118
B. J. Haupt· K. Stattegger . D. Seidov
2.3
Modeled processes depending on scales
Since our knowledge about many geological and biological processes is very limited, we assume a medium grain size homogenous mixture of sediment (Bitzer
and Pflug 1989; Siindermann and K16cker 1983) and that suspended particles in
the water column are transported by currents (Bitzer and Pflug 1989). In order
to calculate the vertical transport, one should take into account not only the vertical velocity w, but also the settling velocity Ws of suspended material. In addition to the vertical transport by upwelling/sinking water, the settling velocity
relative to the water motion is superimposed on the water motion to obtain the
true particle transport. The settling velocity depends on grain size, density and
kinematic viscosity from the surrounding water as well as particle density, form
factor and sedimentological grain diameter, and gravitational acceleration
(Gibbs et al.197l; Gibbs 1985; McCave and Gross 1991; Zanke 1977b). It must be
emphasized that the vertical velocities are spatially variable, and are not preset
fixed values, as in many other models. This is significant, considering that transport and deposition mainly depend on the settling velocity, w s ' which normally
exceeds the vertical velocity of the surrounding water (Fig. 3; McCave 1984). The
movement of sediment is based on mechanical processes (Dietrich et al. 1975;
Miller et al. 1977; Zanke 1982).
Tectonic processes are not considered here because our sedimentation reconstructions cover only geologically short time periods lasting from several hundreds to several thousands of years. During such time intervals, the overall
ocean geometry may be considered stationary for the North Atlantic (Wold
1992). Tectonic subsidence or uplift is much slower than the expected sedimentation rate within a range from a few millimeters to several meters over a 1000year timespan (Shaw and Hay 1989).As a consequence, tectonic movements and
their effects on topography are negligible (Stephenson 1989).
I-- transport length
)
Fig.3 Sketch of different transport length due to different grain size, and different settling
velocity wS' (After Middleton and Southland 1984)
B. J. Haupt· K. Stattegger . D. Seidov
2.3
Modeled processes depending on scales
Since our knowledge about many geological and biological processes is very limited, we assume a medium grain size homogenous mixture of sediment (Bitzer
and Pflug 1989; Siindermann and K16cker 1983) and that suspended particles in
the water column are transported by currents (Bitzer and Pflug 1989). In order
to calculate the vertical transport, one should take into account not only the vertical velocity w, but also the settling velocity Ws of suspended material. In addition to the vertical transport by upwelling/sinking water, the settling velocity
relative to the water motion is superimposed on the water motion to obtain the
true particle transport. The settling velocity depends on grain size, density and
kinematic viscosity from the surrounding water as well as particle density, form
factor and sedimentological grain diameter, and gravitational acceleration
(Gibbs et al.197l; Gibbs 1985; McCave and Gross 1991; Zanke 1977b). It must be
emphasized that the vertical velocities are spatially variable, and are not preset
fixed values, as in many other models. This is significant, considering that transport and deposition mainly depend on the settling velocity, w s ' which normally
exceeds the vertical velocity of the surrounding water (Fig. 3; McCave 1984). The
movement of sediment is based on mechanical processes (Dietrich et al. 1975;
Miller et al. 1977; Zanke 1982).
Tectonic processes are not considered here because our sedimentation reconstructions cover only geologically short time periods lasting from several hundreds to several thousands of years. During such time intervals, the overall
ocean geometry may be considered stationary for the North Atlantic (Wold
1992). Tectonic subsidence or uplift is much slower than the expected sedimentation rate within a range from a few millimeters to several meters over a 1000year timespan (Shaw and Hay 1989).As a consequence, tectonic movements and
their effects on topography are negligible (Stephenson 1989).
I-- transport length
)
Fig.3 Sketch of different transport length due to different grain size, and different settling
velocity wS' (After Middleton and Southland 1984)