Biology Reference
In-Depth Information
huantepec, and periodically it inundates the Yucatán Peninsula, primarily
from tectonic movement of the platform. Baja California has been accreted
to the mainland since about 35.4-23.3 Ma. The spreading center that
formed the Sea of Cortez at 29 Ma continues to open between 23.6 and
6.7 Ma. The Transvolcanic Belt rises in the early Miocene circa 23 Ma,
moderately high elevations form by 17 Ma, and the mountains will continue
to rise into modern times. The Cordillera Central of Central America are
also moderate highlands, and they terminate at about southern Costa Rica,
with isolated islands extending to northwestern South America.
In South America, the Amazon Basin of the Miocene is still an intercon-
nected series of lakes and periodically marine-inundated swamps, but they
are beginning to receive sediments from the rising Andes Mountains in ad-
dition to those from the Guiana and Brazilian highlands. The Orinoco and
Amazon rivers still fl ow westward, and will continue to do so for another
2 million years, when around 15 Ma they will reverse direction. Vast deltas
will then begin to appear along the Caribbean coast, further modernizing
the landscape. In the late Miocene, the Maracaibo River will be dammed
by uplift of the Cordillera Oriental, and Lake Maracaibo will form. The off-
shore Netherlands Antilles will also appear in the Miocene.
Following the Middle Miocene Climactic Optimum, with its possible in-
crease in storms, the change toward lower temperatures and, consequently,
reduced atmospheric moisture, represents a major trend affecting the eco-
systems of the New World. Climates are not becoming drier uniformly at
this time, however, because the distribution of precipitation is determined
by atmospheric circulation, ocean currents, topography, and forcing mech-
anisms that are teleconnected between different parts of the world. Never-
theless, the general result of the cooling temperatures is decreasing precipi-
tation and greater seasonality, gradually spreading over many parts of the
Earth. Recall the paleosol evidence from the Badlands of South Dakota that
indicate a decrease in annual rainfall from 1000 mm in the late Eocene, to
between 250-450 mm in the early Miocene (Retallack 1990), to progres-
sively less and more seasonal precipitation with middle Miocene cooling,
and ultimately to the present 380 mm.
A reason for this shift to a new climate state is an overall slowing of
plate movement (from an annual average of about 12-14 cm in the Jurassic/
Cretaceous to about 2 cm at present) and the associated reduced volcanism
that lowers CO
2
input into the atmosphere. The reduction in CO
2
is being
augmented by positive feedbacks from greater weathering of silicate rocks
resulting from continuing uplift of mountain systems and drainage of con-
tinental margins and interiors, and by the increasing albedo of the drained
