New bathymetry of the Linosa volcanic complex frommultibeam systems (Sicily Channel, Mediterranean Sea)

This paper presents new bathymetric data acquired around Linosa Island, in the Pelagie Archipelago, revealing the submarine extension of the volcanic edifice, more wide and complex than previously known. The seafloor of Linosa, from the coastal area to about 1000 m depth, was mapped with multibeam systems during the ‘Linosa 2016’ and ‘BioGeoLin 2017’ surveys. A bathymetric map of the surveyed area (about 298 km) was drawn at the original 1:30,000 scale. Overall, the submarine portions of Linosa extend on a total area of about 159 km and are preferentially developed in a NW-SE direction, in agreement with the regional main tectonic trend in the area. The new bathymetric data allow to recognize different sectors in the submarine extension of the volcanic edifice where constructional (volcanic) activity alternates with erosive-depositional processes affecting the submarine flanks of the island. ARTICLE HISTORY Received 23 November 2018 Revised 9 July 2019 Accepted 9 July 2019


Introduction
While recent advances in seafloor imagery systems enabled the extensive mapping of the seafloor, enlarging the knowledge of submarine volcanic areas (Casalbore, 2018) we are still far from having a complete coverage of the submarine portions of many volcanic islands, which commonly represent the limited emerged tips of much larger volcanic edifices. The submarine portions of Linosa Island, in the Pelagie Archipelago, were scarcely known up to now, apart from early studies carried out in the 1988-1992 by the University of Bologna with conventional, single-channel echosounder (Romagnoli, 2004;Rossi, Gabbianelli, Romagnoli, & Serri, 1990). These early data showed the uneven extension of the volcanic structure, aligned in NW-SE direction and surrounded by minor submarine eruptive centers, but they only extended to about 500 m of depth, leaving unknown great part of the volcanic edifice. The Pelagie Archipelago, composed by Lampedusa and Linosa Islands and Lampione Islet, is located in the Sicily Channel (central Mediterranean Sea, Italy) at halfway between Sicily and Lybia (35°30 ′ 13 ′′ N-12°36 ′ 25 ′′ E; Figure 1; Innangi, Di Martino, Romagnoli, & Tonielli, 2019;Innangi et al., 2018). Water depths of the Sicily Channel are general less than 400 m, except in the three NW-SE trending grabens of Pantelleria (PG), Linosa (LG) and Malta (MG) formed in the Neogene-Quaternary in a context of continental rifting (Civile et al., 2010;Dart, Bosence, & McClay, 1993;Finetti, 1984;Maldonado & Stanley, 1977). The aim of this work is to present the new bathymetric data acquired around Linosa island, which represents the emerging summit of a larger volcanic complex lying on the western shoulder of the Linosa graben (Grasso et al., 1991; Figure 1). The island has a surface area of about 5.4 km 2 and a maximum elevation of about 195 m above sea level . The subaerial volcanic activity of Linosa ages back to 1.06-0.53 Ma and has been divided into three main eruptive stages, during which hydromagmatic and magmatic eruptions produce tuff rings, tuff cones, scoria and spatter cones and extended lava flows (Grasso & Pedley, 1985Lanzafame, Rossi, Tranne, & Lanti, 1994;Rossi, Tranne, Calanchi, & Lanti, 1996). No data are available, instead, for the age of its wide submarine portions. The distribution of the meso-structures and of the eruptive centers on the island suggests that the NNW-SSE and the WNW-ESE are the main and secondary trends, respectively, along which a significant tectonic control occurred on the development of volcanic activity (Grasso et al., 1991;Lanti, Lanzafame, Rossi, Tranne, & Calanchi, 1988;Lanzafame et al., 1994). According to the paleo-morphological reconstruction and volcano stratigraphy (pyroclastic deposits distribution and provenance) on Linosa, some of its eruptive centers were located at sea, on the shallower SE and NW flanks of the island (Lanti et al., 1988;Lanzafame et al., 1994). This is in agreement with the location of wide insular shelves were observed at depth less than −120 m, giving indications on the original extension of the early volcanic edifice, largely dismantled by marine erosion during Late-Quaternary sea-level fluctuations (Romagnoli, 2004). The new, high-resolution bathymetric map presented here, at 1:30,000 scale (see Main Map), allows to enlarge and better define the submarine structure of the Linosa volcanic edifice, showing a wider and more articulated extension with respect to what previously known.  Table 1.

