Two dredge samples from the Romanche Fracture Zone in the equatorial Atlantic document dolomite formation in pelagic sediments in a deep-sea environment. The samples come from a highly deformed sedimentary succession, the Romanche Sedimentary Sequence, constituting the transverse ridge accompanying the transverse valley of the fracture zone to the north. These sediments were folded and uplifted to or near the seafloor prior to the growth of an early Miocene carbonate platform which unconformably overlies them.

Dolomitized pelagic limestones of Early Cretaceous age preserve solution molds and unaltered tests of radiolaria (opal-A) and ghosts of calcareous nannoplankton in a dolomicritic groundmass. Carbon isotope data indicate a normal marine source of carbon, and oxygen isotope compositions are consistent with precipitation from cool, marine pore waters at or near the sediment-water interface. The 87/Sr86Sr ratio suggests dolomitization at around 25 Ma, presumably when the sediments were exhumed to or near to the seafloor.

In contrast, a dolomite-cemented diatomite of presumably late Eocene age shows no relics of a carbonate precursor. Dolomite crystals grew freely in the originally highly porous rock, cementing it into a tight fabric. The absence of compaction suggests that cementation by dolomite took place soon after deposition and before significant burial of the sediment. Carbon and oxygen isotope compositions of this dolomite suggest that its formation also occurred from cool, marine waters. The 87Sr/86Sr ratio shows the value of seawater around 35 Ma. Under the assumption that dolomite cementation was by seawater, it occurred shortly after deposition at or near the seafloor.

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