ABSTRACT

Petrographic, lithologic, mineralogic, and minor-elemental data from Plio-Pleistocene carbonates at Enewetak Atoll indicate that these sediments have been calcitized in the near-surface meteoric diagenetic environment. Shallow (< 100 m), mostly Pleistocene, sediments remain aragonite-rich with a few discrete meter-scale, calcite-rich intervals developed either at Pleistocene soil zones or in thin (< 5 m) paleophreatic lenses. Deeper ( 100 to 180 m), mostly Pliocene, sediments are characterized by pervasive calcitization of aragonite, extensive dissolution with well-developed moldic porosity, and the occurrence of isopachous bladed and pore-filling equant calcite cements. Pervasively-calcitized Pliocene limestones exhibit a narrow range in ¹⁸O value (±1 = 0.53), a wide range in ¹³C value (±1 = 1.63), and low mean Sr (0.02 wt %) and Mg (0.25 wt %) contents. These data are most consistent with diagenetic alteration of Plio-Pleistocene carbonates by meteoric phreatic calcitization, although in some intervals calcitization in the mixed meteoric-marine environment may have also occurred. Petrographic data document the superposition of subaerial exposure surfaces a d paleophreatic lenses in the subsurface of Enewetak. Sr-isotope data and stratigraphic forward modeling are consistent with the multiphase meteoric phreatic calcitization of Pliocene limestones in response to subaerial exposure shortly after sediment deposition and during Pleistocene sea-level lowstands. Quantitative water-rock interaction modeling of whole-rock chemical trends suggests that calcitization occurred in an "open" diagenetic system at water-rock (W/R) volume ratios of 10³ to 10⁵.

Proxy indicators of climate (e.g., retention of aragonite at depth and the development of caliche at subaerial exposure surfaces) are consistent with an arid to semi-arid Plio-Pleistocene climate. Moreover, a limited supply of meteoric waters in the vadose environment, despite vertically and temporally extensive paleovadose environments, is consistent with limited calcitization of the Pleistocene sequence. Alternatively, previous estimates of the rates of vadose calcitization may be too high to be applicable at Enewetak. Stratigraphic shortening of the Plio-Pleistocene sequence in response to carbonate dissolution during atoll emergence is not consistent with available data.