Cosmogenic nuclide chronology of millennial-scale glacial advances during O-isotope stage 2 in Patagonia

Abstract

The terrestrial glacial record reflects past snowline variability and atmospheric temperature changes. When combined with secure chronologies, these data can be used to test models of ice-age climate. We present new in situ cosmogenic 1 0 Be, 2 6 Al, and 3 He exposure ages, supported by limiting 4 0 Ar/ 3 9 Ar and 1 4 C ages, for seven of the youngest moraines east of Lago Buenos Aires, Argentina, 46.5°S, that were deposited by a large outlet glacier of the Patagonian Ice Cap. Following a major glaciation that deposited extensive moraines prior to 109 ka, paired 1 0 Be- 2 6 Al ages indicate that the next youngest complex of moraines was deposited from 23.0 ′ 1.2 to 15.6 ′ 1.1 ka (1σ). During the last glaciation, ice was at its maximum extent prior to 22 ka and at least five moraines were deposited in less than 10 k.y. These data are in good agreement with three 1 4 C ages of ca. 16 ka from varved sediment banked on top of the youngest of these five moraines and limiting 3 He ages, which range from ca. 33 to 19 ka. The most extensive ice marginal deposits preserved within the last 109 k.y. were formed during marine oxygen isotope stage 2; no moraines dating to stage 4 were found. For stage 2, the distribution of ages at Lago Buenos Aires is similar to cosmogenic nuclide-based glacial chronologies from western North America. In fact, the structure of the last mid-latitude South American ice age-specifically, the overall timing, a maximum ice extent prior to 22 ka, and deglaciation after 16 ka-is indistinguishable from that of the last major glaciation in the Northern Hemisphere, despite a maximum in Southern Hemisphere insolation during this period. The similar mid-latitude glacial history in both hemispheres implies that a global climate forcing mechanism, such as atmospheric cooling, as opposed to oceanic redistribution of heat, synchronizes the ice age climate on orbital time scales.