Identifying key tephra layers for paleoclimate reconstructions in Northern Patagonia

VILLAROSA, GUSTAVO; OUTES, VALERIA

Malargüe, Mendoza, Argentina

Simposio; Reconstrucciones Regionales de las Variaciones Climáticas en América del Sur durante el Holoceno tardío: Una nueva Iniciativa de PAGES; 2006

PAGES (Past Global Changes), International Geosphere-Biosphere Programme-IANIGLA-University of Bern

Several studies based on lacustrine records provide detailed Postglacial paleoclimate reconstructions for Northern Patagonia. Recent evidence of a cool episode occurred during the Late-Glacial, between 11,400 and 10,200 14C yr BP was provided by Ariztegui et al., 1997, Hajdas et al., 2003, Whitlock et al., 2006. This event has been associated to the Younger Dryas and Gerzensee/Killarney oscillation identified in the northern hemisphere. Tephras derived from explosive eruptions of Andean volcanoes can provide precise chronologies and reliable stratigraphic markers allowing correlation and dating of this and other climatic events in records from both sides of the Andes. Five well-defined macroscopic tephra layers identified in Lake Mascardi deposited during the Huelmo/Mascardi Cold Reversal (Hajdas et al., 2003) are considered good regional markers to trace this episode. Their glass separates have been characterized using major, trace and rare earth elements geochemistry and pyroclasts morphology. They range in composition from basaltic andesite to dacite. Dating of these tephras is based on a refined AMS chronological model (Hajdas et al., 2003). MT10-7 layer (9,900 14C yr BP) presents a characteristic bimodal glass composition: a dark grey andesitic fraction and an olive grey dacitic population. Pyroclastic layers with similar petrographic characteristics have been identified in several lakes of the Chilean Lake District, their possible correlation is being studied. A conspicuous white tephra layer was deposited at 8600 14C yr BP, after the culmination of this cold period. Another thick white dacitic lapilli layer marks the beginning of the Late Holocene in Lake Mascardi record at 4,400 14C yr BP. These markers can be traced along the northern Nahuel Huapi area. 14C yr BP was provided by Ariztegui et al., 1997, Hajdas et al., 2003, Whitlock et al., 2006. This event has been associated to the Younger Dryas and Gerzensee/Killarney oscillation identified in the northern hemisphere. Tephras derived from explosive eruptions of Andean volcanoes can provide precise chronologies and reliable stratigraphic markers allowing correlation and dating of this and other climatic events in records from both sides of the Andes. Five well-defined macroscopic tephra layers identified in Lake Mascardi deposited during the Huelmo/Mascardi Cold Reversal (Hajdas et al., 2003) are considered good regional markers to trace this episode. Their glass separates have been characterized using major, trace and rare earth elements geochemistry and pyroclasts morphology. They range in composition from basaltic andesite to dacite. Dating of these tephras is based on a refined AMS chronological model (Hajdas et al., 2003). MT10-7 layer (9,900 14C yr BP) presents a characteristic bimodal glass composition: a dark grey andesitic fraction and an olive grey dacitic population. Pyroclastic layers with similar petrographic characteristics have been identified in several lakes of the Chilean Lake District, their possible correlation is being studied. A conspicuous white tephra layer was deposited at 8600 14C yr BP, after the culmination of this cold period. Another thick white dacitic lapilli layer marks the beginning of the Late Holocene in Lake Mascardi record at 4,400 14C yr BP. These markers can be traced along the northern Nahuel Huapi area.