Description: Highlights • Gas release from wells may counteract efforts to mitigate greenhouse gas emissions. • An approach for assessing methane release from marine decommissioned wells. • This gas release largely depends on the presence of shallow gas accumulations. • Methane release from hydrocarbon wells represents a major source in the North Sea. Abstract Hydrocarbon gas emissions from with decommissioned wells are an underreported source of greenhouse gas emissions in oil and gas provinces. The associated emissions may partly counteract efforts to mitigate greenhouse gas emissions from fossil fuel infrastructure. We have developed an approach for assessing methane leakage from marine decommissioned wells based on a combination of existing regional industrial seismic and newly acquired hydroacoustic water column imaging data from the Central North Sea. Here, we present hydroacoustic data which show that 28 out of 43 investigated wells release gas from the seafloor into the water column. This gas release largely depends on the presence of shallow gas accumulations and their distance to the wells. The released gas is likely primarily biogenic methane from shallow sources. In the upper 1,000 m below the seabed, gas migration is likely focused along drilling-induced fractures around the borehole or through non-sealing barriers. Combining available direct measurements for methane release from marine decommissioned wells with our leakage analysis suggests that gas release from investigated decommissioned hydrocarbon wells is a major source of methane in the North Sea (0.9-3.7 [95% confidence interval = 0.7-4.2] kt yr−1 of CH4 for 1,792 wells in the UK sector of the Central North Sea). This means hydrocarbon gas emissions associated with marine hydrocarbon wells are not significant for the global greenhouse gas budget, but have to be considered when compiling regional methane budgets.

Description: Deep-sea mining for polymetallic nodules is expected to have severe environmental impacts because not only nodules but also benthic fauna and the upper reactive sediment layer are removed through the mining operation and blanketed by resettling material from the suspended sediment plume. This study aims to provide a holistic assessment of the biogeochemical recovery after a disturbance event by applying prognostic simulations based on an updated diagenetic background model and validated against novel data on microbiological processes. It was found that the recovery strongly depends on the impact type; complete removal of the reactive surface sediment reduces benthic release of nutrients over centuries, while geochemical processes after resuspension and mixing of the surface sediment are near the pre-impact state 1 year after the disturbance. Furthermore, the geochemical impact in the DISturbance and reCOLonization (DISCOL) experiment area would be mitigated to some degree by a clay-bound Fe(II)-reaction layer, impeding the downward diffusion of oxygen, thus stabilizing the redox zonation of the sediment during transient post-impact recovery. The interdisciplinary (geochemical, numerical and biological) approach highlights the closely linked nature of benthic ecosystem functions, e.g. through bioturbation, microbial biomass and nutrient fluxes, which is also of great importance for the system recovery. It is, however, important to note that the nodule ecosystem may never recover to the pre-impact state without the essential hard substrate and will instead be dominated by different faunal communities, functions and services.

Description: Highlights • Combining porewater geochemistry, geochemical modeling and subsurface geophysical data in order to understand the fluid flow system of Kerch seep area. • This seep area is not in steady state. • Methane transport is in the form of gas bubbles not porewater advection. • High surface temperatures are the result of hydrate formation and not an indication for elevated geothermal gradients. • Modeling says this seep is young (〈500 years old). Abstract High-resolution 3D seismic data in combination with deep-towed sidescan sonar data and porewater analysis give insights into the seafloor expression and the plumbing system of the actively gas emitting Kerch seep area, which is located in the northeastern Black Sea in around 900 m water depth, i.e. well within the gas hydrate stability zone (GHSZ). Our analysis shows that the Kerch seep consists of three closely spaced but individual seeps above a paleo-channel-levee system of the Don Kuban deep-sea fan. We show that mounded seep morphology results from sediment up-doming due to gas overpressure. Each of the seeps hosts its own gas pocket underneath the domes which are fed with methane of predominantly microbial origin along narrow pipes through the GHSZ. Methane transport occurs dominantly in the form of gas bubbles decoupled from fluid advection. Elevated sediment temperatures of up to 0.3 °C above background values are most likely the result of gas hydrate formation within the uppermost 10 m of the sediment column. Compared to other seeps occurring within the GHSZ in the Black Sea overall only scarce gas indications are present in geoacoustic and geophysical data. Transport-reaction modeling suggests that the Kerch seep is a young seep far from steady state and probably not more than 500 years old.

