Bottom-water Concentrations Research Articles

Nitrate and silicate fluxes at the sediment–water interface of the deep North Pacific Ocean illuminated by 226Ra/230Th disequilibria

By taking advantage of recent analytical advances, we herein develop the 226Ra/230Th isotope systematics as a novel tool for quantifying nitrate and dissolved silicate fluxes across the sediment–water interface of the deep-ocean floor. Sediment cores were retrieved from the seabed between 4927 m and 5951 m in the North Pacific Ocean. Downcore profiles of 230Th and both dissolved and total 226Ra were measured using a high-sensitivity inductively coupled plasma mass spectrometer. At all study sites, a marked deficit of total 226Ra with respect to 230Th was observed between 0 and 20 cm, indicating active migration of soluble 226Ra from the sediment into the overlying seawater. By constructing the mass balance of 226Ra in the sediment column, the flux of dissolved 226Ra across the sediment–water interface was estimated to range from 461 to 1320 dpm m−2 yr−1. When coupled to a diffusive transport model as developed by early investigators, these flux values of 226Ra enabled us to calculate the flux of any dissolved constituent of interest by measuring their bottom water concentrations and pore water “saturation” concentrations. Based on the 226Ra/230Th disequilibrium approach, the derived fluxes vary between 4.1 and 10.5 mmol m−2 yr−1 for nitrate and between 11 and 49 mmol m−2 yr−1 for dissolved silicate. A compilation of nitrate and silicate fluxes from the seabed in the deep Pacific Ocean shows that these values are consistent with historical flux measurements based on the conventional core incubation method in the same study region. In addition, both nitrate and silicate fluxes exhibit a clear depth-dependent trend. Overall, our results suggest that sedimentary diagenetic alterations at the North Pacific Ocean floor below ∼ 5000 m are efficient so that only < 2 % of the particulate organic carbon and < 12 % of the biogenic opal raining to the seafloor are ultimately preserved in the sediment.

Methane dynamics altered by reservoir operations in a typical tributary of the Three Gorges Reservoir

Substantial nutrient inputs from reservoir impoundment typically increase sedimentation rate and primary production. This can greatly enhance methane (CH4) production, making reservoirs potentially significant sources of atmospheric CH4. Consequently, elucidating CH4 emissions from reservoirs is crucial for assessing their role in the global methane budget. Reservoir operations can also influence hydrodynamic and biogeochemical processes, potentially leading to pronounced spatiotemporal heterogeneity, especially in reservoirs with complex tributaries, such as the Three Gorges Reservoir (TGR). Although several studies have investigated the spatial and temporal variations in CH4 emissions in the TGR and its tributaries, considerable uncertainties remain regarding the impact of reservoir operations on CH4 dynamics. These uncertainties primarily arise from the limited spatial and temporal resolutions of previous measurements and the complex underlying mechanisms of CH4 dynamics in reservoirs. In this study, we employed a fast-response automated gas equilibrator to measure the spatial distribution and seasonal variations of dissolved CH4 concentrations in XXB, a representative area significantly impacted by TGR operations and known for severe algal blooms. Additionally, we measured CH4 production rates in sediments and diffusive CH4 flux in the surface water. Our multiple campaigns suggest substantial spatial and temporal variability in CH4 concentrations across XXB. Specifically, dissolved CH4 concentrations were generally higher upstream than downstream and exhibited a vertical stratification, with greater concentrations in bottom water compared to surface water. The peak dissolved CH4 concentration was observed in May during the drained period. Our results suggest that the interplay between aquatic organic matter, which promotes CH4 production, and the dilution process caused by intrusion flows from the mainstream primarily drives this spatiotemporal variability. Importantly, our study indicates the feasibility of using strategic reservoir operations to regulate these factors and mitigate CH4 emissions. This eco-environmental approach could also be a pivotal management strategy to reduce greenhouse gas emissions from other reservoirs.

