500-Century Record Links Northern Hemisphere Cold Spells To Water In South American Salt Flat

Writing in the Feb. 8 issue of the journal Nature, researchers from five universities concluded from their 724 feet of coring that lakes formed on the flat whenever that part of the South American tropics turned exceptionally wet. They also found those high moisture periods coincided with the last glacial age and other cold periods in the Northern Hemisphere. Other scientists have previously argued that the region was dry during those times.

These findings on the salt flat named the Salar de Uyuni corroborate another coring study of the past 25,000 years of water levels on Lake Titicaca, about 185 miles to the north, which some of the same authors described in the Jan. 26 issue of the journal Science.

The salt flat cores provided "a perfect correlation," Paul Baker, the Duke University geology professor who was principal author of both journal reports, said in an interview. "When was there salt and when was there lake mud?" Baker asked. "And, so, when was it dry and when was it wet?" Support for the work came from the National Science Foundation, which is also funding continuing coring studies on Lake Titicaca.

The group of geologists also included Catherine Rigsby of East Carolina University, Geoffrey Seltzer of Syracuse University, Sherilyn Fritz of the University of Nebraska at Lincoln and Tim Lowenstein of the State University of New York in Binghamton.

They made their deductions by analyzing two different components of the cores. The first indicator was natural gamma radiation from potassium, uranium and thorium, chemicals that are plentiful in lake mud residues but practically absent from salt deposits. The other was fossilized diatoms, the remains of silica-encased water-dwelling algae which varied in species depending upon whether the ancient lakes were deeper or shallower and fresher or saltier. Using the core evidence as a time line, the scientists pinpointed an interval 14,900-26,100 years ago, and another 38,100-42,000 years before the present, as periods of "maximum wetness." The more recent interval coincided with the last great period of glaciation and the development of a lake called "Tauca." The earlier one marked the rise and fall of another lake known as "Minchin." Both of those intervals coincided with periods in Earth's orbital cycle when its axis was pitched to bathe the Altiplano with the most sunlight during the Southern Hemisphere's summer, which lasts from December until March.

The authors hypothesize that the high sunlight would reinforce the South American summer monsoon, which transports water vapor from the tropical South Atlantic Ocean across the Amazon basin to the Altiplano and beyond.

Their gamma radiation records also pinpointed shorter lake-building wet periods lasting on the order of 1,000 years. Those intervals were not primed by high-summer-sunlight axial alignments, the authors wrote. Rather, they happened during so-called "Bond Events," times when other scientific studies suggest that the surface of the tropical eastern North Atlantic Ocean was unusually cold.

Using evidence from another study of the modern ocean, their Nature article suggests that ancient Bond Events might have boosted northeast trade winds to move extra moisture over the Amazon and Altiplano.

The report also cited corroborating ice cap studies on top of the Sajama volcano, located between Lake Titicaca and the Salar de Uyuni. There, high precipitation periods are marked by differences in the ratios of two forms – or isotopes – of oxygen in the ice, the authors wrote. The scientists also acknowledged that, although Earth's axis is once again aligned to bathe the Altiplano in extra summer sunlight, the Salar de Uyuni currently has no permanent lake. To explain this exception, they cited other evidence that the area's climate is currently wet.

One study demonstrates that Lake Titicaca has remained at or near its overflow level for the past 3,500 years. A second investigation shows that other lakes have formed on the plateau over much of that period. In fact, the Salar de Uyuni itself now "floods every year," Baker noted in his interview. "It floods and then dries up again."