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D. Seidov . B. J. Haupt
above 2 km was nutrient-depleted during the glacial period. One interpretation
is that there was a lower proportion of northern component water, which, in
turn, means a weaker deep western boundary current and a stronger southern
source water incursion.
In the eastern part of the NA, our sediment model agrees with data that indicate increased flow along the eastern flank of the Mid-Atlantic Ridge. Dowling
and McCave (1993) and Robinson and McCave (1994) provide evidence that the
Feni Drift was substantially enhanced at the last glacial maximum. Generally,
the glacial sediment record indicates an enhanced Holocene bottom current
driven by the Iceland-Greenland overflow characteristic of modern NADW production in the Norwegian-Greenland Seas.
Our trajectory-tracing model reveals very different fates of the water-volumes
in the HM and LGM cases, once exposed to the air-sea interaction and then
mixed downward in convective chimneys. NADW production in the center of the
glacial North Atlantic is strongly supported by the proxy data (Duplessy et al.
1988 1991; Sarnthein et al. 1994 1995). In general, the most recent studies indicate enhanced upper NADW production and decreased lower NADW production (e.g., Oppo et al. 1995). However, there exists evidence that, despite decreased lower NADW production, the glacial conveyor was not, in total, weaker
than today (Yu et al. 1996). This may appear contradictory to the fact that the
glacial northward heat transport was substantially weaker at the LGM. Our trajectory analysis explains this seemingly controversial result. It shows that although the forward conveyor responsible for meridional heat transport became
weaker, the abyssal reversed branch of the conveyor strength increased proportionally to allow the same rate of the southward transport of the tracers in the
deep ocean found by Yu et al. (1996). Mediating the results of Seidov et al. (1996),
Seidov and Haupt (1997) and the global conveyor simulations presented above,
one may give the ratio of LGM to HM conveyor intensity as within 50-70% of
modern magnitude.
Duplessy et al. (1988) argue that during glacial times most of the deep eastern
North Atlantic was filled with southern source water which probably penetrated
up to 45°N. Our experiments, deploying particles in the surface layers in the subpolar gyre and in the deep and abyssal areas in the subtropical gyre, are in complete agreement with this conclusion. Moreover, Michel et al. (1995) point to a
steep gradient in ol3C distribution northward of 30 0 N in the North Atlantic delineating the border between southern and northern source water at the LGM.
We add that this border probably shifted farther to the north in the eastern part
of the basin during that time period.
Moreover, we point out that the trajectory analysis is in agreement with the
notion of a far better ventilated and deeper glacial thermocline in the subtropical North Atlantic (Slowey and Curry 1995). These authors provide indications
that the glacial thermocline was shallower than today with its base raised by
about lOO m. However, the water inside the glacial thermocline was up to 4 °C
cooler than today. Our Fig. 3 confirms that the glacial thermocline was better
ventilated and that there was an enhanced production of subtropical mode wa-
D. Seidov . B. J. Haupt
above 2 km was nutrient-depleted during the glacial period. One interpretation
is that there was a lower proportion of northern component water, which, in
turn, means a weaker deep western boundary current and a stronger southern
source water incursion.
In the eastern part of the NA, our sediment model agrees with data that indicate increased flow along the eastern flank of the Mid-Atlantic Ridge. Dowling
and McCave (1993) and Robinson and McCave (1994) provide evidence that the
Feni Drift was substantially enhanced at the last glacial maximum. Generally,
the glacial sediment record indicates an enhanced Holocene bottom current
driven by the Iceland-Greenland overflow characteristic of modern NADW production in the Norwegian-Greenland Seas.
Our trajectory-tracing model reveals very different fates of the water-volumes
in the HM and LGM cases, once exposed to the air-sea interaction and then
mixed downward in convective chimneys. NADW production in the center of the
glacial North Atlantic is strongly supported by the proxy data (Duplessy et al.
1988 1991; Sarnthein et al. 1994 1995). In general, the most recent studies indicate enhanced upper NADW production and decreased lower NADW production (e.g., Oppo et al. 1995). However, there exists evidence that, despite decreased lower NADW production, the glacial conveyor was not, in total, weaker
than today (Yu et al. 1996). This may appear contradictory to the fact that the
glacial northward heat transport was substantially weaker at the LGM. Our trajectory analysis explains this seemingly controversial result. It shows that although the forward conveyor responsible for meridional heat transport became
weaker, the abyssal reversed branch of the conveyor strength increased proportionally to allow the same rate of the southward transport of the tracers in the
deep ocean found by Yu et al. (1996). Mediating the results of Seidov et al. (1996),
Seidov and Haupt (1997) and the global conveyor simulations presented above,
one may give the ratio of LGM to HM conveyor intensity as within 50-70% of
modern magnitude.
Duplessy et al. (1988) argue that during glacial times most of the deep eastern
North Atlantic was filled with southern source water which probably penetrated
up to 45°N. Our experiments, deploying particles in the surface layers in the subpolar gyre and in the deep and abyssal areas in the subtropical gyre, are in complete agreement with this conclusion. Moreover, Michel et al. (1995) point to a
steep gradient in ol3C distribution northward of 30 0 N in the North Atlantic delineating the border between southern and northern source water at the LGM.
We add that this border probably shifted farther to the north in the eastern part
of the basin during that time period.
Moreover, we point out that the trajectory analysis is in agreement with the
notion of a far better ventilated and deeper glacial thermocline in the subtropical North Atlantic (Slowey and Curry 1995). These authors provide indications
that the glacial thermocline was shallower than today with its base raised by
about lOO m. However, the water inside the glacial thermocline was up to 4 °C
cooler than today. Our Fig. 3 confirms that the glacial thermocline was better
ventilated and that there was an enhanced production of subtropical mode wa-