Abstract. Expanding hypoxia in the Baltic Sea over the past century has led to the development of anoxic and sulfidic (euxinic) deep basins that are only periodically ventilated by inflows of oxygenated waters from the North Sea. In this study, we investigate the potential consequences of the expanding hypoxia for manganese (Mn) burial in the Baltic Sea using a combination of pore water and sediment analyses of dated sediment cores from eight locations. Diffusive fluxes of dissolved Mn from sediments to overlying waters at oxic, hypoxic and euxinic sites are consistent with an active release of Mn from these areas. Although the present-day fluxes are significant (ranging up to ca. 240 μmol m−2 d−1), comparison to published water column data suggests that the current benthic release of Mn is small when compared to the large pool of Mn already present in the hypoxic and anoxic water column. Our results highlight two modes of Mn carbonate formation in sediments of the deep basins. In the Gotland Deep area, Mn carbonates likely form from Mn oxides that are precipitated from the water column directly following North Sea inflows. In the Landsort Deep, in contrast, Mn carbonate and Mn sulfide layers appear to form independently of inflow events, and are possibly related to the much larger and continuous input of Mn oxides linked to sediment focusing. Whereas Mn-enriched sediments continue to accumulate in the Landsort Deep, this does not hold for the Gotland Deep area. Here, a recent increase in euxinia, as evident from measured bottom water sulfide concentrations and elevated sediment molybdenum (Mo), coincides with a decline in sediment Mn content. Sediment analyses also reveal that recent inflows of oxygenated water (since ca. 1995) are no longer consistently recorded as Mn carbonate layers. Our data suggest that eutrophication has not only led to a recent rise in sulfate reduction rates but also to a decline in reactive Fe input to these basins. We hypothesize that these factors have jointly led to higher sulfide availability near the sediment–water interface after inflow events. As a consequence, the Mn oxides may be reductively dissolved more rapidly than in the past and Mn carbonates may no longer form. Using a simple diagenetic model for Mn dynamics in the surface sediment, we demonstrate that an enhancement of the rate of reduction of Mn oxides is consistent with such a scenario. Our results have important implications for the use of Mn carbonate enrichments as a redox proxy in marine systems.

20 Molybdenum (Mo) enrichments in marine sediments are a common indicator of the presence 21 of sulphide near the sediment-water interface and can thereby record historic bottom-water 22 oxygen depletion. Here, we assess the impact of temporal changes in manganese (Mn) cycling 23 and bottom-water oxygen on sedimentary Mo dynamics in a seasonally-hypoxic coastal marine 24 basin (Lake Grevelingen, the Netherlands). High resolution line scans obtained with LA-ICP-25 MS and discrete sample analyses reveal distinct oscillations in Mo with depth in the sediment. 26 These oscillations and high sediment Mo concentrations (up to ~130 ppm) are attributed to 27 deposition of Mo-bearing Mn-oxide-rich particles from the overlying water, the release of 28 molybdate (MoO4 2-) to the pore water upon reduction of these Mn-oxides, and subsequent 29 sequestration of Mo. The latter process only occurs in summer when sulphide concentrations 30 near the sediment-water interface are elevated. We hypothesise that cable bacteria enhance the 31 seasonality in sediment Mo records by contributing to remobilisation of Mo as MoO4 2during 32 oxic periods and by enhancing the pool of Mn-oxides in the system by dissolving Mn-33 carbonates. A sediment record that spans the past ~45 years indicates that sediment Mo 34 concentrations have increased over the past decades, despite less frequent occurrences of anoxia 35in the bottom waters based on oxygen measurements from water column monitoring. We 36 suggest that the elevated Mo in recent sediments reflects both enhanced rates of sulphate 37 reduction and sulphide production in the surface sediment as a result of increased input of 38 organic matter into the basin from the adjacent North Sea since 1999, and an associated 39 enhanced "Mn refluxing" in the marine lake in summer. 40 93 refluxing" is thought to contribute to the high Mo burial fluxes in environments with weakly 94 sulphidic bottom waters (Algeo and Lyons, 2006). To our knowledge, there are no detailed 95 seasonal studies of the dynamics of Mo and Mn in both pore waters and sediments of hypoxic 96 systems to confirm the suggested seasonality in coupled Mn-Mo cycling. 97 Recently, it was discovered that sulphide-oxidising cable bacteria (Nielsen et al., 2010; 98 Pfeffer et al., 2012) may dissolve Fe-sulphides and Mn-carbonates in surface sediments of 99 seasonally-hypoxic systems. Consequently, these bacteria actively contribute to the formation 100 of an oxidised, Fe-and Mn-oxide rich surface layer in winter and spring (Seitaj et al., 2015; 101 Sulu-Gambari et al., 2016b; Sulu-Gambari et al., 2016a). In contrast, sulphur oxidising 102 Beggiatoaceae, present in autumn, had a more limited effect on the formation of Fe-and Mn-103 oxides in the surface sediment (Seitaj et al., 2015; Sulu-Gambari et al., 2016b; Sulu-Gambari 104 et al., 2016a). Due to the coupling of Mn, Fe, S and Mo cycles in hypoxic systems, we 105 hypothesise that the activity of cable bacteria may also be of relevance to the sedimentary 106 dynamics of Mo. More specifical...

