184
W. Ritzrau . H. Fohrmann
assumption which is only valid for the upper bottom nepheloid layer. However,
strong gradients of particle concentration and composition have been observed
in the benthic boundary layer close to the seafloor (Thomsen et al. 1994, 1995;
Townsend et al.1992; Ritzrau 1996). To interpret these field observations and to
answer the question whether aggregate formation and disintegration influence
not only the vertical distribution of the concentration of suspended matter but
also the aggregate size distribution would require an expanded multiple-layer
model with a fine scale resolution close to the seafloor.
A different approach was proposed by Boudreau (1997), who developed a theory to describe the dynamics of exchange processes across the sediment-water
interface. His analytical model comes to one exact solution for the vertical distribution for a single particle size class and a defined hydrodynamic regime.
This approach is comparable to the classical description of the suspension distribution close to the seafloor (Rouse 1937). Here, the ratio of the settling velocity (w s ) of a distinct particle size class over the product of the friction velocity
times the van Karman's constant (w/u*k) determines the shape of the exponential concentration distribution. The combination of the Rouse equation defining
the near-bed concentration distribution and the Karman-Prandtl equation (the
"law of the wall") describing the velocity distribution above the seafloor allows
an estimation of the horizontal flux of distinct particle classes (Muschenheim
1987). This approach led to the idea of hydrodynamic sorting of particles with
different settling velocities. Consequently, distinctly different fluxes of particles
with different densities and hence settling velocities should occur close to the
seafloor. Thus, particle interactions, changing the size and property distribution
of aggregates continuously, would need to be considered as well.
The grain size distribution in the sediments at the seafloor, which is frequently resolved down to the submicron size class, does not necessarily tell how these
particles arrived, let alone describe the mode of their transport in suspension.
Yet the understanding of the dynamics of particle interactions and modification
within the benthic boundary layer is a key parameter to interpreting the sedimentary record in terms of sediment origin and transport paths. Residence
times of particles can be extremely prolonged, since, for example, rebound particles can stay in suspension without any contact with the sediment (Walsh et al.
1988). Resuspended particles may also enter the dynamic modification process
and be redistributed as suspension over wide distances.
During the past few years, the benthic boundary layer in shallow areas
(Ritzrau and Graf 1992) as well as at continental margins (Thomsen et al. 1994,
1995; Ritzrau 1996; Ritzrau and Thomsen 1997; Townsend 1992) has been identified as a distinct layer of water close to the seafloor. Here, not only the composition but also the microbiological modification of particles differs significantly
from the major portion of the water column. In the BBL the concentration of
particulate organic carbon is significantly higher (Fig. 1) and the particles tend
to be richer in organic matter than in the water column above (Thomsen and
Graf 1995). The data presented in Fig. 1 combine samples from 21 stations at the
East Greenland continental margin over a wide depth range from 136 to 2808 m
W. Ritzrau . H. Fohrmann
assumption which is only valid for the upper bottom nepheloid layer. However,
strong gradients of particle concentration and composition have been observed
in the benthic boundary layer close to the seafloor (Thomsen et al. 1994, 1995;
Townsend et al.1992; Ritzrau 1996). To interpret these field observations and to
answer the question whether aggregate formation and disintegration influence
not only the vertical distribution of the concentration of suspended matter but
also the aggregate size distribution would require an expanded multiple-layer
model with a fine scale resolution close to the seafloor.
A different approach was proposed by Boudreau (1997), who developed a theory to describe the dynamics of exchange processes across the sediment-water
interface. His analytical model comes to one exact solution for the vertical distribution for a single particle size class and a defined hydrodynamic regime.
This approach is comparable to the classical description of the suspension distribution close to the seafloor (Rouse 1937). Here, the ratio of the settling velocity (w s ) of a distinct particle size class over the product of the friction velocity
times the van Karman's constant (w/u*k) determines the shape of the exponential concentration distribution. The combination of the Rouse equation defining
the near-bed concentration distribution and the Karman-Prandtl equation (the
"law of the wall") describing the velocity distribution above the seafloor allows
an estimation of the horizontal flux of distinct particle classes (Muschenheim
1987). This approach led to the idea of hydrodynamic sorting of particles with
different settling velocities. Consequently, distinctly different fluxes of particles
with different densities and hence settling velocities should occur close to the
seafloor. Thus, particle interactions, changing the size and property distribution
of aggregates continuously, would need to be considered as well.
The grain size distribution in the sediments at the seafloor, which is frequently resolved down to the submicron size class, does not necessarily tell how these
particles arrived, let alone describe the mode of their transport in suspension.
Yet the understanding of the dynamics of particle interactions and modification
within the benthic boundary layer is a key parameter to interpreting the sedimentary record in terms of sediment origin and transport paths. Residence
times of particles can be extremely prolonged, since, for example, rebound particles can stay in suspension without any contact with the sediment (Walsh et al.
1988). Resuspended particles may also enter the dynamic modification process
and be redistributed as suspension over wide distances.
During the past few years, the benthic boundary layer in shallow areas
(Ritzrau and Graf 1992) as well as at continental margins (Thomsen et al. 1994,
1995; Ritzrau 1996; Ritzrau and Thomsen 1997; Townsend 1992) has been identified as a distinct layer of water close to the seafloor. Here, not only the composition but also the microbiological modification of particles differs significantly
from the major portion of the water column. In the BBL the concentration of
particulate organic carbon is significantly higher (Fig. 1) and the particles tend
to be richer in organic matter than in the water column above (Thomsen and
Graf 1995). The data presented in Fig. 1 combine samples from 21 stations at the
East Greenland continental margin over a wide depth range from 136 to 2808 m