Description: Highlights • Deep-sea mineral exploration and exploitation licenses have been issued recently. • Mining will modify the abiotic and biotic environment. • At directly mined sites, species are removed and cannot resist disturbance. • Recovery is highly variable in distinct ecosystems and among benthic taxa. • Community changes may persist over geological time-scales at directly mined sites. Abstract With increasing demand for mineral resources, extraction of polymetallic sulphides at hydrothermal vents, cobalt-rich ferromanganese crusts at seamounts, and polymetallic nodules on abyssal plains may be imminent. Here, we shortly introduce ecosystem characteristics of mining areas, report on recent mining developments, and identify potential stress and disturbances created by mining. We analyze species’ potential resistance to future mining and perform meta-analyses on population density and diversity recovery after disturbances most similar to mining: volcanic eruptions at vents, fisheries on seamounts, and experiments that mimic nodule mining on abyssal plains. We report wide variation in recovery rates among taxa, size, and mobility of fauna. While densities and diversities of some taxa can recover to or even exceed pre-disturbance levels, community composition remains affected after decades. The loss of hard substrata or alteration of substrata composition may cause substantial community shifts that persist over geological timescales at mined sites.

Description: Natural gas hydrates are considered a potential resource for gas production on industrial scales. Gas hydrates contribute to the strength and stiffness of the hydrate-bearing sediments. During gas production, the geomechanical stability of the sediment is compromised. Due to the potential geotechnical risks and process management issues, the mechanical behavior of the gas hydrate-bearing sediments needs to be carefully considered. In this study, we describe a coupling concept that simplifies the mathematical description of the complex interactions occurring during gas production by isolating the effects of sediment deformation and hydrate phase changes. Central to this coupling concept is the assumption that the soil grains form the load-bearing solid skeleton, while the gas hydrate enhances the mechanical properties of this skeleton. We focus on testing this coupling concept in capturing the overall impact of geomechanics on gas production behavior though numerical simulation of a high-pressure isotropic compression experiment combined with methane hydrate formation and dissociation. We consider a linear-elastic stress-strain relationship because it is uniquely defined and easy to calibrate. Since, in reality, the geomechanical response of the hydrate-bearing sediment is typically inelastic and is characterized by a significant shear-volumetric coupling, we control the experiment very carefully in order to keep the sample deformations small and well within the assumptions of poroelasticity. The closely coordinated experimental and numerical procedures enable us to validate the proposed simplified geomechanics-to-flow coupling, and set an important precursor toward enhancing our coupled hydro-geomechanical hydrate reservoir simulator with more suitable elastoplastic constitutive models.

Description: Highlights • A stack of four BSRs were identified in levee deposits of the Danube deep-sea fan. • The multiple BSRs are not caused by overpressure compartments. • The multiple BSRs reflect stages of stable sealevel lowstands during glacial times. • Gas underneath the previous GHSZ does not start to migrate for thousands of years. Abstract High-resolution 2D seismic data reveal the character and distribution of up to four stacked bottom simulating reflectors (BSR) within the channel-levee systems of the Danube deep-sea fan. The theoretical base of the gas hydrate stability zone (GHSZ) calculated from regional geothermal gradients and salinity data is in agreement with the shallowest BSR. For the deeper BSRs, BSR formation due to overpressure compartments can be excluded because the necessary gas column would exceed the vertical distance between two overlying BSRs. We show instead that the deeper BSRs are likely paleo BSRs caused by a change in pressure and temperature conditions during different limnic phases of the Black Sea. This is supported by the observation that the BSRs correspond to paleo seafloor horizons located in a layer between a buried channel-levee system and the levee deposits of the Danube channel. The good match of the observed BSRs and the BSRs predicted from deposition of these sediment layers indicates that the multiple BSRs reflect stages of stable sealevel lowstands possibly during glacial times. The observation of sharp BSRs several 10,000 of years but possibly up to 300,000 yr after they have left the GHSZ demonstrates that either hydrate dissociation does not take place within this time frame or that only small amounts of gas are released that can be transported by diffusion. The gas underneath the previous GHSZ does not start to migrate for several thousands of years.

