High-resolution morpho-bathymetry of Pozzuoli Bay, southern Italy

Abstract

We present the results of a detailed bathymetric survey of Pozzuoli Bay (Gulf of Naples, Italy). This shallow marine area, along with the Campi Flegrei inland, is a highly active volcanic district in the coastal zone of SW Italy. The area has been active since at least 78 ka B.P., and is structurally dominated by a caldera collapse (∼8 km in diameter) associated with the eruption of the Neapolitan Yellow Tuff (NYT), a 30–50 km³ dense rock equivalent (DRE) ignimbrite dated ∼15 ka B.P. The main cartographic product consists of a 1:10,000 scale morpho-bathymetric map of Pozzuoli Bay, derived from 1 m cell-size, colour hill-shaded, digital terrain model of the seafloor. Multibeam bathymetry data reveal the precise extent of Roman underwater archaeological remains located in the N–NW infralittoral zone of the Bay. Morphometric analysis allowed for the development of thematic representations, including slope and aspect maps. A complete data set of active fluid vents seafloor locations were also recorded during the survey and reported in the final map. The multibeam bathymetric survey illustrated in this study provides an unprecedentedly detailed image of the seafloor morphology of Pozzuoli Bay and represents a contribution to the understanding of the dynamic evolution of the Campi Flegrei caldera, a high-risk volcanic area densely populated by almost one million people.

Keywords:

• multibeam survey
• bathymetry
• seafloor morphology
• digital terrain model
• volcanic risk

1. Introduction

Multibeam swath bathymetry technologies have significantly improved during the last 20 years allowing for the possibility of producing ultra high-resolution images of seafloor morphology. In particular, a new generation of high-resolution bathymetric maps has resulted in a renewed interest in the better understanding of the morphological expression and dynamics of south-eastern Tyrrhenian submarine volcanism over the last few years (e.g. Bosman, Casalbore, Romagnoli, & Chiocci, 2014; Casalbore, Bosman, Romagnoli, Di Filippo, & Chiocci, 2014; Casalbore, Romagnoli, Chiocci, & Frezza, 2010; Chiocci & De Alteriis, 2006; De Alteriis & Violante, 2009; Passaro et al., 2010; Passaro, Milano, Sprovieri, Ruggieri, & Marsella, 2011a; Romagnoli, Casalbore, Bosman, Braga, & Chiocci, 2013; Sacchi et al., 2014; Ventura, Milano, Passaro, & Sprovieri, 2013).

The Campi Flegrei district is an active volcanic area that covers about 200 km² of the coastal zone of SW Italy, a large part of which develops off the Gulf of Naples, and is characterized by at least one large caldera collapse structure. The caldera is represented by a quasi-circular area of ∼8 km in diameter in the central sector of the Campi Flegrei, including the Pozzuoli inland area and Pozzuoli Bay (Carlino & Somma, 2010; Dello Iacono, Zollo, Vassallo, Vanorio, & Judenherc, 2009; De Natale et al., 2006; Orsi, De Vita, & Di Vito, 1996; Rosi & Sbrana 1987; Sacchi et al., 2009; Scandone, Bellucci, Lirer, & Rolandi, 1991; Sacchi et al., 2014).

Despite considerable research work and numerous geophysical surveys conducted in the Campi Flegrei inland area over the last 30 years, the detailed offshore morphology of the Neapolitan Yellow Tuff (NYT) caldera collapse structure and resurgence, and hydrothermal activity are still poorly understood. This is mostly because of the intrinsic limitations associated with the low resolution and poor geo-referencing accuracy of many published bathymetric data sets (e.g. D'Argenio, Pescatore, & Senatore, 2004; Di Vito et al., 1999; Fevola, Fusi, & Mirabile, 1993; Fusi, Mirabile, Camerlenghi, & Ranieri, 1991; Orsi et al., 1996; Milia, 1998; Pescatore, Diplomatico, Senatore, Tramutoli, & Mirabile, 1984).

