Past Climate Variability and Change in the Arctic and at High Latitudes

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The U.S. Climate Science Program Chapter 5
5.3.2b icE-shEEt indications
The cold MIS 6 ice age (about 130–188 ka)
may have produced the most extensive ice
in GreenlAnd (Wilken and Meinert, 2006).
Recently described glacial deposits in East
GreenlAnd support this view (Adrielsson and
Alexanderson, 2005), although more-extensive,
older deposits are known locally (Funder et al.,
2004). Funder et al. (1998) reconstructed thick
ice (greater than 1,000 m) during MIS 6 in areas
of jAMeSon lAnd (East GreenlAnd) that now are
ice-free. However, no confident ice-sheet-wide
reconstructions based on paleoclimatic data are
available for MIS 6 ice.
Both northwest and East GreenlAnd preserve
widespread marine deposits from early in the
MIS 5 interglacial (the interglacial previous to
the present one) (about 74–130 ka), and particularly
from the warmest subdivision of MIS 5,
called MIS 5e (about 123 ka). Depression of
the land from the weight of MIS 6 ice allowed
incursion of seawater as ice melted during the
transition to MIS 5e. The resulting deposits
were not reworked by the subsequent incursion
of seawater during the transition from the most
recent glaciation (MIS 2, which peaked about
24 ka or slightly more recently) to the modern
interglacial (MIS 1, less than 11 ka). Thus, seawater
moved farther inland during the transition
from MIS 6 (glacial) to MIS 5 (interglacial) than
during the transition from MIS 2 (most recent
glacial) to MIS 1 (current interglacial).
Several hypotheses can explain this difference.
Perhaps most simply, there may have been more
ice on GreenlAnd causing greater isostatic depression
during MIS 6 than during MIS 2. However,
if some or all of the older deposits survived
being overridden by cold-based ice of MIS 2,
additional possibilities exist. Because isostatic
uplift occurs while ice is thinning but before
the ice margin melts enough to allow incursion
of seawater, perhaps the MIS 6 ice melted
faster and allowed incursion of seawater over
more-depressed land than was true for MIS 2
ice. Additionally, at the time during MIS 6 that
ice in GreenlAnd receded and thus allowed
incursion of sea-water, global sea level might
have been higher than during the corresponding
part of MIS 2 (perhaps because of relatively
earlier melting of MIS 6 ice on North America
or elsewhere beyond GreenlAnd). More-detailed
modeling of glacial isostatic adjustment will be
required to test these hypotheses. Nonetheless,
the leading hypothesis seems to be that ice was
more extensive in MIS 6 than in MIS 2.
A particularly interesting new result comes
from analysis of materials found in ice cores
from the deepest part of the ice sheet. Willerslev
et al. (2007) attempted to amplify deoxyribonucleic
acid (DNA) in three samples:
1. silty ice at the base of the GreenlAnd
ice Sheet from the dYe-3 drill site (on
the southern dome of the ice sheet) and
the Grip drill site (at the crest of the
main dome of the ice sheet),
2. “clean” ice just above the silty ice of
these sites, and
3. the kAp københAvn formation.
The kAp københAvn, clean-ice, and Grip silty
samples did not yield identifiable quantities of
DNA, but it was possible to prepare extensive
materials from the dYe-3 silty ice. The lack of
DNA at kAp københAvn probably indicates postdepositional
changes, perhaps during roomtemperature
storage following collection. The
lack of DNA at Grip demonstrates that there is
no important wind-blown source. In turn, this
indicates that the material at dYe-3 has a relatively
local source. The dYe-3 material indicates
a northern boreal forest, compared with the
tundra environment that exists in coastal sites
at the same latitude and lower elevation today.
The taxa indicate mean July temperatures then
above 10°C and minimum winter temperatures
above –17°C at an elevation of about 1 km
above sea level (allowing for isostatic rebound
following ice melting). Dating of this warm,
reduced-ice time is uncertain, but a tentative age
of 450–800 ka is probably consistent with the
indications of high sea level in MIS 11.
Nishiizumi et al. (1996) reported on radioactive
cosmogenic isotopes in rock core collected
from beneath the ice at the GiSp2 site (central
GreenlAnd, 28 km west of the Grip site at the
GreenlAnd summit). Joint analysis of beryllium-10
and aluminum-26 indicated a fewmillennia-long
interval of exposure to cosmic
rays (hence ice cover of thickness less than 1 m
or so) about 500±200 ka. This information is
consistent with, and thus provides further support
for, the DNA results of Willerslev et al.
(2007). This work was presented at a scientific