Tropical South America has endured alternating periods of heavy rainfall and severe drought during the last 25,000 years, according a new study in the journal Science.
The report - based on geological evidence from one of South America`s largest lakes - demonstrates how nature can produce sudden, unexpected climate changes that affect the entire planet.
The study, which appears in the Jan. 26 issue of Science, uses sediment samples taken from the bottom of Lake Titicaca - the world`s highest lake navigable to large vessels.
Straddling the border between Bolivia and Peru, Titicaca is 120 miles long, 50 miles wide and has average depth of 500 feet. The lake is located more than 2 miles above sea level on the Altiplano, or High Plateau, of the northern Andes Mountains.
``The Altiplano is like a giant cup, and Titicaca is the deepest point in the vast plateau, so most of the precipitation in the Altiplano drains into the lake,`` says Stanford geologist Robert B. Dunbar, one of the authors of the Science study.
Because very little water drains out of Titicaca, the lake serves as a reliable archive of rainfall patterns over many centuries - not just on the Altiplano, but in a large portion of tropical South America, according to Dunbar and his co-authors.
``Titicaca is the only large and deep freshwater lake in South America, and in deeper portions of the lake, sediment has accumulated continuously for at least the past 25,000 years,`` they add.
The authors point out that earlier studies of Titicaca relied on coring samples from the lake bottom taken at depths of 150 feet or less. To obtain an older and more complete climate record, a team of geologists led by Science co-author Paul A. Baker of Duke University collected three new samples at 270 feet, 450 feet and 690 feet below the surface.
Baker, Dunbar and their colleagues were able to reconstruct a history of precipitation in the Altiplano by determining how water l
'"/>
Contact: Mark Shwartz
mshwartz@stanford.edu
650-723-9296
Stanford University
25-Jan-2001