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Patras Gulf pockmark field (Western Greece) is a tectonically controlled field that has been activated at least twice by strong earthquakes (M5.4, 14 July 1993 and M6.4, 8 June 2008), and episodic gas seepages have been recorded in the past using geophysical means. A distributed temperature sensor (DTS) system was deployed inside a shallow pockmark and along an active fault at the northern end of the field. This ongoing experiment represents the first long-term monitoring ever conducted on gas-bearing pockmarks and active faults by the DTS system. For now, we have acquired and analyzed data regarding about 1.56 years. One of the primary objectives of this study is to establish methodological queues for data processing and analysis, including spectral analysis and incomplete data treatment techniques, to be standardized for use in further stages of the experiment. Spectral analysis was proven capable of separating the temperature footprint of background environmental components, such as sea-atmosphere heat flux, tides, and winds/waves, from high-frequency temperature residuals. Those residuals represent unusual events that might be correlated to seismicity. Monitoring the causal relationship between seismic activity and seabed water temperature changes in the field was thus attempted. No significant local earthquakes occurred during the monitoring period. Although the relation between seismicity and irregular seabed water temperature events was not systematic, we postulate that four thermal events have a causative link with the local seismicity. The DTS system constitutes a low-cost monitoring system, and the promising preliminary results of this experiment suggest that it is worth testing for a longer period. [ABSTRACT FROM AUTHOR]

Ubiquitous pockmarks in continental margins are important to understand the tectonic and sedimentary history and flow activity of their host margins. The study of an extensive field of pockmarks in the South China Sea (SCS) is important both scientifically and economically, for the exploration of oil, gas, and gas hydrate resources and the prevention and control of marine geological disasters. We report our new findings on the characteristics, distribution, and genesis of pockmarks in the SCS based on interpretation of high-resolution multibeam bathymetric data and multichannel seismic profiles collected in the past decade. Three pockmark fields (total area 572,824 km2), are recognized in the northern, western, and southern continental margins of the SCS. The pockmarks are highly variable in shape: circular, elliptical, crescent-shaped, elongated or forming a chain in plan view and U-, V- or W-shaped in cross-section. Pockmarks are normally 10–200 m in diameter, with maximum diameters/axial lengths of 7.3 km for circular, 5.5 km for elliptical, 8.6 km for crescent-shaped, 9.5 km for elongated, and 30 km for chain-type pockmarks. Pockmarks are normally less than 35 m deep, with a maximum depth of 341 m at the southwestern SCS margin. These pockmarks might have originated from methane seepages or from pore fluids from deeper sources, which migrated vertically along gas chimneys, mud diapirs, mud volcanoes, faults or buried channels. In addition to possible structural controls, the pockmark morphologies are affected by bottom currents. [ABSTRACT FROM AUTHOR]

Understanding the dynamics and fate of methane (CH4) release from oceanic seepages on margins and shelves into the water column, and quantifying the budget of its total discharge at different spatial and temporal scales, currently represents a major scientific undertaking. Previous works on the fate of methane escaping from the seafloor underlined the challenge in both, estimating its concentration distribution and identifying gradients. In April 2019, the Envri Methane Cruise has been conducted onboard the R/V Mare Nigrum in the Western Black Sea to investigate two shallow methane seep sites at ∼120 m and ∼55 m water depth. Dissolved CH4 measurements were conducted with two continuous in-situ sensors: a membrane inlet laser spectrometer (MILS) and a commercial methane sensor (METS) from Franatech GmbH. Additionally, discrete water samples were collected from CTD-Rosette deployment and standard laboratory methane analysis was performed by gas chromatography coupled with either purge-and-trap or headspace techniques. The resulting vertical profiles (from both in situ and discrete water sample measurements) of dissolved methane concentration follow an expected exponential dissolution function at both sites. At the deeper site, high dissolved methane concentrations are detected up to ∼45 m from the seabed, while at the sea surface dissolved methane was in equilibrium with the atmospheric concentration. At the shallower site, sea surface CH4 concentrations were four times higher than the expected equilibrium value. Our results seem to support that methane may be transferred from the sea to the atmosphere, depending on local water depths. In accordance with previous studies, the shallower the water, the more likely is a sea-to-atmosphere transport of methane. High spatial resolution surface data also support this hypothesis. Well localized methane enriched waters were found near the surface at both sites, but their locations appear to be decoupled with the ones of the seafloor seepages. This highlights the need of better understanding the processes responsible for the transport and transformation of the dissolved methane in the water column, especially in stratified water masses like in the Black Sea.

