Methane Seep Sites Research Articles

Biogeochemical Reconstruction of Authigenic Carbonate Deposits at Methane Seep Site off Krishna‐Godavari (K‐G) Basin, Bay of Bengal

AbstractActive and relic marine methane‐seep sites are widely distributed globally and are distinguished by distinctive geology, biogeochemistry, and ecosystems. The discovery of methane‐seep sites in the Krishna‐Godavari (K‐G) basin has created exciting new opportunities for methane‐seep research in the Bay of Bengal. In this study, we document the occurrence of authigenic carbonates, including micro‐crystalline aragonite crust (arg‐crusts) admixed with chemosynthetic shells and high‐magnesium carbonate tubular structures (HMC‐tube), from the methane‐seep site SSD‐045/4 in the K‐G basin. The δ13C values of HMC‐tubes (−54.5 to −46.2‰) and arg‐crusts (−57.6 to −34.8‰) indicate biogenic methane as the likely carbon source. Enhanced porewater alkalinity driving carbonate precipitation may be attributed to microbial‐mediated SO₄2−‐AOM processes. Additionally, δ13C values (−35.2 ± 8‰) of the residual organic matter within the carbonates suggest a contribution of methanotrophic bacterial biomass. The δ18Ocarb values of HMC and aragonite indicate methane hydrate degassing and crystallization pathways, respectively. Pelloid‐filled burrows suggest the reworking of shallow HMC deposit by burrowing organisms, whereas the polyphase cementations (aragonite and HMC) within burrows indicate early and burial diagenetic pathways. The wide range in ΣLREE/ΣHREE ratios and Ceanom values in arg‐crusts reflect micro‐spatial variations in redox conditions, likely due to cementation occurring in both open and closed diagenetic systems. In contrast, more constrained Ceanom values and ΣLREE/ΣHREE ratios in HMC tubes suggest persistent sulfidic conditions. Overall, these findings provide insights into the pathways of carbonate formation at the K‐G basin methane‐seep site, highlighting the complex interplay of microbial processes, fluid dynamics, and diagenetic alterations.

Glacial-interglacial sedimentation control on gas seepage exemplified by Vestnesa Ridge off NW Svalbard margin

Vestnesa Ridge is built-up of thick contourites mainly deposited during the last ∼5 million years. Methane leaks from deep gas reservoirs creating pockmarks on its crest, and which have been the focus of numerous studies. Sedimentation patterns in relation to the pronounced changes in oceanography and climate of the last glacial-interglacial cycles and its possible impact of seepage of gas have rarely been studied. Here, we present a detailed history of contourite development covering the last ∼130,000 years with most details for the last 60,000 years. The study is based on 43 marine sediment cores and 1,430 km of shallow seismic lines covering the ridge including methane seep sites, with the purpose of reconstructing changes in depositional patterns in relation to paleoceanographical changes on glacial, interglacial, and millennial time scale in relation to activity of seepage of gas. The results show that thick Holocene deposits occurred below ∼1,250 m water depth in the western part of the ridge. Both in pockmarks at western and eastern Vestnesa Ridge, seepage decreased at ∼10–9 ka in the early Holocene. The fine Holocene mud likely reduced seepage to a slow diffusion of gas and microbial oxidation probably prevented escape from the seafloor. Results also showed that seepage of gas was highly variable during the glacial, and low to moderate during the cold Heinrich stadial H1 (19–15 ka) and Younger Dryas stadial (13–12 ka). Seepage reached a maximum during the deglaciation in the Bølling and Allerød interstadials 15–13 ka and early Holocene 12–10 ka. The deglaciation was a period of rapid climatic, oceanographic, and environmental changes. Seepage of gas varied closely with these events indicating that slower tectonic/isostatic movements probably played a minor role in these millennial scale rapid fluctuations in gas emission.

