Seawater dynamics and environmental settings after November 2002 gas eruption off Bottaro (Panarea, Aeolian Islands, Mediterranean Sea)

Abstract

In November 2002 several gas bursts occurred at sea in the caldera within the islets eastward of Panarea (Aeolian Islands), with degassing of CO₂ lasting several months. In a depression close to Bottaro Islet (PEG1) the gas flowed violently from the depth of $\sim 14\mathrm{m}$ to the surface producing a large plume of gas and fluids. The aims of this paper are to report on the morphological modifications, the water and gas fluxes, and the water dynamics and hydrological properties near the PEG1 site. Bathymetric surveys up to 2006 and divers’ observations up to 2008 revealed that the sinkhole had been partially filled by sediments transported by currents or sliding from the rims. Depth reduced to $\sim 12\mathrm{m}$ and the shape of the depression was becoming similar to the others in the area, suggesting that explosive gas eruptions may have occurred in the past. The gas outflow generated a bubble plume that locally affected the water circulation. Different patterns in the dynamics of the water column were described using rotor current meters, ADCP, ROV and divers’ observations, and high-resolution bathymetry. A divergence about 1 m thick was generated by a surface vortex visible in December 2002 that was not found in September 2003 with reduced gas flow. A sub-surficial layer down to $\sim 1–2\mathrm{m}$ above bottom showed varying speed and directions possibly correlated to tides. A bottom layer of water $\sim 1\mathrm{m}$ thick flowed continuously toward the crater during both surveys. On the nearby sandy bottoms, sand dunes and 2/4 mm size volcanoclastic gravels rolling toward the emitting area were found. The fluxes of water entering at the seafloor were calculated by current velocities, height of the bottom layer, and area of the degassing vent. The input water fluxes were found to be 4.2 and 0.2×10⁸ l/d, for December 2002 and September 2003. The total vertical output flux was also estimated; considering bubble sizes and voids, gas fluxes were 2.6 and 0.3×10⁸ l/d. These values are similar to the fluxes calculated by geochemical and gas sampling methods. CTD data from summer 2003 and spring 2004 recorded pH anomalies with values as low as 6.3 located on main emissions. The low pH water was also distributed laterally for dozens of meters on the bottom, depicting plumes and bottom layers of altered seawater over remarkable distances. The gas burst generated a plume similar to what is described for double core bubble plumes, where the velocity of seawater entering the system is proportional to the vertical gas flux. A new perspective to monitor vent activity is envisaged, and since measuring seawater is easier than measuring gas fluxes, a continuous and real time monitoring of hydrothermal activity could be designed.

Introduction

Fluid discharges in the marine environment take place in many geological settings with large volumes of gas released, mostly in relation to volcanic and hydrothermal activity, and active faults (Prol-Ledesma et al., 2005, Dando et al., 2000, Géli et al., 2008). Studying them is very important in the light of volcanic and seismological surveillance and monitoring, especially as possible precursors (Dando et al., 1995b, Heinicke et al., 2009) and agents for increasing ocean acidification (Hall-Spencer et al., 2008); furthermore, they may be able to produce thermal and chemical weathering of sediments and rocks.

Fluids and gases rising up in venting areas mix with and are entrained by seawater producing large, chemically altered plumes (Resing et al., 2004, Solomon et al., 2009). Strong upward convection can occur at deep sea vents and seeps, when driven by the positive buoyancy which results from the discharge of hot fluids in cold seawater, for example at ridge crests (Lupton et al., 1985) or from the release of large gas columns as reported by Leifer et al. (2000) or Linke et al. (2009). At shallow depths, vent plumes are usually small, but traces of hydrothermal matter, e.g. prokaryotes or sulfur deposits are found at distances of few kms from the source (Bayona et al., 2002).

In shallow waters, where hydrostatic pressure is not the major forcing controlling gas dilution and water boiling, vents are of the gaseohydrothermal type with high flow rates of free gas (Tarasov et al., 1990, Dando et al., 1995a). Water circulation has been described under ‘steady-state’ conditions, that generates many diffusive and sparse small columns of gas bubbles outflowing from the seabed, instead of spots that produce a clear alteration of water dynamics.

