Geochemical study of lime-mud borings and associated coral-reef limestones in the intertidal flat of Kondoi Beach, Taketomi Island, Okinawa, Japan

Abstract

Various types of numerous lime-mud borings have been discovered in the intertidal limestone and coral (Acropora) framestone at Kondoi Beach of Taketomi Island, Okinawa, Japan. Lime-mud borings may be formed by a mechanism similar to that for the micrite envelopes in molluscan shells and other carbonates. The lime-mud borings and surrounding limestone/coral framestone were pairedwisely, sampled and analyzed geochemically. The lime-mud borings show a rather uniform mineralogical composition (about 80% high-Mg calcite (HMC), 10% aragonite, and 10% low-Mg calcite (LMC), whereas the surrounding limestone appears HMC-rich and the coral framestone shows aragonite-rich carbonate mineralogies. Detailed elemental compositional studies suggest precipitation and filling mechanisms of clay-size lime muds in bored void space. Multiple boring of limestones and corals by boring organisms may produce several isolated void spaces that become filled with seawater, and authigenic clay-size crystals of HMC-rich carbonates may precipitate in the bored void space and fill the void space. The HMC-rich and clay-size carbonates eroded from the lime-mud borings may be important sources for the fine fraction of lime mud in the coral reef basins.