Archaeal lipid biomarkers in near-surface sediments at a giant colony of the bivalve Calyptogena: Molecular records of a massive methane release event associated with methane hydrate dissociation

To investigate the possibility that dissociation of subsurface methane hydrate (MH) in the eastern Nankai Trough, offshore of Japan, led to the formation of a giant colony of the bivalve Calyptogena (currently mostly dead), the carbon isotope ratios (δ13C) of archaeal lipids and methane were measured in near-surface core sediments at Daini-Tenryu Knoll. The irregular variation of porewater methane δ13C with depth (from −75 ‰ to −26 ‰) suggested that originally low δ13C microbial methane was degraded in different proportions by anaerobic methane oxidation. Consistent with this inference, biomarkers of anaerobic methanotrophic archaea (ANME), namely, crocetane (2,6,11,15-tetramethylhexadecane), PMI (2,6,10,15,19-pentamethylicosane), and diethers (archaeol and hydroxyarchaeols), were detected in lipid extracts. The low diether δ13C values (−121 ‰ to −104 ‰) were characteristic of ANME, but less variable than the methane δ13C values, and the relationships between diethers and methane δ13C values deviated from regression lines derived using worldwide data from modern methane seep sites. In contrast, δ13C values of the ANME source methane predicted from those regression lines and the diether δ13C values agreed well with methane δ13C values in MH samples obtained by nearby deep drilling. This result strongly suggests that most of the diethers were produced by ANME that proliferated during a past massive methane release event associated with MH dissociation. The crocetane δ13C value, measured in a mixture with phytane and estimated from the correlation of the δ13C of the mixture with the mole fraction of crocetane, was about −127 ‰. More than half of the PMI δ13C values were greater than −100 ‰, suggesting the background presence of fossil PMI from methanogens.

Microbial communities and mineral assemblages in sediments from various habitats at the Haima Cold Seep, South China Sea

Cold seeps create diverse habitats in the deep sea and play an important role in the global carbon cycling. Anaerobic oxidation of methane (AOM) and biogenic mineralization are essential carbon pathways of methane and carbon transformation in cold seeps, however, the effects of habitat heterogeneity on the processes are still poorly understood. In this study, we investigated the microbial communities and mineral assemblages at distinct habitats in the Haima cold seep and their relationships with environmental factors. These habitats were classified as methane seep site (MS), seep-free faunal habitat (FH), and control site (CS). Bacterial communities were significantly different among the three habitats. ANME-3 archaea, Sulfurovum bacteria, and mineralization-associated microbes (e.g., Campylobacterales) were detected in high relative abundances at ROV2. Mineralogical analysis revealed abundant calcite minerals at the seep site, indicating that authigenic carbonate minerals were formed at highly active seep. Multivariate statistical analysis demonstrated that the concentrations of SO42–, Ca2+, and Mg2+ were significantly correlated with the presence of calcite minerals and bacterial communities. These results suggested that AOM-accompanied authigenic carbonate formation is an important factor influencing the mineral assemblages in seep habitats. This finding improves our understanding of marine microbial carbon cycling.

CAGE17-1-Leg 2 cruise report

From the afternoon of May 19th to the early morning of June 2nd 2017, CAGE-WP6 at the Department of Geology Uit, the Arctic University of Norway, arranged a scientific cruise aimed at investigating sediment cores and porewater, and water masses, at the western Svalbard margin, Fram Strait, East Greenland Ridge, and Storfjorden Trough, visiting methane seep sites off Vestnesa Ridge, and in Storfjorden Trough on R/V “Helmer Hanssen”. Investigated areas were (in order of visiting sites on the cruise): Prins Karls Forland (PKF) (overlap with ‘leg 1’ 17th to 19th of May – see Leg 1 report), southern Yermak Plateau, Vestnesa Ridge, East Greenland Ridge, Storfjorden Trough and Storfjorden.
 The scientific sampling was done within the framework of several ongoing projects at the Department of Geology, University of Tromsø: “CAGE – Centre for Arctic Gas Hydrate, Environment and Climate”-WP6: “Methane Release, Ocean Acidification and CO2”. Planned surveys north of Svalbard was abandoned due to extensive sea ice cover and the East Greenland Ridge was surveyed instead. Sea ice over outer Vestnesa prevented surveys there and Storfjorden trough was also completely covered, but the sea ice broke up towards the end of the cruise and thus could be surveyed during the very last part of the cruise.
 A total of 15 gravity cores (c. 60 m), and 20 CTD (conductivity-temperature-depth) casts were performed along the western Svalbard margin (Vestnesa and PKF and Yermak). Data were also collected at East Greenland Ridge and in Storfjorden Trough and Storfjorden, brine overflow was detected. The CTD casts and chirp- and multibeam lines from PKF May 17-19th are reported in the cruise report for Leg 1.
 Chirp profiles and multibeam lines were acquired during transits and in surveys (mapping of East Greenland Ridge, moraine systems and basins in Storfjorden Trough, active and inactive pockmarks at Vestnesa Ridge and over and to find potential core sites.
 The cruise may be known as: CAGE17_1_leg2

CAGE15-1 Cruise Report : CAGE-CO2 Winter Cruise to the western and northwestern Svalbard margin and the Barents Sea

From the afternoon of April 1st to the morning of April 9th 2015, CAGE at the Department of Geology Uit, the Arctic University of Norway, arranged a scientific cruise aimed at investigating the western Barents Sea slope, western and northern Svalbard margin, methane seep sites at Vestnesa and Storfjorden Trough and the Barents Sea on R/V “Helmer Hanssen”. Investigated areas were (in order of visiting sites on the cruise): Malangen Trough, Bjørnøya Trough Mouth Fan south and mid, Storfjorden Fan, Vestnesa Ridge, the northwest Svalbard slope to north of Schmeerenburgfjorden. On the track towards Tromsø we sampled in Storfjorden Trough. The scientific sampling was done within the framework of several ongoing projects at the Department of Geology, University of Tromsø: “CAGE -Centre for Arctic Gas hydrate, Environment and Climate”, in which the sub-projects “Paleo-CIRCUS”, “OA-Ocean Acidification” is included.
 A total of 2 gravity cores, 8 plankton net casts and 22 CTD (conductivity-temperature-depth) casts were performed along the route to Northern Svalbard at the western Barents Sea slope. Data were also collected in Storfjorden Trough and the mouth of Storfjorden. In addition, CTD’s and a small chirp and multibeam survey was performed off Schmeerenburgfjorden, north of fjorden and in its northernmost part.
 Chirp profiles and multibeam lines were acquired during transits and during the survey of outer Schmeerenburgfjorden. The recording started from c. 71 N on the western Barents Sea slope and ended at 70.00 N.
 The cruise may be known as: CAGE15_1_CO2

CAGE15-7 Cruise Report: GEO8144/3144 – Marine Geology and Geophysics Cruise, Greenhouse Gases in the Ocean

From the late afternoon of July 23rd to the morning of August 3rd, 2015, the Department of Geology Uit, the Arctic University of Norway, arranged a AMGG-Research School scientific teaching cruise aimed atinvestigating the western and northern Svalbard margin and the Barents Sea, the Fram Strait to east Greenland, methane seep sites at Vestnesa, Prins Karls Forland and Storfjorden Trough and on R/V “Helmer Hanssen”. Investigated areas were (in order of visiting sites on the cruise): west of Prins Karls Forland, Vestnesa Ridge, Molloy Deep, Fram Strait, East Greenland margin, Smeerenburgfjorden, Hinlopen, Storfjorden, Storfjorden Trough, and Leirdjupet south of Spitsbergenbanken. The scientific sampling was done within the framework of several ongoing projects at the Department of Geology, University of Tromsø: “CAGE – Centre for Arctic Gashydrate, Environment and Climate”, in which the sub-projects “Paleo-CIRCUS”, “OA-Ocean Acidification” is included. The responsible for this year’s course in GEO8144/GEO3144 is Prof. T.L. Rasmussen. A total of 26 gravity cores, 5 piston cores, 12 plankton net casts and 27 CTD (conductivity-temperature-depth) casts were performed at the planned stations. A total of 86.3 km (46 nm) of airgun surveys were run. 108.97 m mud was obtained in total. Chirp profiles and multibeam lines were acquired during transits and during surveys in Storfjorden, Storfjorden Trough and Leirdjupet. In addition, CTD’s and a small chirp and multibeam survey was performed Storfjorden and in Leirdjupet and airgun lines were run in Storfjorden Trough. The recording of echo-sounding started from c. 78 °N on the shelf outside Isfjorden and ended at c. 72 °N, with pauses when passing through protected areas of Svalbard (Smeerenburgfjorden and Hinlopen and respecting the 12 nm zone in Storfjorden). The cruise may be known as: CAGE15_7_AMGG

CAGE14-4 Cruise Report: CAGE-CO2 Cruise and GEO3144 Teaching Cruise to the western Svalbard margin and the Barents Sea

