THE RECORD OF TROPICAL GLACIATION FROM DRILL CORES IN LAKE TITICACA, PERU/BOLIVIA

Snowline depression of alpine glaciers in the tropics is typically used as a proxy for thermal decline during the last glaciation. The timing of tropical glaciation, however, remains poorly constrained. Here we present a drill-core stratigraphy from Lake Titicaca, Peru/Bolivia (16°S, 3810 masl) as a proxy for glaciation in the cordillera surrounding the lake. Continuous sediment accumulation in the lake for at least the last ca. 130 kyr permits comparison with the last glacial cycle of the Northern Hemisphere.

In April-May 2001 we obtained 625 m of drill core from three locations in Lake Titicaca. Rapid accumulation of detrital sediments at the drill-core sites represents the time period when glaciers were more extensive in the surrounding cordillera. Extrapolation of glacial-age radiocarbon dates using a best-fit linear model for each site suggests the last interglacial-glacial transition occurred ~50 14C kyr BP (~57 cal kyr BP) at Site 1, ~55 14C kyr BP (~64 cal kyr BP) at Site 2, and ~62 14C kyr BP (~69 cal kyr BP) at Site 3. The uncalibrated radiocarbon age estimates should be viewed as minimum ages for a given stratigraphic interval. The calibrated radiocarbon ages are provisional ages because of uncertainties in the calendar-age equivalence of radiocarbon ages >18 14C kyr BP and potential uncertainties in old reservoir effects of analyses of samples low in total organic carbon. Therefore we estimate that the last interglacial-to-glacial transition in the tropical Andes commenced at least 64 cal kyr BP, during the equivalent of Marine Isotope Stage 4 (73,910 to 58,960 yr BP; Martinson et al., 1987) when there was a major and rapid drop in sea level and build up of continental ice sheets.

The duration of the last glacial cycle in the tropical Andes is similar to that of alpine glaciers of the eastern Sierra Nevada, U.S. recorded in Owens Lake and of Northern Hemisphere ice sheets, although there is some indication that warming at the last glacial-to-interglacial transition may have started several thousand years earlier in the tropics and Southern Hemisphere. The near-synchronization of the last glacial cycle since at least 64 kyr BP would have required a global cooling mechanism that could impact glaciers in widely differing climatic and geographic settings.