Cryptic microbialite development inside a middle Holocene coralgal reef, Okinawa Jima, the Ryukyu Islands

Abstract Reefal microbialites are crusts and infills of fine-grained, non-skeletal microbial carbonates developed within primary cavities of coralgal reef frameworks. They have been found in reef margins, slopes and caves with ages ranging from the Last Glacial Maximum to the Holocene. Both global and local factors likely control reefal microbialite development, but their respective contributions remain unknown due to limited data on the formation setting, age, and process. Here we report on lithological and biological successions, mesoscale and microscale fabrics, stable isotopes and radiocarbon ages of reefal microbialites in a core recovered on a reclaimed offshore bank reef in Okinawa Jima, the Ryukyu Islands, Northwest Pacific. A total of 13.1-m-thick sedimentary succession is composed of (1) Pleistocene limestone of the Ryukyu Group (> 1.6 m thick), (2) mixed paleosol and bioclastic sediments (1.5 m thick, ca. 7800 mean calibrated years before Present: cal yr BP), (3) Holocene reef deposits (7 m thick, 7300–6300 mean cal yr BP), and (4) reclaimed deposits (3 m thick) in ascending order. This succession reflects a transition from terrestrial to marine environments associated with the middle Holocene transgression. Reefal microbialites are classified into reefal microbial crusts (RMC), mixed encrusting and micritic crusts (MEMC), and intraskeletal and boring-filling microbialites (IBFM). The RMC developed within a narrow stratigraphic interval (1.85–3.55 m below sea level), while the IBFM and the MEMC were common throughout the middle Holocene reef core. The RMC developed within decades to hundreds of years after the death of coralgal communities in the primary cavities of coralgal frameworks at least 0.5 m beneath the living reef surface. The carbon isotope values (mean: 3.3 ‰) in the RMC are close to those expected for high-magnesium calcite precipitated in equilibrium with seawater, while the narrow range of the oxygen isotope values suggests that seawater temperature and salinity did not change greatly during RMC development. Reefal microbialite developments at ca. 7–6 ka in the study area are likely attributed to an expansion of cryptic primary cavities of reef frameworks associated with rising sea levels and vertical reef accretion, enhanced interstitial water circulation under a high-energy setting, and increasing organic matter, nutrient availability, and alkalinity due to occasional terrigenous sediment input.