Icebergs in the North Atlantic . ..
75
uniform distribution of IRD within the ice is consistent with the model results
of Matsumoto (1996), who could not appropriately simulate the IRD sedimentation during isotope stage 5e by confining the sediment load to the base of the
icebergs, but had to use an almost uniform distribution. However, the IRD
layer thickness not only depends on the amount and distribution of sediment
within the ice, but also on the duration of a Heinrich event.
Running the model for 250 years of steady state with constant iceberg meltwater inflow (see notes above on the steady state), that is within the estimates
of Heinrich event durations (Andrews et al. 1994; Manighetti et al. 1995), we
yield the characteristic distribution pattern of IRD shown in Fig. 10. This figure contains also the lO-cm isopachs of Heinrich layer 1 and 2 from Dowdeswell et al. (1995) for comparison. The modelled IRD distribution exhibits a
tongue extending eastward along 50 N, resembling the natural distributions.
The modelled tongue, however, is quite short, especially compared to H-2. According to our other model experiments with icebergs of different initial
heights, the maximum extent of the modelled IRD belt critically depends on
iceberg height. Higher icebergs can travel far more eastward and eventually
reach the European continent. Thus, good estimates of typical postglacial iceberg dimensions are essential for a realistic simulation of Heinrich events. Intercomparison of numerical model results and sediment core data will help to
reconstruct typical dimensions of the icebergs that were released during the
Heinrich event.
5
Conclusions
It has been shown that melting icebergs have effects on the circulation system
similar to pure meltwater inputs from the coasts. Especially the different circulation changes caused by icebergs from Europe or Labrador are comparable.
However, icebergs can be more effective than continental meltwater runoff because they are moving sources of freshwater. They can transport freshwater over
a long distance and subsequently release it within a small area. Thus the region
of meltwater input by icebergs can be more confined than is the case with continental runoff, yielding a more severe influence on density field and circulation.
These first experiments using freely drifting and melting icebergs gave promising results for further modelling of Heinrich events. Both extent and thickness of the Heinrich layers could be approximately reproduced. Sensitivity studies with different iceberg sizes and production rates will yield estimates of
typical iceberg dimensions and freshwater inputs during the Heinrich events.
According to our results, icebergs can drift from a given source region to
almost any place in the North Atlantic. Thus, to fully understand Heinrich
events, it should not only be mapped where IRD was deposited, it is equally
important to determine where the material came from.
Due to the temperature decrease by the icebergs, the heat flux from ocean to
atmosphere must be reduced, and it might be argued that this should be in-
75
uniform distribution of IRD within the ice is consistent with the model results
of Matsumoto (1996), who could not appropriately simulate the IRD sedimentation during isotope stage 5e by confining the sediment load to the base of the
icebergs, but had to use an almost uniform distribution. However, the IRD
layer thickness not only depends on the amount and distribution of sediment
within the ice, but also on the duration of a Heinrich event.
Running the model for 250 years of steady state with constant iceberg meltwater inflow (see notes above on the steady state), that is within the estimates
of Heinrich event durations (Andrews et al. 1994; Manighetti et al. 1995), we
yield the characteristic distribution pattern of IRD shown in Fig. 10. This figure contains also the lO-cm isopachs of Heinrich layer 1 and 2 from Dowdeswell et al. (1995) for comparison. The modelled IRD distribution exhibits a
tongue extending eastward along 50 N, resembling the natural distributions.
The modelled tongue, however, is quite short, especially compared to H-2. According to our other model experiments with icebergs of different initial
heights, the maximum extent of the modelled IRD belt critically depends on
iceberg height. Higher icebergs can travel far more eastward and eventually
reach the European continent. Thus, good estimates of typical postglacial iceberg dimensions are essential for a realistic simulation of Heinrich events. Intercomparison of numerical model results and sediment core data will help to
reconstruct typical dimensions of the icebergs that were released during the
Heinrich event.
5
Conclusions
It has been shown that melting icebergs have effects on the circulation system
similar to pure meltwater inputs from the coasts. Especially the different circulation changes caused by icebergs from Europe or Labrador are comparable.
However, icebergs can be more effective than continental meltwater runoff because they are moving sources of freshwater. They can transport freshwater over
a long distance and subsequently release it within a small area. Thus the region
of meltwater input by icebergs can be more confined than is the case with continental runoff, yielding a more severe influence on density field and circulation.
These first experiments using freely drifting and melting icebergs gave promising results for further modelling of Heinrich events. Both extent and thickness of the Heinrich layers could be approximately reproduced. Sensitivity studies with different iceberg sizes and production rates will yield estimates of
typical iceberg dimensions and freshwater inputs during the Heinrich events.
According to our results, icebergs can drift from a given source region to
almost any place in the North Atlantic. Thus, to fully understand Heinrich
events, it should not only be mapped where IRD was deposited, it is equally
important to determine where the material came from.
Due to the temperature decrease by the icebergs, the heat flux from ocean to
atmosphere must be reduced, and it might be argued that this should be in-