HR: 1330h
AN: PP32B-0285    [PDF]
TI: Cosmogenic Dating of Moraines of the Local Last Glacial Maximum in the Tropical Andes
AU: * Smith, J A
EM: jasmit10@syr.edu
AF: Syracuse University, Dept. of Earth Sciences, 204 Heroy Geology Lab, Syracuse, NY 13244-1070 United States
AU: Seltzer, G O
EM: goseltze@syr.edu
AF: Syracuse University, Dept. of Earth Sciences, 204 Heroy Geology Lab, Syracuse, NY 13244-1070 United States
AU: Rodbell, D T
EM: rodbelld@union.edu
AF: Union College, Geology Department, Olin Building, Schenectady, NY 12308-2311 United States
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550-9234 United States
AU: Farber, D L
EM: farber2@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550-9234 United States
AB: We have used cosmogenic dating ($^{10}$Be) to identify moraines of the local last glacial maximum along two east-west transects in the tropical Andes: the Junin region of central Peru ($\sim$$11\deg$S $76\deg$W) and the Cordillera Real of western Bolivia ($\sim$$16.3\deg$S $68.2\deg$W). The $^{10}$Be ages from boulders on moraines in our study areas suggest that the local last glacial maximum occurred ca. 30,000 $^{10}$Be yr BP (before the inferred peak of Northern Hemisphere glaciation at ca. 21,000 calendar yr BP) and that deglaciation was well underway by 20,000 $^{10}$Be yr BP. Recessional moraines were deposited between about 20,000 and 15,000 $^{10}$Be yr BP. Published $^{14}$C dates from the Cordillera Real indicate that glaciers were within their present limits by about 11,000 calendar yr BP. Asymmetry in the east-west glacial extent and amount of snowline depression was relatively minor in the Junin region, but was more pronounced in the Cordillera Real. In the Junin region, terminal moraines of the local last glacial maximum lie at $\sim$4150-4200 m on the east side of the cordillera and at $\sim$4250-4400 m on the west side. In the Cordillera Real, lateral moraines of the local last glacial maximum lie at $\sim$4600 m on the southwest side of the cordillera (Milluni Valley), while a late-glacial (ca. 12,000 $^{10}$Be yr BP) terminal moraine lies at $\sim$3800 m on the northeast side of the cordillera (Zongo Valley). Snowline depression during the local last glacial maximum in the Andes was $\sim$200-600 m on both sides of the eastern cordillera in the Junin region and on the southwest (Altiplano) side of the Cordillera Real, but closer to $\sim$900-1000 m on the northeast side of the Cordillera Real. The asymmetry likely arose from differences in precipitation (which comes mainly from the east) and from variations in shading, amount of supraglacial material, and topography between the deeply incised eastern valleys and the relatively broad, shallow valleys descending to high-altitude plateau surfaces on the west sides.
DE: 1035 Geochronology
DE: 1620 Climate dynamics (3309)
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 3344 Paleoclimatology
DE: 9360 South America
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting