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

The youngest of these early Quaternary events
of high sea level is the Fishcreekian transgression
(about 2.1–2.4 Ma), suggested to be the
same age as the kAp københAvn Formation on
GreenlAnd (Brigham-Grette and Carter, 1992).
However, age control is not complete, and
Brigham (1985) and Goodfriend et al. (1996)
suggested that the Fishcreekian could be as
young as 1.4 Ma. This deposit contains several
mollusk species that currently are found only to
the south. Moreover, sea otter remains and the
intertidal gastropod Littorina squalida at Fish
Creek suggest that perennial sea ice was absent
or severely restricted during the Fishcreekian
transgression (Carter et al., 1986). Correlative
deposits rich in mollusk species that currently
live only well to the south are reported from
the coastal plain at noMe, AlASkA (Kaufman and
Brigham-Grette, 1993).
The available data clearly indicate episodes of
relatively warm conditions that correlate with
high sea levels and reduced sea ice in the early
Quaternary. The high sea levels suggest melting
of land ice (see Chapter 5, History of the Greenland
Ice Sheet). Thus the correlation of warmth
with diminished ice on land and at sea (see
Chapter 6, History of Arctic Sea Ice)—indicated
by recent instrumental observations, model
results, and data from other time intervals—is
also found for this time interval. Improved time
resolution of histories of forcing and response
will be required to assess the causes of the
changes, but estimates of forcings indicate that
they were relatively moderate and thus that the
strong Arctic amplification of climate change
was active in these early Quaternary events.
3.4.3 The Mid-Pleistocene Transition:
41-k.y. and 100-k.y. Worlds
Since the late Pliocene, the cyclical waxing and
waning of continental ice sheets have dominated
global climate variability. The variations
in sunshine caused by features of Earth’s orbit
have been very important in these ice-sheet
changes, as described in Chapter 2 (Paleoclimate
Concepts).
After the onset of glaciation in North America
about 2.7 Ma (Raymo, 1994), ice grew and
shrank as Earth’s obliquity (tilt) varied in its
41 k.y. cycle. But between 1.2 and 0.7 Ma, the
variations in ice volume became larger and
Past Climate Variability and Change in the Arctic and at High Latitudes
slower, and an approximately 100-k.y. period
has dominated especially during the last
700 k.y. or so (Figure 3.25). Although Earth’s
eccentricity varies with an approximately
100-k.y. period, this variation does not cause
as much change in sunshine in the key regions
of ice growth as did the faster cycles, so the
reasons for the dominant 100-k.y. period in ice
volume remain obscure. Roe and Allen (1999)
assessed six different explanations of this behavior
and found that all fit the data rather well.
The record is still too short to allow the data to
demonstrate the superiority of any one model.
Models for the 100-k.y. variability commonly
assign a major role to the ice sheets themselves
and especially to the lAurentide ice Sheet on
North America, which dominated the total
global change in ice volume (e.g., Marchant and
Denton, 1996). For example, Marshall and Clark
(2002) modeled the growth and shrinkage of
the lAurentide ice Sheet and found that during
growth the ice was frozen to the bed beneath
and unable to move rapidly. After many tens
of thousands of years, ice had thickened sufficiently
that it trapped Earth’s heat and thawed
the bed, which allowed faster flow. Faster flow
of the ice sheet lowered the upper surface, which
allowed warming and melting (see Chapter 5,
History of the Greenland Ice Sheet). Behavior
such as that described could cause the main
variations of ice volume to be slower than the
main variations in sunshine caused by Earth’s
orbital features, and the slow-flowing ice might
partly ignore the faster variations in sunshine
until the shift to faster flow allowed a faster
response. Note that this explanation remains
a hypothesis, and other possibilities exist.
Alternative hypotheses require interactions in
the Southern Ocean between the ocean and sea
ice and between the ocean and the atmosphere
(Gildor et al., 2002). For example, Toggweiler
(2008) suggested that because of the close connection
between the southern westerly winds
and meridional overturning circulation in the
Southern Ocean, shifts in wind fields very
likely control the exchange of CO 2 between the
ocean and the atmosphere. Carbon models support
the notion that weathering and the burial
of carbonate can be perturbed in ways that
alter deep ocean carbon storage and that result
in 100-k.y. CO 2 cycles (Toggweiler, 2008).
Others have suggested that 100-k.y. cycles
and CO 2 might be controlled by variability in
obliquity cycles (i.e., two or three 41-k.y. cycles
The available data
clearly indicate episodes
of relatively warm
conditions that correlate
with high sea levels and
reduced sea ice in the
early Quaternary.
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