LAUSR

Abstract Molybdenum (Mo) enrichments in marine sediments are a common indicator of the presence of sulphide near the sediment-water interface and can thereby record historic bottom-water oxygen depletion. Here, we assess the impact of temporal changes in manganese (Mn) cycling and bottom-water oxygen on sedimentary Mo dynamics in a seasonally-hypoxic coastal marine basin (Lake Grevelingen, the Netherlands). High resolution line scans obtained with LA-ICP-MS and discrete sample analyses reveal distinct oscillations in Mo with depth in the sediment. These oscillations and high sediment Mo concentrations (up to ~ 130 ppm) are attributed to deposition of Mo-bearing Mn-oxide-rich particles from the overlying water, the release of molybdate (MoO 4 2 − ) to the pore water upon reduction of these Mn-oxides, and subsequent sequestration of Mo. The latter process only occurs in summer when sulphide concentrations near the sediment-water interface are elevated. Gravitational focussing of Mn oxides explains the observed increased input of Mo with increasing water depth. Diffusion of MoO 4 2 − from the overlying water contributes only a small amount to the sediment Mo enrichments. Cable bacteria may indirectly impact sediment Mo dynamics by dissolving Mn-carbonates and thereby enhancing the pool of Mn-oxides in the system, and by contributing to remobilisation of sediment Mo during oxic periods. A sediment record that spans the past ~ 45 years indicates that sediment Mo concentrations have increased over the past decades, despite less frequent occurrences of anoxia in the bottom waters based on oxygen measurements from water column monitoring. We suggest that the elevated Mo in recent sediments reflects both enhanced rates of sulphate reduction and sulphide production in the surface sediment as a result of increased input of organic matter into the basin from the adjacent North Sea since 1999, and an associated enhanced “Mn refluxing” in the marine lake in summer.