Short‐Term Arsenic Cycling in a Shallow, Polymictic Lake

AbstractArsenic (As), a harmful contaminant present in many urban lakes, can negatively impact lake ecosystem health when aqueous concentrations are elevated. We observed repeated diel oscillations in As concentrations in the bottom waters of a shallow, temperate lake during a weeklong period. In this work, we explore four mechanistic hypotheses to explain the diel As cycles based on the physical and biogeochemical processes that were investigated during the study. Despite pH being known to control diel As cycles in rivers, we determined that this mechanism was inconsistent with As dynamics observed in Lake Killarney. Instead, we found that iron and manganese concentrations oscillated simultaneously with As concentrations and that redox conditions adjacent to the lakebed likely controlled the near‐bed availability of these three elements. However, based on timescale analysis, we determined that biogeochemical processes at the sediment water interface alone could not have led to the daily oscillations in bottom water concentrations. Rather, turbulence from convective mixing was necessary to transport dissolved species. Notably, we saw that the timing and intensity of peaks in convectively driven turbulence were consistent with observed diel fluctuations in bottom water As. Our results indicate that physical mixing is key in controlling As transport and concentrations on diel timescales within shallow lakes. Understanding the daily cycling of As in shallow lakes is essential for predicting the degree to which lake biota are exposed to this contaminant. Diel oscillations in As concentrations should be considered when designing sampling methods to assess the water quality of contaminated sites.

Near seafloor methane flux in the world's largest human-induced dead zone is regulated by sediment accumulation rate

The vast oxygen-depleted area of the central Baltic Sea is the largest human-induced dead zone in the world with 70,000 km2 or approximately three times the second largest one in the Gulf of Mexico. Methane occurs in high concentrations in bottom waters (3200 nM) and sediments (30 mM), and its dynamics is better constrained for the water column, but still poorly understood on sediments. Here we show that sediment accumulation rate plays a major role in regulating the quantity of organic matter and its residence time in the sulphate reduction and methanogenesis zones and, therefore, affects methane generation, consumption, and diffusive flux in sediments near the seafloor (< 1 m). High fluxes found in high sediment accumulation rate areas and competition for substrate (organoclastic sulphate reduction vs. anaerobic oxidation of methane with sulphate), compromise the ability of the thin microbial filter to consume and prevent methane diffusion through the seafloor.

Artificial reef biofouling community impacts on ecosystem metabolism and biogeochemical cycling in estuarine waters of the northern Gulf of Mexico

Artificial reefs increase hard bottom habitat and bolster shoreline protection in coastal waters. Reefs are rapidly colonized by biofouling communities which can potentially alter ecosystem metabolism. Despite the high biomass and ubiquitous nature of biofouling communities, little is known about their ecosystem metabolism or role in biogeochemical nutrient cycling. Using a combination of laboratory and field-based methods, this study focused on the metabolic activity of artificial reef biofouling communities and their potential to contribute to water column hypoxia in a shallow subtropical estuary. Study objectives aimed to (1) measure in situ dissolved oxygen (DO) concentrations in surface and bottom waters along reef transects during summer months when episodic hypoxic events typically occur, (2) estimate respiration rates in relation to nutrient regeneration in the absence versus presence of biofouling communities, and (3) examine seasonal shifts in trophic structure by relating elemental C:N ratios and stable nitrogen isotope (δ15N) values of the biofouling community across eight sample periods. In situ DO in bottom waters was low near high profile reefs. Water column respiration and nutrient regeneration rates were comparable to other estuaries. Respiration and regeneration rates of ammonium and phosphate were all manifestly higher in the presence of a biofouling community. Biofouling metabolic-driven processes were higher at low profile reefs than at high profile reefs. Seasonal shifts in biofouling elemental composition indicated by higher C:N and lower δ15N during cooler months and lower C:N and higher δ15N during warmer months reflected a transition from a more autotrophic community in winter to a more heterotrophic community during the rest of the year. Our results demonstrate that artificial reef biofouling communities can potentially contribute to hypoxic conditions in coastal waters of the northern Gulf of Mexico.

Top-down control over dissolved organic carbon in the bottom water of the Weddell Sea and its implication for the continental shelf pump

Dense water out of the Antarctic shelves is expected to drive the transport of carbon into the deep Southern Ocean via the formation of Antarctic Bottom Water. However, bottom water formation’s capacity to sequester carbon into the deep ocean is poorly constrained. Here, dissolved organic carbon (DOC), dissolved black carbon (DBC), and particulate organic carbon (POC) were examined to reveal the influence of the Weddell Sea Deep Water (WSDW) on DOC transport during its flowing out of the Weddell Sea. High DOC concentrations (>60 μM-C) and low DBC/DOC ratios (<1.5%) were observed in surface water near the South Orkney Islands, ascribing to sea ice melt-induced phytoplankton blooms. Seawater at the mid-deep depths exhibited a higher DOC concentration (averaging 48.1 ± 3.7 μM-C) than the incoming water source, resulting from the release of DOC from sinking particles. Bottom water had higher DOC concentration compared to the mid-deep layer water (t-test, p < 0.005), while the DBC concentrations were comparable. In addition, the excess DOC (relative to WSDW) in bottom water showed a close relation with POC in surface water. These results reveal a top-down control over the DOC concentration in bottom water through a quick sinking of diatom detritus and subsequently solubilization in bottom water and/or sediment. With an estimate, the WSDW carries 5.1 ± 2.8 Tg-C/yr of excess DOC out of the Powell Basin, representing an important carbon source to the deep Southern Ocean. This study highlights the key role of the Antarctic continental shelf pump in carbon sequestration.

Remediation of marine dead zones by enhancing microbial sulfide oxidation using electrodes

Marine dead zones caused by hypoxia have expanded over the last decades and pose a serious threat to coastal marine life. We tested sediment microbial fuel cells (SMFCs) for their potential to reduce the release of sulfide from sediments, in order to potentially protect the marine environment from the formation of such dead zones. Steel electrodes as well as charcoal-amended electrodes and corresponding non-connected controls of a size of together 24 m2 were installed in a marine harbour, and the effects on water quality were monitored for several months. Both pure steel electrodes and charcoal-amended electrodes were able to reduce sulfide concentrations in bottom water (92 % to 98 % reduction, in comparison to disconnected control steel electrodes). Also phosphate concentrations and ammonium were drastically reduced. SMFCs might be used to eliminate hypoxia at sites with high organic matter deposition and should be further investigated for this purpose.

Terrigenous dissolved organic matter input and nutrient-light-limited conditions on the winter microbial food web of the Beagle Channel

The viral shunt and microbial loop are trophic pathways in the microbial food web, recognized as major drivers of carbon and nutrient cycles in the sea. The Beagle Channel, located at the southernmost extreme of South America, is a subantarctic marine environment influenced by terrestrial runoff and glacial meltwater with high temporal and spatial variability. There is a knowledge gap on the trophic pathways and the microbial food web in the eastern part of the Channel. We herein present for the first time a comprehensive and spatially wide study of the effects of physicochemical and bio-optical parameters on the microbial food web along the Eastern Beagle Channel. The study took place under winter hydrographic conditions, featuring weak vertical stratification of the water column and low chlorophyll a concentrations (0.13 ± 0.07 mg m−3). Biogeochemical gradients were notable between the inner and outer (easternmost) sectors of the Channel, separated by a shallow sill at Mackinlay Strait. As a chemical water mass tracer, the presence of the highest concentrations in bottom waters of NO3 + NO2 in the inner channel and the difference in the T-S diagram indicate the Subantarctic Pacific origin of this water mass. Structural equation models showed that the specific UV absorbance of water (SUVA254, a proxy for the aromatic carbon content) and NO3 + NO2 were the main parameters affecting the microbial food web in surface/subsurface waters suggesting that prokaryotes biomass was partially supported by allochthonous dissolved organic matter (DOM) derived from river discharges. Our results show a major role of the viral shunt in surface/subsurface waters retaining nutrients in the basal levels of the food web. We suggest that the viral shunt in surface/subsurface waters and the water residence time west of Mackinlay Strait allows infected cells and viral lysis products to reach bottom waters propelling the production of recalcitrant and labile DOM there.

