Thermogenic methane injection via bubble transport into the upper Arctic Ocean from the hydrate-charged Vestnesa Ridge, Svalbard

We use new gas-hydrate geochemistry analyses, echosounder data, and three-dimensional P-Cable seismic data to study a gas-hydrate and free-gas system in 1200 m water depth at the Vestnesa Ridge offshore NW Svalbard. Geochemical measurements of gas from hydrates collected at the ridge revealed a thermogenic source. The presence of thermogenic gas and temperatures of similar to 3.3 degrees C result in a shallow top of the hydrate stability zone (THSZ) at similar to 340 m below sea level (mbsl). Therefore, hydrate-skinned gas bubbles, which inhibit gas-dissolution processes, are thermodynamically stable to this shallow water depth. This was confirmed by hydroacoustic observations of flares in 2010 and 2012 reaching water depths between 210 and 480 mbsl. At the seafloor, bubbles are released from acoustically transparent zones in the seismic data, which we interpret as regions where free gas is migrating through the hydrate stability zone (HSZ). These intrusions result in vertical variations in the base of the HSZ (BHSZ) of up to similar to 150 m, possibly making the shallow hydrate reservoir more susceptible to warming. Such Arctic gas-hydrate and free-gas systems are important because of their potential role in climate change and in fueling marine life, but remain largely understudied due to limited data coverage in seasonally ice-covered Arctic environments

A new methodology for quantifying bubble flow rates in deep water using splitbeam echosounders: Examples from the Arctic offshore NW-Svalbard

Quantifying marine methane fluxes of free gas (bubbles) from the seafloor into the water column is of importance for climate related studies, for example, in the Arctic, reliable methodologies are also of interest for studying man-made gas and oil leakage systems at hydrocarbon production sites. Hydroacoustic surveys with singlebeam and nowadays also multibeam systems have been proven to be a successful approach to detect bubble release from the seabed. A number of publications used singlebeam echosounder data to indirectly quantify free gas fluxes via empirical correlations between gas fluxes observed at the seafloor and the hydroacoustic response. Others utilize the hydroacoustic information in an inverse modeling approach to derive bubble fluxes. Here, we present an advanced methodology using data from splitbeam echosounder systems for analyzing gas release water depth (> 100m). We introduce a new MATLAB-based software for processing and interactively editing data and we present how bubble-size distribution, bubble rising speed and the model used for calculating the backscatter response of single bubbles influence the final gas flow rate calculations. As a result, we highlight the need for further investigations on how large, wobbly bubbles, bubble clouds, and multi-scattering influence target strength. The results emphasize that detailed studies of bubble-size distributions and rising speeds need to be performed in parallel to hydroacoustic surveys to achieve realistic mediated methane flow rate and flux quantifications

Mud extrusion dynamics constrained from 3D seismics in the Mercator Mud Volcano. El Arraiche mud volcano field, Gulf of Cadiz

Located on the western Moroccan continental shelf of the Gulf of Cadiz, the Mercator Mud Volcano (MMV) is one of a total of eight mud volcanoes which compose the El Arraiche mud volcano field. We collected a high-resolution P-cable 3D seismic cube during the Charles Darwin cruise 178 in April 2006, covering an area of 25 km2. The data image the upper 500-1000 m of the MMV. El Arraiche mud volcano field is located in the top of the Tortonian accretionary wedge in the Gulf of Cadiz, between 200 and 700 m water deep. Despite of the general compressive trend of the Gulf of Cadiz due to the westward movement of the Gibraltar arc, the local regimen of the western Moroccan margin is extensional in the study area. The MMV is a 2.5 km diameter positive conical structure at 350 m water deep that rises from the flank of a salt diapir. The high-resolution 3D cube shows the main internal structure in the southern flank of an anticline and a secondary structure southwest of it. Parallel and continuous reflections onlapping the anticline structure define the seismic character outside the mud volcano. The body of the main structure shows the typical "Christmas tree" features related to mud flow deposits. The preliminary interpretation of the 3D seismic cube shows four main mud flows southwestward oriented from the main structure and interfingered into the hemipelagic regional sedimentation. From deeper to shallower, the flows are located approximately at 0.870 s, 0.838 s, 0.774 s, and 0.685 s travel time, respectively. The extrusions correlate with the main seismic sequences observed in the surrounding hemipelagic deposits. The maximum run-out distance for the mud flows is approximately 1 km southwestward from the main structure, which corresponds to the third youngest mud flow described. The secondary "Christmas tree" structure penetrates the hemipelagic sediments almost to the seabed. Its seismic character is defined by low amplitude and chaotic signal. Several mud flows are interfingered with the surrounding sediments and, in some cases, overlap the mud flows from the main structure but they are less extensive and thinner but more frequent than those from the main structure. The MMV is an active mud volcano and depends on complex fluid and mud dynamics. The existence of a secondary and apparently "abandoned" structure indicates the variation of mud pathways during the evolution of its plumbing system

Large-scale sedimentation on the glacier-influenced polar North Atlantic Margins: Long-range side-scan sonar evidence

Long-range side-scan sonar (GLORIA) imagery of over 600,000 km² of the Polar North Atlantic provides a large-scale view of sedimentation patterns on this glacier-influenced continental margin. High-latitude margins are influenced strongly by glacial history and ice dynamics and, linked to this, the rate of sediment supply. Extensive glacial fans (up to 350,000 km³) were built up from stacked series of large debris flows transferring sediment down the continental slope. The fans were linked with high debris inputs from Quaternary glaciers at the mouths of cross-shelf troughs and deep fjords. Where ice was slower-moving, but still extended to the shelf break, large-scale slide deposits are observed. Where ice failed to cross the continental shelf during full glacials, the continental slope was sediment starved and submarine channels and smaller slides developed. A simple model for large-scale sedimentation on the glaciated continental margins of the Polar North Atlantic is presented

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 unaccounted 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