Formation of gas Hydrate and Carbonate Nodules Around Active Seeps of Thermogenic Methane at Eastern Margin of Japan Sea

A number of pockmarks, mounds, magnificent flares of gas plumes have been observed on a spur (Umitaka Spur) off Naoetsu in the eastern margin of Japan Sea during the cruises of UT04 (R and T/V Umitaka-maru, 2004) and NT05-09 (Natsushima, 2005). Fifteen piston cores, 5.5 m long, were deployed either on the mounds, into the pockmarks, or on a flat basin floor far from the plumes. Sediment cores are grey to olive black, silty clay. Thirteen cores among 15 are made up of an alternation of bioturbated and laminated units. Piston core PC15 successfully recovered white massive gas hydrate, ca. 2 m long, and carbonate nodules, ca.5 cm in diameter. PC05 and a grab sampler near PC15 also recovered carbonate nodules. Carbonate nodules are composed of calcite and/or aragonite. Microscopic observations have clearly demonstrated aragonite needles. δ13C are from -32.8 to -33.6 ‰ vs. PDB for calcite, from -8.2 to -23.5 ‰ for aragonite and from -11.4 to -16.6 ‰ for the mixture of both. d13C values are a bit heavier than methane of gas hydrate (-38.9 to -39.4 ‰). d18O value are from 3.0 to 3.1 permil for calcite, from 2.9 to 4.9 permil for aragonite and from 3.6 to 5.2 permil for the mixture of both. Ion concentration of the interstitial waters showed that sulfate-methane interface (SMI) range between 1.5 to 3 m. These are remarkably shallow when compared with the SMI on the Blake Ridge (5.0m to 20m, Borowski et al., 1999) and Nankai Trough (4.0m to 63m; Matsumoto and Chen, 2003). Methane flux on and around the spur are thought to be very strong. The shallowest SMI (ca.1.5m) were observed at PC03 and 04, both were located close to gas plumes. Interstitial water chemistry of PC01 decreases with depth, as 550 mM at 0 mbsf and 482 mM at 4 mbsf, whereas at PC03 illustrates an increase of chlorine concentration from 550 mM at 0 mbsf to 750 mM at 4 mbsf. Freshening of I. W at PC01 indicates existence of gas hydrate, Sh=0~12%, in sediments pore space. To the contrary, hyper-salinity is explained as the result of gas hydrate formation in semi-closed system. Gas hydrate excludes salts from the structure, causing salinity increase of ambient waters. The anomaly is usually erased by diffusive mixing of the interstitial water system. Anomalously high concentration of cations and chloride in the PC03 sediments are likely to indicate that the formation of gas hydrate and consumption of waters are not balanced by supply of waters. In other words, rapid supply of methane caused water-deficient environments in shallow sediments. We would conclude that gas hydrate is now being formed in free-gas charged sediments of the Umitaka spur.