Glacial chronology, soil development, and paleoclimate reconstructions for mid-latitude South America, 1 Ma to recent.

摘要

Well-preserved Pleistocene glacial landforms near Lago Buenos Aires, Argentina (71.0°W, 46.5°S), and the soils that developed on them, provide a unique geologic archive of climate fluctuations in southern South America over the last one million years. The timing of glacier advances, constrained with 10Be, 26Al, and 36Cl cosmogenic surface-exposure ages of moraine boulders, is used to reconstruct paleocliniate at several time scales. At Fachinal, Chile, sixteen 10Be and 36Cl exposure ages indicate glaciers advanced at ca. 8.5 and 6.2 ka. The outer moraine may be coeval with the "8.2 ka event" seen in many climate records from the Northern Hemisphere. Forty-nine 10Be and 26Al exposure-ages from the Fenix and Menucos moraines yield ages of ca. 22.7, 21.4, 19.8, 17.0, 15.7, and 14.4 ka. This is generally synchronous with the global ice ages, despite a maximum in local solar insolation at this time, but significant regional variability is likely related to the strength and position of the Southern Westerlies. The Antarctic Cold Reversal age of the Menucos moraine (14.4 +/- 0.9 ka), and the lack of a conclusive Younger Dryas equivalent in southern South America, indicates that the Earth's climate system does not always behave synchronously. Nine 10Be- 36Cl data pairs from Moreno III and Deseado samples indicate that the median boulder erosion rate is 0.2 mm/kyr.; The accumulation of organic carbon, secondary carbonate, and clay-sized particles are the dominant soil forming processes on these moraines. Accumulation rates of carbonate and clay decrease with age, due to either decreased influx during the earliest part of the record, or attainment of equilibrium between influx and loss. Elemental mass balance in nine soils indicates that these weathering products are derived from external sources, and that little chemical weathering occurs in this environment. Calculated soil strain based on the immobility of Zr and Ti is positive for the A and Bk-horizons, and average strain for these soil profiles increases with age. Dramatic accumulation of Ca, without commensurate loss of Si, gain in Al, and Fe mobility, demonstrate that the Ca cannot be derived from within the soil profile.