Hydrothermal fluid venting in the offshore sector of Campi Flegrei caldera: A geochemical, geophysical, and volcanological study

The ongoing unrest at the Campi Flegrei caldera (CFc) in southern Italy is prompting exploration of its poorly studied offshore sector. We report on a multidisciplinary investigation of the Secca delle Fumose (SdF), a submarine relief known since antiquity as the largest degassing structure of the offshore sector of CFc. We combined high-resolution morphobathymetric and seismostratigraphic data with onshore geological information to propose that the present-day SdF morphology and structure developed during the initial stages of the last CFc eruption at Monte Nuovo in AD 1538. We suggest that the SdF relief stands on the eastern uplifted border of a N-S-trending graben-like structure formed during the shallow emplacement of the Monte Nuovo feeding dike. We also infer that the high-angle bordering faults that generated the SdF relief now preferentially allow the ascent of hot brines (with an equilibrium temperature of 1798C), thereby sustaining hydrothermal degassing on the seafloor. Systematic vertical seawater profiling shows that hydrothermal seafloor venting generates a sizeable CO2, pH, and temperature anomaly in the overlying seawater column. Data for the seawater vertical profile can be used to estimate the CO2 and energy (heat) outputs from the SdF area at 50 tons/d (0.53 kg/s) and 80 MW, respectively. In view of the cause-effect relationship with the Monte Nuovo eruption, and the substantial gas and energy outputs, we consider that the SdF hydrothermal system needs to be included in monitoring programs of the ongoing CFc unrest

Geohazard features of the north-western Sicily and Pantelleria

We present maps of geohazard features identified across north-western Sicily and Pantelleria in the framework of the Magic project (MArine Geohazard along Italian Coasts), which involved Italian marine geological researchers in 2007-2013. These seafloor features were recognized using high-resolution bathymetry data and rely on the morphological expression of the seafloor and shallow sub-surface processes. The north-western Sicily is a complex continental margin, affected by morphodynamic, depositional, and tectonic processes. The Egadi offshore is controlled by fault escarpments and alternating retreating and progradational processes. Ustica and Pantelleria submerged edifices show the effect of volcanic activity. The Ustica seafloor is interested in volcanic, tectonic, and gravitational instability processes, while the Pantelleria offshore underwent erosive-depositional processes and the effect of bottom currents. Two levels of interpretation are represented: the physiographic domain at a scale of 1:250.000 and the morphological units and morpho-bathymetric elements at a 1:100.000 scale

Paleoenvironmental and tectonic evolution of the Liassic carbonate platform succession of Mt. Maranfusa (central western Sicily)

The upper part of the Liassic carbonate platform succession (Inici Fm., Hettangian-Sinemurian), outcropping at Mt. Maranfusa (central western Sicily) consists of a sequence of peritidal limestones, overlain by Jurassic to Cenozoic pelagic limestones and their siliciclastic Tertiary covers. The good exposure of this succession, pertaining to the trapanese domain, in the area between Palermo and Sciacca mountains, allowed to study in detail the facies sequences and structural features of the carbonate platform and of the sedimentary dikes, in order to improve knowledge on its paleoenvironmental and tectonic evolution. The Inici Fm. is constituted by subtidal cyclic sequences of wackestone\packstones with gastropods, megalodonts, oncoids, grain aggregates, dasycladacean algae and benthic foraminifers, followed by dismicrites with pisolitic levels of intertidal environment, and at the top, when they are present, bioclastics calcarenites with birdseyes of supratidal environment. At the top of the cycles we recognized often a peculiar type of layers constituted by: 1) dark gray, compact and well cemented limestones with blackish clasts. The microfacies analysis shows packstones consisting of carbonate grains and bioclasts coated with thin regular to highly irregular, laminae of brownish or yellowish micrite, creating small subspherical nodules, or irregular masses, named glaebule, often associated with pisoid. 2) millimetric reddish calcite laminae, locally deformed by elongated cavities, filled with greenish vadose silt, and bordered by radial fibrous cement. The crystals grow toward the inside of the cavity and crystallize at the expense of the silt fills. We classified for the first time the levels 1 as calcrete, while levels 2 were indicated as paleokarst. The facies sequences of the Inici Fm. are shallowing upward; the environment is attributed to a tidal flat and to the neighbour lagoon, pertaining to an inner carbonate ramp. The recognized calcretes are of pedogenic origin and were classified as alpha and beta calcrete (Wright, 1994). The paleokarsts for their characteristics were classified as paleospeleothems, which are important stratigraphic markers because they indicate periods of non-deposition and/or erosion during sea-level changes. Both the layers indicate phases of emersion during the deposition of carbonate platform succession. The study succession is crossed by three fault systems: NNW-SSE trending dextral transtensional fault system, NNE-SSW trending sinistral transtensional fault system, both late Lias\u2013middle-late Miocene in age, often reactivating previous normal faults, and E-W trending transpressional fault system (Miocene). The first two fault systems are cut by neptunian dikes (Fischer, 1964), filled by several generations of Toarcian-early Miocene pelagic sediments layers. Another system of dikes, parallel to the stratification, is created by planar slip in the carbonate rocks; they are filled by pelagic sediments (late Lias-Dogger in age). The neptunian dikes are of tectonic origin with evidence of reactivation of faults, while parallel dikes (injection dike, Castellarin, 1982) are interpreted as to be due to flexure of the platform during the Jurassic tectonic events. So, in this paper we assert, for the first time, that the horizontal layers, previously described by many authors only as horizontal dikes, can be actually interpreted as: a) layers that are effectively horizontal dikes; b) pedogenic calcretes; c) paleospeleothems. As a consequence we identified some episodes of emersion in the evolution of the Trapanese carbonate platform during the Liassic

