New insight on the nature of iron ooid formation - a study from Panarea island (Sicily, Italy)

In spite iron ooids are sedimentary particles widely documented in the fossil record, they are extremely rarein modern settings. The origin and genesis of fossil iron ooids and oolitic ironstones, and their extensive distribution, have long been a matter of debate and controversy (e.g., Ferretti, 2005), invoking both abiotic processes (e.g., Sturesson, 1992) and biologically induced mechanisms (e.g., Burkhalter, 1995). A modern iron ooidal deposit was recently discovered and sampled in the marine shallow-water settings off the coast of Panarea, in the Aeolian Islands, by INGV and ISPRA teams. This exceptional discovery offers an extraordinary opportunity to answer many open questions on fossil analogues. With this contribution we intend to better elucidate the iron ooids origin by studying the ooids-sand from Panarea. To reach this goal, a multi-proxies integrated approach was applied, based on morphological, compositional, and geobiological characterizations. The Panarea deposit consists of coarse-grained, unconsolidated sand with a whitish biogenic component (mostly foraminifera and sponge spiculae) and dark brown rust ooidal grains. The results show that Panarea iron ooids perfectly match their fossil counterparts. They were formed by the deposition of iron oxyhydroxides (mainly goethite) in concentric laminae around nuclei represented by pyroclastic particles and, more rarely, by sponge spicules or other skeletal components. The spherical laminated structure resulted from the constant agitation by degassing of CO2-dominated fluids through the seafloor sediments. Any sound evidences of microbial-mediated micro-texture (such as cell remain or induced structures, as biofilm, already reported from other hydrothermal environments including the Panarea area (Bortoluzzi et al., 2017; Esposito et al., 2018), were found. Our results allow constraining an abiotic mechanism and excluding a microbial contribution in the iron ooid formation process. Esposito, V., Andaloro, F., Canese, S., Bortoluzzi, G., Bo, M., Di Bella, M., Italiano, F., Sabatino, G., Battaglia, P., Consoli, P., Giordano, P., Spagnoli, F., La Cono, V., Yakimov, M.M., Scotti, G. & Romeo, T. (2018): Exceptional discovery of a shallow-water hydrothermal site in the SW area of Basiluzzo islet (Aeolian archipelago, South Tyrrhenian Sea): An environment to preserve. PLoS ONE, 13(1), 1-27. Burkhalter, R.M., (1995): Ooidal ironstones and ferruginous microbialites: origin and relation to sequence stratigraphy (Aalenian and Bajocian, Swiss Jura mountains). Sedimentology, 42, 57-74. Ferretti, A. (2005): Ooidal ironstones and laminated ferruginous deposits from the Silurian ofthe Carnic Alps, Austria. Boll. Soc. Paleontol. Ital., 44, 263-278. Sturesson, U. (1992): Volcanic ash: the source material for Ordovician chamosite ooids in Sweden. J. Sediment. Petrol., 62, 1084-1094.