Climatic periodicity, environmental hazard and atmosphere-ocean circulation in southamerican subtropic during the Holocene

SAYAGO, JOSÉ MANUEL; COLLANTES, M. M.; SAMPIETRO, M. M.; CARIA, M. A.

Oslo

Congreso; International Geological Congress Oslo 2008; 2008

The Great Chaco and Dry Andes (in the transition between South American tropical and temperate belts) is a key region to understand the climatic periodicity in the southern continent. The Polar Front/Westerly northward displacement during winter contrast with the summer influence of South Atlantic anticyclone in addition ITCZ and the periodic ENSO presence. Loess-paleosols sequences in pre-Andean ridges and Chaco plain express alternance of short periods with loess depositation and larger and more humid ones with soil development during Late Pleistocene and Holocene, suggesting cyclicity similar to Daansgard-Oerscher (Zinck & Sayago 1991). It's remarkable that the Holocene humid periods with soil development in the sub-tropic(Sayago et al. 2005.) shows temporal correlation with glacio-chemical sequences detected in the Greenland ice core (O'Brien et al., 1993), also in temporal concordance with glacial expanction in southern Patagonian Andes (Clapperton 1993). A wetter period starting from 3500BP explains the expanction of agrarian cultures over an extensive territory (25°-28° S) from western Chaco to dry Andes (Sayago et al. 2003). The atmosphere-ocean conditions that would have influenced this period would be similar to La Niña phase of ENSO (Moy et al. 2002) with a temporal coincidence with paleosols developed in the western Chaco plain. From 500 AD the growing dryness coincides with a progressive migration of Formative cultures toward more humid lands. Approximately from 1000 to 1300 AD (Garralla 1999) an extreme dryness denoted by development of dunes and xerophytes ecosystem influenced on the extinction of agrarian cultures in northwest Argentina, in similar way that those of Tiawanaku and Maya in Peru and Yucatan (Stine 1994). In this processes conditions like El Niño phase of ENSO would have influenced the weakness of South Atlantic anticyclone and decreasing trade winds over the western side of South America and in the south-tropical Pacyphic. During the XV to XVIII century (coincidently with Little Ice Age) more humid conditions with paleosol development also promote intensive flooding in Chaco plain that destroyed the primitive capital of Túcuman Province. In summary there was a clear correspondence between climatic change, environmental hazard and ocean-atmosphere dynamic that directly affected human settlement in the South American sub-tropic during Holocene. In addition, the paleoclimatic asymmetry with North Atlantic region, would evoke bipolar sea-saw (with shorter temporal resolution), as for instance, wet-cold conditions during Sub-boreal/Sub-Atlantic transition in contrast with agrarian expanction by growing temperature and humidity in the sub-tropic or extreme dryness in western south America while temperature rise in the north during the European Medieval Warm Period, as well soil development in the sub-tropical Chaco plain in contrast with stadial climatic conditions in Europe during Little Ice Age.