Lea Steinle

Temporal Constraints on Hydrate-Controlled Methane Seepage off Svalbard

What Does It All Mean? Strong emissions of methane have recently been observed from shallow sediments in Arctic seas. Berndt et al. (p. 284, published online 2 January) present a record of methane seepage from marine sediments off the coast of Svalbard showing that such emissions have been present for at least 3000 years, the result of normal seasonal fluctuations of bottom waters. Thus, contemporary observations of strong methane venting do not necessarily mean that the clathrates that are the source of the methane are decomposing at a faster rate than in the past. Seasonal gas hydrate destabilization has been releasing methane from marine sediments near Svalbard for at least 3000 years. Methane hydrate is an icelike substance that is stable at high pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may possibly accelerate global warming. Here, we corroborate that hydrates play a role in the observed seepage of gas, but we present evidence that seepage off Svalbard has been ongoing for at least 3000 years and that seasonal fluctuations of 1° to 2°C in the bottom-water temperature cause periodic gas hydrate formation and dissociation, which focus seepage at the observed sites.

Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard

Widespread seepage of methane from seafloor sediments offshore Svalbard close to the landward limit of the gas hydrate stability zone (GHSZ) may, in part, be driven by hydrate destabilization due to bottom water warming. To assess whether this methane reaches the atmosphere where it may contribute to further warming, we have undertaken comprehensive surveys of methane in seawater and air on the upper slope and shelf region. Near the GHSZ limit at ∼400 m water depth, methane concentrations are highest close to the seabed, reaching 825 nM. A simple box model of dissolved methane removal from bottom waters by horizontal and vertical mixing and microbially mediated oxidation indicates that ∼60% of methane released at the seafloor is oxidized at depth before it mixes with overlying surface waters. Deep waters are therefore not a significant source of methane to intermediate and surface waters; rather, relatively high methane concentrations in these waters (up to 50 nM) are attributed to isopycnal turbulent mixing with shelf waters. On the shelf, extensive seafloor seepage at <100 m water depth produces methane concentrations of up to 615 nM. The diffusive flux of methane from sea to air in the vicinity of the landward limit of the GHSZ is ∼4–20 μmol m−2 d−1, which is small relative to other Arctic sources. In support of this, analyses of mole fractions and the carbon isotope signature of atmospheric methane above the seeps do not indicate a significant local contribution from the seafloor source.

Geological settings and seafloor morphodynamic evolution linked to methane seepage

Methane seeps have been shown to be a powerful agent in modifying seabed morphology, amongst others by cementation processes such as the formation of methane-derived authigenic carbonates (MDACs). The cements stabilise mobile sediment particles and thereby promote the formation of edifices such as mounds on various scales. The release of methane from shallow subsurface sources, when concentrated in seeps, has proven hazardous to offshore construction activities. In this paper, methane cycling and MDAC precipitation is explored as a potential “finger on the pulse” for the recognition of shallow gas pockets and active gas seepage. This would provide a valuable planning tool for seabed engineering developments in areas of potential gas seepage. Measurements of methane concentrations in the Irish Sea are correlated with a unique record of longer-term morphological evolution (up to 11 years) of MDAC structures and subsurface geological settings which would favour the build-up of shallow gas. It was found that gas seepage activity associated with fault zones correlates with carbonate mound steepness. Cessation of gas seepage results in a relatively slow process of erosion and burial of the mounds, eventually producing a subdued carbonate mound morphology after several decades. The Quaternary glacial legacy equally seems to define the distribution and geometry of the MDAC structures. In this case, methane gas locally concentrated in sands and gravels capped by clayey glacial sediments may percolate upwards to the seafloor. A link between methane seeps and the formation of unusually large, trochoidally shaped sediment waves observed on continental shelves worldwide is deemed unlikely. However, the observations suggest that gas percolating through sediment waves may be capped by muddy sediments which have deposited on the sediment waves due to anoxic conditions or eroded from a neighbouring cliff. Other sediment waves in the Irish Sea were found to have a step-like morphology similar to that documented in the neighbouring MDAC cemented seafloor. These processes may influence sediment waves dynamics and warrant further investigation.

Life on the edge: active microbial communities in the Kryos MgCl 2 -brine basin at very low water activity

The Kryos Basin is a deep-sea hypersaline anoxic basin (DHAB) located in the Eastern Mediterranean Sea (34.98°N 22.04°E). It is filled with brine of re-dissolved Messinian evaporites and is nearly saturated with MgCl2-equivalents, which makes this habitat extremely challenging for life. The strong density difference between the anoxic brine and the overlying oxic Mediterranean seawater impedes mixing, giving rise to a narrow chemocline. Here, we investigate the microbial community structure and activities across the seawater–brine interface using a combined biogeochemical, next-generation sequencing, and lipid biomarker approach. Within the interface, we detected fatty acids that were distinctly 13C-enriched when compared to other fatty acids. These likely originated from sulfide-oxidizing bacteria that fix carbon via the reverse tricarboxylic acid cycle. In the lower part of the interface, we also measured elevated rates of methane oxidation, probably mediated by aerobic methanotrophs under micro-oxic conditions. Sulfate reduction rates increased across the interface and were highest within the brine, providing first evidence that sulfate reducers (likely Desulfovermiculus and Desulfobacula) thrive in the Kryos Basin at a water activity of only ~0.4 Aw. Our results demonstrate that a highly specialized microbial community in the Kryos Basin has adapted to the poly-extreme conditions of a DHAB with nearly saturated MgCl2 brine, extending the known environmental range where microbial life can persist.

Shallow Gas Migration along Hydrocarbon Wells–An Unconsidered, Anthropogenic Source of Biogenic Methane in the North Sea

Shallow gas migration along hydrocarbon wells constitutes a potential methane emission pathway that currently is not recognized in any regulatory framework or greenhouse gas inventory. Recently, the first methane emission measurements at three abandoned offshore wells in the Central North Sea (CNS) were conducted showing that considerable amounts of biogenic methane originating from shallow gas accumulations in the overburden of deep reservoirs were released by the boreholes. Here, we identify numerous wells poking through shallow gas pockets in 3-D seismic data of the CNS indicating that about one-third of the wells may leak, potentially releasing a total of 3-17 kt of methane per year into the North Sea. This poses a significant contribution to the North Sea methane budget. A large fraction of this gas (∼42%) may reach the atmosphere via direct bubble transport (0-2 kt yr-1) and via diffusive exchange of methane dissolving in the surface mixed layer (1-5 kt yr-1), as indicated by numerical modeling. In the North Sea and in other hydrocarbon-prolific provinces of the world shallow gas pockets are frequently observed in the sedimentary overburden and aggregate leakages along the numerous wells drilled in those areas may be significant.