Green rust formation controls nutrient availability in a ferruginous water column

Abstract

Iron-rich (ferruginous) conditions were a prevalent feature of the ocean throughout much of Earth's history. The nature of elemental cycling in such settings is poorly understood, however, thus hampering reconstruction of paleoenvironmental conditions during key periods in Earth evolution. This is particularly true regarding controls on nutrient bioavailability, which is intimately linked to Earth's oxygenation history. Elemental scavenging during precipitation of iron minerals exerts a major control on nutrient cycling in ferruginous basins, and the predictable nature of removal processes provides a mechanism for reconstructing ancient ocean chemistry. Such reconstructions depend, however, on precise knowledge of the iron minerals formed in the water column. Here, we combine mineralogical and geochemical analyses to demonstrate formation of the mixed-valence iron mineral, green rust, in ferruginous Lake Matano, Indonesia. Carbonated green rust (GR1), along with signifi cant amounts of magnetite, forms below the chemocline via the reduction of ferrihydrite. Further, we show that uptake of dissolved nickel, a key micronutrient required for methanogenesis, is signifi cantly enhanced during green rust formation, suggesting a major control on methane production in ancient ferruginous settings.

Dynamic redox conditions control late Ediacaran metazoan ecosystems in the Nama Group, Namibia

“…Precipitation of this mobilized water column Fe is then potentially induced through a variety M a n u s c r i p t 15 of processes, for example through Fe sulfide (pyrite) formation under euxinic conditions (Canfield et al, 1996;Raiswell and Canfield, 1998), or through precipitation of oxidized or partially oxidized Fe minerals under ferruginous conditions (e.g., Zegeye et al, 2012). These processes have the consequence that Fe HR /Fe T ratios provide a particularly sensitive means to determine whether a depositional setting was oxic or anoxic.…”

Mineralogical Diversity in Lake Pavin: Connections with Water Column Chemistry and Biomineralization Processes

“…(3) Dissimilatory Fe(III)-reducing bacteria (DIRB) have been identified in Lake Pavin [24]. DIRB may be responsible for the formation of green rust (a mixed valence Fe-mineral) in Lake Matano [25][26][27][28][29]. Alternatively, green rust may result from microbial Fe(II) oxidation [30,31].…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…Ferrous iron sorption to those particles may also have given rise to 'green rust'-type deposits that eventually transformed into magnetite or iron silicates (Tamaura et al, 1984;Zegeye et al, 2012;. Recently, greenalite has also been suggested as a primary precipitate that formed in the water column (Rasmussen et al, 2015(Rasmussen et al, , 2016 Europium anomalies in REE+Y patterns have been central in tracing Fe sources.…”

“…Zegeye et al (2012) whether such a conclusion may change in the presence of high concentrations of dissolved Si.…”

“…Carbonated green-rust has been observed in modern ferruginous and sulfate-poor Lake Matano in Indonesia (Zegeye et al, 2012), an environment proposed to be an analogue for the Archaean ocean (Crowe et al, 2008), and it is possible that green-rust was a precursor to magnetite in IFs. In Lake Matano, nanoparticle clusters of ferrihydrite or nanogoethite form at the redoxcline;…”