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Marine waters currently circulate through Eocene and Miocene limestones deposited in slope, fore-reef, and reef-related environments on Enewetak Atoll. Subsurface core samples (375-1,400 m deep) demonstrate that marine waters have extensively altered the original carbonate sediments. Evidence supporting alteration by marine water includes: (1) calcite cements with isotopic compositions characteristic of a marine origin (^dgr13C = 1.3 to 2.5^pmil; ^dgr18O = -1.8 to 0.4^pmil PDB), (2) dolomites with average isotopic compositions that are in equilibrium with cold normal-marine waters (^dgr13C = 2.3^pmil; ^dgr18O = 2.5^pmil PDB), (3) consistent magnesium concentrations in fossil coralline algae, and (4) a lack of diagnostic evidence for meteoric diagenesis. Products of marine diagenesis on Enewetak Atoll are dependent on the CaCO3 saturation state of the diagenetic waters. Above the aragonite saturation depth (350 m), marine diagenesis is dominated by aragonite and high-magnesian-calcite cementation. Between aragonite and calcite saturation depths, marine waters apparently have dissolved aragonite and precipitated low-magnesian, radiaxial calcite cements. Below the calcite saturation depth (1,000 m), deep marine waters have apparently dissolved some calcite and have precipitated dolomite. Effective marine diagenesis requires limestones with good permeability and a hydraulic drive, such as tidal pumping or thermal convection, to move large volumes of marine water through the rock.
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