Microscale evolution of reefal microbialites

Abstract

Reefal microbialites are crusts and infills of microbialites developed within primary cavities of late Quaternary coralgal reef frameworks. In spite of many previous studies, factors controlling mesofabric transitions, particularly from laminated to digitate microbialites but as well as the microscale formation process in relation to microbial communities, have not yet been fully understood. Furthermore, there are still controversies regarding age differences between reef framework growth and microbialite formation. The authors have discussed the macroscale evolution of primary cavities as well as the microscale evolution of reefal microbialites obtained in a middle Holocene reef core drilled off Okinawa Island, south-west Japan, based on petrographic observations using light and scanning electron microscopy with energy dispersive X-ray spectroscopy and radiocarbon (14C) ages of corals and reefal microbialites. The authors’ petrographic observations show that coralgal reef framework growth was followed by encrusting foraminifers and metazoans, coevally with macro-bioerosion and microbioerosion of coralgal frameworks, then finally infilled with an intraskeletal and boring-filling microbialite (IBFM) and encrusted by reefal microbial crusts (RMCs), including structureless and digitate microbialites. The observed macroscale evolution was probably driven in response to progressive changes to reduced light and water circulation in the primary cavities associated with middle Holocene transgression and reef formation. The microfabric of reefal microbialites consists of primary in situ precipitated peloids (spherical micritic clots), voids (primary pores) and allochthonous detrital grains (bioclastic and siliciclastic) fallen from upper reef surfaces. IBFM geopetally infilled inside bioeroded cavities soon after the bioerosion of coralgal frameworks. RMCs have a two-layered microfabric succession, which is composed of the alternation of dense peloidal aggregates and laterally aligned voids in the lower part and the mixing of peloidal aggregates and irregularly and vertically developed voids in the upper part. The authors speculate that the repeated occurrences of laterally aligned voids in the lower part may imply the intermittent pauses of peloidal formation, while peloids accumulated on the irregular outer surfaces may result in the formation of vertically developed voids and digitate surfaces in the upper part. These peloids are probably formed by coccoid sulfate-reducing bacteria in anoxic environments developed by the degradation of bacterial biofilms and organic matters, allowing sulfate reduction and increasing alkalinity, therefore carbonate precipitation. The two-layered microfabric succession generally observed in RMCs may reflect spatial-temporal oxic/anoxic variations in interstitial water and the degree of sulfate ion supply by the gradual closing of primary cavities.