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[BGM02-P04] Origins of spherulites observed in reefal microbial crusts in Kumejima, Okinawa
Microbialites have recorded biological activities and the earth environment for 3.7–3.5 billion years, and are recently drawing attention in terms of oil reservoirs. Microbialites had been widely distributed in Precambrian sea while they are rare in the modern ocean, which hinders studying the formation process of microbialite using their modern analogues. However, reefal microbial crusts (RMCs) and intra-skeletal and boring-filling microbialite (IBFMs) were discovered recently in cavities of reefal carbonates, and they are drawing attention as modern analogues. In this study, RMCs and IBFMs in coral reef carbonates were investigated.
RMCs usually encrust the surface of coralline algae, while IBFMs partly fill borings within skeletons of coralline algae. These microbialites contain and/or associate with spherulites, which are sometimes assumed to be microbial origin due to their common occurrence in microbialites. However, care has to be taken for the origin of Phanerozoic cases because ascidians also produce spherulites as spicules. In this study, therefore, the spherulites in RMCs and IBFMs were compared to ascidian spicules in Kumejima by several methods including polarizing microscope. Ascidian spicules observed in this study have a globular–stellate shape, and extinction patterns showed that they were composed of bundles of parallel acicular crystals. Spherulites showing similar characteristics were also recognized in RMCs and IBFMs, indicating their ascidian origin. However, some spherulites occupying cavities around RMCs and IBFMs were composed of thin acicular and radial crystals, and they were tightly packed to exhibit anhedral appearance. In addition, they contain filamentous structures of about 1 μm in a diameter, and they sometimes radiated from the center of spherulites. These characteristics significantly different from ascidian spicules suggest their microbial origin, which is expected to be confirmed in future study.
RMCs usually encrust the surface of coralline algae, while IBFMs partly fill borings within skeletons of coralline algae. These microbialites contain and/or associate with spherulites, which are sometimes assumed to be microbial origin due to their common occurrence in microbialites. However, care has to be taken for the origin of Phanerozoic cases because ascidians also produce spherulites as spicules. In this study, therefore, the spherulites in RMCs and IBFMs were compared to ascidian spicules in Kumejima by several methods including polarizing microscope. Ascidian spicules observed in this study have a globular–stellate shape, and extinction patterns showed that they were composed of bundles of parallel acicular crystals. Spherulites showing similar characteristics were also recognized in RMCs and IBFMs, indicating their ascidian origin. However, some spherulites occupying cavities around RMCs and IBFMs were composed of thin acicular and radial crystals, and they were tightly packed to exhibit anhedral appearance. In addition, they contain filamentous structures of about 1 μm in a diameter, and they sometimes radiated from the center of spherulites. These characteristics significantly different from ascidian spicules suggest their microbial origin, which is expected to be confirmed in future study.