The isotopic temperature records show 23 interstadial (or Dansgaard/Oeschger)
events first recognized in the GRIP record [ Dansgaard et al., 1993] and
verified in the GISP2 record [ Grootes et al., 1993], between 110 and 15
kyr BP. Intensified formation of North Atlantic Deep Water (NADW) has been
invoked to explain some of these events. NADW forms when waters from the
oceanic thermocline upwell to the surface, cool, and sink in the seas around
Greenland. Heat is transferred from ocean to atmosphere in the process. Partly
because these seas were ice covered, NADW formation was generally slow
during glacial times. Intensification of NADW formation would cause rapid
warmings in Greenland and other land masses adjacent to the North Atlantic,
which can explain the impressive magnitude of the climate changes as well as
their rapidity. These dramatic climate changes were not restricted to Greenland
and nearby boreal areas, as evidenced by the GRIP CH
record [
Chappellaz et al., 1993].
Most of the 23 Greenland interstadial events observed at GISP2 are not yet
associated with major changes in Antarctic climate. However, at least eight of
the nine Greenland events lasting longer than 2 kyr are linked to periods of
warmer climates in East Antarctica [ Bender et al., 1994] as inferred from
the Vostok isotopic temperature record [ Jouzel et al., 1987]. They are
also associated with diminished ice volume, as inferred from variations in the
O of CaCO
in deep sea sediments [ Shackleton and
Pisias, 1985]. Bender et al. [1994] suggested that intensification of
NADW formation resulted in some melting of ice sheets and an increased heat
flux from the Southern Ocean to the atmosphere, both of which caused
Antarctica to warm. Diminution of NADW formation would eventually reverse
the climate amelioration and return Greenland and Antarctica to cold, glacial
conditions.
The GISP2 glaciochemical series provide a sensitive record (see calcium
example from Mayewski et al.
[1994b],
and Bender et al. [1994],
Figure 3) of change in the atmospheric circulation systems affecting Greenland
[ Mayewski et al., 1994b]. Modeling common temporal behavior of these
chemical series reveals a record of change in the relative size and intensity of
the circulation system that transports well-mixed air masses to Greenland (the
measure of relative size and intensity is defined as the polar circulation index
(PCI)). The large-scale atmospheric circulation patterns capable of
transporting the seasalt and dust captured in the GISP2 record are dominated by
westerly or meridional patterns in the circumpolar vortex; therefore, the
periods of increased PCI which mark stadial (cold) intervals indicate stronger
and larger-scale circulation [ Mayewski et al., 1994b]. Massive iceberg
discharge events (previously defined from the marine record and correlated with
certain stadials in the ice core record [ Bond et al., 1993] can also be
interpreted from the glaciochemical record as being accompanied by notable
expansions of ocean ice cover (sea ice and icebergs) and the PCI [
Mayewski et al., 1994b].