Abstract. Expanding hypoxia in the Baltic Sea over the past century has led to anoxic and sulfidic (euxinic) deep basins that are only periodically ventilated by inflows of oxygenated waters from the North Sea. In this study, we investigate the consequences of the expanding hypoxia for manganese (Mn) burial in the Baltic Sea using a combination of pore water and sediment analyses of well-dated sediment cores from 8 locations. Diffusive fluxes of dissolved Mn from sediments to overlying waters at oxic and hypoxic sites are in line with an active release of Mn from these areas. However, this flux of Mn is only small when compared to the large pool of Mn already present in the hypoxic and anoxic water column. Our results highlight two modes of Mn carbonate formation in sediments of the deep basins. In the Gotland Deep area, Mn carbonates likely form from Mn oxides that are precipitated from the water column directly following North Sea inflows. In the Landsort Deep, in contrast, Mn carbonate and Mn sulfide layers form independent of inflow events, with pore water Mn produced in deeper layers of the sediment acting as a key Mn source. While formation of Mn enrichments in the Landsort Deep continues to the present, this does not hold for the Gotland Deep area. Here, increased euxinia, as evident from measured bottom water sulfide concentrations and elevated sediment molybdenum (Mo), goes hand in hand with a decline in sediment Mn and recent inflows of oxygenated water (since ca. 1995) are no longer consistently recorded as Mn carbonate layers. We postulate that the reduction of Mn oxides by hydrogen sulfide following inflows has become so rapid that Mn2+ is released to the water column before Mn carbonates can form. Our results have important implications for the use of Mn carbonate enrichments as a redox proxy in marine systems.

Abstract. In late 2014, a large, oxygen-rich salt water inflow entered the Baltic Sea and caused considerable changes in deep water oxygen concentrations. We studied the effects of the inflow on the concentration patterns of two greenhouse gases, methane and nitrous oxide, during the following year (2015) in the water column of the Gotland Basin. In the eastern basin, methane which had previously accumulated in the deep waters was largely removed during the year. Here, volume-weighted mean concentration below 70 m decreased from 108 nM in March to 16.3 nM over a period of 141 days (0.65 nM d −1 ), predominantly due to oxidation (up to 79 %) following turbulent mixing with the oxygen-rich inflow. In contrast nitrous oxide, which was previously absent from deep waters, accumulated in deep waters due to enhanced nitrification following the inflow. Volume-weighted mean concentration of nitrous oxide below 70 m increased from 11.8 nM in March to 24.4 nM in 141 days (0.09 nM d −1 ). A transient extreme accumulation of nitrous oxide (877 nM) was observed in the deep waters of the Eastern Gotland Basin towards the end of 2015, when deep waters turned anoxic again, sedimentary denitrification was induced and methane was reintroduced to the bottom waters. The Western Gotland Basin gas biogeochemistry was not affected by the inflow.