Highly variable mercury inputs to mountain lakes in Patagonia (Argentina) during the past millennium

RIBEIRO GUEVARA, S.; MEILI, M.; RIZZO, A.; DAGA, R.; ARRIBÉRE, M.

Guiyang, China

Conferencia; 9th International Conference on Mercury as a Global Pollutant; 2009

High Hg levels have been reported in Nahuel Huapi National Park (NHNP) ecosystems (northern Patagonia, Argentina), particularly Hg concentrations 3- to 5- fold above background in deep layers of lacustrine sediments. Two sedimentary sequences were studied to investigate Hg variations during the past millennium. The sequences were extracted from lakes Tonček, a small, high-altitude system, and Moreno, a much larger and deeper lake. The NHNP is situated in the Southern Volcanic Zone, a well-know source of Hg to the environment. Evidence of geothermal activity, another potential Hg source, was found in lakes belonging the NHNP, although it is not expected particularly in lake Tonček. Hg concentrations were measured together with other elements significant to evaluate environmental changes, as well as organic matter, fossil chironomids, and biogenic silica. Tephra layers were also characterized geochemically to identify the sources. The sedimentary sequences were dated based on both 210Pb and 137Cs. In both sequences, two regions were identified where Hg concentrations were elevated about 10 fold over background (50 ng.g-1), ranging from 400 to 500 ng.g-1 in lake Tonček and reaching 650 ng.g-1 in lake Moreno. These are concentrations that are significantly above the background values determined in other fresh water systems, as were also the Hg fluxes, with the higher values in lake Tonček ranging from 120 to 150 μg.m-2.y-1. The two main peaks were dated to the 18th to 19th centuries for the upper, and around the 13th century for the lower. No correlation of Hg with the environmental tracers studied was observed. Volcanic events are recorded in 9 tephra layers in the lake Tonček sequence, and 8 layers in lake Moreno. A distinct increase of Hg concentrations was observed immediately above some tephra layers. Extended fires is another potential source to be considered since records of charcoal profiles determined previously showed the highest peak in the Holocene coinciding with the Hg peaks in the 13th century, and also tree-ring data and historical records gave evidence of extended forest fires during 18th and 19th centuries, potentially associated with the upper Hg peaks. In both periods, the fire records are concurrent with ENSO events suggesting the concomitant incidence of natural geogenic and climatic, and possibly also anthropogenic phenomena.