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

Anthropogenic nutrient input has caused a rapid expansion of bottom water hypoxia in the Baltic Sea over the past century. Two earlier intervals of widespread hypoxia, coinciding with the Holocene Thermal Maximum (HTMHI; 8-4 ka before present; BP) and the Medieval Climate Anomaly (MCA(HI); similar to 1200-750 years BP), have been identified from Baltic Sea sediments. Here we present sediment records from two sites in the Baltic Sea, and compare the trace metal (As, Ba, Cd, Cu, Mo, Ni, Pb, Re, Sb, Tl, U, V, Zn) enrichments during all three hypoxic intervals. Distinct differences are observed between the intervals and the various elements, highlighting the much stronger perturbation of trace metal cycles during the modern hypoxic interval. Both Mo and U show a strong correlation with C-org and very high absolute concentrations, indicative of frequently euxinic bottom waters during hypoxic intervals. During the modern hypoxic interval (Modern(HI)) comparatively less Mo is sequestered relative to C-org than in earlier intervals. This suggests partial drawdown of the water column Mo inventory in the modern water column due to persistent euxinia and only partial replenishment of Mo through North Sea inflows. Molybdenum contents in modern sediments are likely also affected by the recent slowdown in input of Mo in association with deposition of Fe and Mn oxides. Strong enrichments of U in recent sediments confirm that the Modern(HI) is more intense than past intervals. These results suggest that U is a more reliable indicator for the intensity of bottom water deoxygenation in the Baltic Sea than Mo. Sedimentary Re enrichment commences under mildly reducing conditions, but this element is not further enriched under more reducing conditions. Enrichments of V are relatively minor for the MCA(HI) and Modern(HI), possibly due to strong reservoir effects on V in the water column, indicating that V is unreliable as an indicator for the intensity of bottom water hypoxia in this setting. Furthermore, Ba profiles are strongly influenced by post-depositional remobilization throughout the Holocene. The strong relationship between C-org and Ni, Tl and particularly Cu suggests that these trace metals can be used to reconstruct the C-org flux into the sediments. Profiles of As, Sb and Cd and especially Pb and Zn are strongly influenced by anthropogenic pollution.Peer reviewe

Sedimentary molybdenum and uranium : Improving proxies for deoxygenation in coastal depositional environments

Sedimentary molybdenum (Mo) and uranium (U) enrichments are widely used to reconstruct changes in bottom water oxygen conditions in aquatic environments. Until now, most studies using Mo and U have focused on restricted suboxic-euxinic basins and continental margin oxygen minimum zones (OMZs), leaving mildly reducing and oxic (but eutrophic) coastal depositional environments vastly understudied. Currently, it is un-known: (1) to what extent Mo and U enrichment factors (Mo-and U-EFs) can accurately reconstruct oxygen conditions in coastal sites experiencing mild deoxygenation, and (2) to what degree secondary (depositional environmental) factors impact Mo-and U-EFs. Here we investigate 18 coastal sites with varying bottom water redox conditions, which we define by means of five "redox bins", ranging from persistently oxic to persistently euxinic, from a variety of depositional environments. Our results demonstrate that Mo-and U-EF-based redox proxies and sedimentary Mo and U contents can be used to differentiate bottom water oxygen concentration among a range of modern coastal depositional environments. This is underpinned by the contrasting EFs of Mo and U along the redox gradient, which shows a substantial difference of Mo-EFs between redox bins 3-5 (ir/ regularly suboxic - ir/regularly dysoxic - persistently oxic) and of U-EFs between redox bins 1-2 (persistently euxinic - ir/regularly euxinic). Surprisingly, we observe comparatively low redox proxy potential for U in en-vironments of mild deoxygenation (redox bins 3-5). Further, we found that secondary factors can bias Mo-and U-EFs to such an extent that EFs do not reliably reflect bottom water redox conditions. We investigate the impact of limited Mo sedimentary sequestration in sulfidic depositional environments (i.e., the "basin reservoir effect", equilibrium with FeMoS4), Fe/Mn-(oxy)(hydr)oxide "shuttling", oxidative dissolution, the sulfate methane transition zone in the sediment, sedimentation rate, and the local Al background on Mo-and U-EFs.Peer reviewe