Description: The fate of plastic debris entering the oceans is largely unconstrained. Currently, intensified research is devoted to the abiotic and microbial degradation of plastic floating near the ocean surface for an extended period of time. In contrast, the impacts of environmental conditions in the deep sea on polymer properties and rigidity are virtually unknown. Here, we present unique results of plastic items identified to have been introduced into deep-sea sediments at a water depth of 4150 m in the eastern equatorial Pacific Ocean more than two decades ago. The results, including optical, spectroscopic, physical and microbial analyses, clearly demonstrate that the bulk polymer materials show no apparent sign of physical or chemical degradation. Solely the polymer surface layers showed reduced hydrophobicity, presumably caused by microbial colonization. The bacterial community present on the plastic items differed significantly (p 〈 0.1%) from those of the adjacent natural environment by a dominant presence of groups requiring steep redox gradients (Mesorhizobium, Sulfurimonas) and a remarkable decrease in diversity. The establishment of chemical gradients across the polymer surfaces presumably caused these conditions. Our findings suggest that plastic is stable over extended times under deep-sea conditions and that prolonged deposition of polymer items at the seafloor may induce local oxygen depletion at the sediment-water interface.

Description: There has been a steady increase in interest in mining of deep-sea minerals in the Clarion–Clipperton Zone (CCZ) in the eastern Pacific Ocean during the last decade. This region is known to be one of the most eddy-rich regions in the world ocean. Typically, mesoscale eddies are generated by intense wind bursts channeled through gaps in the Sierra Madre mountains in Central America. Here, we use a combination of satellite and in situ observations to evaluate the relationship between deep-sea current variability in the region of potential future mining and eddy kinetic energy (EKE) in the vicinity of gap winds. A geometry-based eddy detection algorithm has been applied to altimetry sea surface height data for a period of 24 years, from 1993 to 2016, in order to analyze the main characteristic parameters and the spatiotemporal variability of mesoscale eddies in the northeast tropical Pacific Ocean (NETP). Significant differences between the characteristics of eddies with different polarity (cyclonic vs. anticyclonic) were found. For eddies with lifetimes longer than 1 d, cyclonic polarity is more common than anticyclonic rotation. However, anticyclonic eddies are larger in size, show stronger vorticity, and survive longer in the ocean than cyclonic eddies (often 90 d or more). Besides the polarity of eddies, the location of eddy formation should be taken into consideration when investigating the impacted deep-ocean region as we found eddies originating from the Tehuantepec (TT) gap winds lasting longer in the ocean and traveling farther distances in a different direction compared to eddies produced by the Papagayo (PP) gap winds. Long-lived anticyclonic eddies generated by the TT gap winds are observed to travel distances up to 4500 km offshore, i.e., as far as west of 110∘ W. EKE anomalies observed in the surface of the central ocean at distances of ca. 2500 km from the coast correlate with the seasonal variability of EKE in the region of the TT gap winds with a time lag of 5–6 months. A significant seasonal variability of deep-ocean current velocities at water depths of 4100 m was observed in multiple-year time series data, likely reflecting the energy transfer of the surface EKE generated by the gap winds to the deep ocean. Furthermore, the influence of mesoscale eddies on deep-ocean currents is examined by analyzing the deep-ocean current measurements when an anticyclonic eddy crosses the study region. Our findings suggest that despite the significant modulation of dominant current directions driven by the bottom-reaching eddy, the current magnitude intensification was not strong enough to trigger local sediment resuspension in this region. A better insight into the annual variability of ocean surface mesoscale activity in the CCZ and its effects on deep-ocean current variability can be of great help to mitigate the impact of future potential deep-sea mining activities on the benthic ecosystem. On an interannual scale, a significant relationship between cyclonic eddy characteristics and El Niño–Southern Oscillation (ENSO) was found, whereas a weaker correlation was detected for anticyclonic eddies.

Description: The thriving interest in harvesting deep-sea mineral resources, such as polymetallic nodules, calls for environmental impact studies, and ultimately, for regulations for environmental protection. Industrial-scale deep-sea mining of polymetallic nodules most likely has severe consequences for the natural environment. However, the effects of mining activities on deep-sea ecosystems, sediment geochemistry and element fluxes are still poorly conceived. Predicting the environmental impact is challenging due to the scarcity of environmental baseline studies as well as the lack of mining trials with industrial mining equipment in the deep sea. Thus, currently we have to rely on small-scale disturbances simulating deep-sea mining activities as a first-order approximation to study the expected impacts on the abyssal environment. Here, we investigate surface sediments in disturbance tracks of seven small-scale benthic impact experiments, which have been performed in four European contract areas for the exploration of polymetallic nodules in the Clarion-Clipperton Zone (CCZ). These small-scale disturbance experiments were performed 1 day to 37 years prior to our sampling program in the German, Polish, Belgian and French contract areas using different disturbance devices. We show that the depth distribution of solid-phase Mn in the upper 20 cm of the sediments in the CCZ provides a reliable tool for the determination of the disturbance depth, which has been proposed in a previous study (Paul et al., 2018). We found that the upper 5–15 cm of the sediments were removed during various small-scale disturbance experiments in the different exploration contract areas. Transient transport-reaction modelling for the Polish and German contract areas reveals that the removal of the surface sediments is associated with the loss of reactive labile organic carbon. As a result, oxygen consumption rates decrease significantly after the removal of the surface sediments, and consequently, oxygen penetrates up to tenfold deeper into the sediments inhibiting denitrification and Mn(IV) reduction. Our model results show that the post-disturbance geochemical re-equilibration is controlled by diffusion until the reactive labile TOC fraction in the surface sediments is partly re-established and the biogeochemical processes commence. While the re-establishment of bioturbation is essential, the geochemical re-equilibration of the sediments is ultimately controlled by the burial rates of organic matter. Hence, under current depositional conditions, the new geochemical equilibrium in the sediments of the CCZ is reached only on a millennia scale even for these small-scale disturbances simulating deep-sea mining activities.