From the afternoon of July 21st to the morning of July 29th 2014, CAGE at the Department of Geology Uit, the Arctic University of Norway, arranged a scientific cruise with teaching (GEO3144/8144- Arctic Marine Geology and Geophysics Cruise) aimed at investigating and teaching students about the western Svalbard margin, methane seep sites at Vestnesa and Storfjorden Trough and the Barents Sea on R/V “Helmer Hanssen”. Investigated areas were (in order of visiting sites on the cruise): Vestnesa Ridge, the west Svalbard slope, Storfjorden Trough, Storfjorden and western Barents Sea. The scientific sampling was done within the framework of several ongoing projects at the Department of Geology, University of Tromsø: “CAGE – Centre for Arctic Gas hydrate Environment and Climate”, in which the sub-projects “Paleo-CIRCUS”, “OA-Ocean Acidification” is included. A total of 11 gravity cores, 7 box-cores (with a total of 18 sub-samples retrieved), 9 plankton net casts and 16 CTD (conductivity-temperature-depth) casts were performed at Vestnesa Ridge, Storfjorden Fan, Storfjorden Trough, Storfjorden and the western Barents Sea, respectively. All gravity cores were logged onboard by magnetic susceptibility loop sensor for stratigraphical purposes and educational purposes. Chirp profiles and multibeam echo sounder lines were acquired during transits and during surveys of Storfjorden and several pockmarks at the Vestnesa Ridge. A total of 129 lines were collected during transits and during surveys. A total of six seismic lines were performed with the airgun. The cruise may be known as: CAGE14_4_CO2_GEO

CAGE16-3 Cruise Report: CAGE-PWC April Cruise to the western Svalbard margin

From the afternoon of April 22nd to the early morning of May 1st 2016, CAGE at the Department of Geology Uit, the Arctic University of Norway, arranged a scientific cruise aimed at investigating sediment cores and pore water, water masses, and planktonic faunas, at the western Svalbard margin and western Barents Sea slope, also visiting methane seep sites off Vestnesa Ridge, Prins Karls Forland (PKF) and in Storfjorden Trough on R/V “Helmer Hanssen”. Investigated areas were (in order of visiting sites on the cruise): PKF, Vestnesa Ridge, PKF again, Storfjorden Trough and Storfjorden. This cruise can be regarded as a continuance of the previous cruise February 8-15, 2016, where all seep sites at Vestnesa Ridge and PKF were abandoned because of bad weather conditions. In April we hope to reach Vestnesa and PKF to get the cores we missed in February. The scientific sampling was done within the framework of several ongoing projects at the Department of Geology, University of Tromsø: “CAGE -Centre for Arctic Gas hydrate, Environment and Climate”, in which the sub-projects Paleo-CIRCUS and “OA-Ocean Acidification” are included.
 A total of 10 gravity cores, 3 multinet casts and 14 CTD (conductivity-temperature-depth) casts were performed along the western Svalbard margin (Vestnesa and PKF). Data were also collected in Storfjorden Trough and Storfjorden, no brine overflow detected in April either.
 Chirp profiles and multibeam lines were acquired during transits and in small surveys at potential core sites over outer Vestnesa, PKF and in Storfjorden filling previous gaps in records from earlier cruises.

CAGE16-1 Cruise Report: CAGE-OA February Cruise to the western Svalbard margin and the western Barents Sea slope

From the afternoon of February 8th to the late morning of February 15th 2016, CAGE at the Department of Geology Uit, the Arctic University of Norway, arranged a scientific cruise aimed at investigating water masses and planktonic faunas at the western Svalbard margin and western Barents Sea slope, also visiting methane seep sites off Vestnesa Ridge, Prins Karls Forland (PKF) and in Storfjorden Trough on R/V “Helmer Hanssen”. Investigated areas were (in order of visiting sites on the cruise): PKF, Storfjorden Fan, Storfjorden Trough, Bjørnøya Trough Mouth Fan, north, mid and south and the slope off Troms (off Malangen). Seep sites at Vestnesa Ridge were abandoned because of bad weather conditions. The scientific sampling was done within the framework of several ongoing projects at the Department of Geology, University of Tromsø: “CAGE – Centre for Arctic Gashydrate, Environment and Climate”. A total of 1 boxcore, 8 plankton net (4 multinet and 4 plankton net) casts and 14 CTD (conductivity-temperature-depth) casts were performed along the route from western Svalbard to Norway along the western Barents Sea slope. Data were also collected in Storfjorden Trough and Storfjorden – no sea ice yet. Chirp profiles and multibeam lines were acquired during transits and in Storfjorden. The cruise may be known as: CAGE16_1

