Armoured sponge spicules from Panarea Island (Italy): Implications for their fossil preservation

A variety of calcareous and siliceous skeletal components are associated with exceptional, still active deposits of modern iron ooids off Panarea, one of the volcanic islands of the Aeolian Arc (Tyrrhenian Sea, Italy). Both ooids and skeletal components occur as loose sediments influenced by submarine hydrothermal fluid vents. Whereas iron ooids are exclusively made of goethite laminae primarily nucleated on volcaniclastic material, sponge spicules – that represent most of the siliceous skeletal component – develop laminated coatings (concretions) of varying sizes. This partial or total coating consists of regularly banded Fe‒rich layers exhibiting the same textural features and mineralogical composition (goethite) of the ooid cortex developed around inorganic cores. Spicules did not reveal any obvious attaching structure or interfingering with the surrounding coating and did not undergo bioerosion or any other evidence of biological intervention during their development. A hydrothermal origin, compatible with the general setting, is therefore proposed for these armoured sponge spicules. We believe that this unique modern case of iron concretions produced on siliceous spicules can contribute to explain other known fossil cases as well as to understand the taphonomy of this type of biogenic silica that seems rarely to have been preserved in such an extreme habitat.