In this study, we present the results of a very high-resolution multibeam bathymetric survey conducted in Pozzuoli Bay in June of 2013. The resulting seafloor map provides an unprecedentedly detailed image of the shallow structure of the NYT caldera collapse and resurgence. In addition, the results of the swath bathymetric survey also enabled the recognition and mapping of active fluid vents (AFV), by using echosounder profiles of the water column (e.g., Passaro et al., 2014). The study of AFV is of particular interest in this area, for the characterization of the feeding magma system as well as for the understanding of genetic processes and rising mechanism of volcanic fluids. In fact, AFV have long been recognized in the emerged Campi Flegrei sectors (e.g. Aiuppa et al., 2013; Chiodini et al., 2010; Giacomelli & Scandone, 2012; Tassi et al., 2013; Vaselli, Tassi, Tedesco, Poreda, & Caprai, 2011). However, occurrence of the spotty marine vents has had less recognition so far (see Giacomelli & Scandone, 2012, and references therein). The morpho-bathymetric map of Pozzuoli Bay provides a first onshore–offshore cartographic basemap for the definition of multi-risk scenarios and coastal zone management actions for the active volcanic area of Campi Flegrei.

2. Geological background

Volcanic activity of the Campi Flegrei spans from the late Quaternary (Di Girolamo et al., 1984; Rosi & Sbrana, 1987; Lirer, Luongo, & Scandone, 1987; Di Vito et al., 1999; Rolandi, Bellucci, Heizler, Belkin, & De Vivo, 2003) and is characterized by at least one large caldera collapse structure, produced during the eruption of the NYT, a 30–50 km³ dense rock equivalent (DRE) ignimbrite dated at ∼15 ka BP (Deino, Orsi, de Vita, & Piochi, 2004).

Several studies have proposed that the onset of the NYT caldera was preceded by a larger caldera collapse during the eruption of the 100–200 km³ DRE Campania Ignimbrite (CI) 39 ka. BP (Lirer et al., 1987; Rosi & Sbrana, 1987; Orsi et al., 1992; De Vivo et al., 2001; Deino et al., 2004), although recent investigations suggest that the CI may have originated instead from fissural eruptions that occurred within the Campania Plain (Rolandi et al., 2003).

The NYT eruption was followed by almost 70 explosive and effusive eruptions among which the most representative Plinian eruptions were Agnano-Pomici Principali, 10 ka (Di Vito et al., 1999) and Agnano-Monte Spina, 4.1 ka (de Vita et al., 1999). Products of hydromagmatic eruptions are mainly monogenetic tuff cones and tuff rings, while effusive eruptions are only represented by a few lava domes (Rosi, Sbrana, & Principe, 1983).

The last eruption was of Monte Nuovo, (1538 A.D) with 0.03 km³ of erupted products (Rosi et al., 1983). After this eruption, the floor of Campi Flegrei caldera was affected by a slow and regular subsidence. In more recent times, two bradyseismic crises accompanied by seismic events of low magnitude occurred in the area. The first was in 1969–1972, and the second was in 1982–1984. These episodes produced an overall 3.5 m of ground uplift at the points of maximum uplift in the town of Pozzuoli (Barberi, Hill, Innocenti, Luongo, & Treuil, 1984; Del Gaudio, Aquino, Ricciardi, Ricco, & Scandone, 2010), with very shallow recorded microseismicity triggered in response to fluid motion episodes (D'Auria et al., 2011).

Since November 2004, a new uplift phase started with about 0.04 m of uplift by the end of October 2006 (Del Gaudio et al., 2010; Troise et al., 2007). The magmatic system is still active as attested by diffuse degassing zones and hydrothermal vents (fumaroles) in the areas of Solfatara crater and Pisciarelli. Petrologic and geophysical evidence (e.g. Battaglia, Troise, Obrizzo, Pingue, & De Natale, 2006; Caliro, Chiodini, & Paonita, 2014; De Natale et al., 2006) suggest (for the Campi Flegrei caldera) the occurrence of a sill-like reservoir of fluids at a depth of 3–4 km, associated with a deeper feeding magma reservoir located at a depth of ca. 8 km.

