Carbon capture and storage is a key mitigation strategy proposed for keeping the global temperature rise below 1.5 degrees C. Offshore storage can provide up to 13% of the global CO2 reduction required to achieve the Intergovernmental Panel on Climate Change goals. The public must be assured that potential leakages from storage reservoirs can be detected and that therefore the CO2 is safely contained. We conducted a controlled release of 675 kg CO2 within sediments at 120 m water depth, to simulate a leak and test novel detection, quantification and attribution approaches. We show that even at a very low release rate (6 kg day(-1)), CO2 can be detected within sediments and in the water column. Alongside detection we show the fluxes of both dissolved and gaseous CO2 can be quantified. The CO2 source was verified using natural and added tracers. The experiment demonstrates that existing technologies and techniques can detect, attribute and quantify any escape of CO2 from sub-seabed reservoirs as required for public assurance, regulatory oversight and emissions trading schemes.

Injection of carbon dioxide (CO2) into subseafloor reservoirs is gaining traction as a strategy for mitigating anthropogenic CO2 emissions to the atmosphere. Yet, potential leakage, migration and dissolution of externally-supplied CO2 from such reservoirs are a cause for concern. The potential impact of CO2 leakage on the biogeochemistry of sediments and overlying waters in the North Sea was studied during a controlled subsurface CO2 release experiment in 2019 at a potential carbon capture and storage site (Goldeneye). This study describes the natural (unperturbed) biogeochemistry of sediments. They are classified as muddy sand to sandy mud with low organic carbon content (∼0.6 %). Distributions of dissolved inorganic carbon (DIC) and total alkalinity (TA) in sediment porewaters are reported in addition to in situ benthic fluxes of dissolved nutrients and oxygen between the sediments and the overlying water. Oxygen fluxes into the sediment, measured using benthic chambers and eddy covariance, were 6.18 ± 0.58 and 5.73 ± 2.03 mmol m−2 d-1, respectively. Diagnostic indicators are discussed that could be used to detect CO2 enrichment of sediments due to reservoir leakage at CCS sites. These include the ratio TA and ammonium to sulfate in sediment porewaters, benthic fluxes and chloride-normalized cation distributions. These indicators currently suggest that the organic carbon at Goldeneye has an oxidation state below zero and is mainly degraded via sulfate reduction. Carbonate precipitation is apparently negligible, whereas decreases in Mg2+ and K+ point toward ongoing alteration of lithogenic sediments by reverse weathering processes.

First direct evidence for ongoing gas seepage activity on the abandoned well site 22/4b (Northern North Sea, 57 degrees 55'N, 01 degrees 38'E) and discovery of neighboring seepage activity is provided from observations since 2005. A manned submersible dive in 2006 discovered several extraordinary intense seepage sites within a 60 m wide and 20 m deep crater cut into the flat 96 m deep seafloor. Capture and (isotope) chemical analyses of the gas bubbles near the seafloor revealed in situ concentrations of methane between 88 and 90%Vol. with delta C-13-CH4 values around -74 parts per thousand VPDB, indicating a biogenic origin. Bulk methane concentrations throughout the water column were assessed by 120 Niskin water samples showing up to 400.000 nM CH4 in the crater at depth. In contrast, concentrations above the thermocline were orders of magnitude lower, with a median value of 20 nM. A dye tracer injection into the gas seeps revealed upwelling bubble and water motion with gas plume rise velocities up to similar to 1 ms(-1) (determined near the seabed). However, the dissolved dye did not pass the thermocline, but returned down to the seabed. Measurements of direct bubble-mediated atmospheric flux revealed low values of 0.7 +/- 0.3 kty(-1), much less than current state-of-the-art bubble dissolution models would predict for such a strong and upwelling in situ gas bubble flux at shallow water depths (i.e. similar to 100 m).Acoustic multibeam water column imaging data indicate a pronounced 200 m lateral intrusion at the thermocline together with high methane concentration at this layer. A partly downward-orientated bubble plume motion is also visible in the acoustic data with potential short-circuiting in accordance to the dye experiment. This observation could partly explain the observed trapping of most of the released gas below the well-established thermocline in the North Sea. Moreover, 3D analyses of the multibeam water column data reveal that the upwelling plume transforms into a spiral expanding vortex while rising through the water column. Such a spiral vortex motion has never been reported before for marine gas seepage and might represent an important process with strong implication on plume dynamics, dissolution behavior, gas escape to the atmosphere, and is considered very important for respective modeling approaches. (C) 2015 The Authors. Published by Elsevier Ltd.

