SEDLOB and PATLOB
137
particles, and sediment porosity). In both experiments using the sedimentation
model SEDLOB, we employed reduced critical velocities to initiate bed load vem,b
and suspension load vern,s' These were set to 0.002 cm s-1 and 0.02 cm S-I, respectively. In control runs we figured out that a reduction is necessary to obtain realistic transports in the bottom layer in the deeper ocean basins. With these altered initial conditions the model was capable of eroding sediment when the
critical velocities were weaker than the velocities predicted by the OGCM. Consequently, it produces more patchy sediment structures. Both simulations were
run over 500 years. Unlike the OGCM, it is not possible to run SED LOB into a
steady-state condition. The forward time integration led to continuous changes
of the bottom slope and therefore the critical velocities for initiating bed load
and suspension load also changed. This is equivalent to the sediment availability
which influences the maximum possible sediment concentration and transport
in the fluid, depending on the bottom slope inclination.
6
Results and Discussion
In the first SEDLOB experiment (E1) only the eolian sediment input from the atmosphere was taken into account (Fig. ISa), whereas the second experiment
(E2) added lateral sediment input from rivers and icebergs (Fig. 17). The sedimentation rate is given in cm/lOOO years. The locations of the main sediment
drifts south of Iceland and south of Greenland are well reproduced (Bohrmann
et al. 1990; McCave and Tucholke 19S6). However, the sedimentation rate is affected by the chosen distribution of the sediment supply. Both experiments show
similar sedimentation patterns. They are mostly formed along the margins of
the current axes. Differences can be found especially in regions where strongly
selective river sediment input was added to the eolian sediment portion (e.g., in
the Bay of Biscay, or in the eastern German Bight). Higher sedimentation rates
occur also in the coastal areas, where currents run approximately parallel to the
shore line. Here the water takes up a relatively low sediment input and accumulates it while moving along the coast. Whenever coastal currents depart from the
coast to the open sea, the speed of the current slows down. Sediment transport
capacity decreases and sediments are deposited. This phenomenon is found especially on continental slopes with a downward steepening bottom topography.
An example of this is found off Southeast-Greenland where the East-Greenland
Current flows through the Denmark Strait into the deep Irminger Basin. This
feature can also be found to the south-southeast of Iceland and to the west of Lofoten, where the Norwegian Coastal Current turns to the east into the Norwegian
Sea. Sediments are also deposited at higher rates on the V 0ring Plateau. The
higher sedimentation rates of these shelf areas are in good agreement with the
recorded sedimentation rates from sediment cores.
The high sedimentation rate area in the northeasternmost part of the model
area has a different origin. In both experiments, El (Fig.lSa) and E2 (Fig.lSb),
the high sedimentation rates are due to the closed northern boundary and are
137
particles, and sediment porosity). In both experiments using the sedimentation
model SEDLOB, we employed reduced critical velocities to initiate bed load vem,b
and suspension load vern,s' These were set to 0.002 cm s-1 and 0.02 cm S-I, respectively. In control runs we figured out that a reduction is necessary to obtain realistic transports in the bottom layer in the deeper ocean basins. With these altered initial conditions the model was capable of eroding sediment when the
critical velocities were weaker than the velocities predicted by the OGCM. Consequently, it produces more patchy sediment structures. Both simulations were
run over 500 years. Unlike the OGCM, it is not possible to run SED LOB into a
steady-state condition. The forward time integration led to continuous changes
of the bottom slope and therefore the critical velocities for initiating bed load
and suspension load also changed. This is equivalent to the sediment availability
which influences the maximum possible sediment concentration and transport
in the fluid, depending on the bottom slope inclination.
6
Results and Discussion
In the first SEDLOB experiment (E1) only the eolian sediment input from the atmosphere was taken into account (Fig. ISa), whereas the second experiment
(E2) added lateral sediment input from rivers and icebergs (Fig. 17). The sedimentation rate is given in cm/lOOO years. The locations of the main sediment
drifts south of Iceland and south of Greenland are well reproduced (Bohrmann
et al. 1990; McCave and Tucholke 19S6). However, the sedimentation rate is affected by the chosen distribution of the sediment supply. Both experiments show
similar sedimentation patterns. They are mostly formed along the margins of
the current axes. Differences can be found especially in regions where strongly
selective river sediment input was added to the eolian sediment portion (e.g., in
the Bay of Biscay, or in the eastern German Bight). Higher sedimentation rates
occur also in the coastal areas, where currents run approximately parallel to the
shore line. Here the water takes up a relatively low sediment input and accumulates it while moving along the coast. Whenever coastal currents depart from the
coast to the open sea, the speed of the current slows down. Sediment transport
capacity decreases and sediments are deposited. This phenomenon is found especially on continental slopes with a downward steepening bottom topography.
An example of this is found off Southeast-Greenland where the East-Greenland
Current flows through the Denmark Strait into the deep Irminger Basin. This
feature can also be found to the south-southeast of Iceland and to the west of Lofoten, where the Norwegian Coastal Current turns to the east into the Norwegian
Sea. Sediments are also deposited at higher rates on the V 0ring Plateau. The
higher sedimentation rates of these shelf areas are in good agreement with the
recorded sedimentation rates from sediment cores.
The high sedimentation rate area in the northeasternmost part of the model
area has a different origin. In both experiments, El (Fig.lSa) and E2 (Fig.lSb),
the high sedimentation rates are due to the closed northern boundary and are