The pluvial debate - Climatic Change in Deserts - page 214

the California coast. The other transect ran from NewMexico to Nevada and included
recently dated strandlines from Lake Lahontan in Nevada (38-42 ° N) and Lake Estan-

cia in NewMexico (34.8 N). What they found was contrary to what might be expected

if the main source of precipitation in the Great Basin Lakes had come from southward
displacement of the winter westerlies during late glacial time. Peak humidity, shown
by times of highest lake level, was earlier in Lake Estancia (24.5 to 15.5 ka, with a
sharp drop in level at 18-17 ka) and later in Lake Lahontan (17.2 to 14.5 ka, preceded
by a sharp drop in lake level between 19.3 and 17.2 ka, after an interval of relatively
high lake level between 25 and 20 ka). Furthermore, the central California coast was
relatively wet between 12.5 and 4.5 ka, which was approximately 5,000 years after
the wet interval evident in southern California. They concluded that the dominant
sources of precipitation for the Great Basin lakes came from the south, notably from
the eastern Pacific and Gulf of Mexico during the summer monsoon season. As more
data come to light in the future, this interpretation will no doubt be tested further,
but two key points deserve emphasis. First, the onset of peak humidity in the Great
Basin lakes was time-transgressive, so that a simple glacial

°

pluvial equation is no
longer tenable. Second, southward displacement of the westerlies can be ruled out as
the major cause of late Pleistocene high lake levels in the Great Basin of the western
United States.

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12.4 Pluvial lakes in South America
The pluvial lakes located at high elevations in the semi-arid Bolivian Altiplano of the
central Andes region have been intensively studied by international research teams
equipped with the necessary drilling equipment (Sylvestre, 2009 ). Many of these
once freshwater lakes are now saline and are referred to locally as salars , or saltpans.
Sylvestre et al. ( 1999 ) used a combination of 14 Cand 230 Th/ 234 U ages to determine
a lake level chronology for the Uyuni-Coipasa basin. She and her colleagues found
that late Pleistocene lake levels began to rise slightly before 16,000 radiocarbon years
ago ( 14 C yr BP) (see Chapter 6 ) and reached maximum levels between 13,000 and
12,000 14 C yr BP. Following a dry spell between about 12,000 and sometime before
9,500 14 C yr BP, the lake rose again to a lower level than in the terminal Pleistocene
between about 9,500 and 8,500 14 C yr BP. They obtained good concordance between
their 14 Cand 230 Th/ 234 U ages for the first and highest lake phase but a lack of accord
for the second lake phase. They attributed this discrepancy to a delayed response of
the groundwater table during the dry phase and used a correction of around 2,000 14 C
years for the reservoir effect.
Geyh et al. ( 1999 ) sought to establish a reliable 14 C chronology for the late glacial/
early Holocene humid phase along a high-altitude transect between 18 S

in the Atacama Desert of northern Chile. They dated a variety of different types
of sample, including non-aquatic, carbon-rich sediments, in order to establish the

° S and 28 °