Large amounts of methane are delivered by fluids through the erosive forearc of the convergent margin offshore of Costa Rica and lead to the formation of cold seeps at the sediment surface. Besides mud extrusion, numerous cold seeps are created by landslides induced by seamount subduction or fluid migration along major faults. Most of the dissolved methane migrating through the sediments of cold seeps is oxidized within the benthic microbial methane filter by anaerobic oxidation of methane (AOM). Measurements of AOM and sulfate reduction as well as numerical modeling of porewater profiles revealed a highly active and efficient benthic methane filter at the Quepos Slide site, a landslide on the continental slope between the Nicoya and Osa Peninsula. Integrated areal rates of AOM ranged from 12.9 ± 6.0 to 45.2 ± 11.5 mmol m−2 d−1, with only 1 to 2.5 % of the upward methane flux being released into the water column. Additionally, two parallel sediment cores from Quepos Slide were used for in vitro experiments in a recently developed sediment-flow-through (SLOT) system to simulate an increased fluid and methane flux from the bottom of the sediment core. The benthic methane filter revealed a high adaptability whereby the methane oxidation efficiency responded to the increased fluid flow within ca. 170 d. To our knowledge, this study provides the first estimation of the natural biogeochemical response of seep sediments to changes in fluid flow.

This chapter contains sections titled: Introduction Methane Hydrate Manifestations Chloride-Anomaly Pattern Floating Hydrates: Evidence from Hydrate Fabric and Low Bulk Densities Hydrate Water Composition Conclusions

Tommeliten is a prominent methane seep area in the Central North Sea. Previous surveys revealed shallow gas-bearing sediments and methane gas ebullition into the water column. In this study, the in situ methane flux at Tommeliten is re-assessed and the potential methane transport to the atmosphere is discussed, with regards to the hydrographic setting and gas bubble modeling. We have compiled previous data, acquired new video and acoustic evidence of gas bubble release, and have measured the methane concentration, and its C-isotopic composition in the water column. Parametric subbottom sonar data reveal the three-dimensional extent of shallow gas and morphologic features relevant for gas migration. Five methane ebullition areas are identified and the main seepage area appears to be 21 times larger than previously estimated. Our video, hydroacoustic, subbottom, and chemical data suggest that ∼1.5×106molCH4/yr (∼26tonsCH4/yr) of methane gas is being released from the seepage area of Tommeliten. Methane concentration profiles in the vicinity of the gas seeps show values of up to 268nM (∼100 times background) close to the seafloor. A decrease in δ13C-CH4 values at 40m water depth indicates an unknown additional biogenic methane source within the well oxygenated thermocline between 30 and 40m water depth. Numerical modeling of the methane bubbles due to their migration and dissolution was performed to estimate the bubble-derived vertical methane transport, the fate of this methane in the water column, and finally the flux to the atmosphere. Modeling indicates that less than ∼4% of the gas initially released at the seafloor is transported via bubbles into the mixed layer and, ultimately, to the atmosphere. However, because of the strong seasonality of mixing in the North Sea, this flux is expected to increase as mixing increases, and almost all of the methane released at the seafloor could be transferred into the atmosphere in the stormy fall and winter time.

This paper is an introduction to and an overview of papers presented in the Special Issue of Marine Geology "Methane seeps at the Hikurangi Margin, New Zealand". In 2006 and 2007, three research cruises to the Hikurangi Margin at the east coast of New Zealand's North Island were dedicated to studying methane seepage and gas hydrates in an area where early reports suggested they were widespread. Two cruises were carried out on RV TANGAROA and one on RV SONNE using the complete spectrum of state-of-the-art equipment for geophysics (seismic, sidescan, controlled source electromagnetics, ocean bottom seismometers and hydrophones, singlebeam and multibeam), seafloor observations (towed camera systems, ROV), sediment and biological sampling (TV-guided multi-corer, gravity-corer, grab, epibenthic sled), deployment of in-situ observatories (landers) as well as water column sampling and oceanographic studies (CTD, moorings). The scientific disciplines involved ranged from geology, geophysics, petrography, geochemistry, to oceanography, biology and microbiology. These cruises confirmed that a significant part of the Hikurangi Margin has been active with locally intense methane seepage at present and in the past, with the widespread occurrence of dead seep faunas and knoll-forming carbonate precipitations offshore and on the adjacent land. A close link to seismically detected fluid systems and the outcropping of the base of the gas hydrate stability zone can be found at some places. Pore fluid and free gas release were found to be linked to tides. Currents as well as density layers modulate the methane distribution in the water column. The paper introduces the six working areas on the Hikurangi Margin, and compiles all seep locations based on newly processed multibeam and multibeam backscatter data, water column hydroacoustic and visual data that are combined with results presented elsewhere in this Special Issue. In total, 32 new seep sites were detected that commonly show chemoherm-type carbonates or carbonate cemented sediment with fissures and cracks in which calyptogenid clams and bathymodiolid mussels together with sibloglinid tube worms live. White bacterial mats of the genus Beggiatoa and dark gray beds of heterotrophic ampharetid polychaetes typically occur at active sites. Bubble release has frequently been observed visually as well as hydroacoustically (flares) and geochemical analyses show that biogenic methane is released. All seep sites, bubbling or not, were inside the gas hydrate stability zone. Gas hydrate itself was recovered at three sites from the seafloor surface or 2.5 m core depth as fist-sized chunks or centimeter thick veins. The strong carbonate cementation that in some cases forms 50 m high knolls as well as some very large areas being paved with clam shells indicates very strong and long lasting seep activity in the past. This activity seems to be less at present but nevertheless makes the Hikurangi Margin an ideal place for methane-related seep studies in the SW-Pacific.

This study combines sediment geochemical analysis, in situ benthic lander deployments and numerical modeling to quantify the biogeochemical cycles of carbon and sulfur and the associated rates of Gibbs energy production at a novel methane seep. The benthic ecosystem is dominated by a dense population of tube-building ampharetid polychaetes and conspicuous microbial mats were unusually absent. A 1D numerical reaction-transport model, which allows for the explicit growth of sulfide and methane oxidizing microorganisms, was tuned to the geochemical data using a fluid advection velocity of 14cmyr−1. The fluids provide a deep source of dissolved hydrogen sulfide and methane to the sediment with fluxes equal to 4.1 and 18.2mmolm−2d−1, respectively. Chemosynthetic biomass production in the subsurface sediment is estimated to be 2.8mmolm−2d−1 of C biomass. However, carbon and oxygen budgets indicate that chemosynthetic organisms living directly above or on the surface sediment have the potential to produce 12.3mmolm−2d−1 of C biomass. This autochthonous carbon source meets the ampharetid respiratory carbon demand of 23.2mmolm−2d−1 to within a factor of 2. By contrast, the contribution of photosynthetically-fixed carbon sources to ampharetid nutrition is minor (3.3mmolm−2d−1 of C). The data strongly suggest that mixing of labile autochthonous microbial detritus below the oxic layer sustains high measured rates of sulfate reduction in the uppermost 2cm of the sulfidic sediment (100–200nmolcm−3d−1). Similar rates have been reported in the literature for other seeps, from which we conclude that autochthonous organic matter is an important substrate for sulfate reducing bacteria in these sediment layers. A system-scale energy budget based on the chemosynthetic reaction pathways reveals that up to 8.3kJm−2d−1 or 96mWm−2 of catabolic (Gibbs) energy is dissipated at the seep through oxidation reactions. The microorganisms mediating sulfide oxidation and anaerobic oxidation of methane (AOM) produce 95% and 2% of this energy flux, respectively. The low power output by AOM is due to strong bioenergetic constraints imposed on the reaction rate by the composition of the chemical environment. These constraints provide a high potential for dissolved methane efflux from the sediment (12.0mmolm−2d−1) and indicates a much lower efficiency of (dissolved) methane sequestration by AOM at seeps than considered previously. Nonetheless, AOM is able to consume a third of the ascending methane flux (5.9mmolm−2d−1 of CH4) with a high efficiency of energy expenditure (35mmolCH4kJ−1). It is further proposed that bioenergetic limitation of AOM provides an explanation for the non-zero sulfate concentrations below the AOM zone observed here and in other active and passive margin sediments.

