VARIATIONS IN GLACIAL FLOUR FLUX TO LAKES IN THE TROPICAL ANDES OVER THE PAST 20 KA

Cores from glacial lakes in the tropical Andes provide continuous, high-resolution proxy records of glacial flour input and paleoglacier extent, storm-induced sedimentation, and hydrologic balance. Here we review cores that span the last 12,000-25,000 cal years from glacial lakes located from 2°45'-13°54'S and from 2500-4920 m. Age-depth models for these cores are based on AMS ¹⁴C dating of macro-vegetal material, ²¹⁰Pb dating, and tephrochronology.

The flux of clastic sediment to glacial lakes varies from <0.01 to 0.4 g cm^-2 yr^--1, and is controlled principally by the proximity of ice margins to lake basins, and by sediment focusing within lake basins. Radiocarbon dated ice margins in central and southern Peru correspond closely with intervals of rapidly increasing clastic sediment flux, and thus this confirms that most of the clastic sediment is indeed glacial flour. Lakes located proximal to and inside of the limits of the last glacial maximum indicate that deglaciation was underway by ~21 kcal yr B.P., earlier than deglaciation in the middle and high latitudes of both hemispheres. These lakes show a monotonic decline in flour flux that reached levels <0.01 g cm^-2 yr^--1 by ~17.5 kcal yr B.P. Lake basins located upvalley from the former began to receive glacial flour ~15 kcal yr B.P., but flour flux dropped to <0.01 g cm^-2 yr^--1 by 12 kcal yr B.P. Those lakes located in catchments with headwall elevations <5500 m never again received glacial flour, but lakes with headwall elevations >5500 m show a pronounced increase in glacial flour flux that began ~ 5 kcal yr B.P., and reached as high as 0.4 g cm^-2 yr^--1, the highest rate of flour input noted to the lakes studied.

A composite record of glacial flour flux for the tropical Andes reveals the highest variance among records 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 among records 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 closely with published records of reduced hydrologic balance in the tropical Andes.