Diagenetic carbonates in the Blake Ridge sediments mostly occur as dolomite and siderite, which may be concentrated in millimeter- to centimeter-sized nodules or laminae (Paull, Matsumoto, Wallace, et al., 1996). These minerals are also present as dispersed crystals, making up a few percent of the sediments. In smear slides, the crystals often exhibit a hexagonal habit and are mostly micron-sized, occasionally in association with individual rhombs with a maximum length of ~5 µm. SEM and TEM observations of the crystal morphologies show dominant hexagonal forms, which occur as single, twinned, or aggregated crystals, and the presence of abundant crystals smaller than 1 µm.
At Site 994, the mineralogy of the bulk sediment determined by XRD indicates that dolomite dominates from the surface down to about 140 mbsf, whereas siderite is present in significant amounts only below this depth (Table 2). The position of the major diffraction peak (d [104]) of dolomite provides information on the nature and amount of the divalent ions that enter into the crystal lattice. This has been extensively used for Fe-free dolomites to estimate their departure from stoichiometry caused by excess CaCO3 (Goldsmith and Graf, 1958; Reeder and Sheppard, 1984). However, calibration for Fe-rich dolomites was unsuccessful because natural ferroan dolomites generally contain excess CaCO3 and minor amounts of MnCO3 (Reeder, 1983). For the Blake Ridge dolomites, the d (104) values vary in the range of 2.886-2.904 Å (i.e., between two end-members with compositions characteristic of stoichiometric dolomite (2.886 Å) and of ankerite [2.904 Å]), but there is no regular trend with depth of the variations of d (104) values. The TEM elemental analysis of a dolomite-rich nodule (Sample 164-994C-6H-4, 68-70 cm; 47.28 mbsf) reveals important variations of the composition of individual diagenetic carbonate crystals, which may be pure calcite, stoichiometric dolomite, or ferroan dolomite with or without Mn. The composition of siderite crystals from a siderite-rich nodule (Sample 164-994C-78X-1, 45-47 cm; 636.66 mbsf) is also variable with different amounts of Mg, Ca, and Mn substituting in the crystal lattice.
The diagenetic dolomites from the Blake Ridge sediments are ferroan dolomites with variable chemical compositions, even within the same depth interval. The Blake Ridge diagenetic carbonates were previously described (Matsumoto, 1983, 1989; Baker and Burns, 1985) at Site 533 of DSDP Leg 76, which was drilled down to ~400 mbsf at 3284 m of water depth, or ~300 m deeper than at Sites 994, 995, and 997. At Site 533, dolomite is mostly present down to 192 mbsf; beneath this level, dolomite becomes very rare while siderite abundance increases. The dolomite/siderite boundary is thus located at different depths in the Blake Ridge; it is deepening from 140 to 192 mbsf as the water depth increases from 3000 to 3284 m. There seems to be a relationship between the depth of this boundary and the position of the upper limit of the gas hydrate-rich layer that lies at about 200 mbsf at Sites 994, 995, and 997, and at about 250 mbsf at Site 533.
At Hole 994C, the comparison of the mineralogical and chemical data (Paull, Matsumoto, Wallace, et al., 1996) shows that siderite becomes more abundant about 60 m above the gas hydrate-rich layer (i.e., in a zone where the salinity and alkalinity gradients increase sharply) (Fig. 2).