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Ferruginous conditions, defined by anoxia and abundant dissolved ferrous iron (Fe2+aq), dominated the Precambrian oceans but are essentially non-existent in a modern, oxygenated world. Ferruginous meromictic lakes represent natural laboratories to ground truth our understanding of the stable Fe isotope proxy, which has been used extensively in interpreting the origins of Fe-rich sedimentary rocks like iron formations (IFs) and the interactions of early life with high-Fe2+aq conditions. Here we report comprehensive geochemical and Fe isotopic analyses of samples collected in May and August 2022, and March 2023, from Deming Lake, Minnesota, a ferruginous meromictic lake that undergoes surface freezing in winter and never becomes euxinic. Through chemical and Fe isotopic analyses of different putative Fe sources to Deming Lake; including eolian input trapped in winter ice cover, nearby bogs, and regional groundwaters sampled at surface springs; we find that a groundwater source provides the best chemical and Fe isotopic match for Deming Lake and can support Fe2+aq-rich waters at depth that maintain a permanent chemocline at ~12 m. The ice-free Deming Lake water column can be split into three layers dominated by distinct Fe cycling regimes. Layer (I) extends from the lake surface to the base of the oxycline at ~6 m, and its Fe cycling is dominated by isotopically light Fe uptake into biomass, likely from stabilized dissolved Fe3+, with variable eolian lithogenic influences. Layer (II) extends between the oxycline and the chemocline at ~12 m and is dominated by partial Fe2+aq oxidation on approach to the oxycline, with the formation of variably isotopically heavy Fe3+-bearing particles. Layer (III) underlies the chemocline and is defined by Fe2+ phosphate (vivianite) and carbonate saturation and precipitation under anoxic, Fe2+aq-rich conditions with little Fe isotopic fractionation. The ice-covered winter water column features more homogenous Fe chemistry above the chemocline, which we attribute to seasonal homogenization of Layers (I) and (II), with suppressed ferric particle formation. Authigenic Fe minerals with non-crustal (light) Fe isotopic compositions only appreciably accumulate in sediments in Deming Lake underlying the chemocline. All sediments deposited above 12 m appear crustal in their Fe isotopic, Mn/Fe, and Fe/Al ratios, likely revealing efficient reductive dissolution of Fe3+-bearing lake precipitates and remineralization of Fe-bearing biomass. We find limited fractionation of Fe isotopes in the ice-covered water column and suggest this provides evidence that substantial delivery of oxidants is required to generate highly fractionated Fe isotopic compositions in Sturtian Snowball era IFs. By comparing Fe isotopic and Mn/Fe fractionation trends in the different Deming Lake layers, we also suggest that correlations between these two parameters in giant early Paleoproterozoic IFs requires the simultaneous deposition of multiple authigenic phases on the ancient seafloor. Finally, high-precision triple Fe isotopic analyses of dissolved Fe impacted by extensive oxidation near the Deming Lake oxycline reveal that the slope of the mass fractionation law for natural, O2-mediated Fe2+aq oxidation is identical to those previously defined for both UV photo-oxidation, and for an array of highly fractionated Paleoproterozoic IFs. 

Abstract Iron (Fe) availability impacts marine primary productivity, potentially influencing the efficiency of the biological carbon pump. Stable Fe isotope analysis has emerged as a tool to understand how Fe is sourced and cycled in the water column; however its application to sediment records is complicated by overlapping isotope signatures of different sources and uncertainties in establishing chronologies. To overcome these challenges, we integrate Fe and osmium isotope measurements with multi‐element geochemical analysis and statistical modeling. We apply this approach to reconstruct the history of Fe delivery to the South Pacific from three pelagic clay sequences spanning 93 million years. Our analysis reveals five principal Fe sources—dust, distal background, two distinct hydrothermal inputs, and a magnesium‐rich volcanic ash. Initially, hydrothermal inputs dominated Fe deposition, but as the sites migrated away from their respective mid‐ocean ridges, other sources became prominent. Notably, from 66 to 40 million years ago (Ma), distal background Fe was the primary source before a shift to increasing dust dominance around 30 Ma. This transition implies that Fe in South Pacific seawater has been dust‐dominated since ≈30 Ma, despite extremely low dust deposition rates today. We speculate that the shift to episodic and low Fe fluxes in the South Pacific and Southern Ocean over the Cenozoic helped shape an ecological niche that favored phytoplankton that adapted to these conditions, such as diatoms. Our analysis highlights how Fe delivery to the ocean is driven by large‐scale tectonic and climatic shifts, while also influencing climate through its integral role in marine phytoplankton and Earth's biogeochemical cycles.