Numerical Study of Glacial and Meltwater Global Ocean Thermohaline Conveyor
89
are found mainly in the NGS, east and south of Iceland, and in the Labrador Sea.
The present -day convection pattern in our calculations is consistent with the
patterns given by Maier-Reimer et al. (1993), Toggweiler et al. (1989), and
Rahmstorf (1995). Shallow convection starts in the Gulf Stream area and marks
a further progression of subtropical water to high latitudes carried by the North
Atlantic Current becoming deeper as it progresses northeastward and westward
around the Greenland coast.
The major feature of glacial convection, as compared to its modern state, was
the far weaker glacial convection in the NGS and the southward shift of the main
convection sites. The maximum ventilation at the LGM occurred in the midNorth Atlantic between 50° and 60 0 N. This result is well supported by proxy data
(Duplessy et al. 1988; Sarnthein et al. 1995). A southward shift of both the convection sites and the polar front led to a decrease in the northward transport of
warm and salty water. Hence a positive feedback worked to establish the glacial
mode of circulation. An even farther southward shift of the convection was limited by Ekman pumping in the subtropics which protected the major anticyclonic gyre from shrinking even more, i.e., providing a negative feedback to balance
the positive one. The MWE convection sites were shifted further southward and
the convection was very shallow, penetrating no deeper than 600 m in the central
North Atlantic. There was no convection to the north of SOON and the Nordic
Seas were completely convection-free during that time slice (Seidov et al. 1996).
S.2
Sediment Accumulation Rates and Sediment Transport
Total sediment accumulation is governed by the requirement that sources and
sinks balance at the sea-surface and be the same for all three time slices. The spatial distribution of sediment is however different, and reveals two distinctly different circulation modes between present, and at the LGM and MWE. Fig. 2
presents the sediment rates for the three chosen time slices. A very high accumulation rate associated with the Holocene/Modern mode of circulation is found in
the vicinity of Iceland in the Irminger Basin, along the Reykjanes Ridge at both
the south and north sides of Iceland, at the Rockall Plateau, and in the NGS
(Fig. 2a). This is in agreement with the present-day concept of sediment trapping in these areas (McCave and Tucholke 1986; Bohrmann et al. 1990; Wold
1992). We find that the modeled sediment accumulation rates are smaller than
those which have been measured. This is because of the low eolian sediment input and also because of the missing lateral input (Haupt 1995). The sediment is
transported by the deep western southward boundary current with local maxima near Newfoundland and farther to the south in and near the Caribbean (McCave and Tucholke 1986). We note that Iceland and the Caribbean are represented by seamounts in the model bottom topography; hence the nonzero accumulation rates are artifacts at the exact positions of these islands.
As the glacial North Atlantic Current diverged from today's northeastern
path, the accumulation rate in the Iceland and Irminger Basins dropped, and
89
are found mainly in the NGS, east and south of Iceland, and in the Labrador Sea.
The present -day convection pattern in our calculations is consistent with the
patterns given by Maier-Reimer et al. (1993), Toggweiler et al. (1989), and
Rahmstorf (1995). Shallow convection starts in the Gulf Stream area and marks
a further progression of subtropical water to high latitudes carried by the North
Atlantic Current becoming deeper as it progresses northeastward and westward
around the Greenland coast.
The major feature of glacial convection, as compared to its modern state, was
the far weaker glacial convection in the NGS and the southward shift of the main
convection sites. The maximum ventilation at the LGM occurred in the midNorth Atlantic between 50° and 60 0 N. This result is well supported by proxy data
(Duplessy et al. 1988; Sarnthein et al. 1995). A southward shift of both the convection sites and the polar front led to a decrease in the northward transport of
warm and salty water. Hence a positive feedback worked to establish the glacial
mode of circulation. An even farther southward shift of the convection was limited by Ekman pumping in the subtropics which protected the major anticyclonic gyre from shrinking even more, i.e., providing a negative feedback to balance
the positive one. The MWE convection sites were shifted further southward and
the convection was very shallow, penetrating no deeper than 600 m in the central
North Atlantic. There was no convection to the north of SOON and the Nordic
Seas were completely convection-free during that time slice (Seidov et al. 1996).
S.2
Sediment Accumulation Rates and Sediment Transport
Total sediment accumulation is governed by the requirement that sources and
sinks balance at the sea-surface and be the same for all three time slices. The spatial distribution of sediment is however different, and reveals two distinctly different circulation modes between present, and at the LGM and MWE. Fig. 2
presents the sediment rates for the three chosen time slices. A very high accumulation rate associated with the Holocene/Modern mode of circulation is found in
the vicinity of Iceland in the Irminger Basin, along the Reykjanes Ridge at both
the south and north sides of Iceland, at the Rockall Plateau, and in the NGS
(Fig. 2a). This is in agreement with the present-day concept of sediment trapping in these areas (McCave and Tucholke 1986; Bohrmann et al. 1990; Wold
1992). We find that the modeled sediment accumulation rates are smaller than
those which have been measured. This is because of the low eolian sediment input and also because of the missing lateral input (Haupt 1995). The sediment is
transported by the deep western southward boundary current with local maxima near Newfoundland and farther to the south in and near the Caribbean (McCave and Tucholke 1986). We note that Iceland and the Caribbean are represented by seamounts in the model bottom topography; hence the nonzero accumulation rates are artifacts at the exact positions of these islands.
As the glacial North Atlantic Current diverged from today's northeastern
path, the accumulation rate in the Iceland and Irminger Basins dropped, and