Magmatic fluids play a role in the development of active gas chimneys and massive gas hydrates in the Japan Sea

The Earth's marine continental margins hold significant amount of carbon in the form of gas hydrate which can interact dynamically with seawater and the atmosphere and potentially influence global climate. It is generally assumed that degradation of organic matter, through either biogenic or thermogenic processes, is responsible for the majority of the gas hydrate presently found in these deposits. We present stable isotope and noble gas results from gas hydrates in the eastern margin of the Japan Sea and compare them with previously published data regarding gases which are found nearby in deep hydrocarbon wells associated with Early Miocene igneous and sedimentary rocks. Our results show that most of the gases which accumulate in the hydrate can be described as a mixture between a deeply-sourced methane-rich thermogenic/crustal source (TC), a methane-rich admixture of biogenic gases with relatively shallow air-saturated seawater (BASW) and a methane-poor magmatic-mantle source (MM). The presence of hydrate with a mantle 3He/4He signatures indicates that fluids enriched in mantle gases are responsible for the mobilization of thermogenic gas within active Japan Sea chimney structures. The growth of primary hydrate at shallow marine sediment depths is ongoing, as is the cycling of gas to hydrate which is eventually buried and dissociated at the base of the gas hydrate stability zone, leading to large pockmark structures. [Display omitted] •Sea of Japan hydrates host mantle helium combined with thermogenic methane.•Helium content is associated with shallow, rapidly-growing massive hydrate.•Deeper, secondary hydrates show extreme helium depletions.•Oki Trough shows a greater biogenic methane component than the other sites.•Stable isotopic composition and helium content appear to be related to heat flow.