Triple sulfur isotope constraints on the sulfur cycling in Lake Sevan, Armenia

Lake Sevan is a meso-eutrophic water body, which was severely impacted by anthropogenic level decrease, pollution and eutrophication during the last century. Starting in the 1970s, a decrease in the water level and an increase in dissolved inorganic nitrogen concentrations resulted in oxygen depletion in the hypolimnion of the lake during summer–autumn thermal stratification of the water column. Our work shows that in October 2019, the redox conditions in the hypolimnion progressed not only to full depletion of oxygen and nitrate, but to the formation of a hydrogen sulfide-rich deep-water layer, which covers 66% of lake’s bottom and accounts for 19% of its water volume. Concentrations of hydrogen sulfide in the hypolimnion of Major and Minor Sevan in October were as high as 9 and 39 μM, respectively.

Triple sulfur isotope composition of sulfate and hydrogen sulfide in the water column of the lake provides further constraints on the biogeochemical processes which result in the formation of hydrogen-sulfide hypolimnion. Values of δ³⁴S for hypolimnetic sulfide are lower by only 7–12 ‰ compared to epilimnetic sulfate, while δ³³S values of sulfide are similar to the δ³³S values of sulfate. These isotopic fingerprints are not consistent either with microbial sulfate reduction in the water column or with its combination with re-oxidative sulfur cycle as the sources of hydrogen sulfide in the hypolimnion. We attribute the formation of a sulfidic deep-water layer to a combination of microbial sulfate reduction in the water column and diffusion of hydrogen sulfide from the sediments.