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warming, typically occurring over a matter of decades, and then gradual
cooling over several centuries. D-O cycles can be stratigraphically
related to layers of ice-rafted detritus released from armadas of melting
icebergs calved from the ice sheets, the most pronounced of which are
termed Heinrich events. These Heinrich events occur in the cold periods
immediately preceding D-O warming. Causes of the D-O cycles are still
incompletely understood.
The Last Glacial Maximum (LGM) occurred from 18-25 ka
(depending on the region and interpretation). Terrestrial ice extent during
the LGM is fairly well known. There were immense ice sheets in North
America (the Laurentide, second in size only to the Antarctic ice sheet
and the Innuitian ice sheet over the Canadian Arctic islands) and parts
of Eurasia. Along with the major Fennoscandian Ice Sheet, ice sheets
covered Svalbard, Franz Josef Land, the Barents Sea, Novaya Zemlya, the
Kara Sea, Iceland and part of the British Isles. The process of deglaciation
after the LGM was very irregular. Of particular note is the Younger Dryas
(YD) event from about 12.8 to 11.5 ka, which represented a temporary
return to near glacial conditions. Regional climate expressions of the YD
were quite varied.
The Holocene epoch (formally defined as the past 11.7 ka) has been
generally warm, and although characterized by a more stable climate
than the Pleistocene, has still been quite variable. Especially mild
conditions during the first part of the Holocene, known as the Holocene
Thermal Maximum, were followed by general cooling, during which the
sea ice cover expanded and glacier extent increased. The Little Ice Age
(LIA) represents the most recent major cooling. Ice core records from
Greenland indicate that temperatures there reached their lowest point in
the past millennium between AD 1579 and 1730. Causes of the LIA are
still debated. Although solar variability appears to have played a role, new
evidence points to effects of an extended period of explosive volcanism
and attendant climate feedbacks. The LIA has been followed by general
warming extending through the present.
10.1
The Distant Past
The present geography of the Arctic is just a brief snapshot in geologic time
(
Figure 10.1
). In 1912, Alfred Wegner proposed that the earth's continents were
once part of a single supercontinent that he named Pangea (meaning “all lands”).
Modern theory explaining continental drift, termed plate tectonics, was developed
during the 1960s. The earth's crust is divided into a number of continental and oce-
anic plates. Because of convection currents in the mantle arising from heat gener-
ated by radioactive decay, the ocean floors are continually moving, sinking at the
edges, spreading from the center, and being regenerated. Convection moves the
