Biology Reference
In-Depth Information
shift of 1500 m. Recalling that since about 2.6 Ma there have been between
eighteen and twenty glacial maxima, the dynamic nature of the biota is
evident. If the current trend in global warming persists, the páramo will
continue to move upward until it fi rst becomes limited and then is forced
off the mountains. If the adjacent forests continue to be lumbered from the
slopes, the consequences for erosion control, fi ltration of drinking water,
and aquifer levels and dependability are obvious.
The Amazon Basin
As northeastern South America separated from Africa around 90 Ma, pull-
apart basins like the Pernambuco Basin formed along the coastal region.
They border the lowlands of Amazonia to the east, which occupies a depres-
sion of about 6 million km
2
. The fl oor of the Amazon Basin follows the broad
contour of the ancient cratonic platform and the later Cretaceous surface. It
is covered by the world's largest extent of lowland rain forest (Richards 1996;
Morley 2000; Furley 2007; Galindo-Leal and Gusmão-Câmara 2003). The
Amazon forest is separated from the similar Atlantic forest by the Brazilian
Highlands region and its extensions to the north (Borborema Plateau) and to
the south (Mato Grosso Plateau). The basin is now fi lled with over 4000 m
of sediment. The oldest was eroded from the bordering Brazilian and Gui-
ana shields and deposited as the 1.6 Ga Roraima Group, which in turn was
eroded to form the distinctive tepuis of Venezuela (fi g. 2.44). These fl at-
topped mesas rise to a height of a mile or more from the lowland mist, and
they provided the setting for Sir Arthur Conan Doyle's novel
The Lost World
.
On the summit there are meadows, some low forests in depressions, and
occasional aquatic vegetation in pools of standing water.
The basin is traversed by a labyrinth of rivers on which small boats can
navigate 100,000 miles of waterway. The Amazon River (Hoorn 2006) is
the second longest in the world at 6450 km. (The Nile is the longest at
6670 km, and the Yangtze is third at 6380 km, although the Amazon may
be counted fi rst at 6800 km if one identifi es its origin in southern rather
than northern Peru.) Large sections of land called cahidas (fallen lands)
become detached along its banks, leaving thousands of furos, or small bays.
The river discharges 175,000 m
3
of water per second, deposits 1 billion tons
of silt into the ocean each year, and during fl oods the river can rise 10 m in
some places. Among the well-known features of the Amazon Basin are its
blackwater and whitewater rivers. Waters that fl ow from the north cross the
hard Precambrian Shield enter as sediment-poor, organic-rich black waters.
Those that drain the Andes Mountains are sediment-rich white waters:
