Location, Location, Location - A Natural History of the New World - page 108

80.0 m). As the Antarctic ice sheet increased, causing the drop in sea level
at 30 Ma and contributing cold meltwater to the world's oceans, it further
cooled the Humboldt Current fl owing northward along the west coast of
South America. Westerly winds blowing across the cold water arrived on
the coastal lands of Peru and Chile as dry desiccating air, and that began the
process of aridifi cation.
The second event was that cooling of ocean waters worldwide reduced
the amount of moisture evaporating into the atmosphere to a point that drier
and more seasonal climates began to expand. The beginning of enhanced
seasonality is shown by spore and pollen evidence from the Norwegian-
Greenland Sea that indicates a drop in cold-month temperature of about
5°C just prior to the Eocene-Oligocene transition (Eldrett et al. 2009). The
trend continued, and by about 15 Ma, it had produced a noticeable reduc-
tion in mesic forests and an increase in grasslands and dry forests. Also by
about 15 Ma, the Andes Mountains had reached suffi cient heights to begin
blocking moisture from the Amazon lowlands. Thus, by the middle Mio-
cene, the combination of these events was (1) favoring organisms adapted
to arid environments, (2) causing coalescence of elements preadapted to
dry habitats, and (3) allowing expansion of formerly restricted dry-habitat
species and communities into a biotic formation recognizable as the Atac-
ama Desert. After that, arid conditions periodically intensifi ed during the
cold dry intervals of the Pleistocene, fl uctuating with the El Niños and La
Niñas that have affected the coast since at least 15 kyr. Prior to that time,
evidence for El Niño events in the form of fl ood debris becomes diffi cult to
distinguish from landslides caused by other factors, such as earthquakes.
The Monte, the Gran Chaco, and the Pampas
In Argentina, as winds rise from the lowlands up the eastern slopes of the
Andes Mountains, moisture is lost at the intermediate elevations so the
higher slopes are dry. Continentality, slope, and coarse soils like those on
alluvial fans play a role, and in addition, rainfall from the west is blocked by
the mountains. The result is a near-desert plain with a MAP of 30-350 mm,
a MAT of 13°C-15° C, and a vegetation similar to that in the basins of the
Basin and Range Province of the western United States and on the Cen-
tral Plateau of Mexico. The region is called the Monte, and it covers about
450,000 km 2 between 24°S and 43°S . Plants similar to those in the dry ar-
eas of North America include Acacia , Caesalpinia , Cassia , Cercidium , Larrea ,
Mimosa , and Prosopis . Molecular data for Larrea suggest a probable South
American ancestor and indicate possible arrival in North America between