Sagartiidae (Cnidaria: Actiniaria) from hydrothermal vents and methane seeps in the Bering Sea

Two species of sea anemones of the genus Sagartiogeton are reported from the reducing environments of the Bering Sea (NW Pacific). One species, Sagartiogeton californicus, is abundant on the Koryak slope in the methane seep sites as well as in the background, and it is also found in hydrothermal vents on the submarine Piip Volcano; another species, Sagartiogeton rufus sp. nov., forms crowded settlements in hydrothermal vent areas on the Northern summit of the submarine Piip Volcano. Taxonomy of Sagartiogeton and related genera is discussed. Genus Antipodactis is synonymized with Sagartiogeton and the family Antipodactidae with Sagartiidae. Genus Artemidactis is suggested to be a senior synonym of Sagartiogeton but formally they are not synonymized. Results of stable isotope analysis showed the absence of site-specific differences between Sagartiogeton californicus from background and methane seep communities and non-chemosynthetic origin of food sources for both Sagartiogeton-species. Zoobank Article LSID: urn:lsid:zoobank.org:pub:3F3C631A-1CAB-4F89-9947-03776AA56096.

CAGE-18-3 Cruise to the Barents Sea, Storfjorden Trough, East Greenland Ridge (Leg 1, 2), Arctic Ocean Vestnesa Ridge, and PKF, (Leg 3)

From the evening of June 29th to the evening of July 10th 2018, CAGE at the Department of Geology Uit, the Arctic University of Norway, arranged a scientific cruise aimed at investigating methane seep sites by surface sediment sampling, sediment core sampling, plankton sampling and water sampling. The areas for investigation were the Barents Sea (Crater area), Storfjorden Trough (Pingo area),East Greenland Ridge (non-seep, for IODP-proposal) (Leg 1 and 2) and the western Svalbard margin, Vestnesa Ridge, Arctic Ocean and Prins Karls Forland (Leg 3) on R/V “Helmer Hanssen”. The scientific sampling was done within the framework of several ongoing projects at the Department of Geology, University of Tromsø: “CAGE -Centre for Arctic Gas Hydrate, Environment and Climate”-WP6: “Methane Release, Ocean Acidification and CO2” hereunder ocean acidification impact on marine calcifiers (planktic foraminifera and pteropods, methane seeping impact on ocean acidification and productivity, the state of the inorganic and organic biological pump from foraminifera, bottom water temperature change and impact on release of methane, living benthic microfauna studies from seep sites for geochemical investigations for calibrations for paleostudies. A total of 15 gravity cores (c. 60 m),and 31 CTD (conductivity-temperature-depth) casts were performed, 11 plankton net (no. 12 failed because the net was torn completely), 22 boxcore samples x 6 surface samples for each. A total of 137 lines of acoustics and seismics together were performed. Chirp profiles and multi-beam lines were acquired during transits and in surveys mapping of new seep sites northeast of “Crater” area, seep sites in Storfjorden Trough, East Greenland Ridge, seeps in Hinlopen trough and Norske Banken, the east slope of Yermak Plateau, active and inactive pockmarks at Vestnesa Ridge and a small patch off PKF for WP4. All acoustic and seismic investigations were performed with soft-start of all instruments (Echo-sounders, deep-penetrating echosounder (‘Chirp’), Multi-beam echosounder and mini-air gun). The cruise may be known as: CAGE18_3_leg1 CAGE18_3_leg2 CAGE18_3_leg3

Quantitative assessment of dissolved inorganic carbon cycling in marine sediments from gas hydrate-bearing areas in the South China Sea

Quantitatively assessing dissolved inorganic carbon (DIC) cycling in methane-rich marine sediments is essential for understanding the contributions of different carbon sources to the global carbon cycle. Here, the data of δ13C-DIC, SO42−, DIC, Ca2+, Mg2+, PO43−, and a reactive transport model were used to analyze the DIC and methane source and calculate the DIC budget in the shallow sediments at two methane seep sites in the Xisha Trough (CL27A) and to the west of the Haima cold seeps in the Qiongdongnan Basin (CL44). Model results show that methane contributed to 92.75% and 79.95% of DIC sources through anaerobic oxidation of methane (AOM) versus only 7.25% and 12.90% by organic matter via organoclastic sulfate reduction (OSR) and methanogenesis at both sites. However, the methane sources vary between the two sites. External thermogenic methane that is decoupled from contemporaneous organic matter deposition drives the AOM at CL27A. A combination of upwardly diffusing external biogenic methane and internal methane derived from in situ methanogenesis drives the AOM at CL44. Internal methane flux from depth reveals the deep methanogenic zone influences the DIC cycling in the shallow sediments at CL44. It is accompanied by the upward diffusive DIC flux that can be identified by the δ13C-DIC data when modeling, which contributes to the remaining 7.15% of the DIC source. Neglecting this DIC source that is set as a fixed value at the lower boundary will misunderstand the DIC cycling in the shallow sediments. This study identified and highlighted the influence of external thermogenic methane and the methanogenic zone on the DIC cycling in marine sediments, and demonstrated assessing DIC cycling accurately in methane-rich marine sediments is important to estimate its contribution to oceanic carbon cycling.

Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments

Although it has been suggested that the trace element patterns of sedimentary pyrite can be affected by early diagenesis, detailed studies on modern marine environments addressing this hypothesis are scarce. Organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM) are the dominant sulfate reduction processes in organic-rich and methane-rich marine sediments, and commonly result in the formation of authigenic pyrite. To better understand how OSR and SD-AOM affect the trace element geochemistry of pyrite, we applied laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) to analyze trace element contents of pyrite from two methane-seep sites (DH-CL11 and HS148) of the South China Sea. The trace element data are complemented by the measurement of iron isotope compositions of authigenic pyrite. This new data set, supplemented by published sulfur isotope data, improves our understanding of the diagenetic processes during early diagenetic pyrite formation. High δ56Fe and δ34S values of pyrite reflect the dominance of SD-AOM within sulfate-methane transition zones (SMTZs) at both sites (i.e., from 705 to 767 cm below seafloor at site DH-CL11 and from 490 to 690 cm below seafloor at site HS148), whereas pyrite from shallow depth characterized by low δ56Fe values points to early OSR processes. Within the SMTZs, pyrite exhibits highly variable but overall high manganese (Mn) contents (12 to 732 mg/kg at site DH-CL11; 12 to 1145 mg/kg at site HS148), compared to OSR-derived pyrite from shallow depth. Similar to this pattern, pyrite formed at the SMTZ shows higher lead (Pb), zinc (Zn), copper (Cu), and vanadium (V) contents than pyrite formed at shallower depths above the SMTZ. Enrichments of these metals in pyrite forming at the SMTZ are best explained by enhanced reductive dissolution of iron and manganese (oxyhydr)oxides under sulfidic conditions induced by SD-AOM, and subsequent trace element incorporation into pyrite. Arsenic (As), molybdenum (Mo), and antimony (Sb) distributions in pyrite vary between the two study sites. Pyrite from shallow depth of site HS148 has high contents of all three elements, indicating effective immobilization of dissolved As, Mo, and Sb during pronounced OSR. In contrast, apparently less intense OSR at shallow depth of site DH-CL11 allowed for more dissolved As, Mo, and Sb to diffuse downward to greater depth, resulting in the enrichments of these elements in pyrite formed at or close to the SMTZ. Positive correlations between As and cobalt (Co), and between As and nickel (Ni) typify pyrite derived from both OSR and SD-AOM for both study sites, and As-rich pyrite from site DH-CL11 is characterized by a distinctly high Co/Ni ratio. Both patterns suggest that the presence of As affects the uptake of Co and Ni into pyrite. Our study documents that pyrite element contents in methane-rich environments are significantly influenced by different sulfate reduction processes during early diagenesis, allowing the differentiation of OSR-derived pyrite from SD-AOM-derived pyrite by means of trace element geochemistry.

Autonomous methane seep site monitoring offshore western Svalbard: hourly to seasonal variability and associated oceanographic parameters

Abstract. Improved quantification techniques of natural sources are needed to explain variations in atmospheric methane. In polar regions, high uncertainties in current estimates of methane release from the seabed remain. We present unique 10- and 3-month time series of bottom water measurements of physical and chemical parameters from two autonomous ocean observatories deployed at separate intense seabed methane seep sites (91 and 246 m depth) offshore western Svalbard from 2015 to 2016. Results show high short-term (100–1000 nmol L−1 within hours) and seasonal variation, as well as higher (2–7 times) methane concentrations compared to previous measurements. Rapid variability is explained by uneven distribution of seepage and changing ocean current directions. No overt influence of tidal hydrostatic pressure or water temperature variations on methane concentration was observed, but an observed negative correlation with temperature at the 246 m site fits with hypothesized seasonal blocking of lateral methane pathways in the sediments. Negative correlation between bottom water methane concentration (and variability) and wind forcing, concomitant with signs of weaker water column stratification, indicates increased potential for methane release to the atmosphere in fall and winter. We present new information about short- and long-term methane variability and provide a preliminary constraint on the uncertainties that arise in methane inventory estimates from this variability.

Distribution of Hydrological Parameters over the Methane Seep Site in the Golubaya Bay (the Black Sea): A Connection with Submarine Freshwater Discharge

Distribution of Hydrological Parameters over the Methane Seep Site in the Golubaya Bay (the Black Sea): A Connection with Submarine Freshwater Discharge

Inter-Comparison of the Spatial Distribution of Methane in the Water Column From Seafloor Emissions at Two Sites in the Western Black Sea Using a Multi-Technique Approach

Understanding the dynamics and fate of methane (CH4) release from oceanic seepages on margins and shelves into the water column, and quantifying the budget of its total discharge at different spatial and temporal scales, currently represents a major scientific undertaking. Previous works on the fate of methane escaping from the seafloor underlined the challenge in both, estimating its concentration distribution and identifying gradients. In April 2019, the Envri Methane Cruise has been conducted onboard the R/V Mare Nigrum in the Western Black Sea to investigate two shallow methane seep sites at ∼120 m and ∼55 m water depth. Dissolved CH4 measurements were conducted with two continuous in-situ sensors: a membrane inlet laser spectrometer (MILS) and a commercial methane sensor (METS) from Franatech GmbH. Additionally, discrete water samples were collected from CTD-Rosette deployment and standard laboratory methane analysis was performed by gas chromatography coupled with either purge-and-trap or headspace techniques. The resulting vertical profiles (from both in situ and discrete water sample measurements) of dissolved methane concentration follow an expected exponential dissolution function at both sites. At the deeper site, high dissolved methane concentrations are detected up to ∼45 m from the seabed, while at the sea surface dissolved methane was in equilibrium with the atmospheric concentration. At the shallower site, sea surface CH4 concentrations were four times higher than the expected equilibrium value. Our results seem to support that methane may be transferred from the sea to the atmosphere, depending on local water depths. In accordance with previous studies, the shallower the water, the more likely is a sea-to-atmosphere transport of methane. High spatial resolution surface data also support this hypothesis. Well localized methane enriched waters were found near the surface at both sites, but their locations appear to be decoupled with the ones of the seafloor seepages. This highlights the need of better understanding the processes responsible for the transport and transformation of the dissolved methane in the water column, especially in stratified water masses like in the Black Sea.

Carbonate-hosted microbial communities are prolific and pervasive methane oxidizers at geologically diverse marine methane seep sites

At marine methane seeps, vast quantities of methane move through the shallow subseafloor, where it is largely consumed by microbial communities. This process plays an important role in global methane dynamics, but we have yet to identify all of the methane sinks in the deep sea. Here, we conducted a continental-scale survey of seven geologically diverse seafloor seeps and found that carbonate rocks from all sites host methane-oxidizing microbial communities with substantial methanotrophic potential. In laboratory-based mesocosm incubations, chimney-like carbonates from the newly described Point Dume seep off the coast of Southern California exhibited the highest rates of anaerobic methane oxidation measured to date. After a thorough analysis of physicochemical, electrical, and biological factors, we attribute this substantial metabolic activity largely to higher cell density, mineral composition, kinetic parameters including an elevated Vmax, and the presence of specific microbial lineages. Our data also suggest that other features, such as electrical conductance, rock particle size, and microbial community alpha diversity, may influence a sample's methanotrophic potential, but these factors did not demonstrate clear patterns with respect to methane oxidation rates. Based on the apparent pervasiveness within seep carbonates of microbial communities capable of performing anaerobic oxidation of methane, as well as the frequent occurrence of carbonates at seeps, we suggest that rock-hosted methanotrophy may be an important contributor to marine methane consumption.

