Abstract
Abstract. The latitudinal position of the southern westerlies has been suggested to be a key parameter for the climate on Earth. According to the general notion, the southern westerlies were shifted equatorward during the global Last Glacial Maximum (LGM: ~24–18 ka), resulting in reduced deep ocean ventilation, accumulation of old dissolved carbon, and low atmospheric CO2 concentrations. In order to test this notion, we applied surface exposure dating on moraines in the southern Central Andes, where glacial mass balances are particularly sensitive to changes in precipitation, i.e. to the latitudinal position of the westerlies. Our results provide robust evidence that the maximum glaciation occurred already at ~39 ka, significantly predating the global LGM. This questions the role of the westerlies for atmospheric CO2, and it highlights our limited understanding of the forcings of atmospheric circulation.
Highlights
The southern westerlies are an important driver for upwelling around Antarctica and deep ocean ventilation (Toggweiler et al, 2006)
We can infer that the maximum ice extent in the Rucachoroi Valley occurred at ∼39 ka, clearly pre-dating the global Last Glacial Maximum (LGM)
The exposure ages of 17.9 ± 1.8 ka and 14.8 ± 1.5 ka from the samples RU31 and RU32 (Fig. 2) show that the glacier continued to occupy the upper part of the Rucachoroi Valley and started to retreat beyond the Rucachoroi Lake only after the global LGM
Summary
This hypothesis builds on the general notion that the westerlies were shifted northward during the global LGM, which has been inferred from terrestrial paleoecological and marine records off Chile (Heusser, 1989; Stuut and Lamy, 2004). Whereas glaciers in Patagonia always received plenty of precipitation and reached their maximum extents at times of low temperatures, i.e. in-phase with the global LGM (Douglass et al, 2006; Kaplan et al, 2008), glacier mass-balances in more arid environments, such as the southern Central Andes, become more precipitation-sensitive, and the glacial chronologies there reflect changes in the position of the westerlies (Kull et al, 2002, 2008, Wager et al, unpublished manuscript)
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