Description: Highlights • High abundance of active anaerobic methanotrophs in sediments of the blowout crater suggests adaptation to methane seepage within at most two decades. • Fast exchange processes in permeable surface sediments prevent sulfate depletion and probably methane-derived carbonate precipitation. • Methane seepage impacts isotopic and assemblage composition of benthic foraminifera. Abstract Methane emissions from marine sediments are partly controlled by microbial anaerobic oxidation of methane (AOM). AOM provides a long-term sink for carbon through precipitation of methane-derived authigenic carbonates (MDAC). Estimates on the adaptation time of this benthic methane filter as well as on the establishment of related processes and communities after an onset of methane seepage are rare. In the North Sea, considerable amounts of methane have been released since 20 years from a man-made gas blowout offering an ideal natural laboratory to study the effects of methane seepage on initially “pristine” sediment. Sediment cores were taken from the blowout crater and a reference site (50 m distance) in 2011 and 2012, respectively, to investigate porewater chemistry, the AOM community and activity, the presence of authigenic carbonates, and benthic foraminiferal assemblages. Potential AOM activity (up to 3060 nmol cm−3 sediment d−1 or 375 mmol m−2 d−1) was detected only in the blowout crater up to the maximum sampling depth of 18 cm. CARD-FISH analyzes suggest that monospecific ANME-2 aggregates were the only type of AOM organisms present, showing densities (up to 2.2*107 aggregates cm−3) similar to established methane seeps. No evidence for recent MDAC formation was found using stable isotope analyzes (δ13C and δ18O). In contrast, the carbon isotopic signature of methane was recorded by the epibenthic foraminifer Cibicides lobatulus (δ13C −0.66‰). Surprisingly, the foraminiferal assemblage in the blowout crater was dominated by Cibicides and other species commonly found in the Norwegian Channel and fjords, indicating that these organisms have responded sensitively to the specific environmental conditions at the blowout. The high activity and abundance of AOM organisms only at the blowout site suggests adaptation to a strong increase in methane flux in the order of at most two decades. High gas discharge dynamics in permeable surface sediments facilitate fast sulfate replenishing and stimulation of AOM. The accompanied prevention of total alkalinity build-up in the porewater thereby appears to inhibit the formation of substantial methane-derived authigenic carbonate at least within the given time window.

Description: Highlights • Polypropylene and biodegradable plastic bags were incubated in marine sediments. • Bacterial colonization was highest on biodegradable plastic bags. • None of the two bag types showed signs of degradation after 98 days. • Marine sediments probably represent a long-term sink for both types of litter. Abstract To date, the longevity of plastic litter at the sea floor is poorly constrained. The present study compares colonization and biodegradation of plastic bags by aerobic and anaerobic benthic microbes in temperate fine-grained organic-rich marine sediments. Samples of polyethylene and biodegradable plastic carrier bags were incubated in natural oxic and anoxic sediments from Eckernförde Bay (Western Baltic Sea) for 98 days. Analyses included (1) microbial colonization rates on the bags, (2) examination of the surface structure, wettability, and chemistry, and (3) mass loss of the samples during incubation. On average, biodegradable plastic bags were colonized five times higher by aerobic and eight times higher by anaerobic microbes than polyethylene bags. Both types of bags showed no sign of biodegradation during this study. Therefore, marine sediment in temperate coastal zones may represent a long-term sink for plastic litter and also supposedly compostable material.

Description: Current estimates suggest that more than 60% of the global seafloor are covered by millions of abyssal hills and mountains. These features introduce spatial fluid-dynamic granularity whose influence on deep-ocean sediment biogeochemistry is unknown. Here we compare biogeochemical surface-sediment properties from a fluid-dynamically well-characterized abyssal hill and upstream plain: (1) In hill sediments, organic-carbon and -nitrogen contents are only about half as high as on the plain while proteinaceous material displays less degradation; (2) on the hill, more coarse-grained sediments (reducing particle surface area) and very variable calcite contents (influencing particle surface charge) are proposed to reduce the extent, and influence compound-specificity, of sorptive organic-matter preservation. Further studies are needed to estimate the representativeness of the results in a global context. Given millions of abyssal hills and mountains, their integrative influence on formation and composition of deep-sea sediments warrants more attention.

