HR: 0800h
AN: U31A-0014    [Abstracts]
TI: A Million-Year Record of Glaciation in the Tropical Andes
AU: * Smith, J A
EM: jasmit10@syr.edu
AF: Syracuse University, Dept. of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse, NY 13244-1070 United States
AU: Seltzer, G O
EM: goseltze@syr.edu
AF: Syracuse University, Dept. of Earth Sciences, 204 Heroy Geology Laboratory, 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: Farber, D L
EM: farber2@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550-9234 United States
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550-9234 United States
AB: We present a longterm record of glaciation in the tropical Andes based on cosmogenic dating ($^{10}$Be) of boulders on moraines. Well-preserved moraines in deglaciated valleys bordering the Junin Plain in central Peru ($\sim$11$\deg$S, 76$\deg$W, 4000 m) were deposited during several glacial cycles extending back more than one million years before present (1 Myr BP). The presence of boulders with zero-erosion $^{10}$Be exposure ages $>$1 Myr constrains boulder erosion rates to relatively low values. For boulders at high altitudes, however, even low boulder erosion rates (0.3 to 0.5 m/Myr) make calculated old exposure ages markedly older [e.g., $\sim$20% older for a zero-erosion age of 400,000 $^{10}$Be years (400 $^{10}$Be kyr)]. Exposure ages recalculated with boulder erosion rates of 0.3 m/Myr straddle interglacial marine isotope stage (MIS) 11 ($\sim$430-390 kyr BP), fall within glacial MIS 12 ($\sim$480-430 kyr BP), but skip over glacial MIS 16 ($\sim$670-630 kyr BP), perhaps the largest ice volume of the past 2 Myr. Increasing the erosion rate used in the calculations to 0.5 m/Myr moves ages into both MIS 11 and MIS 16. If we assume that the older Andean glaciations were indeed synchronous with global ice volume, our data suggest that boulder preservation cannot be treated as a simple linear process. Conversely, the data may be suggesting correctly that glaciation of the tropical Andes was not synchronous with the global glaciations as inferred from the marine isotope record. Our chronology for the last glacial maximum (LGM) in the region supports the idea of asynchrony between the global ice volume record and the terrestrial record of glaciation in the tropical Andes. The LGM in the Junin region of Peru and in the Cordillera Real of Bolivia (16$\deg$S 68$\deg$W) occurred $\sim$34 to 22 $^{10}$Be kyr BP and was less extensive than older glaciations. Asynchrony between the LGM in the Northern Hemisphere ($\sim$21 kyr BP) and the tropical Andes suggests that previous glaciations in the tropical Andes may have been similarly out of step.
DE: 9360 South America
DE: 3344 Paleoclimatology
DE: 1694 Instruments and techniques
DE: 1600 GLOBAL CHANGE (New category)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Union [U]
MN: 2004 AGU Fall Meeting