Experimental evidence for rapid biotic and abiotic reduction of Fe (III) at low temperatures in salt marsh sediments: a possible mechanism for formation of modern sedimentary siderite concretions

Fe (III) reduction is a key component of the global iron cycle, and an important control on carbon mineralization. However, little is known about the relative roles and rates of microbial (biotic) iron reduction, which utilizes organic matter, versus abiotic iron reduction, which occurs without carbon mineralization. This paper reports on the capacity for salt marsh sediments, which typically are rich in iron, to support abiotic reduction of mineral Fe (III) driven by oxidation of sulphide. Sediment was reacted with amorphous FeS under strictly anaerobic conditions at a range of temperatures in biotic and abiotic microcosm experiments. Fe (III) reduction driven by sulphide oxidation occurs abiotically at all temperatures, leading to Fe (II) and elemental sulphur production in all abiotic experiments. In biotic experiments elemental sulphur is also the oxidized sulphur product but higher bicarbonate production leads to FeCO3 precipitation. Abiotic reduction of Fe (III) occurs at rates that are significant compared with microbial Fe (III) reduction in salt marsh sediments. The solid phases produced by coupled abiotic and biotic reactions, namely elemental sulphur and FeCO3, are comparable to those seen in nature at Warham, Norfolk, UK. Furthermore, the rates of these processes measured in the microcosm experiments are sufficient to generate siderite concretions on the rapid time scales observed in the field. This work highlights the importance of abiotic Fe (III) reduction alongside heterotrophic reduction, which has implications for iron cycling and carbon mineralization in modern and ancient sediments.