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The Last 110 kyr and the Dansgaard/Oeschger Events
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].
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* Next:* GISP2 and GRIP *Up:* The GISP2 ice core
* Previous:* The Younger Dryas
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/U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33 Suppl., © 1995 American Geophysical Union/