Many biological seep studies focused on the distribution, structure, nutrition and food web architecture of seep communities as well as on their interaction with the seep geochemistry However, overall respiration at cold seeps received only little attention. We conducted in-situ oxygen flux measurements in combination with ex-situ oxygen micro-profiles, respiration measurements, as well as rate determinations of microbial methane and sulfate turnover to assess respiration pathways as well as carbon turnover at a seep habitat that was recently discovered alongside the Hikurangi Margin offshore northern New Zealand This habitat is dominated by dense beds of tube-building, heterotrophic ampharetid polychaetes. Average total oxygen uptake (TOU) from this habitat was very high (83 7 mmol m(-2) day(-1)) TOU at a non-seep reference site ranged between 27 and 58 mmol m(-2) day(-1). About 37% (30 8 mmol m(-2) day(-1)) of the average TOU was consumed by ampharetids Considering mean diffusive oxygen uptake (85 mmol m(-2) day(-1)) the remaining fraction of similar to 53% of the TOU (44 4 mmol m(-2) day(-1)) might be explained by respiration of epibenthic organisms as well as aerobic methane and sulfide oxidation at the sediment-water interface The strongly negative carbon isotopic signatures (-52.9 +/- 5 parts per thousand VPDB) of the ampharetid tissues indicate a methane derived diet. However, carbon production via anaerobic oxidation of methane (AOM) was too low (0.1 mmol C m(-2) day(-1)) to cover the mean carbon demand of the ampharetid communities (21 mmol C m(-2) day(-1)) Likely, organic carbon generated via aerobic methane oxidation represents their major carbon source This is in contrast to other seep habitats, where energy bound in methane is partly transferred to sulfide via AOM and finally consumed by sulfide-oxidizing chemoautotrophs providing carbon that subsequently enters the benthic food web (C) 2009 Elsevier B.V. All rights reserved.

Natural seepage from the seafloor is a worldwide phenomenon but quantitative measurements of gas release are rare, and the entire range of the dynamics of gas release in space, time, and strength remains unclear so far. To mitigate this, the hydroacoustic device GasQuant (180 kHz, multibeam) was developed to monitor the tempo‐spatial variability of gas bubble releases from the seafloor. GasQuant was deployed in 2005 on the seafloor of the seep field Tommeliten (North Sea) for 36 h. This in situ approach provides much better spatial and temporal resolution of seeps than using conventional ship‐born echo sounders. A total of 52 gas vents have been detected. Detailed time series analysis revealed a wide range of gas release patterns ranging from very short periodic up to 50 min long‐lasting events. The bulk gas seepage in the studied area is active for more than 70% of observation time. The venting clearly exhibits tidal control showing a peak in the second quarter of the tidal pressure cycle, where pressure drops fastest. The hydroacoustic results are compared with video observations and bubble flux estimates from remotely operated vehicle dives described in the literature. An advanced approach for identifying and visualizing rising bubbles in the sea by hydroacoustics is presented in which water current data were considered. Realizing that bubbles are moved by currents helps to improve the detection of gas bubbles in the data, better discriminate bubbles against fish echoes, and to enhance the S/N ratio in the per se noisy acoustic data.

