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Petrographic analysis, electron microprobe analysis, stable isotope geochemistry, and gas analysis of volatile components in fluid inclusions of platform limestones from MIT and Takuyo-Daisan guyots in the northwest Pacific show very little evidence of subaerial exposure. Thus, based on geochemical evidence, meteoric influences associated with subaerial exposure are unlikely to account for the 100to 200-m-deep karstlike depressions observed in SeaBeam bathymetry of the summits of these two guyots. The stable isotope geochemistry of the MIT and Takuyo-Daisan platform limestones is predominantly a marine signature. Electron microprobe analysis of the MIT lower platform limestones reveals early depositional and some early diagenetic environments characterized by high concentrations of sodium, potassium, sulfur, and strontium in the carbonates; these are interpreted as oxidizing marine or saline fluids. Late-stage cements in the lower platform are characterized by sodium, sulfur, iron, and manganese; these are interpreted as saline-reducing fluids. The bulk gas extracted from MIT and Takuyo-Daisan limestone samples included several generations of paleo-pore fluids from different diagenetic environments. Only two samples from MIT Guyot and one from Takuyo-Daisan Guyot contained air inclusions as evidence of intervals of vadose diagenesis. The majority of the samples contained H2S as evidence for the migration of reducing fluids through these limestones during part of their diagenetic history; this concurs with interpretations taken from electron microprobe data. We speculate that generation of the 100to 200-m-deep depressions on the surface of MIT and Takuyo-Daisan guyots could involve two forms of marine dissolution: dissolution of aragonitic components associated with the aragonite compensation depth, and dissolution associated with sulfate reduction deep within the platform. The upper sections of limestone from Limalok and Wodejebato guyots in the Marshall Islands chain differ geochemically from those at MIT and Takuyo-Daisan guyots. In addition, the morphology of the Marshall Islands guyots is also different from MIT and Takuyo-Daisan guyots because of the presence of a well-developed pelagic cap and the absence of deep depressions within the platforms. At Site 873 on Wodejebato Guyot, there is evidence for two emersion events. A 5-m.y. hiatus between platform and pelagic sedimentation on Wodejebato Guyot supports the interpretation of platform exposure before final submersion. Typically, a 1to 2-m.y. hiatus is associated with Marshall Islands guyots, which display no geochemical evidence of subaerial exposure. The 1.5-m.y. hiatus on MIT Guyot sheds doubt on significant subaerial exposure there.

proceedings of the ocean drilling program

Geochemistry of MIT and Takuyo-Daisan Carbonate Platforms Compared with Other Northwest Pacific Guyots Drilled during Leg 144: Implications for Their Diagenetic History