Mercury dynamics in lake sediments

Abstract

Triplicate porewater depth-profiles of pH and concentrations of total Hg (Hg T ), methylmercury (MeHg), Fe, Mn, sulfate, total sulfide, total zero-valent sulfur, organic C and major ions were determined at two sampling dates in a perennially oxygenated basin and a seasonally anoxic basin from Lake Tantaré, a Canadian Shield lake. The vertical distribution of Hg T , MeHg, acid volatile sulfide, total S, Fe, Mn, Al and organic C were also determined in dated sediment cores from the same lake basins and from the deepest site of two other lakes, one also located in the Canadian Shield and the other in the Northeastern part of the Appalachian Mountains. Application of a one-dimensional transport-reaction equation to the dissolved Hg T and MeHg profiles constrains the depth intervals (zones) where these species are produced or consumed in the sedimentary column and yields estimates of net reaction rates of Hg T or MeHg in each of the zones as well as their fluxes at the sediment-water interface.Dissolved Hg T and MeHg diffused from the overlying water into the sediments, except for MeHg at one of the sampling dates in the perennially oxygenated basin. About 97% and 50% of the MeHg flux to the sediments is presently deposited with settling particles in the perennially oxygenated and seasonally anoxic basins, respectively. Removal of porewater Hg T and MeHg occurred at all dates and sampling sites. Comparison of the consumption zones of porewater Hg T and MeHg with the profiles of ancillary parameters, coupled with thermodynamic calculations, suggest that pure Hg mineral phases do not form in the sediments, that Hg T and MeHg adsorption onto authigenic Fe oxyhydroxides occurs in minor proportions, and that the association of Hg T and MeHg to Fe sulfide phases or sulfidized organic matter is possible. Assuming that the net consumption of MeHg in the porewaters was essentially due to demethylation, an apparent first-order rate constant for MeHg demethylation of 0.04-0.8 d À1 was estimated. Production of porewater MeHg occurred only in the perennially oxygenated basin, at sediment depths where SO 4 was consumed. Assuming that the net production of porewater MeHg was essentially due to methylation, an apparent first-order rate constant for Hg methylation ranging between 0.006 d À1 and 0.1 d À1 was calculated. These field-derived Hg methylation and MeHg demethylation rate constant values are within the range of those derived from Hg-spiked experiments. We also show that the post-depositional redistribution of total Hg during the early stages of sediment diagenesis is minor and that the solid-phase Hg T record can be used to reconstruct the evolution of the anthropogenic Hg T deposition.

Iron isotope fractionation in sediments of an oligotrophic freshwater lake

“…More likely, the high dissolved Fe(III) concentrations are due to oxidation artifacts, the formation of strong organic complexes, the presence of Fe(III) colloids, or some combination of these. While precautions were taken to minimize contamination by oxygen (see The build-up of pore water Fe(II) below the SWI is consistent with Fe redox cycling within the upper centimeters of the sediments, as proposed in previous studies conducted in Basin A (Chappaz et al, 2008;Couture et al, 2010b;Feyte et al, 2010;Feyte et al, 2012 …”

Assessment of the spatial distributions of total‐ and methyl‐mercury and their relationship to sediment geochemistry from a whole‐lake perspective

“…The spatial distribution of Hg in whole‐lake basins has previously been studied both from the perspective of understanding total‐Hg mass balance at a catchment or regional scale [ Fitzgerald et al , 2005; Swain et al , 1992; Yang et al , 2002] and to investigate the relation between total‐ and methyl‐Hg and other sediment variables, for example, organic matter, grain size, lake‐basin morphometry, pH and redox conditions [ Croston et al , 1996; Ethier et al , 2010; Feyte et al , 2011; Kainz and Lucotte , 2006; Rhoton and Bennett , 2009]. However, for most studies a relatively low number of sampling sites were used (n = 1–17), and most often these sites were distributed along a single transect from shallow to deep waters.…”

Mercury remobilization in Saguenay Fjord (Quebec, Canada) sediments: Insights following a mass-flow event and its capping efficiency

“…Nevertheless, at SAG-30 and SAG-09, the bell-shaped patterns of the [THg s ] profiles (Figs. 3a and 4c) most likely reflect the history of mercury discharge to the fjord since Hg is known to undergo limited diagenetic remobilization (Lockhart et al, 1995;Feyte et al, 2012). This interpretation is supported by 210 Pb dating of cores collected in 1976 (Smith and Loring, 1981).…”