Description: Highlights • Total amount of generated biogenic methane is estimated at ~3100 Gt. • Total amount of generated thermogenic methane is estimated at ~1,560 Gt. • The Maykop formation is partially productive in the central basin and not yet fully productive towards the basin peripherals. A new numerical model reconstructing the depositional history (98–0 Ma) of the Western Black Sea sub-basin is presented. The model accounts for changing boundary conditions (i.e. water depth, bottom water temperature, heat flow evolution over time) and estimates the rates and total amounts of the in-situ biogenic methane generation and thermally-driven organic matter maturation in the source rocks. The overall thermogenic and biogenic gas generation predicted by the model is estimated at ~1560 Gt and ~3100 Gt, respectively.

Description: Highlights • Physical properties obtained from core and log data at the Danube deep sea fan are reported. • Core-log-seismic integration defines stratigraphic framework at the S2 channel. • All data suggest no gas hydrate is present at drill sites within uncertainties of methods employed. Abstract Drilling, coring, and geophysical logging were performed with the MARUM-MeBo200 seafloor drilling rig to investigate gas hydrate occurrences of the Danube deep sea fan, off Romania, Black Sea. Three sites within a channel-levee complex were investigated. Geophysical log data of P-wave velocity, electrical resistivity, and spectral gamma ray are combined with core-derived physical properties of porosity, magnetic susceptibility, and bulk density. Core- and log physical property data are used to define a time-depth conversion by synthetic seismogram modeling, which is then used to interpret the seismic data. Individual polarity reversed reflectors within the stratigraphic column drilled are linked to reduction in P-wave velocity and bulk density. Those reflectors (and associated reflection packages) are accompanied by distinct and systematic changes in sediment porosity, magnetic susceptibility, and electrical resistivity. Overall, the sediments at drill site GeoB22605 (MeBo-17) represent the younger (upper) levee sequence of the channel, that has been eroded at drill site GeoB22603 (MeBo-16). Splicing seismic data across the channel from the East (MeBo-16) to the West (MeBo-17) demonstrates the continuation of reflectors underneath the channel. The upper ∼50 m below seafloor (mbsf) at site MeBo-16 do not stratigraphically belong to the same sequence of the (deeper) levee-deposits. Above the marked erosional unconformity, sediments at Site MeBo-16 are likely derived by a mixture of repeated slump-events (identified as seismically transparent units) interbedded with hemi-pelagic sedimentation. Similarly, sediments within the upper ∼20 mbsf at Site MeBo-17 are not stratigraphically the same levee-deposits, but are derived by a mixture of slump-events (also seen in the marked seafloor amphitheatre architecture of a large failure complex extending further upslope) and hemi-pelagic sedimentation. All observations combined show that the seismically observed stratigraphic pattern represents a reflectivity sequence mostly driven by variations in density (porosity) and correspondingly by changes in P-wave velocity and electrical resistivity. All observations from the geophysical log- and core, as well as geochemical data do show no evidence for the presence of any significant gas hydrates within the drilled/cored sequences.

Description: The migration of methane through the gas hydrate stability zone (GHSZ) in the marine subsurface is characterized by highly dynamic reactive transport processes coupled to thermodynamic phase transitions between solid gas hydrates, free methane gas, and dissolved methane in the aqueous phase. The marine subsurface is essentially a water-saturated porous medium where the thermodynamic instability of the hydrate phase can cause free gas pockets to appear and disappear locally, causing the model to degenerate. This poses serious convergence issues for the general-purpose nonlinear solvers (e.g., standard Newton), and often leads to extremely small time-step sizes. The convergence problem is particularly severe when the rate of hydrate phase change is much lower than the rate of gas dissolution. In order to overcome this numerical challenge, we have developed an all-at-once Newton scheme tailored to our gas hydrate model, which can handle rate-based hydrate phase change coupled with equilibrium gas dissolution in a mathematically consistent and robust manner.