 

Up the section, the Old Euxinian deposits are gradually replaced by the sediments of the Uzunlarian Formation (end of the Middle Pleistocene). The boundary between these formations is rather conventional it is traced by the reduction in the number of brackish-water mollusk species and the increase in the abundance of the representatives of euryhaline Mediterranean fauna. In the stratotypical section on the coast of Lake Uzunlarian, this formation is represented by two layers. The lower layer is formed by clayey sands and silts, which, along with brackish-water and fresh-water mollusks and benthic foraminifers, contain numerous shells of euryhaline Mediterranean mollusks (Cerastoderma glaucum, Abra ovata, and others). Above, one finds gray-green clays with interlayers of coquina matter mostly formed by marine Mediterranean species. 

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