Paleolimnological records of mercury in Lake Futalaufquen (Patagonian Andean Range)

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

Conferencia; 11th International Conference on Mercury as a Global Pollutant; 2013

High Hg levels were detected in previous research in the pristine lacustrine ecosystems of Los Alerces National Park (LANP; 42º35´-43º11´S; 71º35 ´-72º09´W), a protected zone situated in the Patagonian Andean Range. A short sedimentary sequence was extracted from Lake Futalaufquen (44.6 km2 surface; 168 m max. depth), an oligotrophic system belonging to LANP, aiming to search for past atmospheric Hg inputs to aquatic ecosystems. The sequence, 76 cm long, was extracted in March 2009 at 90 m depth, and sub- sampled every 1 cm to assess the temporal variation of sediment composition from concentration profiles. The sedimentary sequence was dated by 14C, 137Cs and 210Pb techniques. Total Hg (THg) concentrations in bulk sediments were measured with a Direct Mercury Analyzer (Milestone DMA-80). These measurements were performed in duplicate samples of 20 to 40 mg of freeze-dried, homogenized material, from each layer of the sedimentary sequence. In addition, tephra layers were identified and analyzed by Instrumental Neutron Activation Analysis for geochemically characterization, to investigate any association of volcanic eruptions with Hg records. The sedimentary sequence analyzed recorded 1600 y of deposition. Five tephras were visually identified in the sequence, all of them in the upper 30 cm, corresponding to 700 y before present. The upper tephra, included in the top layer at 0.5 cm depth, corresponded to the eruption of volcano Chaiten in 2009. The other four exhibited a similar geochemical characterization but clearly different than that of volcano Chaiten. The THg concentration profile showed low values, with no strong variation, in the deepest 36 cm of the sequence, corresponding approximately to 750 y of sedimentation. The concentration average in this period was 46 (12) ng g-1 (standard deviation in parenthesis). In the upper section of the sequence, THg exhibited increases up to 5 fold the average in deep layers. Higher THg concentrations were found in volcanic ashes (116 to 175 ng g-1) and in bulk sediment from the immediately upper layer (143 to 250 ng g-1) in first, third and fourth tephras of the sequence, suggesting that volcanic eruptions were a significant source of Hg to the aquatic systems of LANP.