Investigating the Formation Mechanisms of Sedimentary Pyrite under Anoxic & Ferruginous Conditions

Abstract

Sedimentary pyrite (FeS2) formation is a major sink in the terrestrial and oceanic sulfur cycle, and its burial is interlinked with modern carbon and iron cycling as well as the oxidation of the atmosphere and oceans on Earth. However, the formation pathways of sedimentary pyrite in the environment are still debated and yet to be elucidated. Pyrite formation is generally thought to proceed from microbial sulfate reduction driven by the presence of organic carbon, which results in the production of H2S (S in the S2- oxidation state), the most reduced form of sulfur. H2S interacts with iron minerals in the environment to precipitate pyrite, where the average oxidation state of S is -1. This indicates that for pyrite to form, either more oxidized forms of sulfur will need to react with reduced iron, or a precursor iron monosulfide mineral, which forms from the interactions between reduced iron and hydrogen sulfide, will need to be oxidized. The pathways of pyrite formation have been extensively studied previously in the laboratory, but due to the cryptic nature of the sulfur cycle, which involves several intermediate redox state sulfur species, it has been difficult to clarify the exact mechanisms in which pyrite precipitates within sediments. Since pyrite formation is limited by the presence and abundance of reactive iron minerals in the environment, observing the behavior of S species in an iron-rich environment could reveal detailed formation mechanisms. To address this, we studied the redox stratified water column and sediments of meromictic and ferruginous Brownie Lake in Minneapolis, Minnesota, USA. The ferruginous nature of Brownie Lake provides the opportunity to directly observe the S intermediates and precursor iron monosulfides that are involved in pyrite formation in a natural environment, since most of the mechanisms of pyrite formation so far have been established via laboratory incubations. We approached this study by investigating the Fe and S speciation and abundances in Brownie Lake water column and bottom sediments using voltammetry, Mössbauer spectroscopy, synchrotron-based X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF). Water column and bottom sediments of Brownie Lake showed aqueous nanoparticulate, as well as solid FeS pyrite precursors. Additionally, the presence of Fe(III)oxyhydroxides and elemental S in the water column suggests that pyrite formation initiates with the sulfidation of Fe(III)oxyhydroxides coupled with FeS dissolution via polysulfides. Brownie Lake sediments hosted pyrite with increasing abundance downcore, and the variations in the abundance of elemental S down the sediment column suggests that pyrite could be forming via the sulfidation of Fe(III)oxyhydroxides in shallow sediments where H2S is produced, which then gets consumed in deeper sediments by the FeS to form pyrite. Our study therefore reveals the transformation of sulfur species in sedimentary pyrite formation in the iron-rich water column and sediments of Brownie Lake.