ALBERT

Description: Submarine mud volcanoes release sediments and gas-rich fluids at the seafloor via deeply-rooted plumbing systems that remain poorly understood. Here the functioning of Venere mud volcano, on the Calabrian accretionary prism in ~1,600 m water depth is investigated, based on multi-parameter hydroacoustic and visual seafloor data obtained using ship-borne methods, ROVs, and AUVs. Two seepage domains are recognized: mud breccia extrusion from a summit, and hydrocarbon venting from peripheral sites, hosting chemosynthetic ecosystems and authigenic carbonates indicative of long-term seepage. Pore fluids in freshly extruded mud breccia (up to 13 °C warmer than background sediments) contained methane concentrations exceeding saturation by 2.7 times and chloride concentrations up to five times lower than ambient seawater. Gas analyses indicate an underlying thermogenic hydrocarbon source with potential admixture of microbial methane during migration along ring faults to the peripheral sites. The gas and pore water analyses point to fluids sourced deep (〉3 km) below Venere mud volcano. An upward-branching plumbing system is proposed to account for co-existing mud breccia extrusion and gas seepage via multiple surface vents that influence the distribution of seafloor ecosystems. This model of mud volcanism implies that methane-rich fluids may be released during prolonged phases of moderate activity.

Description: We investigated gas bubble emissions at the Don-Kuban paleo-fan in the northeastern Black Sea regarding their geological setting, quantities as well as spatial and temporal variabilities during three ship expeditions between 2007 and 2011. About 600 bubble-induced hydroacoustic anomalies in the water column (flares) originating from the seafloor above the gas hydrate stability zone (GHSZ) at ~700 m water depth were found. At about 890 m water depth a hydrocarbon seep area named "Kerch seep area" was newly discovered within the GHSZ. We propose locally domed sediments ('mounds') discovered during ultra-high resolution bathymetric mapping with an autonomous underwater vehicle (AUV) to result from gas hydrate accumulation at shallow depths. In situ measurements indicated spatially limited temperature elevations in the shallow sediment likely induced by upward fluid flow which may confine the local GHSZ to a few meters below the seafloor. As a result, gas bubbles are suspected to migrate into near-surface sediments and to escape the seafloor through small-scale faults. Hydroacoustic surveys revealed that several flares originated from a seafloor area of about 1 km**2 in size. The highest flare disappeared in about 350 m water depth, suggesting that the released methane remains in the water column. A methane flux estimate, combining data from visual quantifications during dives with a remotely operated vehicle (ROV) with results from ship-based hydroacoustic surveys and gas analysis revealed that between 2 and 87 x 10**6 mol CH4 yr-1 escaped into the water column above the Kerch seep area. Our results show that the finding of the Kerch seep area represents a so far underestimated type of hydrocarbon seep, which has to be considered in methane budget calculations.

Description: We mapped, sampled, and quantified gas emissions at the continental margin west of Svalbard during R/V Heincke cruise He-387 in late summer 2012. Hydroacoustic mapping revealed that gas emissions were not limited to a zone just above 396 m below sea level (m b.s.l.). Flares from this depth gained significant attention in the scientific community in recent years because they may be caused by bottom water-warming induced hydrate dissolution in the course of global warming and/or by recurring seasonal hydrate formation and decay. We found that gas emissions occurred widespread between about 80 and 415 m b.s.l. which indicates that hydrate dissolution might only be one of several triggers for active hydrocarbon seepage in that area. Gas emissions were remarkably intensive at the main ridge of the forlandet moraine complex in 80 to 90 m water depths, and may be related to thawing permafrost. Focused seafloor investigations were performed with the remotely operated vehicle (ROV) "Cherokee". Geochemical analyses of gas bubbles sampled at about 240 m b.s.l. as well as at the 396 m gas emission sites revealed that the vent gas is primarily composed of methane (〉 99.70%) of microbial origin (average d13C = -55.7 per mil V-PDB). Estimates of the regional gas bubble flux from the seafloor to the water column in the area of possible hydrate decomposition were achieved by combining flare mapping using multibeam and single beam echosounder data, bubble stream mapping using a ROV-mounted horizontally-looking sonar, and quantification of individual bubble streams using ROV imagery and bubble counting. We estimated that about 53 × 10**6 mol methane were annually emitted at the two areas and allow a large range of uncertainty due to our method (9 to 118 × 10**6 mol yr**-1). These amounts, first, show that gas emissions at the continental margin west of Svalbard were in the same order of magnitude as bubble emissions at other geological settings, and second, may be used to calibrate models predicting hydrate dissolution at present and in the future, third, may serve as baseline (year 2012) estimate of the bubble flux that will potentially increase in future due to ever-increasing global-warming induced bottom water-warming and hydrate dissolution.

