Johnson
Clark M.
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Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history
Johnson
Clark M.
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Preprint
Iron formations (IF) represent an iron-rich rock type that typifies many Archaean and Proterozoic
supracrustal successions and are chemical archives of Precambrian seawater chemistry and postdepositional
iron cycling. Given that IF accumulated on the seafloor for over two billion years of
Earth’s early history, changes in their chemical, mineralogical, and isotopic compositions offer a
unique glimpse into environmental changes that took place on the evolving Earth. Perhaps one of
the most significant events was the transition from an anoxic planet to one where oxygen was
persistently present within the marine water column and atmosphere. Linked to this progressive
global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally
influenced continental weathering processes, the supply of nutrients to the oceans, and, ultimately,
diversification of the biosphere and complex life forms. Many of the key recent innovations in
understanding IF genesis are linked to geobiology, since biologically assisted Fe(II) oxidation,
either directly through photoferrotrophy, or indirectly through oxygenic photosynthesis, provides a
process for IF deposition from mineral precursors. The abundance and isotope composition of
Fe(II)-bearing minerals in IF additionally suggests microbial Fe(III) reduction, a metabolism that is
deeply rooted in the Archaea and Bacteria. Linkages among geobiology, hydrothermal systems, and
deposition of IF have been traditionally overlooked, but now form a coherent model for this unique
rock type. This paper reviews the defining features of IF and their distribution through the
Neoarchaean and Palaeoproterozoic. This paper is an update of previous reviews by Bekker et al.
(2010, 2014) that will improve the quantitative framework we use to interpret IF deposition. In this
work, we also discuss how recent discoveries have provided new insights into the processes
underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans.