Cold Seeps Research Articles

Trace and Rare Earth Element Fingerprints of Aerobic Oxidation of Methane in Seep‐Dwelling Bivalves

AbstractAerobic methanotrophic bacteria are pivotal in the global carbon cycle by converting methane into biomass and inorganic carbon species. Light rare earth elements (light‐REE; La, Ce) are part of the metalloenzymes mediating the biochemical processes in methanotrophs. However, the partitioning of trace metals and REE in chemosymbiotic megafauna with methanotrophic endosymbionts remains largely unknown. Here we determine stable isotope compositions (δ13C, δ15N) of soft tissues (gill, mantle, foot) as well as trace metal and REE contents of soft tissues and shells of chemosymbiotic bivalves dwelling at methane seeps (Site F and Haima seeps) of the South China Sea. Isotopic compositions of soft tissues are found to reflect the mode of chemosymbiosis (i.e., methanotrophy vs. thiotrophy). Particularly gill tissues of bivalves with methanotrophic endosymbionts display marked light‐REE and trace metal enrichments, which is interpreted to reflect the enzymatic activity of endosymbionts. Additionally, correlations between light‐REE and copper (Cu), zinc (Zn), and molybdenum (Mo) contents in soft tissues of chemosymbiotic mussels are ascribed to the uptake of these elements by the methanotrophic symbionts. An observed higher trace metal content in the tissue of the semi‐infaunal and infaunal bivalves with thiotrophic endosymbionts is believed to reflect the uptake of metals associated with sulfide particles. This study documents diagnostic enrichment of trace metals and light‐REE in the soft tissues of bivalves with chemotrophic endosymbionts and provides new constraints for future identification of chemosymbiosis at ancient seeps.

Structural control on carbon emissions at the Nirano mud volcanoes – Italy

The Nirano Salse in Italy is a well-studied site where natural gas seepage (NGS) and other hydrocarbon fluids and gases are emitted at the earth's surface. A novel integrated approach is applied to define a comprehensive structural interpretation of the gas seepage and flow dynamic in the mud volcano system of the Nirano Salse Regional Nature Reserve (Modena, Northern Apennines). The paper investigates the relationship between gas emissions and local structures, particularly faults and fractures, in the shallow subsurface (down to 500–600 m depth) to understand the control that structures have on fluid ascent from deep leaky hydrocarbon traps. We performed continuous monitoring of mud levels within vents; carried out geological surveys to characterize the main stratigraphic and structural discontinuities; measured the carbon emissions (CH4 and CO2) seepage both from volcanoes and the surrounding soil by a portable gas fluxmeter; and integrated the results with available geophysical surveys. The authors argue that the transgressive Pleistocene-Pliocene Argille Azzurre Formation hides the complex and highly structured pre-Pliocene geology of the area, in which faults and fractures act as pathways for deep fluid ascent. The emissions are aligned along a NE-SW trend at the intersection of a NE-SW fracture system and NW-SE-oriented normal faults, which are both associated to the local tensional stress field of a likely left-lateral strike-slip transfer structure or in the extrados of a fold. By examining both natural gas macroseepage and diffuse flux, it is shown that local structures control the fluid ascent and contribute to the emission of hydrocarbon gases and fluids at the Earth's surface. Understanding the structural control of carbon emissions at the Nirano Salse is also important for evaluating the carbon budget at the site, particularly in areas where there are detectable surface emissions. The study has implications for geologic, environmental, and economic issues, including hydrocarbon exploration, hazard assessment, and impact on the atmospheric carbon budget. Furthermore, the outcomes have an important implication to evaluate the potential for dangerous abrupt mud eruptions, and the site safety in proximity to the mud volcanoes.

Trophic relationship between mussels and scale worms under various seepage intensities in the haima cold seep: Insights from stable isotopes (δ13C and δ15N) and C:N:P stoichiometry

The deep-sea mussels Gigantidas haimaensis (Mytilidae: Bathymodiolinae) usually contain one scale worm Branchipolynoe pettiboneae in their mantle cavity in the Haima cold seep, South China Sea. To explore their environmental adaptation and coexistence mechanisms, the stable isotopes (δ13C and δ15N) and C:N:P ecological stoichiometry of G. haimaensis and the associated B. pettiboneae were investigated under different methane seepage intensities. In the presence of seepage, most mussels harbored one scale worm in their mantle cavity. However, under seepage cease, the physiological status of mussels looked unhealthy, and no scale worm appeared in their bodies. The variation in δ13C values was great among different mussel tissues, ranging from −49.7‰ to −57.8‰. The δ13C values of mussel tissues followed the order of foot > gill > mantle under active seepage, while no regular trend was found under seepage cease. The δ13C and δ15N of scale worms were averagely enriched by 2.9 ‰ and 3.2 ‰ relative to their mussel hosts, and the trophic niche separation between scale worms and mussels was more significant under active seepage. The δ13C value of mussel foot was significantly higher under active seepage (average −50.8‰) than under seepage cease (average −55.1‰), indicating that mussels might ingest and assimilate more 13C-rich suspended particulate organic matter (POM) under active seepage. In mantle tissues, the high C:N ratio and low δ13C value should be attributed to the high content of energy storage substances. The C:N ratio of mantle dramatically declined with the exhaustion of energy storage materials under seepage cease. The variation of P content was most significant in the gill, which might be regulated by the abundance of symbiotic bacteria. It can be speculated that the dramatic decline of P content in the gill was attributed to the loss of high-P bacterial symbionts under seepage cease. Both the C:P and N:P ratios of gill tissues significantly increased under seepage cease, which suggested the decline of food quality for scale worms. Mussel-dwelling scale worms might detect the change of food quality and abandon their starving mussel host. Our results help to better understand the environmental adaptation of chemoautotrophic mussels and scale worms under unstable seepage intensity in cold seep ecosystems.

Investigation on damage-permeability model of dual-porosity coal under thermal-mechanical coupling effect

Understanding the permeability characteristics of coal under mining disturbance is crucial for ensuring coal mine safety and efficient gas extraction. Coal is a dual porosity medium consisting of matrix and fracture, and there are some differences in mechanics and permeability between matrix pores and fracture. However, research on the damage and gas seepage characteristics of dual-porosity coal is limited. In this paper, a coupled damage-permeability model considering matrix pores and fractures in coal is established, incorporating the influences of effective stress, gas adsorption, thermal expansion, and thermal cracking. And the reliability of the newly developed model was verified using experimental data of temperature and pressure variations under different stress boundary conditions. The results indicate that during the elastic stage, the total permeability and fracture permeability of coal exhibit a trend of decreasing and then increasing with pore pressure, while matrix permeability gradually increases. In the full stress-strain stage, the overall damage-permeability and fracture damage-permeability of coal exhibit an “S"-shaped variation trend with axial strain, while matrix damage-permeability shows a slow increasing trend. To accurately characterize the evolution of gas seepage in coal, contributions from matrix pores and fractures must be comprehensively considered, and the effects of gas adsorption, effective stress, thermal cracking, and thermal expansion on permeability cannot be ignored. Furthermore, through parameter sensitivity analysis, it is found that during the elastic stage, coal permeability decreases with an increase in the adsorption sensitivity coefficient and thermal expansion coefficient. In the full stress-strain process, coal permeability increases with an increase in the damage-permeability coefficient. The results of this study are of great significance for predicting the evolution of coal permeability in the working face ahead under mining disturbance.

Exploring the depths: Metagenomic profiling of cold seep bacterial communities and isolation of hydrocarbon-degrading microbes from the Cauvery-Mannar basin

Cold seeps are unique ecosystems characterized by the presence of endemic communities reliant on microorganisms that survive through chemosynthesis, utilizing sulfide, methane, hydrocarbons, and other reduced compounds. Recently, a cold seep was discovered in the Cauvery-Mannar basin during an expedition aboard the research vessel ORV Sindhu Sadhana which led to the investigation into the distribution and isolation of microorganisms within the sediment layers (0–6 cm). Metagenomic analysis based on V3-V4 region of the sediment's bacterial community revealed a prevalence of bacteria associated with methane and hydrocarbons. Dominant phyla included Campilobacterota (20–25%), Proteobacteria (15–25%), Firmicutes (15-20%), Desulfobacterota (10–18%), Chloroflexi (5–10%), and Actinobacterota (5–7%). In-vitro isolation of hydrocarbon-degrading bacteria using minimal media supplemented with 1% crude oil gave rise to 15 bacterial strains. Molecular identification of six isolates (chosen based on the dense growth obtained in agar plates) revealed their affiliation with the genera Acinetobacter, Rhodococcus, Acinetobacter, and Bacillus. This comprehensive study constitutes the first report on the bacterial community structure found in sediments from a cold seep of Cauvery-Mannar basin, along with the successful isolation and identification of hydrocarbon-degrading bacteria.

Bioaccumulation and Trophic Transfer of Short-, Medium-, and Long-Chain-Chlorinated Paraffins in Benthic Biotas of the Haima Cold Seep, South China Sea

Bioaccumulation and Trophic Transfer of Short-, Medium-, and Long-Chain-Chlorinated Paraffins in Benthic Biotas of the Haima Cold Seep, South China Sea

Microeukaryotes have unexpected importance in cold seep food webs through predation and parasitism

Cold seeps are inhabited by a high abundance of chemosynthetic prokaryotes and invertebrates. However, a complete food web between invertebrates and prokaryotes cannot be established without the important link of microeukaryotes. Yet, question such as which microeukaryotes are at work in cold seeps is still mostly open. We, therefore, compared the biological communities in cold seeps and adjacent areas of the South China Sea by the metabarcoding and quantitative PCR (qPCR) of nuclear 18S and 16S rRNA genes, and estimated the potentially ecological roles of microeukaryotes by cross-domain microbial network analysis. The anaerobic methane oxidizing archaea (ANME), sulfate-reducing bacteria (SRB) and Radiolaria accounted for most variations between microbial communities from the cold seeps and adjacent areas. A significant correlation was detected between the copy numbers of 18S rRNA gene and communities of the ANME and SRB in cold seeps, indicating close links between microeukaryotes and chemosynthetic prokaryotes. After adding microeukaryotic nodes microbial network became more complex, and the negative correlation proportion and average path length increased, suggesting increased predation and parasitism. The ANME and SRB groups were mainly negatively correlated with and potentially preyed by Acantharea and Cercozoa, which were then negatively correlated with annelids and mollusks. Parasitic microeukaryotes of Syndiniales widely existed in cold seeps and had close relationships with Polycystinea. Parasitism might cause the death of Polycystinea and the release of Syndiniales spores, which may be an important carbon source for the cold seep biota. Using Mantel analyses, we also demonstrated that trophic relationships are closer in cold seeps than in the adjacently deep-sea sediments, and that microeukaryote-metazoans linkages in cold seeps are closer than those between prokaryotes and metazoans. Overall, our study outlines the main components of food webs in cold seeps and highlights that microeukaryotes have an unexpected importance in the carbon cycle.

Biogeochemical investigations of methane seepage at the ultraslow-spreading Arctic mid-ocean ridge: Svyatogor ridge, Fram Strait

The Arctic continental shelves are important reservoirs of methane stored in gas hydrates and deeper geological formations. However, little is known about methane dynamics in deeper oceanic settings such as mid-ocean ridges, mainly due to operational challenges related to their remoteness. This study investigates a recently discovered methane seepage environment at Svyatogor Ridge, a sediment-covered transform fault on the western flank of the Arctic mid-ocean ridge west of Svalbard. Svyatogor Ridge was previously hypothesized to host deep gas hydrates and active fluid flow systems, which is a unique combination for this setting and requires geochemical evidence. Based on sediment and foraminiferal geochemistry, we demonstrate that Svyatogor Ridge hosts a shallow methane cycle with anaerobic oxidation of microbial methane, sustaining chemosynthetic communities at the seafloor. Our geochemical datasets also suggest that methane fluxes were higher in the past than today and long-lasting, with episodes of intense methane oxidation recorded in pre-Holocene sediments. Methane seepage is still ongoing at this location, although no evidence exists that methane is reaching the sea surface. The results provide the first evidence of a methane seepage environment ever reported from the ultra-slow spreading Arctic mid-ocean ridge, thus calling for a reevaluation of the role of this type of ridge in the ocean methane cycle.

Characterizing the Oxygen Minimum Zone (OMZ) in the Costa Rican Eastern Tropical Pacific using in situ data from field campaigns

For conservation and sustainable fisheries, it is important to characterize the Oxygen Minimum Zones or OMZ in and around the methane seeps of the Eastern Tropical Pacific (ETP), Costa Rica, through the analysis of temperature, salinity, density, and oxygen profiles. The data used in this work were collected during several oceanographic research campaigns in the Pacific continental margin and offshore of Costa Rica, between 2009 and 2019, using a CTDs, as the profiler of physical parameters of the water column. In general, it was observed that dissolved oxygen gradually decreases with depth to the thermocline, then its concentration decreases more rapidly and remains low, indicating the presence of the OMZ and tends to increase slightly at greater depths. Mean vertical extension of the OMZ near and around the seeps was 763 m and the mean depth for the minimum dissolved oxygen value was 393 m. Spatial differences of measurements taken at stations near the methane seeps were calculated with respect to the measurements at the station located above them. Overall, a greater variability of the oxygen anomalies was observed within the mixed layer, while under the thermocline their values remain stable and around zero.

Monitoring underwater volcano degassing using fiber-optic sensing

Continuous monitoring of volcanic gas emissions is crucial for understanding volcanic activity and potential eruptions. However, emissions of volcanic gases underwater are infrequently studied or quantified. This study explores the potential of Distributed Acoustic Sensing (DAS) technology to monitor underwater volcanic degassing. DAS converts fiber-optic cables into high-resolution vibration recording arrays, providing measurements at unprecedented spatio-temporal resolution. We conducted an experiment at Laacher See volcano in Germany, immersing a fiber-optic cable in the lake and interrogating it with a DAS system. We detected and analyzed numerous acoustic signals that we associated with bubble emissions in different lake areas. Three types of text-book bubbles exhibiting characteristic waveforms are all found from our detections, indicating different nucleation processes and bubble sizes. Using clustering algorithms, we classified bubble events into four distinct clusters based on their temporal and spectral characteristics. The temporal distribution of the events provided insights into the evolution of gas seepage patterns. This technology has the potential to revolutionize underwater degassing monitoring and provide valuable information for studying volcanic processes and estimating gas emissions. Furthermore, DAS can be applied to other applications, such as monitoring underwater carbon capture and storage operations or methane leaks associated with climate change.

The diversification and potential function of microbiome in sediment-water interface of methane seeps in South China Sea.

The sediment-water interfaces of cold seeps play important roles in nutrient transportation between seafloor and deep-water column. Microorganisms are the key actors of biogeochemical processes in this interface. However, the knowledge of the microbiome in this interface are limited. Here we studied the microbial diversity and potential metabolic functions by 16S rRNA gene amplicon sequencing at sediment-water interface of two active cold seeps in the northern slope of South China Sea, Lingshui and Site F cold seeps. The microbial diversity and potential functions in the two cold seeps are obviously different. The microbial diversity of Lingshui interface areas, is found to be relatively low. Microbes associated with methane consumption are enriched, possibly due to the large and continuous eruptions of methane fluids. Methane consumption is mainly mediated by aerobic oxidation and denitrifying anaerobic methane oxidation (DAMO). The microbial diversity in Site F is higher than Lingshui. Fluids from seepage of Site F are mitigated by methanotrophic bacteria at the cyclical oxic-hypoxic fluctuating interface where intense redox cycling of carbon, sulfur, and nitrogen compounds occurs. The primary modes of microbial methane consumption are aerobic methane oxidation, along with DAMO, sulfate-dependent anaerobic methane oxidation (SAMO). To sum up, anaerobic oxidation of methane (AOM) may be underestimated in cold seep interface microenvironments. Our findings highlight the significance of AOM and interdependence between microorganisms and their environments in the interface microenvironments, providing insights into the biogeochemical processes that govern these unique ecological systems.

Deep-large faults controlling on the distribution of the venting gas hydrate system in the middle of the Qiongdongnan Basin, South China Sea

Many locations with concentrated hydrates at vents have confirmed the presence of abundant thermogenic gas in the middle of the Qiongdongnan Basin (QDNB). However, the impact of deep structures on gas-bearing fluids migration and gas hydrates distribution in tectonically inactive regions is still unclear. In this study, the authors apply high-resolution 3D seismic and logging while drilling (LWD) data from the middle of the QDNB to investigate the influence of deep-large faults on gas chimneys and preferred gas-escape pipes. The findings reveal the following: (1) Two significant deep-large faults, F1 and F2, developed on the edge of the Songnan Low Uplift, control the dominant migration of thermogenic hydrocarbons and determine the initial locations of gas chimneys. (2) The formation of gas chimneys is likely related to fault activation and reactivation. Gas chimney 1 is primarily arises from convergent fluid migration resulting from the intersection of the two faults, while the gas chimney 2 benefits from a steeper fault plane and shorter migration distance of fault F2. (3) Most gas-escape pipes are situated near the apex of the two faults. Their reactivations facilitate free gas flow into the GHSZ and contribute to the formation of fracture - filling hydrates.©2024 China Geology Editorial Office.

Evolution of Permeability and Gas Seepage in Deep Coal Under 3D Stress

Evolution of Permeability and Gas Seepage in Deep Coal Under 3D Stress

Potential Underestimate in Reported Bottom-up Methane Emissions from Oil and Gas Operations in the Delaware Basin

Methane is a greenhouse gas and identified as a key driver of near-term climate change. Bottom-up approaches estimate annual methane loss from US natural gas production and transport at 6 Tg, but recent studies suggest this may be an underestimate. To investigate this possibility, an equipment-based emissions inventory, using EPA emission factors, was developed to calculate methane emissions from oil and gas operations in the Delaware basin, USA. Emission factors and activity data were then updated using contemporary and region-specific measurement data. The original inventory estimated emissions at 315 Gg CH4 y−1 (gas production-normalized rate of 0.6% loss), while the updated inventory estimated emissions of 1500 Gg CH4 y−1 (2.8% loss). The largest changes resulted from large fugitive emissions from oil production (+430 Gg CH4 y−1), updating maintenance activity emissions (+214 Gg CH4 y−1), considering flaring inefficiency (+174 Gg CH4 y−1), and the inclusion of associated gas venting (+136 Gg CH4 y−1). This study suggests that a systematic underestimate probably exists in current bottom-up inventories and identifies sources currently missing or may be incorrect. We also strongly recommend that emission factors should be validated through direct comparison against measurement campaigns that include long-tail distributions typical of oil and gas activities.

Fracture Network Influence on Rock Damage and Gas Transport following an Underground Explosion

Simulations of rock damage and gas transport following underground explosions that omit preexisting fracture networks in the subsurface cannot fully characterize the influence of geo-structural variability on gas transport. Previous studies do not consider the impact that fracture network structure and variability have on gas seepage. In this study, we develop a sequentially coupled, axi-symmetric model to look at the damage pattern and resulting gas breakthrough curves following an underground explosion given different fracture network realizations. We simulate 0.327 and 0.164 kT chemical explosives with burial depths of 100 m for 90 stochastically generated fracture networks. Gases quickly reach the surface in 30% of the higher yield simulations and 5% of the lower yield simulations. The fast breakthrough can be attributed to the formation of connected pathways between fractures to the surface. The formation of a connected damage pathway to the surface is not clearly correlated with the fracture intensity (P32) in our simulations. Breakthrough curves with slower transport are highly variable depending on the fracture network sample. The variability in the breakthrough behavior indicates that ignoring the influence of fracture networks on rock damage, which strongly influences the hydraulic properties following an underground explosion, will likely lead to a large underestimation of the uncertainty in the gas transport to the surface. This work highlights the need for incorporation of fracture networks into models for accurately predicting gas seepage following underground explosions.

