Figure 11. The ¹⁸O values of siderite samples from Sites 994, 995, and 997 are shown relative to the calculated equilibrium ¹⁸O values of siderite (solid line). This equilibrium line was calculated from the equation for fractionation between siderite and water (Carothers et al., 1988), assuming that the ¹⁸O of water is 0 PDB and that the geothermal gradient on the Blake Ridge is ~35ºC/km (Ruppel, 1997). The ¹⁸O values of siderite found at depths greater than ~500 mbsf suggest formation shallower in the sediment column, where temperatures are cooler. Note that siderite ¹⁸O values have not been corrected for their phosphoric acid-liberated CO₂ fractionation (Carothers et al., 1988). The ~-1.4 shift for siderite that would result from applying a siderite acid-fractionation factor would still support our assertion that siderite formation occurs within, and above, the gas hydrate-bearing section of the sediment column on the Blake Ridge (120-450 mbsf).