Trace metal cycling and ^(238)U/^(235)U in New Zealand's fjords: Implications for reconstructing global paleoredox conditions in organic-rich sediments

Abstract

Reconstructing the history of ocean oxygenation provides insight into links between ocean anoxia, biogeochemical cycles, and climate. Certain redox-sensitive elements respond to changes in marine oxygen content through phase shifts and concomitant isotopic fractionation, providing new diagnostic proxies of past ocean hypoxia. Here we explore the behavior and inter-dependence of a suite of commonly utilized redox-sensitive trace metals (U, Mo, Fe, and Mn) and the emerging "stable" isotope system of U (^(238)U/^(235)U, or δ^(238)U) in New Zealand fjords. These semi-restricted basins have chemical conditions spanning the complete redox spectrum from fully oxygenated to suboxic to intermittently anoxic/euxinic. In the anoxic water column, U and Mo concentrations decrease, while Fe and Mn concentrations increase. Similarly, signals of past euxinic conditions can be found by U, Mo, Fe, and Mn enrichment in the underlying sediments. The expected U isotopic shift toward a lower δ^(238)U in the anoxic water column due to U(VI)–U(IV) reduction is not observed; instead, water column δ^(238)U profiles are consistent in fjords of all oxygen content, falling within previously reported ranges for open ocean seawater (δ^(238)U = −0.42 ± 0.07‰). Additionally, surface sediment δ^(238)U results show evidence for competing U isotope fractionation processes. One site indicates increased export of ^(238)U from seawater to the underlying sediments (fractionation between aqueous seawater U and particulate sediment U, or Δ_(U(aq)−U(solid)) = −0.25‰), consistent with redox-driven fractionation. Another site suggests potential U(VI) adsorption-driven fractionation, reflecting increased export of ^(235)U from seawater to sediments (Δ_(U(aq)−U(solid)) = 0.25‰). We discuss several potential factors that could alter δ^(238)U in waters and sediments beyond redox-driven shifts, including adsorption to organic matter in waters of high primary productivity, reaction rates for competing processes of U adsorption and release, and isotopic constraints of U coming into the system from terrestrial environments. These potential complications should be understood and constrained through observations, experiments, and models before future application of δ^(238)U as a global paleoredox tracer can achieve its full potential.

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