April 2011

Most large deposits of iron ore are associated with iron formations for the simple reason that they have the highest iron concentrations of any “normal” rock type. Iron formations are found in all Precambrian shields. Iron was preconcentrated in iron formations by surface processes in Precambrian marginal-marine environments, as outlined elsewhere in this issue.

Post-depositional chemical reactions involving iron are important in shallow-marine sediments. They play a significant role in governing the types of minerals that precipitate in such settings. The level of iron supply to marine sediments creates contrasting chemical pathways, each producing distinctive mineral assemblages. An understanding of these processes not only offers insights into past sedimentary environments on Earth but also a greater appreciation of the nature of mineral–water–bacteria interactions throughout the shallow-marine realm.

The reconstruction of oceanic paleoredox conditions on Earth is essential for investigating links between biospheric oxygenation and major periods of biological innovation and extinction, and for unravelling feedback mechanisms associated with paleoenvironmental change. The occurrence of anoxic, iron-rich (ferruginous) oceanic conditions often goes unrecognized, but refined techniques are currently providing evidence to suggest that ferruginous deep-ocean conditions were likely dominant throughout much of Earth’s history. The prevalence of this redox state suggests that a detailed appraisal of the influence of ferruginous conditions on the evolution of biogeochemical cycles, climate, and the biosphere is increasingly required.

The biogeochemical cycle of iron plays a key role in the ocean by delivering bioavailable iron that controls plankton productivity. Transport through the iron cycle occurs mainly as nanoparticulate (oxyhydr)- oxides, which are physically and chemically intermediate between aqueous and particulate forms and can be directly or indirectly bioavailable. Iron nanoparticles transform with time to more stable forms by increased crystallinity, aggregation and growth, and they also alter to other nanominerals. These age transformations can be inhibited or reversed. The resulting aging– rejuvenation cycle first produces stability during long-distance transport and then reverses the process such that bioavailable and labile iron can be produced and delivered to the open ocean.

The rapid redox cycling of iron is one of the most pervasive geochemical processes catalyzed by microbial organisms. Numerous microbial metabolisms rely on transferring electrons to and from iron, even in “extreme” environments considered challenging for life due to high acidity, high alkalinity, high temperature, low organic content, or low water abundance. Recent efforts to explore the iron biogeochemistry of extreme systems, such as hydrothermal vents, seafloor basalts, serpentinizing systems, and acid mine drainage, have significantly expanded our expectations regarding the distribution and activity of iron-dependent life on Earth, and potentially other iron-rich silicate planets, such as Mars.

Microbes are intimately involved in the iron cycle. First, acquisition of iron by microorganisms for biochemical requirements is a key process in the iron cycle in oxygenated, circumneutral pH environments, where the solubility of Fe (III) (oxyhydr)oxides is extremely low. Second, a number of aerobic (using O2) and anaerobic (living in the absence of O2) autotrophic bacteria gain energy for growth from the oxidation of dissolved and solid-phase Fe(II) compounds to Fe(III) (oxyhydr)oxides. Third, heterotrophic Fe (III)-reducing bacteria close the chemical loop by reducing solid-phase Fe (III) minerals back to dissolved and solid-phase Fe(II). Together these metabolic processes control the partitioning of the Earth’s fourth most abundant crustal element, and they are additionally tied to the cycling of several major nutrients (e.g. carbon, oxygen, nitrogen, sulfur) and trace elements (e.g. phosphorus, nickel) in modern and ancient environments.

As an essential nutrient and energy source for the growth of microbial organisms, iron is metabolically cycled between reduced and oxidized chemical forms. The resulting flow of electrons is invariably tied to reactions with other redox-sensitive elements, including oxygen, carbon, nitrogen, and sulfur. Therefore, iron is intimately involved in the geochemistry, mineralogy, and petrology of modern aquatic systems and their associated sediments, particulates, and porewaters. In the geological past, iron played an even greater role in marine geochemistry, as evidenced by the vast deposits of Precambrian iron-rich sediments, the “banded iron formations.” These deposits are now being used as proxies for understanding the chemical composition of the ancient oceans and atmosphere.