Hydrothermal gas outflows are largely reported in the Mediterranean (Dando et al., 2000, Aliani et al., 2004). Major emissions have been found in the Aeolian Islands for centuries (Dekov and Savelli, 2004 and references therein) and fumarolic activities have been reported since the Roman Age, in addition to occasional outbursts.

On 3 November 2002 a violent burst of gas occurred E of Panarea (Fig. 1), lasting for years with a consistent flux from fractures and sinkholes on the seafloor, mostly near the islet of Bottaro. The phenomenon was investigated in detail from geological and geochemical points of view (Caliro et al., 2004, Capaccioni et al., 2005, Anzidei et al., 2005, Caracausi et al., 2005, Esposito et al., 2006). It immediately attracted the attention of researchers in the light of volcanological surveillance and risk, who focused on the hydrothermal or magmatic origin (Chiodini et al., 2006, Capaccioni et al., 2006), and on the possible connection to regional tectonics. Heinicke et al. (2009) inferred a common link along NE striking faults between Panarea and Stromboli, within the extensional domain of the Eastern sector of the Aeolian Arc (Acocella et al., 2009). The gas burst of Panarea occurred concurrently with an increase of seismic activity in the Southern Tyrrhenian Sea and strong eruptions of Etna and Stromboli (Billi and Funiciello, 2008, Walter et al., 2009).

This event provided a unique chance for studying the oceanographic setting during a large gas outflow in shallow water and the modification of the environment and surrounding water.

This paper aims to report on the morphological changes of the seabed and to describe the water dynamics and hydrological properties over time during the massive gas eruption at the PEG1, Bottaro. An attempt to obtain data on water and gas fluxes was made using current velocity data, ROV, divers’ and topographic investigations.

The Aeolian Islands form a volcanic arc that lies North of Sicily and borders the SE portion of the Tyrrhenian Sea. The archipelago is formed by seven islands and minor islets, including the present active volcano of Stromboli. Among them Vulcano and Panarea present fumarolic activities and gas vents at sea.

The geological, geophysical and petro-geochemical settings of this major geotectonic feature, formed by the convergence of the African and Eurasian plates and by the subduction and southeastward rollback of the Ionian lithosphere, have been discussed by several authors (Barberi et al., 1973, Barberi et al., 1974, Argnani and Savelli, 1999, Argnani, 2000, Calanchi et al., 2002, De Astis et al., 2003). Panarea is now considered inactive, however Gamberi et al. (1997) have shown possible recent volcanic outcrops near Basiluzzo; present deformation patterns are likely connected to NE-SW trending faults (Lucchi et al., 2007).

The gas release of 3 November 2002 in the caldera E of Panarea, a marine area that had already experienced gas eruptions, so much so that it was called bollitore (boiler) by local inhabitants (Italiano and Nuccio, 1991), produced visibly impressive eruptions and generated a sustained column of bubbles from the sea bed to the sea surface (Fig. 2, Fig. 6, Fig. 7). Several active spots were identified by direct observations and bathymetric investigations (Anzidei et al., 2005), including a major one just SW of Bottaro, named PEG1, with gas reaching the surface from a depth of 15 m.

An explosive collapse of the seafloor was triggered and a NW oriented subelliptic depression was formed, with a sinkhole at the southern tip, where the main gas outflow was located. This sinkhole showed a funnel-like morphology to the S, and vertical and subvertical walls to the N, exposing cemented breccias and cobblestones likely of Holocenic age (Esposito et al., 2006). A large plume of fine sediments was present at the surface for some days after the burst.

During the most active degassing phase lasting several months up to mid 2003, the emissions were found to be an emulsion of CO₂-dominated gas with suspended sediments, colloidal sulfur and water-condensated microdroplets acidified by dissolution of compounds such as SO₂, HCl and HF (Capaccioni et al., 2005, Capaccioni et al., 2006). Caliro et al. (2004) estimated a gas output of 10⁹ (November 2002, all emissions) and of 4 to 2×10⁷ l/d (May–July 2003, PEG1), which represents orders of magnitude higher than the total gas output of 10⁶ l/d measured within the Islets in the 1980's (Italiano and Nuccio, 1991). Eruptive fluids dispersed at sea produced plumes that greatly affected the marine environment with changes in the biota (Gugliandolo et al., 2006, Manini et al., 2008) and in the water properties (Tassi et al., 2009). Fig. 2e shows the location of the major emissions (named PEG1,2,3,8).