Aim of this work is to show evidences for pre-earthquake processes and to set up earthquake forecasting experiments, based on two hypotheses: 1) extreme change of the activity of underwater methane seeps appears days before earthquakes; 2) effects from increase of shallow water seeps emissions are visible on satellite images. [ABSTRACT FROM AUTHOR]

Abstract: Multibeam echosounder systems (MBES) have long provided bathymetric data with high temporal and spatial resolution. In the last couple of decades, MBES observations of scattering in the water column have been finding increasing use in oceanographic studies. Here we review the wealth of studies using water column multibeam data to address questions in fisheries, marine mammal and zooplankton research as well as seeps and hydrothermal vents. We also summarize some of the tantalizing new oceanographic applications of water column MBES, such as kelp ecosystems, near surface bubbles, suspended sediment, mixing and internal waves, as well as the proper determination of the extent of shipwreck above the sea floor. We highlight the many advantages of using water column MBES and discuss the challenges. [Copyright &y& Elsevier]

Abstract: A high-resolution seismic survey, piston coring, and water column measurements were conducted on the continental shelf off southeastern Korea to examine the gas-charged sediments and gas seepage in the water column. We examined the factors controlling the spatial distribution of the acoustic blanking zone in recent Korea Strait shelf mud (KSSM) deposits along the southeastern shelf. The molecular composition, methane, hydrogen, and carbon dioxide isotope values of headspace gases, and the geochemical composition of core sediments, were measured in 11 piston cores. Dissolved methane concentrations in the water column were observed at 6 of the 11 coring sites. Acoustic blanking of the recent KSSM deposits was limited to areas with water depths between 30 and 105m along the continental shelf, except line L-13 (133m depth). The flanks of the acoustic blanking zone can be classified as distinct, disturbed, and step types based on their configurations. The highest methane concentration in the headspace gases was about 2.7%, 2–4m below the seafloor, which matched the top of the acoustic blanking zone. No obvious methane was found in piston cores obtained from the gas-free zone. The δ13CCH4 ranged from −56.5 to −106.3‰ relative to Vienna Pee Dee Belemnite (V-PDB), and the δDCH4 ranged from −194 to −207‰ relative to Vienna Standard Mean Ocean Water (V-SMOW). These isotope values show that methane is produced by the microbial reduction of CO2. CH4 oxidation results in a significantly heavier carbon isotope fraction (from –91.2 to –56.5‰ in the 2m below the seafloor) with constant δ13CCO2 isotope fractionation. No obvious differences were seen in the total carbon (TC; 2.438–3.378%), total nitrogen (TN; 0.159–0.548%), total organic carbon (TOC; 1.139–2.078%), or CaCO3 (2.825–15.258%) in the continental shelf, whereas the TOC was high and the CaCO3 low distant from the Nakdong River. Distinct dissolved methane anomalies (21.87ppm) were seen in the water column in the acoustic blanking and gas seepage zones. This level was about 15 times higher than the dissolved methane level (1.5ppm) in the water column in the gas-free zone. These results indicate that the acoustic blanking zone accumulates biogenic methane. Methane seepage was observed in the water column throughout the acoustic blanking zone of the continental shelf off southeastern Korea. [Copyright &y& Elsevier]

