Biology Reference
In-Depth Information
reached in the declining atmospheric concentration of CO 2 . The two cen-
ters frequently fuse into a single sheet, and at the earliest and most extensive
advance, the ice extends to Nebraska where the Elk Creek Till is dated at
2.14 Ma. In the intervening areas to the north and along the margin, there
is a zone of permafrost 80-200 km wide supporting a herbaceous com-
munity of lichens, mosses, grasses, and sedges, and a dwarfed woody com-
munity of alder, birch, and willow. This is the early tundra, and elements
and patches have been present locally since the middle to late Pliocene. As
an ecosystem, it dates primarily from 3-2 Ma and becomes most extensive
just prior to and following the glacial periods in polar regions when there
are maximum extents of ice-free permafrost surfaces. At the Last Glacial
Maximum circa 18 kyr, ice will cover 40 million km 2 , or 30 percent of the
Earth's surface, compared to the rapidly diminishing 15 million km 2 at pres-
ent, and sea level was about 120 m lower. Among other consequences, each
of the eighteen to twenty lowerings will enlarge the Florida Peninsula by
twofold, primarily by extending its western coast, and expand the Zapata
(Cuba) and other coastal swamps that are presently near their minimum
(interglacial) extent.
Winters in Greenland at the LGM were 11°C-15°C colder and summers
2°C colder than at present, and these have probably been the approximate
conditions between eighteen and twenty times since 2.6 Ma. Around 2 Ma,
the northern ice sheet for the fi rst of many times in the Quaternary will
divide the polar jet stream, shifting the southern arm southward and dis-
placing the high pressure system located beneath the descending arm of the
Hadley circulation cell and the Bermuda-Azores High at 30°N-35°N. The
resulting cooler and wetter pluvial climates will prevail for 90 percent of
the next 2 million years, or about 90,000 of each of the 100,000-year Milan-
kovitch cycles. Toward the south, it is 6°C-8°C colder than at present; it is
5°C-6°C colder at the latitude of Barbados and in the lowlands of Brazil at
each glacial maximum; and it is noticeably drier. The record as preserved in
ice cores documents that during glacial intervals conditions are cold, windy,
dry, and dusty (Lambert et al., 2008).
The early Pleistocene fauna of North America is grouped into the Irving-
tonian, or Mammuthus -pre- Bison NALMA, and includes new immigrants
from Asia and South America. The Asian component includes cold-habitat
steppe species such as mammoth and musk-oxen, and microtid rodents
like lemmings, mice, and voles. The Rancholabrean fauna after 0.5 Ma will
add goats and bison from Asia. Many of the Miocene faunal elements are
gone. Among the carnivores lost are amphicyonines, hemicyonines, and
borophagine canids; among the herbivores, dromomerycids and most anti-
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