Decoupled Cr, Mo, and U records of the Hongshuizhuang Formation, North China: Constraints on the Mesoproterozoic ocean redox

Abstract

Multiple lines of evidence have suggested a mix of oxic, ferruginous, and euxinic conditions in the Mesoproterozoic oceans. It is important to constrain the relative proportions of these different redox environments because they might have governed the availability of some bio-essential elements and influenced the evolution of early eukaryotes. Here, we present a suite of geochemical data from the Mesoproterozoic Hongshuizhuang Formation, North China. Sampled shales were deposited under ferruginous to intermittently euxinic bottom waters, with heavy organic matter loading. Such environments are often characterized by co-enrichments of redox-sensitive trace elements. However, contrary to expectations, Cr contents of the Hongshuizhuang shales show almost no discernible authigenic enrichment relative to the upper continental crust, while the enrichments of U are muted as well. Molybdenum, in contrast to Cr and U, is moderately enriched. Since these elements respond differently to anoxia with or without sulfide, the global redox landscape can therefore be estimated. By integrating the budgets of Cr, Mo, and U, we infer that during the deposition of the Hongshuizhuang Formation, at least ~8% (up to ~46%) of seafloor was covered by anoxic waters, but euxinia accounted for only ~1–3% of the total seafloor area.