Evidence of a shallow water submarine hydrothermal field off Zannone Island from morphological and geochemical characterization: Implications for Tyrrhenian Sea Quaternary volcanism

Abstract

Discoveries from multibeam bathymetry and geochemical surveys performed off Zannone Island (western Pontine Archipelago, Tyrrhenian Sea) provide evidence of an undocumented hydrothermal field characterized by ongoing fluid emissions and morphologically complex giant depressions located in shallow water (<150 m water depth). Based on a detailed morpho‐bathymetric study we identify the seabed morphologies produced by hydrothermal fluid emission activity. We recognize five giant depressions (length >250 m) that host pockmarks, mounds, small cones, and active fluid vents, which are interpreted as complex fluid‐escape features developed both through vigorous‐explosive events and steady seepage. Their spatial distribution suggests that the NE‐SW trending faults bounding the Ponza‐Zannone structural high and the shallow fractured basement are favorable conditions for the upward migration of hydrothermal fluids. Moreover, we performed a detailed geochemical study to investigate the source of the hydrothermal fluids. The geochemical signature of the collected fluids provides information of active CO2‐dominated degassing with a significant contribution of mantle volatiles, with measured 3He/4He values > 3.0 Ra that are similar to those recorded at Stromboli and Panarea volcanoes. The hydrothermal system produces volatiles that may originate from residual magma batches, similar to the Pleistocene trachytes cropping out in the SE sector of Ponza Island that were probably intruded in the shallow crustal levels and never erupted. The discovery of the Zannone hydrothermal field updates the record of active hydrothermal areas of the Mediterranean Sea. Moreover, the recognition of several giant hydrothermal depressions characterized by a complex morphology is peculiar for the Mediterranean Sea.

Magmatic Signature in Submarine Hydrothermal Fluids Vented Offshore Ventotene and Zannone Islands (Pontine Archipelago, Central Italy)

“…The vented gases were collected by Remote Operated Vehicles (ROV) at Zannone and by diving at Ventotene. The gases offshore Zannone areas were collected in 2014 [27] and in 2017 (this work) using a ROV equipped with a gas sampling system made up of four small funnels connected to 240 ml serum-type bottles. The system was fitted to the front of the ROV with the inverted funnels slipped inside the bottle necks ( Figure 3).…”

“…The Pontine Archipelago is located in the Central Tyrrhenian Sea, less than 30 km to the west of the Italian coast, and is composed of five major islands, from NW to SE: Palmarola, [27]). See Ingrassia et al [26] and Martorelli et al [27] for further information; (c) location map of the VEN site lying south to the island of Ventotene.…”

“…According to Martorelli et al [27], the mantle component identified in the hydrothermal fluids currently vented offshore Zannon originates in residual magma batches intruded into the crust and never erupted. Direct mantle degassing has been excluded [27] due to permeability limitations induced by the local crustal thickness of about 25 km (see Italiano et al [66] for further information).…”

“…The current status of the Pontine Island Volcanism is only expressed by a persistent submarine hydrothermal activity. Several wide and depressed hydrothermal areas have recently been discovered offshore the island of Zannone during morphobathymetric investigations [26,27] and to the SW of the Ventotene coast during deep diving activity. The hydrothermal activity takes place mainly along NE-SWand NW-SE-oriented tectonic structures, dissecting the Latium and Campanian continental margins in structural highs (e.g., Ponza-Zannone high) and deep sedimentary basins (e.g., Ventotene and Palmarola basins).…”

High potential for temperate viruses to drive carbon cycling in chemoautotrophy‐dominated shallow‐water hydrothermal vents

“…Recent investigations offshore Zannone Island (Mediterranean Sea) led to the discovery of a new shallow-water hydrothermal field, in which CO 2 -dominated degassing supports teeming microbial life and white benthic mats developing at the fluids' outflows in a highly heterogeneous system of pockmarks, sulfidic mounds, cones and lithified crusts (Ingrassia et al, 2015;Martorelli et al, 2016).…”