The evolution of the Earth's surface iron cycle

Formation and burial of pyrite (iron sulfide) in marine sediments exert a fundamental control on atmospheric oxygenation and seawater buffering over geological timescales. However, little is known about how the formation and delivery of its precursor, reactive iron (Fe) oxide minerals, have evolved throughout Earth history. Secular variability in reactive Fe (including Fe oxides and pyrite) preserved in marine sediments is commonly attributed solely to redox changes. Here, we develop an approach to distinguish redox-driven influences from other controls on sedimentary reactive Fe, including the intensity of continental silicate weathering and the transfer of terrigenous particles to the ocean. We apply this framework to a compilation of reactive Fe data spanning 1,200 My of Earth history. Our results reveal persistently low proportions of reactive Fe from the Mesoproterozoic through the Cambrian, followed by a pronounced mid- to late Paleozoic rise and a subsequent decline in the late Cenozoic. This temporal pattern is inconsistent with a purely redox-driven control. Comparison with independent proxies for continental weathering and land-ocean sediment and solute fluxes suggests a strong coupling between reactive Fe burial, oxidative silicate weathering, and tectonically driven erosion. Notably, the mid- to late Paleozoic rise in reactive Fe coincided with the proliferation of land plants and increasing atmospheric oxygen. We propose a positive feedback where Earth-surface Fe cycling was both amplified by atmospheric oxygenation and contributed to it through its influence on pyrite burial in marine sediments.