Gene-Based Modeling of Methane Oxidation in Coastal Sediments: Constraints on the Efficiency of the Microbial Methane Filter

Methane is a powerful greenhouse gas that is produced in large quantities in marine sediments. Microbially mediated oxidation of methane in sediments, when in balance with methane production, prevents the release of methane to the overlying water. Here, we present a gene-based reactive transport model that includes both microbial and geochemical dynamics and use it to investigate whether the rate of growth of methane oxidizers in sediments impacts the efficiency of the microbial methane filter. We focus on iron- and methane-rich coastal sediments and, with the model, show that at our site, up to 10% of all methane removed is oxidized by iron and manganese oxides, with the remainder accounted for by oxygen and sulfate. We demonstrate that the slow growth rate of anaerobic methane-oxidizing microbes limits their ability to respond to transient perturbations, resulting in periodic benthic release of methane. Eutrophication and deoxygenation decrease the efficiency of the microbial methane filter further, thereby enhancing the role of coastal environments as a source of methane to the atmosphere

Anaerobic methanotrophy is stimulated by graphene oxide in a brackish urban canal sediment

Anthropogenic activities are influencing aquatic environments through increased chemical pollution and thus are greatly affecting the biogeochemical cycling of elements. This has increased greenhouse gas emissions, particularly methane, from lakes, wetlands, and canals. Most of the methane produced in anoxic sediments is converted into carbon dioxide by methanotrophs before it reaches the atmosphere. Anaerobic oxidation of methane requires an electron acceptor such as sulphate, nitrate, or metal oxides. Here, we explore the anaerobic methanotrophy in sediments of three urban canals in Amsterdam, covering a gradient from freshwater to brackish conditions. Biogeochemical analysis showed the presence of a shallow sulphate–methane transition zone in sediments of the most brackish canal, suggesting that sulphate could be a relevant electron acceptor for anaerobic methanotrophy in this setting. However, sediment incubations amended with sulphate or iron oxides (ferrihydrite) did not lead to detectable rates of methanotrophy. Despite the presence of known nitrate-dependent anaerobic methanotrophs (Methanoperedenaceae), no nitrate-driven methanotrophy was observed in any of the investigated sediments either. Interestingly, graphene oxide stimulated anaerobic methanotrophy in incubations of brackish canal sediment, possibly catalysed by anaerobic methanotrophs of the ANME-2a/b clade. We propose that natural organic matter serving as electron acceptor drives anaerobic methanotrophy in brackish sediments

Sedimentary molybdenum and uranium: Improving proxies for deoxygenation in coastal depositional environments

Sedimentary molybdenum (Mo) and uranium (U) enrichments are widely used to reconstruct changes in bottom water oxygen conditions in aquatic environments. Until now, most studies using Mo and U have focused on restricted suboxic-euxinic basins and continental margin oxygen minimum zones (OMZs), leaving mildly reducing and oxic (but eutrophic) coastal depositional environments vastly understudied. Currently, it is unknown: (1) to what extent Mo and U enrichment factors (Mo- and U-EFs) can accurately reconstruct oxygen conditions in coastal sites experiencing mild deoxygenation, and (2) to what degree secondary (depositional environmental) factors impact Mo- and U-EFs. Here we investigate 18 coastal sites with varying bottom water redox conditions, which we define by means of five “redox bins”, ranging from persistently oxic to persistently euxinic, from a variety of depositional environments. Our results demonstrate that Mo- and U-EF-based redox proxies and sedimentary Mo and U contents can be used to differentiate bottom water oxygen concentration among a range of modern coastal depositional environments. This is underpinned by the contrasting EFs of Mo and U along the redox gradient, which shows a substantial difference of Mo-EFs between redox bins 3–5 (ir/regularly suboxic – ir/regularly dysoxic – persistently oxic) and of U-EFs between redox bins 1–2 (persistently euxinic – ir/regularly euxinic). Surprisingly, we observe comparatively low redox proxy potential for U in environments of mild deoxygenation (redox bins 3–5). Further, we found that secondary factors can bias Mo-and U-EFs to such an extent that EFs do not reliably reflect bottom water redox conditions. We investigate the impact of limited Mo sedimentary sequestration in sulfidic depositional environments (i.e., the “basin reservoir effect”, equilibrium with FeMoS4), Fe/Mn-(oxy)(hydr)oxide “shuttling”, oxidative dissolution, the sulfate methane transition zone in the sediment, sedimentation rate, and the local Al background on Mo- and U-EFs