Automatic target recognition and geolocalisation of natural gas seeps using an autonomous underwater vehicle

Autonomous Underwater Vehicles (AUVs) have proven to be a precious resource for oceans preservation thanks to their ability to accomplish survey missions on wide areas without the need of human supervision. This also includes the detection and mapping of underwater gas emissions, whether these are due to damaged offshore structures or naturally released from the seafloor. Indeed, submarine seepage have severe repercussions on the surrounding marine habitat and contribute to greenhouse gas emissions, hence the need to identify and monitor them. This work focuses on the online Automatic Target Recognition (ATR) and geolocalisation of underwater gas leakages utilising an AUV equipped with a Forward Looking Sonar (FLS). With the aim to accurately position the seeps, it was necessary to address the problem of navigating the vehicle in areas with the presence of gas bubbles. In fact, bubbles adversely affect acoustic sensors typically exploited by underwater robots for navigation. The paper investigates the effects that seepage have on the performance of navigation solutions based on two different acoustic sensors: a Doppler Velocity Log (DVL) sensor and an Ultra-Short BaseLine (USBL) device. Afterwards, a solution to autonomously recognise in real-time gas seeps on FLS acoustic imagery utilising a Single Shot MultiBox Detector (SSD) is presented. The relative position of the detected seep with respect to the vehicle is then retrieved and combined with the estimated AUV position to obtain the geodetic location of the seep. Finally, the proposed algorithm was tested during at sea experiments, where gas leaks were artificially reproduced, and the achieved results prove the validity of the proposed method to autonomously detect and accurately geolocate underwater seeps in real time using an AUV.

Physiological and metabolic insights into the first cultured anaerobic representative of deep-sea Planctomycetes bacteria.

Planctomycetes bacteria are ubiquitously distributed across various biospheres and play key roles in global element cycles. However, few deep-sea Planctomycetes members have been cultivated, limiting our understanding of Planctomycetes in the deep biosphere. Here, we have successfully cultured a novel strain of Planctomycetes (strain ZRK32) from a deep-sea cold seep sediment. Our genomic, physiological, and phylogenetic analyses indicate that strain ZRK32 is a novel species, which we propose be named: Poriferisphaera heterotrophicis. We show that strain ZRK32 replicates using a budding mode of division. Based on the combined results from growth assays and transcriptomic analyses, we found that rich nutrients, or supplementation with NO3- or NH4+ promoted the growth of strain ZRK32 by facilitating energy production through the tricarboxylic acid cycle and the Embden-Meyerhof-Parnas glycolysis pathway. Moreover, supplementation with NO3- or NH4+ induced strain ZRK32 to release a bacteriophage in a chronic manner, without host cell lysis. This bacteriophage then enabled strain ZRK32, and another marine bacterium that we studied, to metabolize nitrogen through the function of auxiliary metabolic genes. Overall, these findings expand our understanding of deep-sea Planctomycetes bacteria, while highlighting their ability to metabolize nitrogen when reprogrammed by chronic viruses.

Physiological and metabolic insights into the first cultured anaerobic representative of deep-sea Planctomycetes bacteria

Planctomycetes bacteria are ubiquitously distributed across various biospheres and play key roles in global element cycles. However, few deep-sea Planctomycetes members have been cultivated, limiting our understanding of Planctomycetes in the deep biosphere. Here, we have successfully cultured a novel strain of Planctomycetes (strain ZRK32) from a deep-sea cold seep sediment. Our genomic, physiological, and phylogenetic analyses indicate that strain ZRK32 is a novel species, which we propose be named: Poriferisphaera heterotrophicis. We show that strain ZRK32 replicates using a budding mode of division. Based on the combined results from growth assays and transcriptomic analyses, we found that rich nutrients, or supplementation with NO3- or NH4+ promoted the growth of strain ZRK32 by facilitating energy production through the tricarboxylic acid cycle and the Embden-Meyerhof-Parnas glycolysis pathway. Moreover, supplementation with NO3- or NH4+ induced strain ZRK32 to release a bacteriophage in a chronic manner, without host cell lysis. This bacteriophage then enabled strain ZRK32, and another marine bacterium that we studied, to metabolize nitrogen through the function of auxiliary metabolic genes. Overall, these findings expand our understanding of deep-sea Planctomycetes bacteria, while highlighting their ability to metabolize nitrogen when reprogrammed by chronic viruses.

