Review
Carbon dioxide emission and heat release estimation for Pantelleria Island (Sicily, Italy)

https://doi.org/10.1016/j.jvolgeores.2014.02.011Get rights and content

Highlights

  • Restricted areas of Pantelleria emit huge amount of CO2 from the soil.

  • Soil temperature and soil CO2 flux patterns suggest a common origin for fluids and heat.

  • The minimum geothermal potential of the shallow reservoir of Pantelleria is 10–12 MW.

Abstract

Detailed surveys of diffuse CO2 flux, soil temperature, thermal gradients, and sampling of high-T fumaroles were carried out in the Favare area and Lake Specchio di Venere on Pantelleria Island. Spatial patterns of diffuse CO2 emissions in the Favare area reflect structural discontinuities (faults, fractures or cracks in the soil) associated with the volcano-tectonic structures of the young Monastero Caldera (NNE–SSW to NE–SW trending). The estimated diffuse CO2 output from two adjacent sites in the Favare area (~ 93,000 m2) is 7.8 t d 1 (equivalent to 2.62 kt a 1), whereas that from the west shore of the lake (450 m2) is 0.041 t d 1 (or 0.015 kt a 1). The extrapolation of diffuse CO2 fluxes across the entire altered area of Favare suggests that CO2 emissions are ~ 19.3 t d 1. The diffuse CO2 flux correlates with shallow soil temperatures, indicating a similar source for both the heat and volatiles from the underlying geothermal reservoir. Gas equilibria applied to fumarolic effluents define P–T conditions for this reservoir at 2–6 bar and 120–160 °C, in good agreement with measurements from exploratory wells in these areas (e.g., 135 °C at a depth of 290 m). Using the CO2 flux as a tracer for steam output, and consequently for heat flow, the calculated thermal energy for the shallow reservoir is 10–12 MW; this represents the minimum geothermal potential of the reservoir on Pantelleria island.

Introduction

The geochemistry of Pantelleria fluids has been studied extensively in the context of drilling programs aiming to assess the potential for geothermal development. Pantelleria has been renown since Roman times for its hot springs, thermal wells and gaseous emissions, locally associated with emissions of warm air (~ 40 °C) and steam from fractures in the soil (historically known as “calidaria”). Fumaroles with temperatures close to the boiling point of water (~ 100 °C) are concentrated in the central part of the island (Montagna Grande–Mount Gibele–Passo del Vento areas, Fig. 1). A volcanic endorheic basin, Lake Specchio di Venere (literally “Venus' Mirror”, also known as “Bagno dell'Acqua”; hereafter Lake SV) lies within a caldera depression in the northern sector of the island (Fig. 1). The lake is characterized by an intense release of gas bubbles (essentially CO2) from the surface of the blue-green waters. The lake's features have been documented in the scientific literature since the 19th century (Foerstner, 1881, Washington, 1913).

Geothermal investigations at Pantelleria began in the late 60s, when some thermometric wells were drilled in different sectors of the island (Barbier, 1969). In the early 90s new geothermal exploration was undertaken on the island, with progressive improvement in the quality and quantity of observations. Four shallow exploratory wells were drilled in 1992 to depths of 150 to 300 m (wells PT1, PT2, PT3 and PT4, Fig. 1). Two other wells, sunk in 1993 (PPT1, PPT2) to depths of about 1200 m, are in areas of the island previously identified as sites of major geothermal interest. Most studies, based on borehole data and gas geothermometric data on natural manifestations, envisage a geothermal system consisting of two distinct, vertically stacked layers, the shallower one characterized by an almost pure liquid phase (marine–meteoric waters) and the deeper one by a vapor-dominated or liquid-vapor phase (Squarci et al., 1994, Chierici et al., 1995a, Parello et al., 2000). The two layers are likely interconnected through faults or fractures in the central sector of the young “Monastero” Caldera (Cornette et al., 1983), also known as the “Cinque Denti” Caldera (Mahood and Hildreth, 1986), where intense tectonic activity uplifted the floor of the caldera. Most thermal manifestations on the island (hot springs, thermal wells, and emissions of warm air from the soil) are probably fed by the lower-temperature superficial aquifer. In contrast, the persistent, hotter fumaroles within the Monastero Caldera and the thermal manifestations in Lake SV (along the northern edge of the Monastero Caldera) are fed by hot fluids rising from the deeper aquifer and are variably mixed with the superficial marine–meteoric component.

