A paleolimnological reconstruction of the last millenium from Lago Lepué ( ≈43ºS), Isla Grande de Chiloé, based on diatom análisis: preliminary results

DIAZ, C. A.; MORENO, P.; MAIDANA, N. I.

MALARGÜE, MENDOZA

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

PAGES

Here we report a high-resolution diatom record from a NW Patagonian lake spanning the last 1000 years. The record from Lago Lepué (~43°S) shows 104 taxa, with many species known only from subantarctic areas, along with a significant number (29) of unidentified and/or presumably new taxa. The available information on the ecological attributes of the most representative taxa allows some preliminary inferences on past fluctuations in pH, trophic status and water level. The diatom stratigraphy over the last 1000 years is represented mainly by the alternation of assemblages dominated by the benthic Encyonopsis difficilis-Brachysira brebissonii and the planctonic Aulacoseira distans-A. alpigena. We identify three stages in the recent paleolimnological history of Lago Lepué: (i) the interval between 1000-900 yr BP is characterized by a progressive increase lake level and a reconstructed decrease in nutrient availabilitity, (ii) a high lake level stands occurred between 900-700 yr maintaining an stable oligotrophic condition, and (iii) a declining lake level trend since 700 yr BP showing an increase in nutrient concentrations, mainly since the most recent ~200 years and probably related to human activities. The inferred low stand in lake levels between 1000-900 yr BP are contemporaneous with peak fire activity, as indicated by a high-resolution charcoal record from the same lake sediment core. On the other hand, the inferred high-lake level stand in Lago Lepué between 900-700 yr BP was coeval with a prominent decline in fire activity. While, diminished lake level indicated by diatoms from 700 yr BP (specially the last ~200 yr BP) is correlated with the increased fire activity by human colonization.Considering that Lago Lepué is a small closed-basin lake, fed almost exclusively by rainfall, we suggest variations in the amount of precipitation brought by the westerly winds over the last millennium, with an apparent maximum between 900-700 yr BP just before the onset of the Little Ice Age.