Data collected from Enewetak Atoll suggest multi-generated synsedimentary cements of the patch-reef facies. The most common cement occurs as micritic aragonite in coralgal boundstones. Isopachous fibrous cements consist of parallel to subparallel crystals uniformly lining all sides of a cavity. These cements are present in subtidal marine environments and suggest early precipitation.

Organic matter occurs coincident with the precipitation of these marine cements. The source of the organic matter may be due partly to neomorphic replacement of aragonite to low Mg calcite. The ^dgr¹³C_PDB values range from -6 to -12 ppm. Previous work by Gross and Tracey concluded that abnormally light ^dgr¹³C values were produced by a 1:1 reaction of isotopically light (^dgr¹³C-22 ppm) soil gases dissolved in meteoric water and transported throughout the rock with the cements. Our data suggest that soil gases in the carbonate soils of Enewetak are isotopically much heavier than terrigenous soils. We conclude that the isotopically light ^dgr¹³C values are partly attributed to the organic matter trapped in the aragonite lattice.

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