Pathways of methane removal in the sediment and water column of a seasonally anoxic eutrophic marine basin

Methane (CH4) is a key greenhouse gas. Coastal areas account for a major proportion of marine CH4 emissions. Eutrophication and associated bottom water hypoxia enhance CH4 production in coastal sediments. Here, we assess the fate of CH4 produced in sediments at a site in a seasonally anoxic eutrophic coastal marine basin (Scharendijke, Lake Grevelingen, the Netherlands) in spring (March) and late summer (September) in 2020. Removal of CH4 in the sediment through anaerobic oxidation with sulfate (Formula presented.) is known to be incomplete in this system, as confirmed here by only slightly higher values of δ13C-CH4 and δD-CH4 in the porewater in the shallow sulfate-methane-transition zone (~5-15 cm sediment depth) when compared to deeper sediment layers. In March 2020, when the water column was fully oxygenated, CH4 that escaped from the sediment was at least partially removed in the bottom water through aerobic oxidation. In September 2020, when the water column was anoxic below ~35 m water depth, CH4 accumulated to high concentrations (up to 73 µmol L-1) in the waters below the oxycline. The sharp counter gradient in oxygen and CH4 concentrations at ~35 m depth and increase in δ13C-CH4 and δD-CH4 above the oxycline indicate mostly aerobic water column removal of CH4. Water column profiles of particulate and dissolved Fe and Mn suggest redox cycling of both metals at the oxycline, pointing towards a potential role of metal oxides in CH4 removal. Water column profiles of (Formula presented.) and (Formula presented.) indicate removal of both solutes near the oxycline. Analyses of 16S rRNA gene sequences retrieved from the water column reveal the presence of aerobic CH4 oxidizing bacteria (Methylomonadaceae) and anaerobic methanotrophic archaea (Methanoperedenaceae), with the latter potentially capable of (Formula presented.) and/or metal-oxide dependent CH4 oxidation, near the oxycline. Overall, our results indicate sediment and water column removal of CH4 through a combination of aerobic and anaerobic pathways, which vary seasonally. Some of the CH4 appears to escape from the surface waters to the atmosphere, however. We conclude that eutrophication may make coastal waters a more important source of CH4 to the atmosphere than commonly assumed

Solid phase speciation controls copper mobilisation from marine sediments by methanobactin

Although marine environments represent huge reservoirs of the potent greenhouse gas methane, they currently contribute little to global net methane emissions. Most of the methane is oxidized by methanotrophs, minimizing escape to the atmosphere. Aerobic methanotrophs oxidize methane mostly via the copper (Cu)-bearing enzyme particulate methane monooxygenase (pMMO). Therefore, aerobic methane oxidation depends on sufficient Cu acquisition by methanotrophs. Because they require both oxygen and methane, aerobic methanotrophs reside at oxic-anoxic interfaces, often close to sulphidic zones where Cu bioavailability can be limited by poorly soluble Cu sulphide mineral phases. Under Cu-limiting conditions, certain aerobic methanotrophs exude Cu-binding ligands termed chalkophores, such as methanobactin (mb) exuded by Methylosinus trichosporium OB3b. Our main objective was to establish whether chalkophores can mobilise Cu from Cu sulphide-bearing marine sediments to enhance Cu bioavailability. Through a series of kinetic batch experiments, we investigated Cu mobilisation by mb from a set of well-characterized sulphidic marine sediments differing in sediment properties, including Cu content and phase distribution. Characterization of solid-phase Cu speciation included X-ray absorption spectroscopy and a targeted sequential extraction. Furthermore, in batch experiments, we investigated to what extent adsorption of metal-free mb and Cu-mb complexes to marine sediments constrains Cu mobilisation. Our results are the first to show that both solid phase Cu speciation and chalkophore adsorption can constrain methanotrophic Cu acquisition from marine sediments. Only for certain sediments did mb addition enhance dissolved Cu concentrations. Cu mobilisation by mb was not correlated to the total Cu content of the sediment, but was controlled by solid-phase Cu speciation. Cu was only mobilised from sediments containing a mono-Cu-sulphide (CuSx) phase. We also show that mb adsorption to sediments limits Cu acquisition by mb to less compact (surface) sediments. Therefore, in sulphidic sediments, mb-mediated Cu acquisition is presumably constrained to surface-sediment interfaces containing mono-Cu-sulphide phases

