Sedimentary processes acting in the hydrothermal area off the Pontine Archipelago (western Mediterranean Sea): Possible Microbialites Growing

Microbialites are organo-sedimentary deposits forming as result of the interaction between benthic microbial communities and detrital or chemical sediments, occurring in terrestrial, lacustrine, and marine environments. These organo-sedimentary deposits are characterized by different morphologies and are considered to be proxies for understanding the origins of life on our planet.Recently, possible microbialites deposits, characterized by different morphologies, were found off the western Pontine Archipelago (western Mediterranean Sea) in water depth of about 130 m. Such deposits were investigated by acoustic techniques (multibeam, side scan sonar and very-high resolution seismic profiles) and ground-truth data (Remote Operated Vehicle and seafloor samples), during different research cruises from 2012 to 2017. The deposits are characterized by different morphological expressions (i.e. crusts and mounds) and were found in a hydrothermal area located off the Zannone Island in water depths between 110 and 138 m. This area is characterized by a very complex seafloor morphology mainly due to the occurrence of several fluid-related features (i.e. hydrothermal chimneys, crusts and mounds, various size pockmarks, widespread bacterial assemblages, and hydrothermal altered rocks). In this work we show the main morphological, mineralogical, and geochemical characteristics of possible modern microbialites deposits (crusts and mounds) sampled on the seafloor of the Zannone Giant Pockmark, in a sector of the Zannone hydrothermal field characterized by vigorous CO fluid emissions and widespread white bacterial aggregations. Scanning electron microscope (SEM) and X-RAY diffraction analyses show as these authigenic deposits are mainly formed by quartz, feldspar, sulphur, minor barite and gypsum, and organomineral aggregate of benthos microbial communities.The obtained results coupled with the already available information, led us to interpret these features as possible growing siliceous microbialites deposits. This interpretation updates the general overview on the sedimentary processes acting within the hydrothermal seafloor off Zannone Island. Moreover it represents one of a few cases of siliceous microbialites deposits growing in extreme sedimentary environment of the Mediterranean Sea.