Gas-hydrates are ice-like compounds consisting of water molecules and guest gas molecules such as methane. Gas-hydrates are formed at temperatures well above 0°C if the gas pressure is enough high to stabilize gas-hydrates. Therefore, organic-rich continental margin sediments provide favorable conditions for the formation of methane hydrate.
Authigenic carbonates formed during burial in the non-hydrated sediments off northeast Japan demonstrate V-shaped pattern in the variation of carbon and oxygen isotopic composition, whereas those in the hydrated sediments of the Blake Outer Ridge off Florida provide quite different pattern with extraordinary heavy oxygen siderites. This anomaly is likely to have been related with decomposition of methane hydrate at the depth of 500 to 600 meters.
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