The University of Arizona Desert Laboratory Past Research Highlights

Christa Placzek, Jay Quade, and P. Jonathan Patchett, Desert Laboratory and Department of Geosciences, University of Arizona

Geologists at work on the Salar de Uyuni, Southern Bolivian Altiplano.

Large, deep lakes approaching the size of modern Lake Baikal and Lake Michigan once occupied basins on the arid southern Bolivian Altiplano, which is now covered by parched salt pans (Fig. 1). These salt pans, known as salars, are remnants of ancient lakes that were up to 140 meters deep. Fortunately, these salt pans are not the only evidence attesting to the presence of old lakes; carbonate encrustations and erosional benches mark paleoshorelines and provide materials that can be dated. An accurate chronology of these lake highstands is required to assess the climate mechanisms responsible for such major changes in lake size - mechanisms likely to also have affected the American Southwest. A research report of this work by Desert Lab and University of Arizona scientists recently appeared in The Geological Society of America Bulletin.

Fig 1: Map of the Titicaca/Poopo/Coipasa/Uyuni hydrographic basin and the
major climate features of South America.
Fig 1: a) Map of the Titicaca/Poopo/Coipasa/Uyuni hydrographic basin. Modern lakes are in gray and salars are dotted. The dashed line represents the approximate maximum extent of paleo-Lake Tauca at ˜3780 m. b) The major climate features of South America. The Titicaca/Poopo/Coipasa/Uyuni drainage basin is hatched. The ITCZ is the Intertropical Convergence Zone.

Fig. 2: massive carbonate encrustations mark ancient shorelines and occur up to 140 meters above modern salt pans or very shallow lakes (Lake Poopo in distance).

Scientists sampled and mapped lake sediments and carbonate encrustations (Fig. 2) to determine the age and extent of these ancient lakes; a new and robust 120,000 year old chronology of lake events in these parched basins features two major and several minor lake episodes (Fig. 3). Many U-Th and ¹⁴C dates of shoreline deposits track the rise and fall of a late-glacial age (18,100-14,100) deep lake (Tauca) and recognize a new 120,000 to 98,000 year old deep lake (Ouki). This newly recognized lake Ouki has spuriously young (28-45 ka) radiocarbon dates; these new dates call for a fundamental shift in explanations of climate change in the region. Smaller lakes filled the basins between 11,000 and 13,000, between 20,000 and 24,000, around 46,000 and between 80,000 and 95,000 years ago.

Fig. 3: Lake cycles on the Bolivian Altiplano.

At least 88 radiocarbon and more than 90 U-Th dates were used to constrain the timing of lakes on the southern Bolivian Altiplano. The radiocarbon dating method was used to determine the ages of younger samples and the U-Th method was used for older samples. U-Th dates are processed at a Thermal Ionization Mass Spectrometer facility at the University of Arizona. The large number of U-Th dates produced by this study allowed assessment of carbonate textures for potential diagenetic effects, evaluation of the isotopic composition and quantity of initial Th incorporated into the carbonate, and assessment of carbonate textures with respect to the precision of resulting dates. These efforts resulted in a number of precise U-Th dates and affirmed the precision of younger radiocarbon dates. A strategy for successful U-Th dating of paleolake carbonates is outlined in a paper that will soon appear in Geochemistry, Geophysics, Geosystems; this strategy has implications for other global lake basins, as well as speleothems and travertines.

Such major changes in lake size demand equally large shifts in ancient climate, which are likely linked to changes in moisture content over the Amazon basin and changes in the well-known El Niño/La Niña cycle in the adjacent Pacific Ocean. Temperature may influence Altiplano lake levels indirectly, as potentially dry glacial periods in the Amazon Basin are linked to dry conditions on the Altiplano. Intensification of the trade winds associated with La Niña-like conditions currently brings increased precipitation on the Altiplano, and deep-lake development during the most recent deep lake cycle coincides with apparently intense and persistent La Niña-like conditions in the central Pacific.

Placzek, C., Quade, J., Patchett, P.J., 2006. Geochronology and stratigraphy of Late Pleistocene lake cycles on the Southern Bolivian Altiplano: Implications for causes of tropical climate change. Geological Society of America Bulletin, 118, 515-532.

Placzek, C., Patchett, P.J., Quade, J., and Wagner, J.D.M., 2006. Strategies for successful U-Th dating of paleolake carbonates: an example from the Bolivian Altiplano. Geochemistry, Geophysics, Geosystems, 7, (in press).