3. Data and methods

The bathymetric survey of Pozzuoli Bay was carried out in June of 2013 on board the ‘Idrosfera’ catamaran using a Reson Seabat 8101 multibeam. This echosounder is characterized by a 240 kHz acoustic source frequency, 101 beams and 150° of pulse width, and is particularly suitable for high-resolution mapping in shallow water (up to 100 m depth). The use of the V101 Hemisphere differential global positioning system receiver coupled with FOG-200 motion sensor ensured sub-metre precision for beam positioning during data acquisition. A tidal correction derived from the national tide gauge network was also applied to increase vertical resolution. A set of 18 sound velocity profiles were acquired during the survey. In addition, a hull-mounted sound velocity profiler sensor was used for continuous, real-time measurements of sound velocity and temperature for the uppermost part of the water column.

After data processing, depth measurements were interpolated to a digital terrain model (DTM) with 1-m grid resolution. During multibeam acquisition, fluid emissions at the seafloor were mapped using the 50/200 kHz double frequency Furuno FCV620 5.6-Inch Waterproof Fishfinder that can detect acoustic anomalies inside the water column. The location of all major AFV recognized at the seafloor during the survey was included in the cartographic representation presented in this study.

In order to complete the Main Map, the newly obtained DTM was integrated with another marine DTM (5X5 m grid cell size) acquired by the IAMC-CNR in 2002 on board of R/V ‘Tethis', with a Reson Seabat 8111 (100 kHz) echosounder for sectors deeper than 80 m below sea level (bsl), off Pozzuoli Bay. Finally, the marine DTM was merged with a 5X5 m grid of the Campi Flegrei coastal area, acquired by the Campania Region in 2004 (Main Map).

For the interpolated marine/terrestrial DTM standard geomorphologically derived products were computed, including (a) a slope map (Figure 2(a)); (b) an aspect map (Figure 2(b)).

Seafloor areas reported in the main morpho-bathymetric map, as well as in Figures 1–2, locally display an apparently rugged-shaped, ripple-like morphology, possibly as a consequence of an ineffective motion reference units correction. This is, however, an artefact, resulting from small vertical oscillations of depth measurements that produce fake seafloor undulations aligned along swath, perpendicularly to the ship heading (Figure 2(c)). Shaded relief is computed using gradient over contiguous cells, which may produce noise due to gradient computation on relatively flat areas. Indeed, elevation profiles extracted from the DTM reveal that the magnitude of ripple-like noise does not exceed 0.05 m (0.02 on average; Figure 2(d)).

Figure 1. (a) DTM of Pozzuoli Bay. Projection is UTM (33); Datum is WGS84. (Lakes: AL = Averno, LL = Lucrino; Craters: MN = Monte Nuovo, GA = Gauro; AS = Astroni; CI = Cigliano; SO = Solfatara; AG = Agnano; AS = Astroni); Inset (b) Archaeological remains in the marine sector in front of Baia (VdP = Villa dei Pisoni; BeC = Baianus Lacus entry channel); Inset (c) Archaeological remains in the marine sector in front of Pozzuoli (PI = Portus Iulius; PIEC = Portus Iulius Entry Channel).

Figure 2. (a) Slope map; (b) Aspect map; (c) Seafloor DEM sample displaying apparent rugged-shaped, ripple-like morphology, actually due to an artefact (fake seafloor undulations aligned along swath, perpendicular to the ship heading); (d) bathymetric profile P1–P2 showing shape and magnitude of ripple-like noise.

The final map was created using universal transverse mercator (UTM) conformal projection and WGS84 reference ellipsoid. The vertical datum (0 m elevation) has been referred to the average level of the low spring tides of Pozzuoli Bay.

4. Results, discussion and conclusions.

The cartographic product illustrated in this study is based on a 1-m grid resolution marine DTM of Pozzuoli Bay at 1:10,000 scale and provides a detailed image of the onshore–offshore morphology of the Campi Flegrei coastal volcanic district (Figure 1). The final marine DTM covers about 30 km², in a water depth range of 1–110 m bsl. In the area, slope varies between 0° and 80° (Figure 2(a)), with higher values mostly concentrated around younger volcanic cones (e.g. Monte Nuovo), while in the marine sector, gentle slopes prevail, and several terraced surfaces mostly oriented N130°E occur (Figure 2(b)). Stepped terraced areas on the easternmost side of the Bay are up to 1.5 km wide, while in the west, their width is as small as 0.5 km.

