THE TIMING OF GLACIATION IN THE TROPICAL ANDES FROM SURFACE EXPOSURE DATING AND CORES FROM GLACIAL LAKES

Cosmogenic radionuclide (CRN) dating of moraines, and cores from glacial lakes provide complementary records of glaciation. As few lakes predate the local last glacial maximum (LLGM), CRN dates are important for the reconstruction of glacial activity early in the last glacial cycle. In contrast, as the moraine record of glaciation is fragmented and cannot record intervals of reduced ice extent, glacial lakes provide the continuity of record and increased precision in age control to augment CRN-based chronologies for the past ~20 ka.

We have developed a chronology of glaciation for the the Junin Plain (4000 masl) of central Peru (11°S; 76°W). CRN dating indicates that moraines of the last glacial cycle stabilized during three intervals: 34-22, 20-16, and 15-12 ka (Smith et al.,2005, Science 308, 678+). Importantly, maximum ice volume in the tropical Andes occurred 30-34 ka, and ice retreat was underway by ~21 ka, which contrasts markedly with the record of global ice volume.

The flux of clastic sediment to glacial lakes varies from <0.01 to 0.4 g cm^-2 yr^-1. Lakes located proximal to and inside of the limits of the LLGM indicate that deglaciation was underway by ~21 ka, thus confirming the CRN-based moraine chronology. These lakes show a monotonic decline in sediment flux that reached minimum levels by ~17.5 ka. Lake basins located upvalley from the former began to receive glacial flour ~15 ka, but flour flux dropped to minimum levels by 12 ka. Those lakes located in catchments with headwall elevations <5500 masl never again received glacial flour, but lakes with headwall elevations >5500 masl show a pronounced increase in glacial flour flux that began ~ 5 ka, and reached maximum levels by ~2 ka.

A composite record of glacial flour flux reveals the highest variance during the late glacial, which reflects the variable timing of deglaciation of lakes at various elevations. Likewise, high variance among records occurs during the late Holocene, when low elevation catchments remained ice-free and high-elevation catchments were reoccupied by ice. The lowest variance occurs during the early Holocene when lakes at all elevations reveal the lowest rates of glacial flour flux. This early Holocene interval of low flour flux corresponds with records of reduced hydrologic balance in the tropical Andes.