Impact of strong redox shifts and manganese cycling on sedimentary molybdenum isotopes in stratified hypoxic/euxinic basins

Molybdenum concentrations and stable isotope compositions have been extensively applied to marine sediments for palaeo-redox reconstruction. While the open ocean Mo isotope signature is conservative on time scales of several million years, and depends on the changing global isotopic mass balance between input and output fluxes, large Mo isotope variability over smaller time scales has been identified in organic-rich sediments. In addition to changing dissolved sulphide levels and deep-water ventilation times, the varying extent of Fe and Mn oxide shuttling is also considered to be a cause of short-term variability in Mo isotopes. Here we present the Mo isotope composition of well-dated sediments from the redox-stratified Landsort Deep (Baltic Sea) covering the current warm period and the medieval climate anomaly. These modern sediments provide the unique opportunity to directly link water column redox conditions, derived from instrumental time-series, to the sedimentary Mo isotope record. We observe an overall lower Mo isotopic composition (av. δ 98 Mo −0.21 ‰) during longer-lasting bottom water hypoxia between ∼ 1955 and 2000 CE than during weakly sulfidic interruptions and sapropel formation since ∼ 2000 CE (av. δ 98 Mo + 0.63 ‰). Such lower δ 98 Mo signatures during hypoxia benefitted from intense Mn oxide-shuttling near the sediment/water interface. Sedimentary δ 98 Mo values as low as −0.94 ‰ compared to seawater, most likely required intense Mo isotope fractionation via repeated Mn oxide shuttling, aided by incomplete thiolation at a weakly sulfidic sediment/water interface, before Mo was finally sequestered from strongly sulfidic porewaters. Such combined Mo isotope fractionation is also seen in the weakly sulfidic and hypoxic water columns of the Landsort and Gotland Deeps, which suggests significant changes in Mo cycling depending on the presence of dissolved sulfide and Mn oxide. Based on comparable geochemical variations, similar redox dynamics could be also attested for the Landsort Deep sediments deposited during the medieval climate anomaly. In comparison to other modern O 2 -deficient marine settings ranging from mildly reducing to strongly sulfidic conditions, the Landsort Deep sediments deposited during fluctuating sulfidic-hypoxic periods with intense shuttling of Mn oxides act as a distinct sink of isotopically light Mo from the ocean with overall lower δ 98 Mo compared to the marine Mo input flux. Widespread occurrence of Mn ores in organic matter-rich sediments throughout Earth’s history implies that dynamic redox conditions comparable to those in the Landsort Deep were more frequent. This could be particularly true for past periods of global warming, when the flooding of shelf areas promoted expansion of reducing bottom waters in epicontinental seas. Finally, the coupled Mo-Mn-S cycling during fundamental redox changes in such epicontinental settings, analogue to those observed in the modern Landsort Deep, could have significantly impacted the global marine Mo budget.