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

5.2.2b biological indicators and
rElatEd fEaturEs
Living things are sensitive to climate. The
species found in a tropical rain forest differ
from those found on the tundra. By comparing
modern species from different places that have
different climates, or by looking at changes
in species at one place for the short interval
of the instrumental record, the relation with
climate can be estimated. Assuming that this
relation has not changed with time, longer
records of climate then can be estimated from
occurrence of the remains of different species
in older sediments (e.g., Schofield et al., 2007).
These climate records then can be tied, to some
degree, to the state of the ice sheet.
Lake sediments are especially valuable as
sources of biotic indicators, because sedimentation
(and thus the record) is continuous and the
ecosystems in and around lakes tend to be rich
(e.g., Bjorck et al., 2002; Andresen et al., 2004;
Ljung and Bjorck, 2004). Pollen (e.g., Ljung and
Bjorck, 2004; Schofield et al., 2007), microfossils,
and macrofossils (such as chironomids,
also called midge flies (Brodersen and Bennike,
2003)) are all used to great advantage in
reconstructing past climates. The isotopic composition
of shells or of inorganic precipitates in
lakes records some combination of temperature
and of the isotopic composition of the water.
Physical aspects of lake sediments, including
those linked to biological processes (e.g., loss on
ignition, which primarily measures the relative
abundance of organic matter in the sediment)
are also related to climate. In places where the
weight of the ice previously depressed the land
below sea level and subsequent rebound raised
the land back above sea level and formed lakes
(see section 5.2.2C, below), the time of onset of
lacustrine conditions and the modern height of
the lake together provide key information on
ice-sheet history (e.g., Bennike et al., 2002).
Raised marine deposits in GreenlAnd and surroundings
provide an additional and important
source of biological indicators of climate
change. Many marine deposits now reside above
sea level, because of the interplay of changing
sea level, geological processes of uplift and
subsidence, and isostatic response (ice-sheet
growth depressing the land and subsequent
ice-sheet shrinkage allowing rebound, with a
lagged response; see section 5.2.2C, below).
Past Climate Variability and Change in the Arctic and at High Latitudes
Biological materials within those deposits, and
especially shells, can be dated by radiocarbon
or uranium-thorium techniques (see section
5.2.2d, below). Those dates then help fill in
the history of relative sea level that can be
used to infer ice-sheet loading histories and to
reconstruct climates on the basis of the species
present (e.g., Dyke et al., 1996).
5.2.2c glacial isostatic adjustmEnt
and rElatiVE sEa-lEVEl indicators
nEar thE icE shEEt
Within the geological literature, sea level is generally
defined as the elevation of the sea surface
relative to some adjacent geological feature.
(This convention contrasts with the concept of
an absolute sea level whose position (the sea
surface) is measured relative to some absolute
datum, such as the center of Earth.) This definition
of sea level is consistent with geological
markers of past sea-level change (such as ancient
shorelines, shells, and driftwood), which
reflect changes in the absolute height of either
the sea surface or the geological feature (i.e., an
ancient shoreline can be exposed because the
surface of the ocean dropped, or land uplifted,
or a net combination of land and ocean height
changes). During the time periods considered in
this report, the dominant processes responsible
for such changes, at least on a global scale, have
been the mass transfer between ice reservoirs
and oceans associated with the ice-age cycles,
and the deformational response of Earth to this
transfer of mass. This deformational response is
formally termed glacial isostatic adjustment.
The growth and shrinkage of ice have generally
been sufficiently slow that glacial isostatic
adjustment of the solid Earth is characterized
by both immediate elastic and slow viscous
(i.e., flow) effects. As an example, if a large ice
sheet were to form instantly and then persist for
more than a few thousand years, the land would
respond by nearly instantaneous elastic sinking,
followed by slow subsidence toward isostatic
equilibrium as deep, hot rock moved outward
from beneath the ice sheet. Roughly speaking,
the final depression would be about 30% of the
thickness of the ice. Thus the ancient Lauren-
tide ice Sheet, which covered most of cAnAdA
and the northeastern United States and whose
peak thickness was 3–4 km, produced a crustal
depression of about 1 km. (For comparison,
The ancient Laurentide
Ice Sheet, which covered
most of Canada and the
northeastern United
States and whose peak
thickness was 3–4 km,
produced a crustal
depression of about
1 km.
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