Fluid escape structures in the north Sicily continental margin

High resolution and multichannel seismic profiles coupled with multibeam echosounder (seafloor relief) data, acquired along the northern Sicily continental margin (southern Tyrrhenian Sea), document the occurrence of mound and pockmark features, revealing fluid escape processes. Along this margin, morphology of the high-gradient continental slope is irregular due to the presence of structural highs, slope failures and canyons, and is interrupted by flat areas at a mean depth of 1500 m. Seismostratigraphic analysis tools and methods were used to identify fluid escape structures and to work out a classification on the basis of their morpho-acoustic characteristics. The detailed 3D bathymetric chart was used to define the top view morphologic features and their areal distribution. With the aim to evaluate the geochemical content of fluids, we collected a 2.3 m long sediment core in correspondence of a pockmark at a depth of 414 m. Pore waters were sampled every 10 cm and analysed in relation to their conductivity (EC) and composition (d18O, dD, Li, Na, K, Mg, F, Cl, Br, NO3, SO4). The new data show the occurrence of different types of structures with highly contrasting seismic and morphologic signatures, both dome-type and concave-upward structures. The latter have a characteristic circular shape and are known as pockmarks. Morphobathymetric, stratigraphic and structural data suggest that these structures occur along fault planes, mainly associated with diagenetic carbonates and fluid venting activity. Pockmarks could be the result of both fault and landslide structures, as they appear aligned along a straight direction and occur in proximity of the slope, and are associated with slope instabilities. The structural features are possibly associated with the recent tectonics mapped on-land as well as the widespread seismicity of the margin. Geochemical features reveal that pore water is slightly enriched in heavy isotopes with respect to Mediterranean seawater, while the distribution profiles of EC, ion concentration (Cl, SO4, Na, K, Mg, Ca), ion/Chloride ratios (Na/Cl, K/Cl, Ca/Cl, Mg/Cl and Alk/Cl) seem to indicate the existence of an external source of fluids and the occurrence of sediment-fluids interaction processes. A possible mechanism causing pore water freshening could be the destabilisation of gas hydrate

Fluid escape structures in the north Sicily continental margin

High resolution and multichannel seismic profiles coupled with multibeam echosounder (seafloor relief) data, acquired along the northern Sicily continental margin (southern Tyrrhenian Sea), document the occurrence of mound and pockmark features, revealing fluid escape processes. Along this margin, morphology of the high-gradient continental slope is irregular due to the presence of structural highs, slope failures and canyons, and is interrupted by flat areas at a mean depth of 1500 m. Seismostratigraphic analysis tools and methods were used to identify fluid escape structures and to work out a classification on the basis of their morpho-acoustic characteristics. The detailed 3D bathymetric chart was used to define the top view morphologic features and their areal distribution. With the aim to evaluate the geochemical content of fluids, we collected a 2.3 m long sediment core in correspondence of a pockmark at a depth of 414 m. Pore waters were sampled every 10 cm and analysed in relation to their conductivity (EC) and composition (δ18O, δD, Li, Na, K, Mg, F, Cl, Br, NO3, SO4). The new data show the occurrence of different types of structures with highly contrasting seismic and morphologic signatures, both dome-type and concave-upward structures. The latter have a characteristic circular shape and are known as pockmarks. Morphobathymetric, stratigraphic and structural data suggest that these structures occur along fault planes, mainly associated with diagenetic carbonates and fluid venting activity. Pockmarks could be the result of both fault and landslide structures, as they appear aligned along a straight direction and occur in proximity of the slope, and are associated with slope instabilities. The structural features are possibly associated with the recent tectonics mapped on-land as well as the widespread seismicity of the margin. Geochemical features reveal that pore water is slightly enriched in heavy isotopes with respect to Mediterranean seawater, while the distribution profiles of EC, ion concentration (Cl, SO4, Na, K, Mg, Ca), ion/Chloride ratios (Na/Cl, K/Cl, Ca/Cl, Mg/Cl and Alk/Cl) seem to indicate the existence of an external source of fluids and the occurrence of sediment-fluids interaction processes. A possible mechanism causing pore water freshening could be the destabilisation of gas hydrates