Cyanate dynamics under algal blooms and sediment resuspension events in a shallow micro-tidal estuary in the lower Chesapeake Bay

Although an emerging component in the marine nitrogen (N) cycle, cyanate concentrations and cycling have not been examined in estuarine systems to date. To better understand controls on cyanate concentrations in estuaries, time series data of cyanate and nutrient concentrations in the Lafayette River, a micro-tidal, sub-tributary of the lower Chesapeake Bay, were examined between June and September 2018, and May to September 2019. Cyanate concentrations ranged from near the detection limit (0.4 nmol L−1) to 82.9 nmol L−1 in 2018, and 6.8–207.4 nmol L−1 in 2019. Variations in cyanate concentrations were highly correlated with chlorophyll biomass in the summer, biomass degradation in early fall, and sediment resuspension that occurred in response to meteorological forcing. Cyanate concentrations increased after Chl a concentrations decreased suggesting algal decomposition as a source of cyanate. High cyanate concentrations in bottom waters, corresponded to wind-induced sediment resuspension events in the Lafayette River, again suggesting organic matter decomposition as a source of cyanate. Cyanate concentrations in sediment pore water varied between years; in summer 2018, cyanate concentrations were up to 150 nmol L−1, while in 2019, they were three times lower. To confirm an algal source for cyanate, a degradation experiment was conducted using Lafayette River water collected during a bloom of Margalefidinium polykrikoides in 2018. In dark incubation bottles, cyanate was one of the first labile organic nitrogen products produced, suggesting the contention that high concentrations of cyanate in late summer and fall were the result of organic matter decomposition. Neither cyanate nor ammonium accumulated in light bottles suggesting production and uptake are tightly coupled and microbes have a high affinity for cyanate in the light. In dark bottles, cyanate production rates were 6.8 nmol L−1 d−1, while microbial removal rates during the late phase of degradation were 1.5 nmol L−1 d−1, suggesting that cyanate may not be a preferred nitrogen substrate for microbes (including nitrifiers) in dark bottles or that microbes have a lower affinity for cyanate in the dark, allowing cyanate to reach steady state at concentrations greater than the detection limit.

The Peruvian oxygen minimum zone was similar in extent but weaker during the Last Glacial Maximum than Late Holocene

Quantifying past oxygen concentrations in oceans is crucial to improving understanding of current global ocean deoxygenation. Here, we use a record of pore density of the epibenthic foraminifer Planulina limbata from the Peruvian Oxygen Minimum Zone to reconstruct oxygen concentrations in bottom waters from the Last Glacial Maximum to the Late Holocene at 17.5°S about 500 meters below the sea surface. We found that oxygen levels were 40% lower during the Last Glacial Maximum than during the Late Holocene (about 6.7 versus 11.1 µmol/kg, respectively). A comparison with other reconstructions of oxygen concentrations in the region reveals a shallow Oxygen Minimum Zone during the Last Glacial Maximum that was similar in water depth and extent but weaker than during the Late Holocene. Increased glacial oxygen concentrations are probably related to lower temperatures (higher oxygen solubility), decreased nutrient and increased oxygen supply by source waters, and a decrease in coastal upwelling.

Occurrence, partition, and risk of four adjacent transition metals in seawater, sediments and demersal fish from the Pearl River Estuary, South China Sea.

The spatio-seasonal variation, bioaccumulation and potential ecological risk of four metals (Co, Ni, Cu and Zn) in the multi-media of the Pearl River Estuary (PRE) were assessed. The degree of contamination and ecological risk of the four metals followed this pattern: Cu>Zn>Ni>Co, upstream > downstream, wet season > dry season. There were non-linear and linear correlations for Co, linear and non-linear correlations for Ni, and non-linear and no correlation for both Cu and Zn, between metals in bottom water or bioavailable concentrations in sediment and fish muscle. Co and Ni showed lightly polluted levels; Cu and Zn showed heavily polluted levels. This study demonstrates a tendency of cross-accumulation in fish of these four transition metals within specific habitats of the PRE.

