TIMING AND MAGNITUDE OF QUATERNARY LACUSTRINE PHASES ON THE BOLIVIAN ALTIPLANO AS RECORDED IN SUBSURFACE STRATA OF THE RIO DESAGUADERO VALLEY, BOLIVIA

Laterally complex fluvial and lacustrine strata in five fifty-meter drill cores from the central Rio Desaguadero (RD) valley record at least nine periods of increased moisture on the Bolivian Altiplano. The strata reveal the northernmost extent of paleolakes previously identified in outcrops and cores from the southern RD valley and Salar de Uyuni. They also provide evidence that paleolakes on the Bolivian Altiplano were fed by overflow from Lake Titicaca (LT).

The best-dated of the 9 lacustrine phases were deposited at ~40,000 to 32,000, ~18,000 to 15,000, and ~8700 to 7800 cal yrs BP and are correlative with LT highstands and with lacustrine strata in cores from the southern RD valley and the Salar de Uyuni. The lower strata of the ~40,000 cal yrs BP paleolake (“Minchin”?) are present as far north as Nequela (17º 27' 50" S, 68 02' 30" W), near the northern extent of the study area. The upper strata of this unit, however, are correlative with fluvial strata at Tejopa (17º 38' 60" S, 67 33' 13" W), the southernmost drill core in this study, documenting the drying of this lake as LT flow decreased. The ~18,000-15,000 cal yrs BP lake strata occur as far north as Manquiri (17º 34' 44" S, 67 52' 57" W), suggesting that at its maximum extent this paleolake (“Tauca”?) reached far into the central RD valley. The ~8700-7800 cal yrs BP lake strata provide the first evidence of increased moisture in the RD valley during this time. The lack of correlative lacustrine strata in other parts of the RD valley is accounted for by a disconformity in the southern cores. Interestingly, unlike the northern and southern RD valley strata, the central RD cores contain no record of lacustrine sedimentation at ~2000 cal yrs BP, suggesting that LT overflow was not sufficient to create a basinwide lake.

Continued analysis of the lacustrine strata in these cores (inorganic and organic C analyses, grain size analysis, diatom paleoecology, and additional radiocarbon dating) will allow us to more precisely document the nature of these paleolakes, their significance to regional changes in effective moisture, and their correlation with global-scale circulation changes.