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O. Ct a H — re Ci ers o rc » o center, and being regenerated. Convection moves the plates in different directions. We now believe that the supercontinent Pangea broke up roughly 200 Ma during the early Jurassic period. Continental drift is of course still occurring, but by about 2 Ma the continents looked largely as they do today.

Just prior to the breakup of Pangea, the present Arctic land masses were located in mid-latitudes. Displacement of the region to north of the Arctic Circle occurred around 100 Ma during the Cretaceous. The Canadian Basin and Makarov Basin seem to have originated during the Cretaceous period. Sea floor spreading increased in the early Tertiary (about 57 Ma) when Greenland separated from Eurasia and the Eurasian and Nordic basins formed (Vogt, 1986). This changing distribution of lands and ocean and mountain-building events (orogenies) had strong impacts on climate. In the Paleocene and Eocene epochs (60-50 Ma), Ellesmere Island seems to have supported many subtropical vertebrate fauna in a swamp-forest environment (McKenna, 1980). Deciduous conifer species grew in West Greenland and Spitsbergen. A deepwater connection to the Nordic Seas via Fram Strait developed around 35 Ma. This was about the time when temperatures of the global ocean surface and bottom waters began to decrease sharply. Global sea level lowering in the late Miocene (6.5-5.2 Ma) led to isolation of the Arctic Ocean and exposure of the shelf areas (Danilov, 1989). However, a major marine transgression (sea level 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 Basin 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, 1989). The formation of permafrost in the Arctic probably occurred about 1.65 Ma. However, evidence from Fairbanks, Alaska, suggests an earlier onset of 2.22.5 Ma (Harris, 2001). The time required for ground freezing to the depth observed today, assuming only conductive heat transfer, can be estimated from the long-term geothermal gradient, the paleotemperature history of the upper surface of permafrost, and the soil thermal properties. Calculations by Lunardini (1993) for Prudhoe Bay, Alaska, suggest that with a constant surface temperature of -11 °C, development of 90% 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 surface temperature 2.5 °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 1400 m observed in eastern Siberia (Duchkov and Balobaev, 2001) might have required the complete Quaternary period to form. Such a thickness is not in equilibrium with the modern surface temperature and must be slowly thawing.

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