Description: Evidence for twelve sites with gas bubble emissions causing hydroacoustic anomalies in 18 kHz echosounder records ('flares') was obtained at the convergent Makran continental margin. The hydroacoustic anomalies originating from hydrocarbon seeps at water depths between 575 and 2870 m disappeared after rising up to 2000 m in the water column. Dives with the remotely operated vehicle 'Quest 4000 m' revealed that several individual bubble vents contributed to one hydroacoustic anomaly. Analyzed gas samples suggest that bubbles were mainly composed of methane of microbial origin. Bubble size distributions and rise velocities were determined and the volume flux was estimated by counting the emitted bubbles and using their average volume. We found that a low volume flux (Flare 1 at 575 mbsl: 90 ml/min) caused a weak hydroacoustic signal in echograms whereas high volume fluxes (Flare 2 at 1027 mbsl: 1590 ml/min; Flare 5 C at 2870 mbsl: 760 ml/min) caused strong anomalies. The total methane bubble flux in the study area was estimated by multiplying the average methane flux causing a strong hydroacoustic anomaly in the echosounder record with the total number of equivalent anomalies. An order-of-magnitude estimate further considers the temporal variability of some of the flares, assuming a constant flux over time, and allows a large range of uncertainty inherent to the method. Our results on the fate of bubbles and the order-of-magnitude estimate suggest that all of the ?40 ± 32 ? 106 mol methane emitted per year within the gas hydrate stability zone remain in the deep ocean.

Description: Submarine mud volcanoes develop through the extrusion of methane-rich fluids and sediments onto the seafloor. The morphology of a mud volcano can record its extrusive history and processes of erosion and deformation affecting it. The study of offshore mud-volcano dynamics is limited because only few have been mapped at resolutions that reveal their detailed surface structures. More importantly, rates and volumes of extruded sediment and methane are poorly constrained. The 100 m high twin cones of Venere mud volcano are situated at ~1600 m water depth within Squillace Canyon along the Ionian Calabrian margin, Mediterranean Sea. Seafloor bathymetry, and backscatter data obtained by a ship-based system and an autonomous underwater vehicle (AUV) allow mapping of mudflow deposits of the mud volcano and bedforms in the surrounding canyon. Repeated surveying by AUV document active mud movement at the western summit in between 2014 and 2016. Through sediment coring and tephrochronology, ages of buried mudflow deposits are determined based on the sedimentation rate and the thickness of overlying hemipelagic sediments. An average extrusion rate of 27000 m^3/year over the last ~882 years is estimated. These results support a three-stage evolutionary model of Venere mud volcano since ~4000 years ago. It includes the onset of quiescence at the eastern cone (after ~2200 years ago), erosive events in Squillace Canyon (prior to ~882 years ago), and mudflows from the eastern cone (since ~882 years). This study reveals new interactions between a mud volcano and a canyon in the deep sea.

Description: In 2003, the Chapopote asphalt flow was discovered in the southern Gulf of Mexico at a depth of 2,900 m. Subsequent exploration has expanded the known extent of asphalt volcanism across abyssal depths in much of this region. Aspects of asphalt flow morphology are analogous to ropy pāhoehoe flows known from eruptions of basaltic lava on land, but the timing and formation sequence of asphalt flows has been difficult to infer because limited visibility in the deep ocean makes it challenging to image large areas of the seafloor. Combining data from autonomous underwater vehicle mapping and remotely operated vehicle navigation with powerful optical mosaicking techniques, we assembled georeferenced images of the Chapopote asphalt flows. The largest image captured an area of 3,300 m² with over 15 billion pixels and resolved objects at centimeter scale. Augmenting this optical resolution with microbathymetry led to the recognition that very large asphalt pavements exhibiting highly varied morphologies and weathering states comprised a series of at least three separate flow units, one on top of another. The Chapopote asphalt volcano likely erupts during phases of intensified activity separated by periods of reduced activity. After extrusion, chemical and physical changes in the asphalt generate increasing viscosity gradients both along the flow path and between the flow's surface and core. This allows the asphalt to form pāhoehoe lava-like shapes and to support dense chemosynthetic communities over timescales of hundreds of years.