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Many studies use sedimentary biogenic silica (bSiO₂) stable isotopes (e.g., δ³⁰Si) as paleoproxies but neglect signals from other sedimentary reactive SiO₂phases. We quantified δ³⁰Si for multiple reactive Si pools in coastal river‐plume sediments, revealing up to −5‰ difference between acid‐leachable and alkaline‐digestible amorphous SiO₂. Thus, previous studies have missed valuable information on early diagenetic products and, in cases where sediments were not cleaned, potentially biased bSiO₂δ³⁰Si values. Acid‐leachable δ³⁰Si, that is, from authigenic products, are the result of either multistep fractionation from a bSiO₂source or an ~2‰ fractionation (consistent with metal hydroxide formation) from slowly dissolving lithogenic SiO₂. This analysis also suggests that sedimentary diatom bSiO₂, which has increased regionally in the last half‐century, is the critical substrate of early authigenic Si precipitates. Regional eutrophication, which has stimulated sedimentary bSiO₂accumulation, may have facilitated additional sequestration of both sedimentary Si and cations from early diagenetic products.

Uranium isotopes (²³⁸U/²³⁵U) have been used widely over the last decade as a global proxy for marine redox conditions. The largest isotopic fractionations in the system occur during U reduction, removal, and burial. Applying this basic framework, global U isotope mass balance models have been used to predict the extent of ocean floor anoxia during key intervals throughout Earth's history. However, there are currently minimal constraints on the isotopic fractionation that occurs during reduction and burial in anoxic and iron‐rich (ferruginous) aquatic systems, despite the consensus that ferruginous conditions are thought to have been widespread through the majority of our planet's history. Here we provide the first exploration of δ²³⁸U values in natural ferruginous settings. We measured δ²³⁸U in sediments from two modern ferruginous lakes (Brownie Lake and Lake Pavin), the water column of Brownie Lake, and sedimentary rocks from the Silurian‐Devonian boundary that were deposited under ferruginous conditions. Additionally, we provide new δ²³⁸U data from core top sediments from anoxic but nonsulfidic settings in the Peru Margin oxygen minimum zone. We find that δ²³⁸U values from sediments deposited in all of these localities are highly variable but on average are indistinguishable from adjacent oxic sediments. This forces a reevaluation of the global U isotope mass balance and how U isotope values are used to reconstruct the evolution of the marine redox landscape.

Atmospheric oxygen levels control the oxidative side of key biogeochemical cycles and place limits on the development of high‐energy metabolisms. Understanding Earth's oxygenation is thus critical to developing a clearer picture of Earth's long‐term evolution. However, there is currently vigorous debate about even basic aspects of the timing and pattern of the rise of oxygen. Chemical weathering in the terrestrial environment occurs in contact with the atmosphere, making paleosols potentially ideal archives to track the history of atmospheric O₂levels. Here we present stable chromium isotope data from multiple paleosols that offer snapshots of Earth surface conditions over the last three billion years. The results indicate a secular shift in the oxidative capacity of Earth's surface in the Neoproterozoic and suggest low atmospheric oxygen levels (<1% PALpO₂) through the majority of Earth's history. The paleosol record also shows that localized Cr oxidation may have begun as early as the Archean, but efficient, modern‐like transport of hexavalent Cr under an O₂‐rich atmosphere did not become common until the Neoproterozoic.