The Mo- and U-isotope signatures in alternating shales and carbonate beds of rhythmites: A comparison and implications for redox conditions across the Cambrian-Ordovician boundary

The Green Point Formation (GPF) of the Cow Head Group in western Newfoundland (Canada) includes the Global Boundary Stratotype Section and Point (GSSP) for the Cambrian-Ordovician (Є-O) boundary on Laurentia. This formation is composed primarily of thick, deep subtidal to slope deposits (rhythmites) of alternating lime mudstones and shale interbeds. Here, we employ isotopic and elemental data of Mo and U of these shales and carbonates to better understand their depositional environments and the open ocean redox conditions at the Є-O transition. The variability of δ98Moauth (−0.40 to +3.16‰) indicates variable aqueous sulfide concentrations ([H2S]aq) in bottom waters or porewaters during deposition of the shales. A significant positive correlation of shale [Mo]auth with [U]auth coupled with low Mo concentrations compared to Phanerozoic euxinic shales further points to weakly reducing (~dysoxic/suboxic to anoxic) benthic conditions with H2Saq primarily confined to porewaters. For the lime mudstones, the δ98Mo variability (−0.30 to +2.34‰) suggests variable [H2S]aq in porewaters during early diagenesis. Low Mo concentrations ([Mo]carb = 0.25 ± 0.80 μg/g, 2σ) and a poor correlation of [Mo]carb with δ98Mocarb indicate weak sulfidic ([H2S]aq < 11 μM) porewater conditions and possibly dysoxic to suboxic bottom environments. Thus, the δ98Mocarb may not track the δ98Mo of basin waters because of low [H2S]aq in porewaters and insignificant accumulation of authigenic Mo during early diagenesis. This explanation is consistent with a poor correlation between δ98Mocarb and the δ238U of basin waters (estimated from δ238Ucarb). A cautious examination of MoEF/UEF, δ98Moauth, and δ238Uauth of the shales suggests that the GPF was likely deposited in a basin with topographic barriers. During deposition of the lower and middle part of the studied interval, the barriers in combination with sea-level fluctuations may have caused partial isolation of the local basin from the open ocean. Rapid sea-level rises in the earliest Ordovician greatly increased the connectivity between the local basin and the open ocean, resulting in remarkable increases in MoEF/UEF, [Mo]auth, and [U]auth of the shales. The δ238U of the earliest Ordovician open oceans, inferred from δ238Ucarb, exhibited perturbations and an average value of −0.78 ± 0.33‰ (2σ), reflecting oceanic redox oscillations and expanded ocean euxinia compared to the modern oceans.

Effects of temperature and oxygen on 137Cs desorption from bottom sediment of a dam lake

The temperature and oxygen environment play important roles in the desorption of 137Cs from freshwater lake sediment to lake water. In this study, 12 quarterly surveys were performed to measure the dissolved 137Cs concentration in surface and bottom lake water, the vertical distribution of water temperature, and the dissolved oxygen (DO) concentration at the upstream, midstream, and downstream sites of the Yokokawa Dam Lake in Fukushima Prefecture, Japan. Higher concentrations of dissolved 137Cs were detected in the bottom water than in the surface water, especially in the summer and midstream lake regions at depths of 8–21 m owing to higher temperatures, which activated the bacterial decomposition of organic matter, and anaerobization, which enhanced the NH4+ in the pore water and 137Cs desorption from mineral particles. To compare the effects of anaerobization and increasing temperature on 137Cs desorption from sediment particles, intact sediment core samples were collected from the lake midstream and incubated for 1–14 days in a chamber under three controlled temperature and oxygen environment conditions: aerobic +10 °C; anaerobic +10 °C; and anaerobic +20 °C. The vertical distribution of 137Cs in the sediment pore water showed a similar profile as NH4+ and K+, and both the increased temperature and reduced DO concentration enhanced the 137Cs desorption. A comparison of the standard partial regression coefficients of temperature and DO concentration in the multiple regression equation for the 137Cs concentration in pore water shows that the reduction of DO from saturation to zero at the water–sediment interface accelerated the desorption of 137Cs more strongly than did the temperature rise from 10 to 20 °C. The experimental results show a nearly inverse proportional relationship between NH4+ and the distribution coefficients of 137Cs after thermodynamic correction, except in the surface layer. These findings indicate that the 137Cs concentration in pore water can be explained by the temperature-dependent chemical distribution between the solid–aqueous phase and its equilibrium with NH4+.

