LGM and Late Glacial glacier advances in the Cordillera Real and Cochabamba (Bolivia) deduced from 10Be surface exposure dating

Surface exposure dating (SED) is an innovative tool already being widely applied for moraine dating and for Late Quaternary glacier and climate reconstruction. Here we present exposure ages of 28 boulders from the Cordillera Real and the Cordillera Cochabamba, Bolivia. Our results indicate that the local Last Glacial Maximum (LGM) in the Eastern Cordilleras occurred at ~22–25 ka and was thus synchronous to the global temperature minimum. We were also able to date several Late Glacial moraines to ~11–13 ka, which likely document lower temperatures and increased precipitation ("Coipasa" humid phase). Additionally, we recognize the existence of older Late Glacial moraines re-calculated to ~15 ka from published cosmogenic nuclide data. Those may coincide with the cold Heinrich 1 event in the North Atlantic region and the pronounced "Tauca" humid phase. We conclude that (i) exposure ages in the tropical Andes may have been overestimated so far due to methodological uncertainties, and (ii) although precipitation plays an important role for glacier mass balances in the tropical Andes, it becomes the dominant forcing for glaciation only in the drier and thus more precipitation-sensitive regions farther west and south.

Surface exposure ages of samples obtained from Reedy Glacier, Antarctica

Fresh deposits above the margins of Reedy Glacier show that maximum ice levels during the last glaciation were several hundred meters above present near the glacier mouth and converged to less than 60 m above the present-day surface at the head of the glacier. Exposure ages of samples from five sites along its margin show that Reedy Glacier and its tributaries thickened asynchronously between 17 and 7 kyr BP At the Quartz Hills, located midway along the glacier, maximum ice levels were reached during the period 17-14 kyr BP. Farther up-glacier the ice surface reached its maximum elevation more recently: 14.7-10.2 kyr BP at the Caloplaca Hills; 9.1-7.7 kyr BP at Mims Spur; and around 7 kyr BP at Hatcher Bluffs. We attribute this time-transgressive behavior to two different processes: At the glacier mouth, growth of grounded ice and subsequent deglaciation in the Ross Sea embayment caused a wave of thickening and then thinning to propagate up-glacier. During the Lateglacial and Holocene, increased snow accumulation on the East Antarctic Ice Sheet caused transient thickening at the head of the glacier. An important result of this work is that moraines deposited along Reedy Glacier during the last ice age cannot be correlated to reconstruct a single glacial maximum longitudinal profile. The profile steepened during deglaciation of the Ross Sea, thinning at the Quartz Hills after 13 kyr BP while thickening upstream. Near its confluence with Mercer Ice Stream, rapid thinning beginning prior to 7-8 kyr BP reduced the level of Reedy Glacier to close to its present level. Thinning over the past 1000 years has lowered the glacier by less than 20 m.