Description: Highlights • PetroMod is the 1st basin modelling software including methane hydrate simulation. • The Gas hydrate module includes physical, thermodynamic, and kinetic properties. • PetroMod simulates the evolution over time of the GHSZ. • PetroMod includes a kinetic for the organic matter degradation at low temperature. Abstract Within the German gas hydrate initiative SUGAR, a new 2-D/3-D module simulating the biogenic generation of methane from organic matter and the formation of gas hydrates has been developed and included in the petroleum systems modelling software package PetroMod®. Typically, PetroMod® simulates the thermogenic generation of multiple hydrocarbon components (oil and gas), their migration through geological strata, finally predicting oil and gas accumulations in suitable reservoir formations. We have extended PetroMod® to simulate gas hydrate accumulations in marine and permafrost environments by the implementation of algorithms describing (1) the physical, thermodynamic, and kinetic properties of gas hydrates; and (2) a kinetic continuum model for the microbially mediated, low temperature degradation of particulate organic carbon in sediments. Additionally, the temporal and spatial resolutions of PetroMod® were increased in order to simulate processes on time scales of hundreds of years and within decimetres of spatial extension. In order to validate the abilities of the new hydrate module, we present here results of a theoretical layer-cake model. The simulation runs predict the spatial distribution and evolution in time of the gas hydrate stability field, the generation and migration of thermogenic and biogenic methane gas, and its accumulation as gas hydrates.

Description: Takahe seep, located on the Opouawe Bank, Hikurangi Margin, is characterized by a well-defined subsurface seismic chimney structure ca. 80,500 m2 in area. Sub-seafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure. Reaction-transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m. Model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas. The dissolution of gas (up to 3761 mmol m−2 yr−1) dwarfed the rate of methanogenesis within the simulated sediment column (2.6 mmol m−2 yr−1). Dissolved methane is mainly consumed by anaerobic oxidation of methane (AOM) at the base of the sulfate reduction zone and trapped by methane hydrate formation below it, with maximum rates in the central part of the chimney (946 and 2420 mmol m−2 yr−1, respectively). A seep-wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of AOM rates, gas hydrate formation and benthic dissolved methane fluxes of 3.68 × 104 mol yr−1, 73.85 × 104 mol yr−1and 1.19 × 104 mol yr−1, respectively. A much larger flux of methane probably escapes in gaseous form through focused bubble vents. The approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales.

Description: The weathering of silicate minerals exposed on the continents is the largest sink of atmospheric CO2 on time scales of millions of years. The rate of this process is positively correlated with global mean temperature and atmospheric CO2 concentration, resulting in a negative feedback that stabilizes Earths’ climate (Berner, 2004). Detrital silicates derived from the physical denudation of the continents are a major component of marine sediments (Li and Schoonmaker, 2003). However, their geochemical behaviour is poorly understood and they are considered to be unimportant to the long-term carbon cycle. We show that in organic matter-rich sediments of the Sea of Okhotsk detrital silicates undergo intense weathering. This process is likely favoured by microbial activity, which lowers pore water pH and releases dissolved humic substances, and by the freshness of detrital silicates which originate from the cold, poorly weathered Amur River basin. Numerical simulations of early diagenesis show that submarine weathering rates in our study area are comparable to average continental weathering rates (Gaillardet et al., 1999). Furthermore, silicate weathering seems to be widespread in organic matter-rich sediments of continental margins, suggesting the existence of a significant CO2 sink there. These findings imply a greater efficiency of the silicate weathering engine also at low surface temperatures, resulting in a weakening of the negative feedback between pCO2, climate evolution and silicate weathering.