Identification of Hg-bearing phases and fluxes in the sedimentary record of laguna del Plata, central Argentina

STUPAR, Y.; GARCÍA M. G; SCHÄFER, J; SCHMIDT S; PIOVANO, E.; BLANC, G; HUNEAU, F; LE COUSTUMER PH

REVISTA MEXICANA DE CIENCIAS GEOLóGICAS

CENTRO GEOCIENCIAS UNAM

Lugar: México; Año: 2014 vol. 31 p. 104 - 115

1026-8774

In this work the variations in the concentrations of Hg, its carrying phases and fluxes in the last ~80 years are analyzed for the sedimentary record of the Laguna del Plata. Chemical, mineralogical and sedimentological analysis were performed as well as the radiometric dating of a 120 cm sediment core extracted from Laguna del Plata and sediments collected from the riverbed along the Suquía River basin that discharges into the mentioned lake. Total mercury (HgT) was determined through cold vapour (O2 flux) Atomic Absorption Spectrometry previous incineration and amalgamation, using a direct mercury analyzer. The results suggest that variations in Hg levels respond mainly to hydrological changes registered in the system in the last ~80 years. During the dry period that affected the region before 1968, the main Hg sources were the sediments transported from the upper Suquía River watershed. Constant Hg concentrations measured in the base of the sedimentary core, are similar to those measured in the sediments of the basin, which support the hypothesis of a contribution from terrigenous Hg. The main Hg-bearing phase determined in these sediments is pyrite and in lesser extent, particulate organic matter. The rise in the regional precipitation from 1972 to 2003 coincide with an augmentation in HgT concentrations, probably associated to a higher wash down from the upper part of the basin and a higher atmospheric Hg input from precipitation. In that period, Hg probable reached the lake adsorbed into pyrite and Fe and Mn-(hydr)oxides present in the riverbed sediments. Once in the lake a subsequent remobilization and transport from various biogeochemical processes would have occurred, which explains its association with organic matter in sediments accumulated in that period. The peak of Hg concentrations registered in the sediments accumulated between 1990 and 1995 is attributed to the contribution from volcanic ash that reached the region after the eruption of the Lascar volcano in 1993. Finally, in the most recent sediments (accumulated from 2003) the steady increase in Hg concentrations is attributed to the mentioned geogenic sources, but also to the increase of global atmospheric Hg fluxes.