Ironstone as a proxy of Paleozoic ocean oxygenation

Marine ironstone is a Phanerozoic biochemical sedimentary rock that contains abundant primary iron. Although rare, ironstone is conspicuous in the Paleozoic sedimentary record. Its iron source remains contentious, with traditional models invoking a continentally derived source. Increasing sedimentologic evidence suggests that many Paleozoic ironstones formed along favourably oriented continental margins where coastal upwelling delivered ferruginous waters, with the postulated source of iron being deep-ocean hydrothermal fluids. Thus, deep-water redox conditions are potentially a critical control on the concentration of iron in early Phanerozoic seawater and the accumulation of ironstone. We report novel in situ strontium isotopic and europium anomaly data from five geographically and temporally distinct Ordovician-Silurian ironstones from the Rheic and Iapetus oceans to test the emerging ferruginous upwelling model. These data provide robust evidence that ironstone formed from marine fluids with a component of mid-ocean ridge hydrothermal waters that remained largely unmixed during transport to the locations of ironstone accumulation. These results imply that the deep oceans must have been oxygen deficient to allow local or regional enrichment in hydrothermal iron. Such conditions are interpreted to have been associated with a spatiotemporally variable redox-stratified global ocean with anoxia beneath an oxic surface layer. Thus, sedimentologic and geochemical data presented herein agree with other paleoredox proxies that suggest rising oxygen levels in the Neoproterozoic gave way to early Paleozoic oceans with dynamic redox conditions. The temporal record of Paleozoic ironstone suggests redox stratification, poor deep-ocean ventilation, and upwelling-related ironstone genesis persisted until the Devonian but not into the Carboniferous.