Acoustic data acquisition and processing
Bathymetric data were collected and processed using Teledyne PDS 4.1; the software received positioning information from an Oministar Differential Global Positioning System (DGPS) and attitude data from an Ixea Octans 3000 motion sensor. Sound velocity values at multibeam transducers were provided by a Valeport mini-SVS probe, while a sound velocity profiler was systematically used to get the velocity profile in the water column. Both measurements are required for a proper depth computation. During the surveys various filter settings were applied to the multibeam data and quality control displays were used to check the data quality; data logging progress was shown in real-time using a color-coded Digital Terrain Model (DTM). The PDS Editing combines in a single module 3D swath editing, multibeam calibration, DTM editing and SVP editor and it allows to clean swath data and to update DTM model in real-time. Data de-spiking was carried out manually without the application of automatic filters in order to preserve data accuracy and resolution. Bathymetric data, previously collected in very shallow-water in the framework of a mapping project, were also available (Coastal Consulting Exploration s.r.l., 2008; Figure 1, blue area). This latter dataset was acquired in 2008 using Reson SeaBat 8125 and was integrated with the new data in order to expand the bathymetric map towards the coast (less than 15 m of the depth). These data were acquired only within the Pelagie Marine Protected Area . Therefore, in the southern sector, which is not included in the MPA, the bathymetric data are missing in the range from 20 m to the coast (Figure 1).

Results and discussion
All available data were merged to create a bathymetric map of Linosa, drawn at the original scale of 1:30.000 (see Main Map). The final DTM (10 × 10 m cell grid size) shows that the volcanic edifice is mostly extended below sea level and much larger than the small area of the island (Figure 2). The depth of the base of the volcanic complex extends, in fact, down to at least −1000 m to the north and north-east, −750 m to the west and east, and about −400/−450 m to the south. This is due to the different setting of the submarine flanks of the volcanic edifice, hereafter described according to an informal division in four main sectors (Figure 3). The overall extension of the submarine portions of Linosa is the widest along the NW-SE direction, reaching about 20 km. Sector NW (Figure 3) is characterized by a number of minor, submarine eruptive centers, mostly aligned along a volcanic belt, and it is composed by more than 20 individual eruptive centers lying at different depth (Belvisi, 2018;3D view in Figure 4(a)). Two of these have the summit in relatively shallow-water, i.e. 'Secca Maestra' and 'Secca di Tramontana', the two main shoals in Figure 4(a). The former is a conical eruptive center with the summit at −38 m, while the second is a sub-circular morphological volcanic structure strongly eroded, lying between 40 and 20 m of depth on the insular shelf surface (Figure 4(a)). On the opposite side of the island (Sector S in Figure 3), the southern submarine flank of Linosa has a sub-rounded shape and is less deep than the northern one (see 3D view in Figure  4(b)). A number of eruptive centers are present on the southern submarine flank of the island, rising from depth between −250 and −570 m (Figure 4(b)) and damming small intra-slope basins (Belvisi, 2018;Romagnoli, 2004). They are roughly arranged in a semi-circular pattern around the island, although local NNW-SSE to WNW-ESE alignments are recognizable in their distribution. The former alignment can be observed, for instance, in the flattened and strongly eroded remnants of minor eruptive centers (with top at −33 and −30 m, respectively, in Figure 4(b) and Innangi et al., 2018) observed in shallow-water, on the insular shelf to the SE of the island, having here an outer edge at about −100 m. Previous works Romagnoli, 2004) described the shelf in the southern submarine sector as being covered by volcaniclastic deposits arranged in terraced, prograding depositional bodies. The smoothness of the shelf surface reflects, in fact, the sedimentary coverage, likely fed by reworked volcaniclastic material due to the erosion of pyroclastic and lava units on the island, a process commonly observed in the post-eruptive, degradational stage of insular volcanoes (Romagnoli & Jakobsson, 2015). Finally, on the southeastern flank of the island, between about −300 and −450 m, a lava field is present, giving place to an irregular seabed with relatively fresh morphology (Figure 4(b)). A peculiar feature to the south-west of Linosa island is a main scarp in the seabed, about 9 km long, developed between −500 and −700 m in an NW-SE direction (Figure 4(b)). It delimits the volcanic edifice westwards, being likely tectonically controlled (see also Figure 2). The north-eastern and western submarine flanks of Linosa Island (sectors NE and W in Figure 3) are, instead, characterized by erosive processes, as witnessed by the presence of active canyons with their heads in shallow-water (where insular shelves are lacking, see the 3D views in Figure 4(c,d)), alternating with sub-radial volcanic structures and outcrops, representing the eroded remnants of constructional volcanic features (Chadwick et al., 2005;3D view in Figure 4(c)). An irregular, small lava field with irregular morphology is observed to the north-east of the island, at depth varying between −856 and −988 m, covering a total area of 2.27 km 2 (Belvisi, 2018;Figure 4(c)). For what regards the canyons, the occurrence in their bottom of crescent-shaped bedforms with crest-lines trending roughly perpendicular to the slope direction (Figure 4(c)) suggests that they are seat of active reworking processes, as also observed in other submarine volcanic context such as in the Aeolian Islands (Casalbore, Bosman, Romagnoli, & Chiocci, 2017). Finally, in the southern area of Linosa from the base of the volcanic edifice towards the south, the seafloor decreases in a flat, gradually sloping area (  Further investigations are necessary to better understand the nature of these features.

