Sedimentary molybdenum and uranium: Improving proxies for deoxygenation in coastal depositional environments

Sedimentary molybdenum (Mo) and uranium (U) enrichments are widely used to reconstruct changes in bottom water oxygen conditions in aquatic environments. Until now, most studies using Mo and U have focused on restricted suboxic-euxinic basins and continental margin oxygen minimum zones (OMZs), leaving mildly reducing and oxic (but eutrophic) coastal depositional environments vastly understudied. Currently, it is unknown: (1) to what extent Mo and U enrichment factors (Mo- and U-EFs) can accurately reconstruct oxygen conditions in coastal sites experiencing mild deoxygenation, and (2) to what degree secondary (depositional environmental) factors impact Mo- and U-EFs. Here we investigate 18 coastal sites with varying bottom water redox conditions, which we define by means of five “redox bins”, ranging from persistently oxic to persistently euxinic, from a variety of depositional environments. Our results demonstrate that Mo- and U-EF-based redox proxies and sedimentary Mo and U contents can be used to differentiate bottom water oxygen concentration among a range of modern coastal depositional environments. This is underpinned by the contrasting EFs of Mo and U along the redox gradient, which shows a substantial difference of Mo-EFs between redox bins 3–5 (ir/regularly suboxic – ir/regularly dysoxic – persistently oxic) and of U-EFs between redox bins 1–2 (persistently euxinic – ir/regularly euxinic). Surprisingly, we observe comparatively low redox proxy potential for U in environments of mild deoxygenation (redox bins 3–5). Further, we found that secondary factors can bias Mo-and U-EFs to such an extent that EFs do not reliably reflect bottom water redox conditions. We investigate the impact of limited Mo sedimentary sequestration in sulfidic depositional environments (i.e., the “basin reservoir effect”, equilibrium with FeMoS4), Fe/Mn-(oxy)(hydr)oxide “shuttling”, oxidative dissolution, the sulfate methane transition zone in the sediment, sedimentation rate, and the local Al background on Mo- and U-EFs.