Early last glacial maximum in the southern Central Andes reveals northward shift of the westerlies at ~39 ka

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 CO<sub>2</sub> 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 CO<sub>2</sub>, and it highlights our limited understanding of the forcings of atmospheric circulation

GLACIATIONS | Late Pleistocene in South America

“…High-stand levels, which occurred before 43.5 cal ka BP and particularly between 40 and 21.5 cal ka BP, were interpreted as resulting from precipitation increase in response to enhanced westerly wind activity. Similar results were recently presented by Zech et al (2011), who provided evidence that full-glacial conditions occurred ca. Similar results were recently presented by Zech et al (2011), who provided evidence that full-glacial conditions occurred ca.…”

The Southern Westerlies in Central Chile during the two last glacial cycles as documented by coastal aeolian sand deposits and intercalating palaeosols

“…In central Chile, the water levels of Laguna Tagua-Tagua (34°30′) were highest during the LGM and prior to 42 ka (Valero-Garcés et al, 2005). Glacial records in the southern central Andes show major high stands prior to the LGM (Denton et al, 1999;Lowell et al, 1995;Zech et al, 2008Zech et al, , 2011. During the LGM, marine sedimentary records at 33°S and 41°S suggest SSTs of about 12°C and 9°C lower than today (Lamy et al, 2002.…”

“…Lamy et al, 2001Lamy et al, , 2002Stuut and Lamy, 2004;Stuut et al, 2006;Toggweiler et al, 2006;Kaiser et al, 2008) studies point to a northward shift of the SWW during the LGM and a southward retreat during the Holocene which support the palynological assumptions. Glacier advances, in Central Chile, however, predate the global LGM (Zech et al, 2006(Zech et al, , 2008(Zech et al, , 2011, indicating an earlier northward shift of the SWW and a dry LGM. Results from modelling studies are controversial, including southward shifts during the LGM (Valdes, 2000;Wyrwoll et al, 2000;Wainer et al, 2005), northward shifts (Caviedes, 1990) and no shifts at all with only intensity changes (Kull et al, 2002;Rojas et al, 2009).…”

Impact of geomagnetic excursions on atmospheric chemistry and dynamics

“…As glaciers are not only sensitive to temperature changes, but also to precipitation (and particularly so under arid conditions), this has been interpreted as indicating a northward shift of the southern hemispheric westerly winds and increased precipitation at ∼ 40 kyr. A possible link to the Laschamp event has been suggested (Zech et al, 2011), which has its reasoning in the fact that GCRs have a significant influence on the upper tropospheric and stratospheric chemistry through ionization and the production of NO x and HO x , which can lead to catalytic destruction of stratospheric ozone and production of tropospheric ozone (Calisto et al, 2011). A massive ozone hole during the Laschamp event has in fact recently been suggested as potential factor favouring the extinction of the Neanderthal (Valet and Valladas, 2010).…”

“…Possible effects for the atmosphere and the ecosystems have not attracted much scientific attention so far, although it is foreseeable that geomagnetic excursions will also occur in the future. The initial motivation for this study stems from the finding that glaciers in the relatively arid southern central Andes at ∼ 30-40 • S reached their maximum extents at ∼ 40 kyr, long before the last global glacial maximum at ∼ 20 kyr (Lowell et al, 1995;Denton et al, 1999;Espizua, 2004;Zech et al, 2007Zech et al, , 2008Zech et al, , 2011. As glaciers are not only sensitive to temperature changes, but also to precipitation (and particularly so under arid conditions), this has been interpreted as indicating a northward shift of the southern hemispheric westerly winds and increased precipitation at ∼ 40 kyr.…”