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 (-1.6‰ to +12.5‰ δ13C) and heavy-oxygen (+0.1‰ to +7.9‰ δ18O) values. Blake Outer Ridge sediments are impregnated with methane hydrate, which is generally enriched in 18O 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 18O-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.

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