Temporal and spatial variations in benthic nitrogen cycling in a temperate macro-tidal coastal ecosystem: Observation and modeling

We used field observations, laboratory measurements and a reactive transport model (RTM) to investigate temporal and spatial variations in benthic nitrogen (N) cycling in the eutrophic temperate macro-tidal Vilaine Bay (VB), France. A time series of benthic flux measurements and pore-water profiles of dissolved inorganic N (DIN: ammonium, nitrate, nitrite) and dissolved organic N (DON) was conducted at a single station between April and September 2015 (six times). A spatial investigation of the benthic fluxes was performed in July 2016 at this station and three other stations in the VB. All measurements were accompanied by a large panel of physical, chemical and biological descriptors in the water column. In 2015, benthic ammonium fluxes at the monitoring station varied between 75 μmol m−2 h−1 in spring and were less than 10 μmol m−2 h−1 in summer. The benthic DON fluxes co-varied with the ammonium fluxes, ranging from 100 μmol m−2 h−1 in spring to zero in summer. In the summer of 2016, a phytoplankton bloom occurred and as a result the benthic ammonium and DON fluxes reached higher values than in the spring of 2015, accompanied by bottom water hypoxia at one measured station. Benthic nitrate and nitrite fluxes varied between −31 (towards the sediments) and 22 μmol m−2 h−1 and were explained by the bottom water concentration and nitrification rates. After fitting the existing pore-water profiles, the applied RTM correctly simulated the temporal and spatial variations in the benthic DIN fluxes and predicted that a large part of the deposited organic matter (OM) is remineralized aerobically at the sediment-water interface (SWI). The overall results showed a synthetic pattern of benthic N cycling in the VB, based on the occurrence of diatom blooms as the main source of OM in the sediments. The rapid decomposition of this deposited diatom material at the SWI releases large amounts of ammonium and DON to the water column and rapidly consumes oxygen at the sediment surface. When blooms occur in summer, their decomposition can be followed by hypoxia/anoxia in the bottom water. When blooms are absent, benthic N fluxes are weak and mainly fed by the diffusion from the pore-water. By integrating the present results in a 3D ecological model, it should be possible to more accurately predict the development of bottom water hypoxia in the VB.

Copper-binding ligands in deep-sea pore waters of the Pacific Ocean and potential impacts of polymetallic nodule mining on the copper cycle

The release of potentially toxic metals, such as copper (Cu), into the water column is of concern during polymetallic nodule mining. The bioavailability and thus toxicity of Cu is strongly influenced by its speciation which is dominated by organic ligand (L) complexation in seawater, with L-complexes being considered less bioavailable than free Cu2+. The presence of CuL-complexes in deep-sea sediments has, however, not been systematically studied in the context of deep-sea mining. We thus analyzed the Cu-binding L concentration ([L]) in deep-sea pore waters of two polymetallic nodule provinces in the Pacific Ocean, the Peru Basin and the Clarion-Clipperton-Zone, using competitive ligand equilibration–adsorptive stripping voltammetry. The pore-water dissolved Cu concentration ([dCu]) ranged from 3 to 96 nM, generally exceeding bottom water concentrations (4–44 nM). Based on fitting results from ProMCC and Excel, Cu was predominantly complexed by L (3–313 nM) in bottom waters and undisturbed pore waters. We conclude that processes like deep-sea mining are unlikely to cause a release of toxic Cu2+ concentrations ([Cu2+]) to the seawater as > 99% Cu was organically complexed in pore waters and the [Cu2+] was < 6 pM for 8 of 9 samples. Moreover, the excess of L found especially in shallow pore waters implied that even with a Cu release through mining activities, Cu2+ likely remains beneath toxic thresholds.

