Multiple sulfur isotope constraints on sulfur cycling in monomictic Lake Kinneret

Sulfur cycling in the water column of Lake Kinneret with emphasis on the triple sulfur isotope (δ32S, δ33S, δ34S) fractionation between sulfate and hydrogen sulfide was investigated. In addition to providing quantitative constraints, the triple sulfur isotope composition of sulfur species in a stratified water body offers insights into sulfur cycling in the ancient oceans. Another goal of this work was evaluation of the rates and mechanisms of hydrogen sulfide oxidation during the mixing of the lake water column. To achieve these goals, five sampling campaigns were carried out, during which temperature, pH, and concentrations of oxygen and sulfur species were measured, along with the triple isotope composition of hydrogen sulfide and sulfate. It was found that the value of isotopic fractionation between sulfate and hydrogen sulfide increases from the early stratification in spring to the advanced stages of stratification in autumn, reaching ϵ34S = -34 ± 4 ‰ in November. Combination of δ34S and δ33S values suggests that the extent of recycling of hydrogen sulfide through disproportionation of zero-valent sulfur increases in summer due to its phototrophic oxidation of hydrogen sulfide and further increases in autumn due to the mixing of sulfidic and oxic waters. After the full mixing of the lake water column, hydrogen sulfide is fully oxidized in less than sixty hours. This observation agrees with kinetic calculations and suggests that dissolved Fe(II) and Mn(II) are present at concentrations high enough to catalyze oxidation of hydrogen sulfide.