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GISP2 and GRIP Records Prior to 110 kyr BP

The climatic significance of the deeper part of the GISP2 ice core,
below 2790 m depth and 110 kyr age, is a matter of considerable
investigation and controversy. The isotopic temperature records and
electrical conductivity records of GISP2 and GRIP, so similar for ice
*<*110 kyr in age, are very different in the lower part [/ Grootes et
al/., 1993; / Taylor et al/., 1993a]. Ice in GISP2 below 2790 m depth is
folded and tilted, and shows evidence of unconformities [/ Gow et al/.,
1993]. The O of O in GISP2 above 2790 m matches almost perfectly with
the Vostok record [/ Sowers et al/., 1993]; below that depth, it is far
noisier and cannot be aligned with the smoothed Vostok signal [/ Bender
et al/., 1994]. These features all suggest that ice age changes
discontinuously in the deepest part of GISP2 as a result of folding,
extensive boudinage (squeezing out of layers of ice), and/or intrusion.
/ Bender et al/. [1994] concluded that the bottom 200 m of ice at GISP2
may be correctly ordered but discontinuous and extremely condensed,
perhaps extending back to several hundred kyr BP. Alternatively, the
core may contain a disordered sequence of much younger ice, perhaps
largely from about 115-130 kyr BP.

GRIP (1993) interpreted the climate proxy records from the deepest part
of their core as being properly ordered and continuous. They observed
large and rapid changes in isotopic temperature with depth and concluded
that these features represented rapid changes in climate during marine
isotopic Stage 5e, the warmest part of the previous interglacial period.
Such a conclusion has extremely important implications for climate
because, together with the Dansgaard-Oeschger events, it suggests that
rapid cooling events are possible during the current interglacial period.

Evidence for rapid climate change during Stage 5e rests on the
assumption that the deep part of the GRIP ice core is continuous, in
contrast to GISP2. GRIP is located over the present ice divide, while
GISP2 is 30 km to the west. Therefore GRIP may in fact be more likely to
be continuous, although a recent modeling study has shown that the
divide itself has probably migrated [/ Anandakrishnan et al/., 1993].
Continuity of the deep part of the GRIP core has not yet been
definitively demonstrated. Until this is done, by showing that gas
composition records at GRIP are identical with those at Vostok or by
some other approach, the evidence for rapid climate change in Greenland
during the last interglacial remains equivocal.

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/U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33 Suppl., © 1995 American Geophysical Union/