Three Pleistocene carbonate samples from a core from Eniwetok Atoll, Marshall Islands, were analyzed in order to describe and determine the origin of clay found extensively in the subsurface of the atoll. The samples represent a lagoonal depositional environment as evidenced by relatively high abundances of coral and Halimeda , and the lack of coralline algae. Sample depths of 51, 63, and 79 m below MSL occur below three distinct solution unconformities at 22, 60 and 70 m Consequently, each sample represents a zone with a different diagenetic history. Eight size fractions of the samples were grouped into three size intervals based upon the following compositional characteristics observed on scanning electron microscope micrographs: (1) generally biogenic carbonate grains (>5 mu ), (2) significant amounts of calcite cement (2-5 mu ), and (3) abundant aragonite needles (.1-2 mu ), some of which are believed to be aragonitic cement. The mineralogy of the >5 mu fractions is determined largely by the mineralogic composition of the biogenic grains and their degree of neomorphism, whereas the mineralogy of the 2-5 and .1-2 mu fractions is determined largely by the addition of calcitic and aragonitic cements, respectively, superimposed upon neomorphosed remnants. Each sample has some calcite cement (most in the 79 m sample) which probably precipitated in the vadose and/or meteoric phreatic zones during periods of atoll emergence. On the other hand, differences in the strontium and magnesium concentration values for 100% aragonite suggest that: (1) the aragonite cement in the 51 m sample precipitated in a closed system, e.g., from hypersaline solutions during times of atoll emergence, and (2) the aragonite cement in the 63 and 79 m samples precipitated in an open system, e.g., from the marine phreatic zone during times of atoll submergence. Evidence for cycles of atoll emergence and submergence consists of growth relations between equant calcitic and aragonitic cement grains.