Conclusions
The volcanic island of Linosa has an area of 6 km 2 and represents only a small percentage (about 3.6%) of the entire volcanic complex (about 165 km 2 ). This first bathy-morphological analysis of the multibeam data recently acquired around the island allows to improve the knowledge of the submarine extension of the Linosa volcanic complex and of its submarine development. In particular, the new data enhance the fundamental role, in the growth of the submarine portions of the volcanic edifice, played by the NW-SE oriented regional tectonic trend in controlling the subaerial and submarine volcanic activity (Lanti et al., 1988;Rossi et al., 1990). Overall, it is possible to define different sectors in the submarine extension of the volcanic edifice, having distinctive characteristics: . The north-western and southern submarine flanks are characterized by the widespread occurrence of secondary eruptive centers: these in the NW of the island are distributed along a volcanic belt that extends for over 8 km in an NW-SE direction, while to the south they are distributed in a semi-circular arrangement with respect to the island, being partly aligned along the same trend. . In the eastern/north-eastern and western submarine flank, on the other hand, erosive and depositional processes prevail, as witnessed by the presence of a network of active canyons from shallow-water to the submarine base of the volcanic edifice. This is partly due to the absence of a well-developed insular shelf in this sector disconnecting the subaerial and the submarine portions, while reworking processes are promoted on these steep submarine flanks.
An important lineament of likely tectonic origin delimits the south-western submarine flank at depth of 500-700 m; its NW-SE orientation is in agreement with the main regional structural systems in the Sicily Channel and in the nearby Linosa graben (Grasso et al., 1991). Similar preferential alignment is shown by several fluid-escape features (pockmarks) observed in the area to the south of the island base, suggesting a structural control on these processes. These features   Figure 2). Below, the bathymetric section a-a ′ crosses two pockmarks (about 40-50 m deep, and width between 250 and 500 m).
will thus require further investigation in order to better characterize the interaction with active tectonics.
To conclude, the new bathymetric map of Linosa presented here represents a first base for future investigation on the growth and evolution of this volcanic island, whose wide submarine portions are still largely unknown.

Software
Bathymetric data were acquired and processed with Teledyne PDS 4.1, as well as the final DTM. The 3D views were produced with Global Mapper cartographic software. The Main Map was produced using QGIS NOOSA 3.6.2.

Acknowledgements
Thanks to the crew of the R/V 'Minerva Uno', for all the support during board and research operations.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This study benefited from contribution of the project 'Implementation of research activity and monitoring around Pelagie Islands Marine Protected Area', within the project 'CAmBiA -Contabilità Ambientale e Bilancio Ambientale' funded by the Ministry of the Environment and Protection of Land and Sea (also known as MATTM -Ministero dell'Ambiente e della Tutela del Territorio e del Mare), Directive n°5135 of March 2015. This study also benefited from the contribution of the RITMARE (La Ricerca ITaliana per il MAREhttp://www.ritmare.it/) Flagship Project, funded by Ministry of Education, University and Research (also known as MIUR -Ministero dell'Istruzione dell'Università e della Ricerca) [NRP 2011[NRP -2013.