It is worth noting that Pozzuoli Bay is particularly important in terms of underwater archaeology. The whole coastal area was subject to remarkable anthropic influence during Roman times, resulting in the establishment of two settlements, Portus Iulius to the East (the present-day Pozzuoli area) and Baianus Lacus to the West (Baia). The remains of these settlements are presently lying under the sea, at water depths between a few meters and 15 m bsl (Figure 1(a)–1(c), as a consequence of both subsidence of the NYT caldera floor and sea-level rise. Baianus Lacus included villas and luxury buildings, like Villa dei Pisoni (VdP, Figure 1(b)), and was the site of several thermal complexes (Carlino, Somma, Troise, & De Natale, 2012). In the easternmost sector of the Bay (the ancient Puteoli), underwater remains are represented by the Portus Iulius complex (Figure 1(c)) that was originally a military district later converted into a commercial settlement. The abandonment of the military site was likely a consequence of progressive burial of the old harbour area, due to the seaward accumulation, by the river network, of loose volcaniclastic material that was mantling the slopes of Campi Flegrei during the first two centuries after Christ.

A long breakwater, called the Via Herculanea connected Portus Julius to Baianus Lacus, and separated the anthropic embayment of Lucrino (presently Lake Lucrino) from the open sea (Passaro et al., 2013). Later, the morphology of the entire area underwent dramatic change after the Monte Nuovo eruption, in 1538 A.D.

A histogram of elevation was computed in order to highlight the average distribution of sub-horizontal (terraced) surfaces that, in this type of plot, show positive peaks in the distribution (Passaro et al., 2011a; Passaro, Ferranti, & De Alteriis, 2011b). Terraced surfaces occur at water depths of 116, 101, 76, 37, 22 and 7 m and also above sea level at 4, 19, 34, 64 and 98 m in altitude (Figure 3). These surfaces are likely the result of phases of dynamic equilibrium between seafloor erosion, ground deformation (uplift and/or subsidence), sea-level fluctuations and sedimentary input from the coast.

Figure 3. Elevation histogram derived from the DTM data, illustrating statistical distribution and average elevation of planar (terraced) surfaces. Red line indicates the envelope of grid cell elevations computed at 1 m; black line shows the same distribution computed at 3 m.

The spatial distribution of AFV detected at the seafloor during the bathymetric survey suggests that AFV are preferentially distributed along linear patterns, locally parallel to the coastline segments, and may mirror slope and/or aspect changes.

ORCID

Salvatore Passaro http://orcid.org/0000-0002-3045-9002

Software

PDS2000 (Teledyne RESON) was used to process the multibeam data and generate the marine DTM. DraftSight 32 (Dassault Systèmes) freeware was used to edit vector maps, export thematic layers and add toponyms and symbols (e.g. breakwaters, jetties, buoys). Quantum GIS ‘Dufour’ (ver. 2.0.1) was adopted for managing and editing geographical data, computing slope and aspect maps and creating the final map design.

Acknowledgements

The high-resolution bathymetric survey of Pozzuoli Bay was carried out in June of 2013 by Geomarine srl, Senigallia (AN), Italy, under the coordination of INGV, Napoli and IAMC-CNR, Napoli. The final DTM was integrated with a marine DTM (5X5 m grid cell size) acquired by the IAMC-CNR in 2002 within the frame of the CARG Project (Regione Campania and ISPRA) along with a DTM of the inland area (5X5 m grid cell size), acquired for the ORCA Project (2004) of the Campania Region.

Funding

Financial support for this research was provided by the Programma Operativo Nazionale (PON) – Ricerca e Competitivita’ 2007–2013 funded by the Italian Ministry of University and Research – Project MONICA (PON01_01525).