Diagenetic production, accumulation and sediment-water exchanges of methylmercury in contrasted sediment facies of Lake Titicaca (Bolivia)

Stéphane Guédron (1, 2) , Stéphane Audry (3) , Dario C. Acha (2) , Sylvain Bouchet (4) , David Point (3) , T. Condom (5) , Carlos Heredia (1, 2) , Sylvain Campillo (1) , Pascale Anabelle Baya (3) , A. Groleau (6) , Erwan Amice (7) , David Amouroux (4, 2)

1 ISTerre - Institut des Sciences de la Terre
2 UMSA - Universidad Mayor de San Andrés
3 GET - Géosciences Environnement Toulouse
4 IPREM - Institut des sciences analytiques et de physico-chimie pour l'environnement et les materiaux
5 IGE - Institut des Géosciences de l’Environnement
6 IPGP (UMR_7154) - Institut de Physique du Globe de Paris
7 LEMAR - Laboratoire des Sciences de l'Environnement Marin (LEMAR)

Monomethylmercury (MMHg) concentrations in aquatic biota from Lake Titicaca are elevated although the mercury (Hg) contamination level of the lake is low. The contribution of sediments to the lake MMHg pool remained however unclear. In this work, seven cores representative of the contrasted sediments and aquatic ecotopes of Lake Titicaca were sliced and analyzed for Hg and redox-sensitive elements (Mn, Fe, N and S) speciation in pore-water (PW) and sediment to document early diagenetic processes responsible for MMHg production and accumulation in PW during organic matter (OM) oxidation. The highest MMHg concentrations (up to 12.2 ng L−1 and 90% of THg) were found in subsurface PWs of the carbonate-rich sediments which cover 75% of the small basin and 20% of the large one. In other sediment facies, the larger content of OM restricted MMHg production and accumulation in PW by sequestering Hg in the solid phase and potentially also by decreasing its bioavailability in the PW. Diagenetically reduced S and Fe played a dual role either favoring or restricting the availability of Hg for biomethylation. The calculation of theoretical diffusive fluxes suggests that Lake Titicaca bottom sediments are a net source of MMHg, accounting for more than one third of the daily MMHg accumulated in the water column of the Lago Menor. We suggest that in the context of rising anthropogenic pressure, the enhancement of eutrophication in high altitude Altiplano lakes may increase these MMHg effluxes into the water column and favor its accumulation in water and biota.

Mots clés: Lake sediment; Methylmercury; Early diagenesis; High altitude ecosystem; Diffusive fluxes; Acl

Domaines: Polymères; Matériaux; Chimie théorique et/ou physique; Chimie analytique; Biodiversité et Ecologie

Stéphane Guédron, Stéphane Audry, Dario C. Acha, Sylvain Bouchet, David Point, et al.. Diagenetic production, accumulation and sediment-water exchanges of methylmercury in contrasted sediment facies of Lake Titicaca (Bolivia). Science of the Total Environment, 2020, 723, pp.138088. ⟨10.1016/j.scitotenv.2020.138088⟩. ⟨hal-02543070⟩