Molybdenum dynamics in sediments of a seasonally-hypoxic coastal marine basin

Abstract

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...

Ocean Circulation in the Toarcian (Early Jurassic): A Key Control on Deoxygenation and Carbon Burial on the European Shelf

“…These sedimentary rocks are also characterized by high molar TOC/P ratios (>150), high Fe/Al, and low Mn/Al. Such a geochemical signature is typical for sediments overlain by low‐oxygen bottom waters (Algeo & Maynard, ; Mort et al, ; Sulu‐Gambari et al, ). Sediments at these sites also show evidence for a high primary productivity and photic zone euxinia as deduced from micropaleontological records, Rock‐Eval pyrolysis and biomarker analysis (supporting information Table S1).…”

Transient bottom water oxygenation creates a niche for cable bacteria in long‐term anoxic sediments of the Eastern Gotland Basin

“…Their activity can generate a centimetres thick suboxic zone devoid of both O 2 and H 2 S. The separation of the two half-redox reactions results in a characteristic pH signature, with a subsurface pH maximum due to the acid-consuming cathodic O 2 or NO 3 − reduction and a deeper pH minimum due to acid-forming anodic H 2 S oxidation (Nielsen et al, 2010). Such acidification stimulates the dissolution of FeS and Ca-and Mn-carbonates (Risgaard-Petersen et al, 2012;Rao et al, 2016;Sulu-Gambari et al, 2016a), with the possible corelease of trace metals like As, Co and Mo (Sulu-Gambari et al, 2017;van de Velde et al, 2017). The liberated Fe 2+ , Ca 2+ and Mn 2+ re-precipitate at the sediment surface as Feand Mn-oxides and carbonates.…”

Hypoxia in the Holocene Baltic Sea: Comparing modern versus past intervals using sedimentary trace metals

“…Another key difference is that water column Mo is actively scavenged by particle Mn-(and sometimes Fe) oxides, which may carry Mo to the sediment-water interface (e.g. Turekian, 1977;Adelson et al, 2001;Sulu-Gambari et al, 2017), whereas U is generally enriched via diffusion into sediments (e.g. Klinkhammer and Palmer, 1991).…”