Molybdenum (Mo) enrichments in marine sediments are a common indicator of the presence of sulphide near thesediment-water interface and can thereby record historic bottom-water oxygen depletion. Here, we assess theimpact of temporal changes in manganese (Mn) cycling and bottom-water oxygen on sedimentary Mo dynamicsin a seasonally-hypoxic coastal marine basin (Lake Grevelingen, the Netherlands). High resolution line scansobtained with LA-ICP-MS and discrete sample analyses reveal distinct oscillations in Mo with depth in thesediment. These oscillations and high sediment Mo concentrations (up to ~130 ppm) are attributed to depositionof Mo-bearing Mn-oxide-rich particles from the overlying water, the release of molybdate (MoO42−) tothe pore water upon reduction of these Mn-oxides, and subsequent sequestration of Mo. The latter process onlyoccurs in summer when sulphide concentrations near the sediment-water interface are elevated. Gravitationalfocussing of Mn oxides explains the observed increased input of Mo with increasing water depth. Diffusion ofMoO42− from the overlying water contributes only a small amount to the sediment Mo enrichments. Cablebacteria may indirectly impact sediment Mo dynamics by dissolving Mn-carbonates and thereby enhancing thepool of Mn-oxides in the system, and by contributing to remobilisation of sediment Mo during oxic periods. Asediment record that spans the past ~45 years indicates that sediment Mo concentrations have increased overthe past decades, despite less frequent occurrences of anoxia in the bottom waters based on oxygen measurementsfrom water column monitoring. We suggest that the elevated Mo in recent sediments reflects both enhancedrates of sulphate reduction and sulphide production in the surface sediment as a result of increased inputof organic matter into the basin from the adjacent North Sea since 1999, and an associated enhanced “Mnrefluxing” in the marine lake in summer.
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