Submarine mud volcanism represents an important. pathway for methane from deeper reservoirs to the surface, where it enters the benthic carbon cycle. To quantify overall methane release from the Captain Arutyunov mud volcano (CAMV) and to assess the contribution of macrobenthic seep organisms to the regulation of the benthic methane flux, we linked water column methane concentrations, seabed methane emission and pore water geochemistry to the spatial distribution of seep biota. Prominent organisms of the CAMV seep biota were 3 different species of frenulate tubeworms. Seabed methane emission ranged from 0.001 to 0.66 mmol m(-2) d(-1). Dense patches of tubeworms were associated with the lowest seabed methane emission. Elevated methane emission was associated with a sporadic distribution of tubeworms and the occurrence of numerous mud clasts. Despite the presence of a large subsurface methane reservoir, the estimated total methane release from CAMV was low (0.006 x 10(6) mol yr(-1)). In addition to direct methane consumption by Siboglinum poseidoni, the tubeworms likely contribute to the retention of methane carbon in the sediment by affecting bacterial communities in the proximity of the tubes. The siboglinids create new meso-scale habitats on the sediment Surface, increasing habitat heterogeneity and introducing niches for bacterial communities.

In situ methane emission measurements from sediments are combined with water column backscatter anomalies recorded with an Acoustic Doppler Current Profiler (ADCP) integrated on a benthic observatory. During cruise SO191 to the Hikurangi Margin (New Zealand), the Fluid Flux Observatory (FLUFO) was deployed at a cold seep site at Omakere Ridge. The first 26 hours of in situ incubation revealed low to moderate methane fluxes (0.01 to 0.4 mmol m-2 d-1). In the following sampling sequence, however, the methane concentration in the flux chamber reached 3-fold higher concentrations. Simultaneous to the sudden methane increase, a significant backscatter anomaly was recorded and persisted for 30 min and covered the entire depth range (100 m) of the upward looking ADCP. Data analyses revealed that a plume outburst likely occurred during this time. A second data set was obtained during lander deployments at Rock Garden where visual observations by ROV confirmed the transient pattern of free gas injection into the water column. Acoustic flares and methane concentration increases in the bottom water hint towards a pressure (tidal) induced discharge mechanism. The presented data demonstrate the temporal and spatial variability of seabed methane emission, and very short methane signal lifetime in the water column (hours) due to turbulent diffusion. Both have to be considered when methane budgets are extrapolated from single methane emission rates.

Mud volcanism is a widespread phenomenon in the Gulf of Cadiz (GoC) and provides a window into deep structural and diagenetic processes [1]. Sediment pore fluids from the Mercator mud volcano (MMV), located in the El Arraiche mud volcano field offshore Morocco, are extremely enriched in chloride reaching up to 5.3 M (thus, exceeding normal seawater values by a factor of 9). Na/Cl ratios are close to 1 suggesting halite dissolution by the ascending fluid. This is corroborated by 3D seismic data that shows an active anticline below the eastern flank of the MMV, which can only be explained by a rising salt diapir. Additionally, the fluids are highly enriched in Li and B indicating a deep fluid source from mineral dewatering reactions at elevated temperatures (>100 °C). A deep fluid source is also supported by a radiogenic Sr/Sr porewater signal of 0.7106 as well as the morphology of quartz and gypsum crystals transported within the ascending mud matrix. These crystals are probably of Triassic origin. Hence, the geochemical and the 3D seismic data suggest that the Triassic salt province 500 km further south on the Moroccan Margin might extend further north than previously known and that halokinesis has to be considered as a driving force for mud volcanism in the GoC. Finally, a 1-D transport-reaction model has been applied to constrain the fluid advection rates. The numerical simulations of conservative porewater compounds Cl, Li, and B reveal upward fluid flow rates of ~6 cm/a at the top of the MMV, gradually decreasing to 0.3 cm/a towards the rim. In addition, a CTD mounted onto a video sled system provided bottom water salinity and temperature information (about 2 m above the seafloor) from tracks crossing the MMV. In combination with the numerical porewater analysis, this unique spatial data set allows to constrain the overall budgets for the release of water, methane, and other porewater constituents as well as the heat flow of the MMV.