Methane anomalies, its flux on the sea–atmosphere interface and their relations to the geological structure of the South-Tatar sedimentary basin (Tatar Strait, the Sea of Japan)

Ocean Dynamics and Methane Plume Activity in Tatar Strait, Far Eastern Federal District, Russia as Revealed by Seawater Chemistry, Hydroacoustics, and Noble Gas Isotopes

“…As has been documented previously (Shakirov et al, 2019), methane concentrations can exceed saturation around the Tatar Strait plume sites. To determine to what degree oversaturation is present, the oversaturation ratio (SR) for methane (Kudo et al, 2018) was determined as follows:…”

“…If the most intense red and orange areas within the hydroacoustic flares (Figure 3) do in fact represent such gas hydrate which forms layers around rising gas bubbles as has been observed elsewhere in the Japan Sea (Aoyama et al, 2007), it is clear that the gas hydrate bubbles could extend into lower intermediate (IIIb) and even upper intermediate (IIIa) water masses, above the predicted range of gas hydrate stability (Figure 7A) as calculated from the equation of Dickens and Quinby-Hunt (1994). Shakirov et al (2019) interpreted some of the shallower gas anomalies as being due to the dissociation gas hydrate bubbles, based on observations by Yapa et al (2001), whereby gas hydrate from seeps which are relatively rich in ethane and propane, with C 1 /(C 2 +C 3 )<6, can remain stable at higher temperatures and shallower depths, even shallower than intermediate waters (IIIa and IIIb) found in Tatar Strait. Recovered shallow seafloor sediments near the plume sites in Tatar Strait yield headspace gases which are primarily thermogenic with measured C 1 /(C 2 +C 3 ) ratios <10 and as low as 2.8 (Yatsuk et al, 2020), so the upper limit of hydrate stability could potentially be shallower than the ~300 m depth indicated by pure methane, although the rising gas bubbles would have to be collected and analyzed to directly confirm this.…”

“…Our study area is located between 47.5 o N and 48.5 o N in Tatar Strait (Figure 1C), including the eastern margin of the strait where abundant gas seep plumes are located, deeper central portion of the trough, and seep-free portion of the trough on the eastern margin (Shakirov et al, 2019;Yatsuk et al, 2020). The water depth is sufficient in the study areas to preserve the vertical water mass structure (Danchenkov, 2004), and the study area also includes the northern portion of the deep Tatar Trough.…”

“…The samples were then analyzed onboard the R/V Akademik M.A. Lavrentyev for dissolved gases (methane, hydrocarbon gases, carbon dioxide, nitrogen, and oxygen) using shipboard gas chromatographic instrumentation as described in Vereshchagina et al (2013) and Shakirov et al (2019). In order to create headspace gas in the 68 ml bottles, 12 ml high-purity helium was injected through the rubber septum.…”

“…In these cases, it has been shown that the ascending gas hydrate melts and gas bubbles disperse within the water column before reaching the subsurface (II) and surface (I) waters (e.g., Greinert et al, 2006;Salomatin et al, 2014). Gravity coring during both LV70 and LV85 as well as the previous cruises has revealed thin laminar gas hydrate layers in the shallow sediments beneath the plumes (Jin et al, 2013;Shakirov et al, 2019), while no sediment-hosted gas hydrate has been recovered from the western margin of Tatar Strait where the reference site LV85-C2ref. is located.…”