Geoscience Reference
In-Depth Information
rise) followed during the early Pliocene (5-3.5 Ma). Ice-rafted sediment began to
appear in the Nordic Basin at about 4 Ma. The Bering Strait appears to have opened
in the Late Miocene and earliest Pliocene, about 5.5 to 4.8 Ma (Marincovitch and
Gladenkov,
1999
).
Progressive cooling during the late Tertiary period is well-documented. Tectonic
isolation of the Arctic from the North Atlantic and Pacific occurred during the
Pliocene (3.5-3.0 Ma). This promoted cooling and accumulation of land ice in
Alaska, Iceland, and Greenland (Harris,
2001
). Seasonal freezing of the Arctic
Ocean may have begun in the late Pliocene, with a perennial sea cover present
by 0.85 Ma (Herman,
1983
). The formation of permafrost in the Arctic probably
occurred about 1.65 Ma. However, evidence from Fairbanks, Alaska suggests an
earlier onset of 2.2-2.5 Ma (Harris,
2001
). The time required for ground freezing
to the depth observed today, assuming only conductive heat transfer, can be esti-
mated from the long-term geothermal gradient, the paleotemperature history of the
upper surface of permafrost, and the soil thermal properties. Calculations by V.
Lunardini (
1993
) for Prudhoe Bay, Alaska, suggest that with a constant surface tem-
perature of −11
o
C, development of 90 percent of the observed equilibrium thickness
of permafrost (600 m) would have been achieved within around 0.5 million years.
Alternative temperature histories support more rapid growth. For example, a sur-
face temperature 2.5
o
C lower would enable permafrost 570 m thick to form in only
120,000 years. Lunardini also calculates that the maximum permafrost thickness of
around 1,400 m observed in eastern Siberia (Duchkov and Balobaev,
2001
) might
have required the complete Quaternary period to form.
10.2
Paleoclimate Records for the Quaternary
10.2.1
Primary Sources
The remainder of this chapter focuses on the Quaternary. As mentioned, the
Quaternary started around 2.6 Ma and comprises the Pleistocene (2.6 Ma to 11.7 ka)
and Holocene (11.7 ka to present) epochs (
Figure 10.1
). The Quaternary is recent
enough so that many different types of paleoclimate evidence are preserved, espe-
cially from the last interglacial onwards.
A wealth of information has been obtained by coring the polar ice sheets, such
as from GISP 2, GRIP and Antarctic ice core projects (e.g., the Vostok and EPICA
cores). The new North Greenland Eemian Ice Drilling (NEEM) ice core records
extend to 130 ka, back to the last interglacial (the Eemian), and Antarctic ice cores
extend back much further (430 to 750 ka). Arctic ice cores have also been obtained
from smaller ice caps in the Canadian Arctic Archipelago (Devon Island, Ellesmere
Island, Baffin Island). Thousands of marine sediment cores have been collected
across the world's oceans, many providing records for the complete Quaternary and
even longer. Other paleo-environmental information for the Quaternary comes from
pollen, diatoms, and plant and animal macrofossils preserved in lake sediments and