Molybdenum isotope composition of seep carbonates – Constraints on sediment biogeochemistry in seepage environments

Methane seepage causes significant changes in the local diagenetic environment and impacts the marine sedimentary cycles of sulfur, carbon, and molybdenum (Mo), which are intimately linked to each other. Molybdenum isotopic compositions were determined for authigenic carbonate nodules and bulk sediments of a 230-m long sediment core collected at a methane seep site in the South China Sea. Authigenic carbonate nodules occur at different sediment depths and display low δ13C values around −50‰ revealing that carbon predominantly derived from biogenic methane. Moreover, seep carbonate nodules are substantially enriched in Mo (Al-normalized enrichment factors MoEF between 27 and 70), and display nearly constant δ98Mo values around an average of +1.9‰ (relative to NIST3134 = +0.25‰), close to the Mo isotopic composition of modern seawater (+2.3‰). Linear positive correlations between δ98Mo values and pyrite sulfur contents and Mo contents were identified for seep carbonate samples. This implies that Mo scavenging into pyrite is likely controlled by hydrogen sulfide availability derived from sulfate-driven anaerobic oxidation of methane (SD-AOM). The essentially constant and high δ98Mo values suggest that Mo species were nearly quantitatively incorporated into the authigenic carbonates under highly sulfidic conditions, probably at times when the sulfate-methane transition zones (SMTZ) was situated at shallow sediment depth at times of high seepage activity. In contrast, bulk sediments that exhibit typical marine δ13C values close to 0‰ (versus VPDB), hence devoid of methane-derived seep carbonates, are characterized by low Mo contents (MoEF = ~0.9). Their lower δ98Mo values mostly ranging from +0.2 to +0.8‰, in accord with low Mo contents, likely reflect input of detrital Mo. Bulk sediments bearing seep carbonate nodules display variable δ13C values lower than −5‰ and are characterized by a wide range in Mo contents (MoEF from 1.5 to 45.3) and δ98Mo values (+0.7 to +3.0‰). The high δ98Mo values of some sediment samples, relative to the seep carbonate signature, might indicate an additional accumulation of a 98Mo enriched pool of molybdenum during later SD-AOM at a deeper SMTZ postdating initial seep carbonate formation; such 98Mo enrichment was probably caused by preceding preferential removal of lighter Mo isotopes from pore fluids. Our results indicate that Mo isotope systematics of seep deposits can provide constraints on the dynamics of the biogeochemical cycling of carbon, sulfur, and trace elements in methane seep environments. A potential for authigenic carbonates to archive the Mo isotopic composition of seawater is indicated, but will have to be assessed in future studies.

Fluid conduits formed along burrows of giant bivalves at a cold seep site, Sounthern Taiwan

At a Pliocene methane seep site in Taiwan characterized by numerous tubular carbonate bodies and bivalves, a half meter-long dolomitic concretion containing a lucinid bivalve Anodontia goliath reveals the impact of chemosymbiotic bivalves on fluid migration through shallow sediments. The concretion consists of a cortex encasing a central channel filled with sparite and varying amounts of brecciated material derived from the channel walls. The studied bivalve is preserved with the hinge upward. When restored to life position, the central channel is connected to the anterior side of the bivalve. In the lower longer segment, the channel is connected to the posterior sector of the bivalve. The channels are interpreted as the anterior tube for supply of respiration water and the posterior tube for mining sulphide generated in the sulfate-methane transition zone of the sediment. The dolomitic cortex surrounding the central channel is characterized by δ13C values as low as −29‰ VPDB, indicating that the rate of anaerobic oxidation of methane was enhanced around the conduit. At the base of the central channel, upward soft sediment deformation structures likely resulted from the dragging effect when fluid migrated out of the sediment and entered the channel. Brecciation of the channel walls indicates pulsating seepage during the incipient stages of cementation. The burrow, thus, acted as a highly permeable conduit channeling upstreaming methane-charged fluid for a substantial period of time. Therefore, burrows can be efficient in pre-determining the formation of fluid conduits at seep sites, having potentially a large impact on fluid mixing and diagenetic reaction rates in sediments, and ultimately the amount of methane released. The reported bivalve with its conserved burrow fossilized as seep carbonate concertion provides direct evidence that fluid conduits form along animal burrows.