Sunil Vadakkepuliyambatta

Ice-sheet-driven methane storage and release in the Arctic

It is established that late-twentieth and twenty-first century ocean warming has forced dissociation of gas hydrates with concomitant seabed methane release. However, recent dating of methane expulsion sites suggests that gas release has been ongoing over many millennia. Here we synthesize observations of ∼1,900 fluid escape features—pockmarks and active gas flares—across a previously glaciated Arctic margin with ice-sheet thermomechanical and gas hydrate stability zone modelling. Our results indicate that even under conservative estimates of ice thickness with temperate subglacial conditions, a 500-m thick gas hydrate stability zone—which could serve as a methane sink—existed beneath the ice sheet. Moreover, we reveal that in water depths 150–520 m methane release also persisted through a 20-km-wide window between the subsea and subglacial gas hydrate stability zone. This window expanded in response to post-glacial climate warming and deglaciation thereby opening the Arctic shelf for methane release.

Extensive release of methane from Arctic seabed west of Svalbard during summer 2014 does not influence the atmosphere

We find that summer methane (CH4) release from seabed sediments west of Svalbard substantially increases CH4 concentrations in the ocean but has limited influence on the atmospheric CH4 levels. Our conclusion stems from complementary measurements at the seafloor, in the ocean, and in the atmosphere from land-based, ship and aircraft platforms during a summer campaign in 2014. We detected high concentrations of dissolved CH4 in the ocean above the seafloor with a sharp decrease above the pycnocline. Model approaches taking potential CH4 emissions from both dissolved and bubble-released CH4 from a larger region into account reveal a maximum flux compatible with the observed atmospheric CH4 mixing ratios of 2.4–3.8 nmol m−2 s−1. This is too low to have an impact on the atmospheric summer CH4 budget in the year 2014. Long-term ocean observatories may shed light on the complex variations of Arctic CH4 cycles throughout the year.

Abiotic methane from ultraslow-spreading ridges can charge Arctic gas hydrates

Biotic gas generation from the degradation of organic carbon in marine sediments supplies and maintains gas hydrates throughout the worlds oceans. In nascent, ultraslow-spreading ocean basins, methane generation can also be abiotic, occurring during the high-temperature (>200 °C) serpentinization of ultramafic rocks. Here, we report on the evolution of a growing Arctic gas- and gas hydrate–charged sediment drift on oceanic crust in eastern Fram Strait, a tectonically controlled, deep-water gateway between the subpolar North Atlantic and Arctic Oceans. Ultraslow-spreading ridges between northwest Svalbard and northeast Greenland permit the sustained interaction of a mid-ocean ridge transform fault and developing sediment drift, on both young ( 10 Ma) oceanic crust, since the late Miocene. Geophysical data image the gas-charged drift and crustal structure and constrain the timing of a major 30 km lateral displacement of the drift across the Molloy transform fault. We describe the buildup of a 2 m.y., long-lived gas hydrate– and free gas–charged drift system on young oceanic crust that may be fed and maintained by a dominantly abiotic methane source. Ultraslow-spreading, sedimented ridge flanks represent a previously unrecognized carbon reservoir for abiotic methane that could supply and maintain deep-water methane hydrate systems throughout the Arctic.

Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor

Massive methane blow-outs may be responsible for clusters of kilometer-wide craters in the Barents Sea. Methane takes the quick way out Accounting for all the sources and sinks of methane is important for determining its concentration in the atmosphere. Andreassen et al. found evidence of large craters embedded within methane-leaking subglacial sediments in the Barents Sea, Norway. They propose that the thinning of the ice sheet at the end of recent glacial cycles decreased the pressure on pockets of hydrates buried in the seafloor, resulting in explosive blow-outs. This created the giant craters and released large quantities of methane into the water above. Science, this issue p. 948 Widespread methane release from thawing Arctic gas hydrates is a major concern, yet the processes, sources, and fluxes involved remain unconstrained. We present geophysical data documenting a cluster of kilometer-wide craters and mounds from the Barents Sea floor associated with large-scale methane expulsion. Combined with ice sheet/gas hydrate modeling, our results indicate that during glaciation, natural gas migrated from underlying hydrocarbon reservoirs and was sequestered extensively as subglacial gas hydrates. Upon ice sheet retreat, methane from this hydrate reservoir concentrated in massive mounds before being abruptly released to form craters. We propose that these processes were likely widespread across past glaciated petroleum provinces and that they also provide an analog for the potential future destabilization of subglacial gas hydrate reservoirs beneath contemporary ice sheets.

Postglacial response of Arctic Ocean gas hydrates to climatic amelioration

Significance Shallow Arctic Ocean gas hydrate reservoirs experienced distinct episodes of subglacial growth and subsequent dissociation that modulated methane release over millennial timescales. Seafloor methane release due to the thermal dissociation of gas hydrates is pervasive across the continental margins of the Arctic Ocean. Furthermore, there is increasing awareness that shallow hydrate-related methane seeps have appeared due to enhanced warming of Arctic Ocean bottom water during the last century. Although it has been argued that a gas hydrate gun could trigger abrupt climate change, the processes and rates of subsurface/atmospheric natural gas exchange remain uncertain. Here we investigate the dynamics between gas hydrate stability and environmental changes from the height of the last glaciation through to the present day. Using geophysical observations from offshore Svalbard to constrain a coupled ice sheet/gas hydrate model, we identify distinct phases of subglacial methane sequestration and subsequent release on ice sheet retreat that led to the formation of a suite of seafloor domes. Reconstructing the evolution of this dome field, we find that incursions of warm Atlantic bottom water forced rapid gas hydrate dissociation and enhanced methane emissions during the penultimate Heinrich event, the Bølling and Allerød interstadials, and the Holocene optimum. Our results highlight the complex interplay between the cryosphere, geosphere, and atmosphere over the last 30,000 y that led to extensive changes in subseafloor carbon storage that forced distinct episodes of methane release due to natural climate variability well before recent anthropogenic warming.