The geographic information in the assessment of geohazard in the marine environment

Slope instability and erosion, mass transport, volcanic and tectonic activity, fast sediment accumulation fluid escape are the main processes responsible for the geohazard in marine environment. A major knowledge about the geological setting of the offshore areas and related processes can be crucial to assess and manage the potential geological risks. High resolution morphobathymetric surveys, yielded in the frame of the MaGIC project (Marine Geohazards along the Italian Coasts), integrated with previously acquired data, single-channel seismic reflection profiles, backscatter data and sediments sampling, allow to define the geomorphological, stratigraphic and structural features in the offshore sector located in the north-western Sicilian continental margin and in the Ustica offshore (Southern Tyrrhenian). The main goals are: 1) to identify and map the main morphological-structural lineaments of the studied area; 2) to correlate the main structural lineaments on-and offshore; 3) to highlight the instability phenomena of the continental slope, being the area seismically active; 4) to draw up risk maps and to individuate the main dangerous sites; Morphostructural features recognized along the continental shelf to slope system highlighted erosional and depositional morphologies, structural elements at regional and local scale, submerged volcanic edifices, structures related to fluid escape as mud volcanoes and pockmarks. On the basis of the mapping of morphostructural elements and their integration with seismic data and sampling we identified the most likely elements of geological hazard. In the study area we individuated: 1) in the Gulf of Palermo, canyons making the continental slope very rough, also due to the occurrence of widespread mass wasting; 2) between the Gulf of Castellammare and the offshore of the Palermo Mountains, channels and erosional furrows, slumpings, turbiditic fans, landslides, pockmarks and tectonic lineaments; 3) in the offshore of Ustica island, volcanic structures, some of which aligned parallel to tectonic lineaments as Arso fault. The quantitative, GIS analysis allowed a better assessment of the real geohazard. Further investigation on the geological processes, ages of paroxysmal phenomena (eruption, landslides and earthquakes) are necessary to define the most appropriate monitoring strategies in order to assess possible risks for the coastal area and its infrastructures

Volcano- and neoectonic-related slope failures in the north-western Sicily Channel (central Mediterranean Sea): Implications for understanding and assessing geohazard risk

The southern Sicily coasts represent an important contribution to Italian tourism and marine geological processes in the Sicily Channel could pose a significant risk to neighbouring populations and goods. In this work, we are presenting the first results of the data collection that allowed us to identify and map several geological elements that can be used to assess submarine geohazards in the Sicily Channel. By using multibeam data and high-resolution seismic reflection profiles acquired during the ACUSCAL 2015 Cruise, we defined the characteristics of the morphostructural highs, and the morphology of slope failures and the stratigraphy of the mass transport deposits (MTD). In particular, we studied in detail the Graham Bank, which is located in a shallow sector of the north-western Sicily Channel at a distance of 45 km from the Sicilian coastline, where seven seamounts (M1-M7) have been identified and studied in detail within a small area, between 10 and 350 m deep. Their morphometric parameters allowed classification to be implemented on a shape basis. The volcanoes are 115-180 m high and 500-1500 m wide. M2 and M3 (3.5 km X 2.8 km) form the Graham Bank. Most of them show strongly inclined flanks with an average slope of 30°. Most of these seamounts are aligned along two trends (NW-SE and N-S), parallel to the main tectonic structures of the Sicily Channel. The identified structures show physical characteristics, which are very similar to several submarine volcanoes described elsewhere on the seafloor, allowing to conclude that they are volcanic seamounts. In this regard, it is important to highlight that the Graham Bank was affected in the last 100 years, by many eruptions (Colantoni et al. 1975). Furthermore, we distinguished slope failures relating to different mechanisms. In the western flanks of the M2 and M3, volcanic activity and concurrent up-slope triggered mass failures. In the eastern flank M2 gravitational collapse of volcanic edifices is mainly linked to neotectonic activity and volcanism. In the central part of the study area, a MTD is linked to neotectonic activity and to the rise up of volcanic rocks. These MTDs were mapped and described as potential tsunamigenic elements and their volumes were estimated. This work allowed us to understand geological features and processes in a tectonic-volcanic environment, which may represent a threat for coastal areas of the southern Sicily