Tephrochronology of recent events in the Andean Range (northern Patagonia): spatial distribution and provenance of lacustrine ash layers in the Nahuel Huapi National Park

Tephrochronology is a powerful tool for dating sedimentary sequences, especially in Patagonia, where a large number of active volcanoes have produced frequent historical eruptions. Short lacustrine sedimentary sequences were extracted from the lakes Moreno Oeste and Toncek (Nahuel Huapi National Park). Seventeen volcanic ash layers were identified in both cores, Pb-210 and Cs-137 were used for dating techniques, and historical volcanic records were employed for correlation. White pumice and glass shards from the tephras were characterised by measuring major and trace element contents by instrumental neutron activation analysis. Two volcanic sources - the Cordon Caulle and the Calbuco volcanoes - were recognised as the prevailing systems that impacted the area in the past 800-1000 a. The Calbuco volcano was mainly responsible for the nine tephras identified in the Lake Toncek sequence. Four of these nine tephras also contained material from the Cordon Caulle complex, and could be interpreted as composite tephras or possible reworked ones. The Calbuco volcano showed predominant influences in the Lake Moreno Oeste sequence. Six of the eight tephras identified in such a sequence were from the Calbuco volcano, and three of these six contained material from the Cordon Caulle complex. The other two were from the Cordon Caulle complex, with contributions from the Calbuco volcano in one of them. These findings show that the high frequency of volcanic events in the study region demands an accurate characterisation of the products generated by each source, as well as an evaluation of their spatial distribution, to obtain a consistent framework for dating recent environmental changes. Copyright (C) 2010 John Wiley & Sons, Ltd.