Iceberg ploughmarks in the SW Barents Sea imaged using high-resolution P-Cable 3D seismic data

Iceberg ploughmarks are common features of glaciated continental margins worldwide. They form when iceberg keels plough through sediments on the ocean floor. They occur in various sizes and shapes, but generally exhibit a linear or curvilinear geometry. The SW Barents Sea is a heavily glaciated margin and its seafloor shows numerous ploughmarks among other glacial landforms (Andreassen et al. 2008). We present seafloor features formed during the late Weichselian glaciation (Solheim et al. 1990) imaged using high-resolution P-Cable 3D seismic data. P-Cable 3D seismic data have a spatial resolution of 6×6 m which is better than many hull-mounted multibeam systems, particularly in deep water. However, the cumulative seismic response of the seafloor is slightly different due to the lower-frequency bandwidth (30–350 Hz) of P-Cable compared to multibeam systems (up to 500 kHz). In addition, amplitude information is readily available and complements grid-based interpretations. Numerous linear and curvilinear depressions occur on the seafloor near …

Bottom-simulating reflector dynamics at Arctic thermogenic gas provinces: An example from Vestnesa Ridge, offshore west Svalbard

The Vestnesa Ridge comprises a > 100 km long sediment drift located between the western continental slope of Svalbard and the Arctic mid-ocean ridges. It hosts a deep-water (>1000 m) gas hydrate and associated seafloor seepage system. Near-seafloor headspace gas compositions and its methane carbon isotopic signature along the ridge indicate a predominance of thermogenic gas sources feeding the system. Prediction of the base of the gas hydrate stability zone for theoretical pressure and temperature conditions and measured gas compositions, results in an unusual underestimation of the observed bottom simulating reflector (BSR) depth. The BSR is up to 60 m deeper than predicted for pure methane and measured gas compositions with > 99 % methane. Models for measured gas compositions with > 4% higher order hydrocarbons result in a better BSR approximation. However, the BSR remains > 20 m deeper than predicted in a region without active seepage. A BSR deeper than predicted is primarily explained by unexpected spatial variations in the geothermal gradient and by larger amounts of thermogenic gas at the base of the gas hydrate stability zone. Hydrates containing higher order hydrocarbons form at greater depths and higher temperatures and contribute with larger amounts of carbons than pure methane hydrates. In thermogenic provinces, this may imply a significant upward revision (up to 50 % in the case of Vestnesa Ridge) of the amount of carbon in gas hydrates.

The history and future trends of ocean warming‐induced gas hydrate dissociation in the SW Barents Sea

The Barents Sea is a major part of the Arctic where the Gulf Stream mixes with the cold Arctic waters. Late Cenozoic uplift and glacial erosion have resulted in hydrocarbon leakage from reservoirs, evolution of fluid flow systems, shallow gas accumulations, and hydrate formation throughout the Barents Sea. Here we integrate seismic data observations of gas hydrate accumulations along with gas hydrate stability modeling to analyze the impact of warming ocean waters in the recent past and future (1960–2060). Seismic observations of bottom-simulating reflectors (BSRs) indicate significant thermogenic gas input into the hydrate stability zone throughout the SW Barents Sea. The distribution of BSR is controlled primarily by fluid flow focusing features, such as gas chimneys and faults. Warming ocean bottom temperatures over the recent past and in future (1960–2060) can result in hydrate dissociation over an area covering 0.03–38% of the SW Barents Sea.

Buried subglacial landforms in the SW Barents Sea imaged using high-resolution P-Cable seismic data

A large grounded ice sheet covered the epicontinental Barents Sea multiple times during the Late Pleistocene and left its erosional imprint (Solheim et al. 1990; Knies et al. 2009). Erosion involved the removal of large amounts of sediments from the Tertiary and Cretaceous sedimentary successions of the Barents Sea. The Upper Regional Unconformity (URU) represents the erosional base for continental-shelf glaciations (Vorren et al. 1988). Here, we present palaeo-landforms from this erosional base imaged using high-resolution P-Cable seismic data acquired along the border between the Ringvassoya Fault Complex and the Loppa High in the SW Barents Sea (Fig. 1). P-Cable seismic data have a frequency bandwidth of up to 300 Hz and spatial resolution of 6×6 m, which is much better than conventional 3D seismic data. Hence, interpretation of P-Cable data allows visualization of buried geomorphic landscapes in exceptional detail. In addition, amplitude information is readily available and complements grid-based interpretations. Fig. 1. Buried subglacial landforms in …

Shallow carbon storage in ancient buried thermokarst in the South Kara Sea

Geophysical data from the South Kara Sea reveal U-shaped erosional structures buried beneath the 50–250 m deep seafloor of the continental shelf across an area of ~32 000 km2. These structures are interpreted as thermokarst, formed in ancient yedoma terrains during Quaternary interglacial periods. Based on comparison to modern yedoma terrains, we suggest that these thermokarst features could have stored approximately 0.5 to 8 Gt carbon during past climate warmings. In the deeper parts of the South Kara Sea (>220 m water depth) the paleo thermokarst structures lie within the present day gas hydrate stability zone, with low bottom water temperatures −1.8 oC) keeping the gas hydrate system in equilibrium. These thermokarst structures and their carbon reservoirs remain stable beneath a Quaternary sediment blanket, yet are potentially sensitive to future Arctic climate changes.