Geochemistry of iron and trace metals in seep carbonates of the middle Okinawa Trough impacted by hydrothermal plumes

Geochemistry of iron and trace metals in seep carbonates and its dependence on anaerobic oxidation of methane (AOM)-driven diagenesis in cold seeps are not well documented. Here we characterize geochemistry of Fe and trace metals in carbonate nodules collected at an active cold seep in the middle Okinawa Trough (OT) impacted by hydrothermal plumes. The carbonate nodules in the studied core are dominated by aragonite even at sulfate-low core bottom. Most aragonite at the core bottom may have formed much earlier close to the seafloor under high sulfate concentration. Additionally, porewater at the depth may not be developed to a condition for the formation of calcite dominating over aragonite due to constant downward replenishment of sulfate across through shallow SMTZ (40 cm below sediment surface). As indicated by δ13C and δ18O of the carbonates, anaerobic oxidation of thermogenic methane from gas hydrate dissociation is the dominant fluid source for carbonate precipitation. It can be inferred from the existence of vivianite that Fe reduction coupled to AOM (Fe-AOM) may have ever been prevalent at the hydrothermal Fe-impacted seep site. Enhanced inputs of Mo, Co, Cu, Ni, and Zn associated with hydrothermal Fe-oxyhydroxides-organic colloids is the main source for their high enrichment in the carbonate nodules, and clay minerals and AOM-related microbial organics may have played an important role in their partitioning in the nodules. Uranium enrichment in the carbonate nodules is mainly through enhanced U delivery associated with Fe-oxyhydroxides-scavenged organic colloids and additional U enrichment induced by U(VI) reduction. U(IV) incorporation into carbonate lattices dominates U partitioning in the carbonate nodules over the upper 60 cm of the seep sediments, but the incorporation is substantially dampened at depth due to an increase in concentration of dissolved inorganic carbon, which consequently may have rendered U(IV) adsorption on clay minerals as the important U sink in the nodules. Our results suggest that the exceptional enrichments of Co, Cu, Ni, Zn, Mo, and U in seep carbonates may be used as a paleo-proxy to differentiate cold-seep carbonates whose formation has been co-impacted by hydrothermal plumes.

Manifestation of Gas Seepage from Bottom Sediments on the Sea Surface: Theoretical Model and Experimental Observations

The key area of the Arctic Ocean for atmospheric venting of CH4 is the East Siberian Arctic Shelf (ESAS). Leakage of methane through shallow ESAS waters needs to be considered in interactions between the biogeosphere and a warming Arctic climate. The development of remote sensing techniques for gas seepage detection and mapping is crucially needed for further applications in the ESAS and other areas of interest. Given the extent of the seepage areas and the magnitude of current and potential future emissions, new approaches are required to effectively, rapidly, and quantitatively survey the large seepage areas. Here, we consider the main features of gas seep detection on the sea surface in the characteristics of wind waves and radar signals. The kinematics of wave packets based on the kinetic equation for the spectral density of the wave action of surface waves is described. The results of a full-scale experiment on the remote radar observation of a model gas seep to the sea surface in the radar equipment signals are considered. The characteristic radar signatures of the gas seep in a wide range of hydrometeorological conditions, the parameters of which were recorded synchronously with the radar mapping, were determined. The results of the first radar observations of natural methane seeps on the ESAS are presented, and their radar contrasts are evaluated. The theoretical conclusions are in good qualitative agreement with the results of the model experiment and field studies and can be used for further research in aquatic areas with potential gas seepage, both of natural or anthropogenic origin, such as bubbling release from broken underwater gas pipelines.