LONG PALEOCLIMATE RECORD (>275 KYR) OF TROPICAL SOUTH AMERICA FROM SALAR DE UYUNI, BOLIVIA

The 220 m Salar de Uyuni salt-mud core contains a record of closed-basin tropical paleoenvironments and paleoaridity that shows oscillations between three basic settings: perennial lakes (laminated to massive muds), perennial saline lakes ((primary gypsum and chevron/cumulate halite) and desiccated saline pans and mudflats (diagenetic gypsum/halite cements and massive muds). Fourteen halites have been dated by the Th-230/U-234 disequilibrium method to 275 ka at a depth of 116.5 m. Extrapolation, assuming constant sedimentation rates, gives ages >500 kyr at the base of the core. During the latest 75 kyr (0 to 55 m core depth), Salar de Uyuni experienced continuous lacustrine conditions fluctuating between perennial saline (halite) lakes and shallow to deep, freshwater to saline lakes, including paleolakes Tauca, Minchin, and two older unnamed units. In contrast, at core depths >90 m (>~140 ka), Salar de Uyuni was predominantly desiccated (saline pan and mudflat salts and muds), with minor perennial lake phases none of which were deep or freshwater. The transition from desiccated paleoenvironments to perennial lake settings occurs between 90 and 55 m (140 to 75 ka). In contrast to the large lakes of the last glacial, desiccated salt flats alternating with rare perennial lakes prevailed during the penultimate glacial period and probably the three previous glacial cycles at Uyuni. These results suggest two important controls on paleoenvironments at Salar de Uyuni, operating at different time scales. Wet-dry cycles, occurring at ~20 kyr time scales, exist in the entire core. These may be regulated by precessional cycles and insolation variations producing long-term changes in summer precipitation. Superimposed on these cyclical changes in effective moisture is the long-term shift toward much wetter conditions, which may be due to the opening of a hydrologic connection between Salar de Uyuni and the wetter northern Altiplano (Lake Titicaca). Extrabasinal inflow from Lake Titicaca could explain the succession of large perennial lakes at Salar de Uyuni in the last 75 kyr that could not be sustained in earlier times from local inflows.