Precambrian banded iron formations: paleoecological and paleobiological aspects

Banded iron formations (BIF) are economically important sedimentary deposits in Earth’s Precambrian rock record. They are marine chemical sediments consisting of alternating iron-rich and silica-rich bands, which were deposited throughout much of the Precambrian. BIF sare important proxies for the geochemical composition of Precambrian seawater and provide evidence for early microbial life. Iron in BIFs was likely precipitated in the form of Fe3+ minerals, such as ferrihydrite (Fe(OH)3), either through the metabolic activity of anoxygenic photoautotrophic Fe2+-oxidizing bacteria (photoferrotrophs), by microaerophilic bacteria, or by the oxidation of dissolved Fe2+by oxygen produced by early cyanobacteria. BIFs were deposited, albeit intermittently, for more than 4 billion years, but as the Earth system changed fundamentally, so did the style of iron precipitation. Earlier studies have invoked a continental source of iron for BIFs, derived via intense chemical weathering. The discovery of modern seafloor-hydrothermal systems, supplying Fe and Mn to seawater, shifted attention to the hydrothermal processes in the deep ocean as the most likely source. A biological role in iron precipitation has been suggested for over a century, however only recently the importance of microorganisms has received wider acceptance. Components of the Earth system that are most relevant to deposition of BIFs include magmatism, biosphere, and sea-water redox state and composition.