On the Biogeochemistry of Modern Euxinia: From the Origin and Controls of Sulfurization Pathways to Trace Elements as Indicators of Environmental Changes

Abstract

Euxinic lakes are analogues of the chemical and microbial sulfur (S) cycling that was prevalent in the anoxic conditions of ancient Earth. Mahoney Lake (Canada) and Green Lake (USA) were studied for their sulfate reducing and sulfide oxidizing bacteria that respectively produce high concentrations of sulfide and organic matter (OM) that are preserved as pyrite and organo-sulfur compounds (OSCs) in the sediments. Isotope and elemental proxies were used to evaluate the origin and controls of the sulfurization reactions that drive pyrite and OSCs burial, and the elements that influence localized environmental changes. The wide range in sulfate and sulfide concentrations make the lakes ideal settings for comparing S cycling between a hyper-euxinic (Mahoney) and a moderate (Green) end member of euxinia. The S isotope offset between dissolved sulfate and sulfide is identical (~50‰) showing that sulfate availability did not influence the isotope fractionation. The S isotopes of pyrite and OSCs in Mahoney sediments did not exhibit diagenetic effects observed in other studies. The S isotopes of these two phases are nearly identical, suggesting that pyrite and OSCs are formed within the water column. In contrast, diagenetic reactions preferentially formed pyrite in Green Lake sediments with an average 10‰ S isotope offset from OSCs. Reactive Fe and trace element patterns are consistent with euxinic conditions in both lakes. Redox sensitive trace metals (i.e., Mo) can track temporally and spatially localized changes in redox and broader climatic changes during the Holocene. These climate changes, including tephra from an eruption, might have influenced the variability of OM and ecology in Mahoney Lake. Molecular analysis of Mahoney Lake water showed a diverse stoichiometry of OSCs that suggests the sulfurization rates of iron and OM are competitive. These OM compounds can contribute to rapid rates of OSCs formation. Kinetic modelling supports our hypothesis that high concentrations of reactive OM play a significant role to competitive sulfurization reactions, which subsequently influenced the observed unconventional isotope patterns within the sedimentary record of Mahoney Lake.