Iron-rich (ferruginous) conditions were a prevalent feature of the ocean throughout much of Earths history. The nature of elemen- tal cycling in such settings is poorly understood, however, thus ham- pering reconstruction of paleoenvironmental conditions during key periods in Earth evolution. This is particularly true regarding con- trols on nutrient bioavailability, which is intimately linked to Earths oxygenation history. Elemental scavenging during precipitation of iron minerals exerts a major control on nutrient cycling in ferrugi- nous basins, and the predictable nature of removal processes provides a mechanism for reconstructing ancient ocean chemistry. Such recon- structions depend, however, on precise knowledge of the iron miner- als 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 mag- netite, 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.
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