Calcified filaments of an endolithic alga in recent Bermuda reefs

In Recent Bermuda reefs, the filaments of the endolithic green alga Ostreobium extend into the cavities within the reef frame. There the filaments are calcified by two precipitative cements: an outward growing crust of equant, bladed, or fibrous crystals, and an inward growing core of equant or bladed crystals, both of magnesium calcite. The filaments are worm-like threads of 15 to 300 µ diameter; they commonly form irregular mesh works lining or filling cavities; the meshwork frequently traps and cements sediment particles. These calcified filaments are part of the intensive synsedimentary submarine cementation of these reefs. Introduction of the filaments into a given cavity is a simple mechanism of affecting the micro-environment and changing the polymorph of the cement; in this way sequences of two or more different cement generations may be formed. Similarities in appearance between filament cross sections and pellets found in the cavities suggest that the pellets may be derived from calcified filaments by fragmentation. The combination of boring canals and calcified algal filaments provides a time mark which is useful in establishing time and sequence of internal sedimentation and cementation. In addition, the combination of algal boring and precipitative encrustation of the algal filaments is proposed as potential criterion for distinguishing fresh-water and submarine from subaerial early diagenetic environments.