Characterization of marine microbial communities around an Arctic seabed hydrocarbon seep at Scott Inlet, Baffin Bay

Seabed hydrocarbon seeps present natural laboratories for investigating responses of marine ecosystems to petroleum input. A hydrocarbon seep near Scott Inlet, Baffin Bay, was visited for in situ observations and sampling in the summer of 2018. Video evidence of an active hydrocarbon seep was confirmed by methane and hydrocarbon analysis of the overlying water column, which is 260 m at this site. Elevated methane concentrations in bottom water above and down current from the seep decreased to background seawater levels in the mid-water column >150 m above the seafloor. Seafloor microbial mats morphologically resembling sulfide-oxidizing bacteria surrounded areas of bubble ebullition. Calcareous tube worms, brittle stars, shrimp, sponges, sea stars, sea anemones, sea urchins, small fish and soft corals were observed near the seep, with soft corals showing evidence for hydrocarbon incorporation. Sediment microbial communities included putative methane-oxidizing Methyloprofundus, sulfate-reducing Desulfobulbaceae and sulfide-oxidizing Sulfurovum. A metabolic gene diagnostic for aerobic methanotrophs (pmoA) was detected in the sediment and bottom water above the seep epicentre and up to 5 km away. Both 16S rRNA gene and pmoA amplicon sequencing revealed that pelagic microbial communities oriented along the geologic basement rise associated with methane seepage (running SW to NE) differed from communities in off-axis water up to 5 km away. Relative abundances of aerobic methanotrophs and putative hydrocarbon-degrading bacteria were elevated in the bottom water down current from the seep. Detection of bacterial clades typically associated with hydrocarbon and methane oxidation highlights the importance of Arctic marine microbial communities in mitigating hydrocarbon emissions from natural geologic sources.

Hydrocarbon-seep deposits in the lower Permian Angie Formation, Central Lhasa Block, Tibet

Methane seepage along continental margins has been linked to climatic warming, yet it is uncertain whether such seepage took place during the Permian, a period that witnessed a transition from an icehouse to a greenhouse world. To address this issue, we investigated calcareous concretions and layers within mudstones/shales of the Kungurian Angie Formation in the Xainza area, central Tibet. A combination of petrographic, mineralogical, elemental, and carbon‑sulfur isotopic analyses provide strong evidence for a hydrocarbon-seep origin of these carbonates, with thermogenic methane and crude oil being the most likely carbon sources. Our dataset also indicates that, although fueled by a common deep-buried hydrocarbon reservoir, these layers and concretions experienced different biogeochemical processes due to variable rates of cold-seep fluid flux combined with persistently low sulfate concentrations in bottom waters of the Bangong-Nujiang Tethyan Ocean. A biogeochemical switch from sulfate-dependent to iron oxide-coupled anaerobic oxidation of methane in the sulfate-methane transition zone, likely triggered by sulfate exhaustion, was inferred to affect carbonate layer formation near the sediment-water interface. If true, this switch, together with common vent structures, would have resulted in escape of large amounts of methane directly to the ocean-atmosphere system. The hydrocarbon seeps in the central Lhasa Block was approximately coeval with rapid climate warming in the peri-Gondwanan region during the early Kungurian. Although further investigations are required in the future, this study demonstrates a possible causal relationship between regional tectonic evolution, hydrocarbon seepage, and global climate change.

Heavy Metals in Macrozoobenthos and Sediments of the Coastal Zone of the Eastern Gulf of Finland

The Zn, Cd, Pb, Cu, Mn, and Fe concentrations in sediments, bottom waters, and benthic macroinvertebrates (Amphipoda, Bivalvia, Gastropoda, Hirudinea, and Oligochaeta) collected at six coastal stations in the Eastern Gulf of Finland were determined with the aim to reveal the features of heavy metals bioaccumulation in the macrozoobenthos. It was shown that benthic macroinvertebrates accumulate Mn, Fe, and Zn more actively than Pb, Cu, and Cd, while Hirudinea and Oligochaeta worms have enhanced accumulative capacities as compared to amphipods and mollusks. A close relationship was found between the Zn and Pb concentrations in the sediments and amphipods, but for the other elements no reliable relationship was observed. It was recommended to use organisms of other trophic levels along with zoobenthos and data on heavy metal concentrations in sediments for chemical monitoring of coastal waters.

