Iron minerals as archives of Earth's redox and biogeochemical evolution

Iron minerals provide sedimentary repositories of chemical information pertaining to Earth’s redox and biogeochemical evolution, from before the Great Oxidation Event some 2.5 billion years ago, to more recent events occurring up to and into the Cenozoic Era. The most powerful chemical information recorded in iron minerals comes in the form of trace-element signatures, most notably their concentrations and stable isotope compositions. Here we provide an introduction to iron mineralogy and the processes responsible for the accumulation and preservation of trace-element signatures in iron minerals, focusing on the deposition of iron minerals in three key ancient sedimentary archives: banded iron formations, ferromanganese crusts and black shales. We introduce the theory and practical use of non-traditional trace-element stable-isotope systems in redox and biogeochemical research, focusing on the recent use of iron, molybdenum and chromium stable isotopes to shed light on the redox and biogeochemical information stored in iron-rich sediments. By analysing both trace-element concentrations and stableisotope compositions recorded in iron minerals, iron-rich sedimentary archives are providing a unique window into the past, where changes in trace-element signatures shed light on major transitions in Earth’s redox and biogeochemical evolution.