Explosive Vlcanism during the Holocene in the Upper Limay River Basin. The effects of ash falls on human societies. Northern Patagonia, Argentina.
by
Gustavo Villarosa
CRUB, Universidad Nacional del Comahue, Quintral 1250. 8400 Bariloche. Argentina
Coauthors: Valeria Outes (CONICET, Grupo de estudios Ambientales GEA. CRUB, Universidad Nacional del Comahue, Argentina), Adam Hajduk (CONICET, Museo de la Patagonia F. P. Moreno, Administración de Parques Nacionales, Bariloche, ARGENTINA), Mabel Fernández (Universidad Nacional de La Pampa and Universidad Nacional de Luján, Argentina) and Eduardo Crivelli Montero (CONICET, CiIAFIC, Argentina)

Evidence of explosive volcanism during the Holocene has been found in the Archaeological record (ca. 10,000 BP - ca. 1.350 BP) of various sites along the upper Limay River basin, Northern Patagonia, Argentina. Tephras were sampled and fingerprinted. In various cases they were bracketed with radiocarbon dates and correlated with well-studied tephra layers from sedimentary cores from nearby lakes and even with representative profiles from well-preserved outcrops.

The aim of this study is to understand possible human response to the postglacial ash fall events that generated sudden stress. Some of them might have been triggered by seismic activity as it happened during the 1960 earthquake in Valdivia, Chile, the greatest seismic moment and energy release ever measured (statistical recurrence interval is estimated in about hundred of years). Evidence is found in several of the studied sites.

Landscape and general conditions after these events must have changed rapidly. The dwellers of the region were nomadic hunters and gatherers and therefore the ash fall events may have forced them to abandon their site at least for a short-term until the adverse conditions disappeared. The dwindling pastures and the corresponding dental attrition caused by the pumiceous particles must have produced a reduction of the local population of herbivores, mainly guanaco, which was their primary subsistence prey.

Some of these events may have taken to an end the well-defined and well-structured human occupation of some rock shelters. Occupations immediately after the events of great magnitude were limited to relatively smaller areas with short, occasional visits, which have left few archaeological evidences.

On the short-term, the sudden changes in the environment must have produced stress and the significant impact on human societies was the abandonment of their affected sites. Once natural conditions previous to the volcanic event were reestablished the hunters-gatherers re-colonized the areas.

Date received: November 23, 2004