Frutexites from microbial/metazoan bioconstructions of recent and Pleistocene marine caves (Sicily, Italy)

• Description of arborescent Frutexites-like structures in metazoan/microbial bioconstructions
• First report of ferromanganesiferous microbial structures in submerged caves
• Relationship between Frutexites and Fe-Mn autotrophic/chemoheterotrophic bacteria
• Depositional model of Frutexites formation

Frutexites-like structures were recorded in small bioconstructions from Recent and Pleistocene marine caves. The skeletal/microbialite frameworks hosting Frutexites form in cryptic environments characterized by poor light availability and reduced water circulation leading to confinement, oligotrophic conditions and the development of typical cave communities. Biostalactites and biotic crusts develop in the more confined parts of the both Recent and Pleistocene caves. They are composed of skeletal organisms engulfed in fine micrite sediments deposited in situ via microbial metabolic activity. The Frutexites, composed of Iron and Manganese oxides, are confined to microcavities or microfractures of the bioconstructions. They grow from an original discontinuity surface into the surrounding consolidated micritic sediments. A model based on the increase of the microporosity of micritic sediments due to the dissolution of the original material and precipitation of ferromanganesiferous compounds was proposed. This process occurs during the syndepositional diagenetic stage due to the intrusion in the bioconstruction framework of acidic continental water rich in Iron and Manganese. Mesophilic Fe–Mn autotrophic and chemoheterotrophic bacteria flourish on the microcavities and induce the precipitation of Fe and Mn oxides.