Authigenic siderite from the Miocene and Pliocene sediments of the Blake Outer Ridge formed in a methane fermentation zone in equilibrium with anomalously heavy oxygen water, as evidenced by its heavy-carbon (-1.6ppm to +12.5ppm delta (super 13) C) and heavy-oxygen (+0.1ppm to +7.9ppm delta (super 18) O) values. Blake Outer Ridge sediments are impregnated with methane hydrate, which is generally enriched in (super 18) O relative to ambient water. This suggests that decomposition of methane hydrate and subsequent release of large amounts of methane and heavy oxygen water affected the formation of the (super 18) O-enriched siderite. Gas hydrate reaction in sediments is an important process for causing a dramatic variation of oxygen isotopic composition of the interstitial water and of authigenic carbonates formed during burial diagenesis.