Seasonal dynamics of the activities of dissolved 137Cs and the 137Cs of fish in a shallow, hypereutrophic lake: Links to bottom-water oxygen concentrations

Remobilization of radiocesium from anoxic sediments can be an important mechanism responsible for long-term contaminations of lakes. However, it is unclear whether such remobilization occurs in shallow lakes, where concentrations of dissolved oxygen in the hypolimnion (bottom DO) change temporally in response to meteorological conditions, and whether remobilized radiocesium influences the activity in fish. We examined the seasonal dynamics of the activities of dissolved 137Cs and 137Cs in fish (pond smelt and crucian carp) from Lake Kasumigaura, a shallow, hypereutrophic lake, five years after the Fukushima Daiichi Nuclear Power Plant accident. The activities of both dissolved 137Cs and 137Cs in fish declined during that time, but the declines showed a clear seasonal pattern that included a summer peak of 137Cs activity. The activity of dissolved 137Cs increased when the bottom DO concentration decreased, and a nonlinear causality test revealed significant causal forcing of dissolved 137Cs activity by bottom DO. The fact that NH4-N concentrations in bottom waters were higher in the summer suggested that remobilization of 137Cs from sediments could result from highly selective ion-exchange with NH4-N. Despite the shallow depth of Lake Kasumigaura, winds had little influence bottom DO concentrations or dissolved 137Cs activities. The fact that seasonal means of 137Cs activities in pond smelt and crucian carp were positively correlated with the seasonal means of dissolved 137Cs activities suggested that remobilized 137Cs may have influenced the seasonal dynamics of radiocesium in fish through food-chain transfer, but higher feeding rates in warm water could may have also contributed to the seasonal dynamics of 137Cs activity in fish. Our findings suggest that in shallow lakes, intermittent but repeated hypoxic events may enhance remobilization of radiocesium from sediments, and remobilized radiocesium may contributed to long-term retention of radiocesium in aquatic organisms.

Effects of hypolimnetic oxygenation on fish tissue mercury in reservoirs near the new Almaden Mining District, California, USA.

Almaden, Calero, and Guadalupe reservoirs (San Jose, CA, USA) are small (<13 million m3) surface water reservoirs polluted by the former New Almaden Mining District, North America's most productive historical mercury (Hg) mine. Stevens Creek Reservoir (Cupertino, CA, USA) also has elevated fish Hg concentrations, but no historical mining source. We report a 15-year dataset to evaluate the effectiveness of line diffuser hypolimnetic oxygenation systems (HOSs) in reducing methylmercury (MeHg) concentrations in reservoir water and fish after four consecutive years of operation. HOSs were installed in each reservoir to increase dissolved oxygen concentrations in bottom water, thereby suppressing the activity of anaerobic bacteria (e.g., sulfate-reducing bacteria) known to produce MeHg. Before HOS operation, MeHg concentrations increased in bottom waters of all four reservoirs during periods of thermal stratification and profundal hypoxia. MeHg concentrations decreased significantly in bottom waters during HOS operation, with mean reductions of 63%-85% below pre-oxygenation concentrations. However, MeHg concentrations were unchanged or increased in surface waters. This could be the result of enhanced mixing between surface and bottom waters as a result of line diffuser oxygenation, or continued Hg methylation occurring in the oxic water column and littoral sediments. Despite little change in whole water column MeHg concentrations, we observed modest but significant declining trends in fish tissue Hg in Guadalupe and Stevens Creek reservoirs. Results suggest that oxygenation, rather than directly lowering MeHg in water, may have mixed nutrients into surface waters, thereby enhancing primary productivity and indirectly affecting Hg bioaccumulation by diluting concentrations in phytoplankton.