Abstract: Based on high-resolution Chirp seismic, multibeam bathymetry and side scan sonar data collected in the İzmir Gulf, Aegean Sea in 2008 and 2010, gas-related structures have been identified, which can be classified into three categories: (1) shallow gas accumulations and gas chimneys, (2) mud diapirs, and (3) active and inactive pockmarks. On the Chirp profiles, shallow gas accumulations were observed along the northern coastline of the outer İzmir Gulf at 3–20 m below the seabed. They appear as acoustic turbidity zones and are interpreted as biogenic gas accumulations produced in organic–rich highstand fan sediments from the Gediz River. The diapiric structures are interpreted as shale or mud diapirs formed under lateral compression due to regional counter–clockwise rotation of Anatolian microplate. Furthermore, the sedimentary structure at the flanks suggests a continuous upward movement of the diapirs. Several pockmarks exist close to fault traces to the east of Hekim Island; most of them were associated with acoustic plumes indicating active degassing during the survey period in 2008. Another Chirp survey was carried out just over these plumes in 2010 to demonstrate if the gas seeps were still active. The surveys indicate that the gas seep is an ongoing process in the gulf. Based on the Chirp data, we proposed that the pockmark formation in the area can be explained by protracted seep model, whereby sediment erosion and re–distribution along pockmark walls result from ongoing (or long lasting) seepage of fluids over long periods of time. The existence of inactive pockmarks in the vicinity, however, implies that gas seepage may eventually cease or that it is periodic. Most of the active pockmarks are located over the fault planes, likely indicating that the gas seepage is controlled by active faulting. [Copyright &y& Elsevier]

Abstract: Methane concentrations and carbon stable isotopic ratios (δ13CCH4), oxygen concentrations and hydrographic parameters (CTD) were investigated in the water column of the north-western Black Sea during the summers of 2003 and 2004. Water samples were collected along a transect which crosses three methane seep areas in 90, 220, and 600m water depth. These active seeps strongly influence methane distribution in the overlying anoxic and oxic water column. Methane concentration and stable isotope (δ13CCH4) patterns indicate that water column stratification and microbial methane oxidation efficiently hamper the transfer of methane to the sea surface. Only the shelf seep site in 90m water depth acts as a direct source of atmospheric methane. Microbial methane oxidation and/or gas stripping seem to cause oxygen depletion above the two shallower seep areas. The methane flux from the 90m site into the water column is estimated to have been 0.599×106 molyr−1 (9.6 t yr−1) in 2003 and 0.347×106 molyr−1 (5.6 t yr−1) in 2004. Comparison of results from the two years shows different water column methane inventories in the deep part of the transect, implying a variable methane source strength at the 600m deep site. The flux from this area is estimated to have been 11.35×106 molyr−1 in 2003. [Copyright &y& Elsevier]

Terrestricythere crimaea Schornikov sp. nov. is described from the north-west part of the Black Sea in the Crimean Peninsula. The new species is found associated with zones of gas seepage on the sea floor at two sites: Kruglaya Bight in the Sevastopol area and on the southern coast of Cape Tarkhankut. It is the first truly marine species of the genus. [ABSTRACT FROM AUTHOR]

Abstract: Methane (CH4) concentrations were measured in the water column, in sediment porewaters, and in atmospheric air, in the Ría de Vigo, NW Spain, during both the onset (April 2003) and at the end of (September 2004) seasonal upwelling. In addition, CH4 concentration and stable isotopic signatures (δ 13CH4) were measured in porewaters, and sediment methanogenesis and aerobic oxidation of CH4 were determined in sediment incubations. Surface water column CH4 (2 m depth) was in the range 3–180 nmol l−1 (110–8500% saturation) and followed a generally landward increase but with localised maxima in both the inner and middle Ría. These maxima were consistent with CH4 inputs from underlying porewaters in which CH4 concentrations were up to 3 orders of magnitude higher (maximum 350 μmol l−1). Surface water CH4 concentrations were approximately three times higher in September than in April, consistent with a significant benthic CH4 flux driven by enhanced sediment methanogenesis following the summer productivity maximum. CH4 and δ 13CH4 in sediment porewaters and in incubated sediment slurries (20 °C) revealed significant sediment CH4 oxidation, with an apparent isotopic fractionation factor (r c) of ∼1.004. Using turbulent diffusion models of air–sea exchange we estimate an annual emission of atmospheric CH4 from the Ría de Vigo of 18–44×106 g (1.1–2.7×106 mol). This estimate is approximately 1–2 orders of magnitude lower than a previous estimate based on a bubble transport model. [Copyright &y& Elsevier]