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Title:
Evidence of Early Holocene Glacial Advances in Southern South America from Cosmogenic Surface Exposure Dating
Authors:
Douglass, D. C.; Singer, B. S.; Kaplan, M. R.; Ackert, R. P.; Mickelson, D. M.; Caffee, M. W.
Affiliation:
AA(University of Wisconsin - Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States ; ), AB(University of Wisconsin - Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States ; ), AC(University of Wisconsin - Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States ; University of Edinburgh - School of GeoSciences, Drummond Street, Edinburgh, EH8 9XP United Kingdom ; ), AD(Harvard University - Earth and Planetary Sciences, 20 Oxford Street, Cambridge, MA 02138 United States ; ), AE(University of Wisconsin - Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States ; ), AF(Purdue University - PRIME Lab, 525 Northwestern Ave., West Lafayette, IN 47907 United States ; )
Publication:
American Geophysical Union, Fall Meeting 2004, abstract #PP33B-0927
Publication Date:
12/2004
Origin:
AGU
AGU Keywords:
9360 South America, 3344 Paleoclimatology, 1824 Geomorphology (1625), 1827 Glaciology (1863), 1620 Climate dynamics (3309)
Bibliographic Code:
2004AGUFMPP33B0927D

Abstract

10Be and 36Cl cosmogenic nuclide surface exposure dating of erratic boulders reveal two glacier advances in southern South America (46°S) during the Early Holocene. Seven of ten boulders from the outer moraine yield a weighted mean of 8.5±0.7 ka and five of six boulders from the inner moraine yield a weighted mean age of 6.2±0.8 ka (2σ uncertainties). The four outliers are anomalously old (interpreted to contain inherited cosmogenic isotopes from prior exposure) and are identified on the basis of Chi-Squared statistics and bi-modal probability distribution curves. These glacial advances are likely the result of a northward migration of the southern westerlies causing an increase in precipitation and/or a decrease in temperature at this latitude. Reconstructions of equilibrium line altitudes (ELA) at the times of moraine deposition based on Accumulation Area Rations (AARs) are about 300 m lower than modern. This ELA depression is not particularly sensitive to the AAR used, and corresponds to conditions 2.4° C cooler (if no change in precipitation), or 1000 mm/a wetter (if no change in temperature) than the modern climate. The older advance precedes the currently accepted initiation of Holocene glacial activity in southern South America by about 3000 years, and appears to be temporally synchronous with the "8.2 ka event" recorded in Greenland and many other parts of the world. The younger advance is slightly older than, but indistinguishable from, documented neoglacial advances and climate changes in southern South America and four other continents. If there are causal links between these temporally synchronous, globally distributed events, then millennial scale climate changes appear to involve reorganization of global weather systems (such as migration of the southern westerlies), or may be externally forced (e.g. solar variability).
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