HR: 1340h
AN: U33A-0014    [Abstracts]
TI: Cosmogenic Surface-Exposure Dating of Boulders on Last-Glacial and Late-Glacial Moraines, Lago Buenos Aires, Argentina: Interpretive Strategies and Paleoclimate Implications
AU: * Douglass, D C
EM: douglass@geology.wisc.edu
AF: Department of Geology and Geophysics University of Wisconsin-Maidson, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Singer, B S
EM: bsinger@geology.wisc.edu
AF: Department of Geology and Geophysics University of Wisconsin-Maidson, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Kaplan, M R
EM: mkaplan@geo.ed.ac.uk
AF: Department of Geology and Geophysics University of Wisconsin-Maidson, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Kaplan, M R
EM: mkaplan@geo.ed.ac.uk
AF: School of GeoSciences University of Edinburgh, Drummond St, Edinburgh, EH9 3JW United Kingdom
AU: Mickelson, D M
EM: davem@geology.wisc.edu
AF: Department of Geology and Geophysics University of Wisconsin-Maidson, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Caffee, M
EM: mcaffee@physics.purdue.edu
AF: PRIME Lab Purdue University, 525 Northwestern Ave, West Lafayette, IN 47907 United States
AB: The most substantial and least quantifiable source of uncertainty in cosmogenic surface-exposure datasets is the variable exposure histories of boulders from the same landform. The development of precise and accurate chronologies requires distinguishing boulders that best reflect the age of the landform from those which are outliers. We use the Mean Square of Weighted Deviates statistic and cumulative frequency plots to identify groups of samples that have statistically similar ages based on the number of samples and the uncertainty associated with the analyses. This group of samples most likely represents the best estimate of the landform age. We use these tools to interpret 49 surface-exposure ages from six last-glacial and late-glacial moraines at Lago Buenos Aires, Argentina (LBA; 71.0W, 46.5S). Seven of the orty-nine samples are identified as anomalously young, and are interpreted to have been exhumed after moraine deposition. The remaining samples indicate that glacial advances or still-stands of the ice margin occurred at 22.7±0.9, 21.4±1.9, 19.8±1.1, 17.0±0.8, 15.7±0.6, and 14.4±0.9 ka (±2 σ). This maximum ice extent is roughly synchronous with maximum global ice volume and several of the re-advances are contemporaneous with Heinrich events and other Northern Hemisphere cold periods. The late-glacial readvance at ca. 14.4 ka is contemporaneous with the Antarctic Cold Reversal (ACR), and precedes the Younger Dryas Chronozone (YD). No evidence for a Younger Dryas glacial advance has been found in the Lago Buenos Aires basin. This precise glacial chronology indicates there were significant and important differences in climate across southern South America. The timing of maximum ice extent and onset of deglaciation at LBA occur ~4000 years later than in the Chilean Lake District (41S). Fossil pollen from the CLD area indicates cooler conditions between ca. 14.2 and 11.2, and increased silt in a nearby lake core provides indirect evidence for glacial advances at this time. The onset of this late-glacial cool period precedes the YD, but post-dates the ACR. The LBA glacial record is in better accord with the Strait of Magellan (SM; 52S) than with the CLD. There ice reached its maximum around 25 ka, and a significant late-glacial re-advance occurred between ca. 15 and 11.5 ka. Both LBA and the SM have climate records similar to Antarctica, whereas the climate records from the CLD are combinations of Antarctic and Northern Hemisphere signals.
DE: 0720 Glaciers
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1605 Abrupt/rapid climate change (4901, 8408)
DE: 9360 South America
SC: Union [U]
MN: Fall Meeting 2005