Deoxygenation and organic carbon sequestration in the Tethyan realm associated with the middle Eocene climatic optimum

The middle Eocene climatic optimum (ca. 40 Ma) stands out as a transient global warming phase of ~400 k.y. duration that interrupted long-term Eocene cooling; it has been associated with a rise in atmospheric CO2 concentrations that has been linked to a flare-up in Arabia-Eurasia continental arc volcanism. Increased organic carbon burial in the Tethys Ocean has been proposed as a carbon sequestration mechanism to bring the middle Eocene climatic optimum to an end. To further test these hypotheses, we assessed the sedimentary and geochemical expression of the middle Eocene climatic optimum in the northern Peri-Tethys, specifically, the organic-rich Kuma Formation of the Belaya River section, located on the edge of the Scythian Platform in the North Caucasus, Russia. We constructed an age-depth model using nannofossil chronobiostratigraphy. Throughout the studied middle Eocene interval (41.2–39.9 Ma), we documented sea-surface temperatures of 32–36 °C based on the tetraether index of tetraethers consisting of 86 carbons (TEX86), depending on proxy calibration, and during the early middle Eocene climatic optimum, we observed sea-surface warming of 2–3 °C. Despite the proximity of the section to the Arabia-Eurasia volcanic arc, the hypothesized source of volcanic CO2, we found no evidence for enhanced regional volcanism in sedimentary mercury concentrations. Sedimentary trace-element concentrations and iron speciation indicate reducing bottom waters throughout the middle Eocene, but the most reducing, even euxinic, conditions were reached during late middle Eocene climatic optimum cooling. This apparent regional decoupling between ocean warming and deoxygenation hints at a role for regional tectonics in causing basin restriction and anoxia. Associated excess organic carbon burial, extrapolated to the entire regional Kuma Formation, may have been ~8.1 Tg C yr–1, comprising ~450 Pg C over this ~55 k.y. interval. Combined with evidence for enhanced organic carbon drawdown in the western Peri-Tethys, this supports a quantitatively significant role for the basin in the termination of the middle Eocene climatic optimum by acting as a large organic carbon sink, and these results collectively illustrate that the closing Tethys Ocean might have affected global Paleogene climate

Seasonal dynamics of the microbial methane filter in the water column of a eutrophic coastal basin

In coastal waters, methane-oxidizing bacteria (MOB) can form a methane biofilter and mitigate methane emissions. The metabolism of these MOBs is versatile, and the resilience to changing oxygen concentrations is potentially high. It is still unclear how seasonal changes in oxygen availability and water column chemistry affect the functioning of the methane biofilter and MOB community composition. Here, we determined water column methane and oxygen depth profiles, the methanotrophic community structure, methane oxidation potential, and water–air methane fluxes of a eutrophic marine basin during summer stratification and in the mixed water in spring and autumn. In spring, the MOB diversity and relative abundance were low. Yet, MOB formed a methane biofilter with up to 9% relative abundance and vertical niche partitioning during summer stratification. The vertical distribution and potential methane oxidation of MOB did not follow the upward shift of the oxycline during summer, and water–air fluxes remained below 0.6 mmol m−2 d−1. Together, this suggests active methane removal by MOB in the anoxic water. Surprisingly, with a weaker stratification, and therefore potentially increased oxygen supply, methane oxidation rates decreased, and water–air methane fluxes increased. Thus, despite the potential resilience of the MOB community, seasonal water column dynamics significantly influence methane removal