AN ~15,000-YEAR LONG RECORD OF TROPICAL GLACIATION FROM THE CORDILLERA BLANCA PERU

Glacial valleys in the tropical Andes with headwall elevations <5000 m asl have been ice free for the entire Holocene and glacial lakes in these valleys possess sediment records that are dominated by massive, organic-rich gyttja. Few records have been acquired from glacial lakes directly downvalley from presently glaciated peaks (>5200 m asl) yet these lakes offer the potential to record continuous changes in Holocene ice volume through proxy indicators of the flux of glacier flour. Upper Laguna Gueshque is a moraine-dammed lake located at ~4300 m asl on the west side of the Cordillera Blanca, Peru (9°35’S; 77°25’W). The lake is located beyond the Holocene ice limit directly downvalley from two presently glaciated drainage basins encompassing ~25 km². The accumulation zone of active glaciers are underlain by granodiorite that yields an average magnetic susceptibility (MS) of ~2.4x10^-5 m³kg^-1 whereas most unglaciated portions of the drainage basin are underlain by quartzite, which yields an average MS of 3.9 x10^-7 m³kg^-1. Holocene moraines are dominated by granodiorite clasts. Three cores up to 13.5 meters long reveal large amplitude swings in MS and sedimentation rate, which reflect large changes in ice volume; 10 AMS radiocarbon dates provide the basis for a preliminary age model. Sedimentation flux was slowest during the late glacial and early Holocene (0.3 mm/yr) and fastest during the late Holocene (1.1 mm/yr). Peaks in MS suggest intervals of expanded ice cover from: 15.5-12.5, 6.0-3.25, 1.75-0.1 k cal yr BP. Between these intervals ice cover was substantially reduced; the most ice-free interval of the last 15 ka occurred from 12-8 and 3.25-1.75 k cal yr BP. During these intervals, ice cover was probably substantially less than at present. Reduced flux of glacier flour during the late glacial is consistent with nearby evidence of rapidly retreating ice margins during the Younger Dryas in response to cold but dry conditions. The expansion of glaciers beginning ~ 6 k cal yr BP is synchronous with the trend toward steadily depleting d18O values in ice from Nevado Huascaran~75 km north of the Gueshque Valley. Finally, ice expansion 1700 cal yr BP is supported by radiocarbon-dates on peat incorporated into a moraine in a nearby drainage.