A 17 kyr Record of the Southern Westerlies From Lago Cardiel, Patagonia, Argentina

MARKGRAF, V.; SCHWALB, A; CUSMINSKY, G; GILLI, A; BERNASCONI, S; ANSELMETTI, F; ARISTEGUI, D

San Francisco

Otro; American Geophisical Union Fall Meeting 2008; 2008

American Geophysical Union

The location of the Southern Westerlies (SW) stormtracks is related to the pressure gradient between the position and strength of the South Pacific High Pressure and the Circum Antarctic Low Pressure that also determines the location of the Subpolar oceanic front. Inferences on late Quaternary behavior of the SW's stormtracks in southern South America were made on the basis of precipitation-sensitive sensitive proxies such as records of past changes in vegetation, fire frequency, lake levels, river-runoff, glacier extent and marine conditions. Thus, higher levels of precipitation have been taken to imply higher westerly cyclonic activity. However, east of the Andes, higher levels of precipitation can be also related to enhanced incursions of easterly moisture during times when SW's are weak. Throughout southern South America conditions were markedly colder and drier during fullglacial times, indicating that the SW's were weaker and that westerly as well as easterly moisture was reduced given the much lower SST's. Multi-proxy evidence from Lago Cardiel (49°S, 76 m water depth, 379 km2 area, ~ 10km east of the Andean cordillera) sediments show that the lake at the end of the Last Glacial Maximum (LGM) was small, shallow and saline, according to ostracode assemblages represented by Limnocythere rionegroensis. By 13 cal kyr BP the lake had nearly dried out indicating that despite overall rapidly increasing temperatures after 17 cal kyr BP precipitation did not increase south of latitude 40°S. After 13 cal kyr BP lake level rose rapidly, culminating in a +55 m shoreline at 10,7 cal kyr BP. Presence of planktic diatoms and the highest ostracode diversity represented by Limnocythere patagonica, Kapcypridopsis megapodus, Eucypris fontana and E. virgata indicate a large and deep lake which suggests that the SW were weak allowing easterly moisture to advance towards the Andes. By 7,5 cal kyr BP lake level dropped to a level close to present. Benthic diatoms became abundant, and the modern ostracode assemblages, consisting of mainly E. fontana and L. patagonica in minor abundance, was established indicating reduction in precipitation to present-day levels. The occurrence of drift deposits after ~ 6,8 cal kyr BP reveals the presence of a wind- driven lake current, which indicates strong and persistent SW at a latitude of 49°S during the middle and late Holocene.