A few recent studies have evidenced the huge amount of CO2 emitted at Pantelleria through diffuse soil degassing, bubbling, dissolved gas and, subordinately, fumaroles. Favara et al. (2001) estimated that the total output of CO2 from the island is 390 kt a 1, with diffuse soil degassing as the major contributor (320 kt a 1). They identified the western and southwestern shores of Lake SV and the Favare area (near to Montagna Grande) as two sites of enhanced soil degassing (Favara et al., 2001). D'Alessandro (2007) confirmed the major contribution of the Lake SV and the Favare area to the total output of diffuse CO2.

The present work reports on an in-depth study of the process of diffuse degassing of volcanic–hydrothermal fluids from the Lake SV and the Favare area based on combined measurements of soil CO2 flux, soil temperature and gas composition from high-temperature fumaroles. Geostatistical procedures are used to quantify the amount of CO2 emitted and to distinguish between biogenic and hydrothermal sources. Resulting maps of soil CO2 flux and soil temperature are related to volcano-tectonic discontinuities, along which deep gases are able to migrate towards the surface. Moreover, we discuss the possible origin of the released fluids and define the P–T conditions at depth by analyzing the chemical and isotopic composition of the high-temperature (~ 100 °C) fumaroles. Lastly, we estimate the thermal energy associated with the degassing process and compare this energy with the heat released by steam condensation in the soil.

Section snippets

Pantelleria Island: geostructural setting and geothermal features

The Island of Pantelleria is the emergent part of a larger submarine volcanic edifice extending below sea-level to a depth of about 1300 m in the Sicily Channel continental rift system (inset in Fig. 1). Geophysical and geological investigations revealed that the offshore area of Pantelleria is characterized by crustal stretching (Colombi et al., 1973, Boccaletti et al., 1984), high heat flow (Della Vedova et al., 1995) and a positive Bouguer anomaly (Morelli et al., 1975).

The island consists

Geothermal aquifers of Pantelleria

Extensive sampling of almost all the known thermal manifestations on the island were carried out prior to or during drilling. Only exploratory wells PT3 and PPT1 (Fig. 1), with a maximum in-hole temperature of 135 °C at 290 m depth for PT3 (Squarci et al., 1994) and 260 °C at 1000 m depth for PPT1 (Chierici et al., 1995b) produced thermal fluids.

Compositions of water from wells and gas geothermometry data from natural manifestations indicate that the geothermal system comprises a deep, hot confined

Study areas

We undertook a detailed investigation of three areas of the island (Fig. 3) that have long been identified as sites of anomalous degassing. The first area lies on the west shore of Lake SV, very near the northernmost rim of the Monastero Caldera (Fig. 3a). This shore of the lake comprises a mofette with a T = 35 °C, weak but persistent gas emissions, and two of the main thermal springs that feed the lake (Aiuppa et al., 2007). The investigated area is located ~ 200 m south of the mofette and covers

Soil CO2 flux and soil temperature

Soil CO2 flux measurements in all areas yielded similar mean values between 90 and 100 g m 2 d 1 (Table 2). The distributions of CO2 flux measurements from Lake SV and Favara Grande indicate the presence of three populations: high-flux A, intermediate-flux B and low-flux C (Table 2). Only two statistically distinct flux populations were identified in the Favara Piccola area, populations A and C (Table 2).

The high-flux populations from Lake SV and Favara Grande (A populations) comprise 18% and 10%

Soil CO2 degassing: quantification and spatial patterns

The carbon dioxide flux at Pantelleria is relatively low (90 to 100 g m 2 d 1) if compared to fluxes generally measured at other quiescent Italian volcanoes such as Solfatara or Vulcano (ranging from a few hundred to 1000 g m 2 d 1, Granieri et al., 2006, Granieri et al., 2010) but quite similar to soil CO2 degassing values at Vesuvio (Frondini et al., 2004, Granieri et al., 2013) and Ischia (Chiodini et al., 2005). Data interpretation suggests that CO2 degassing is fed both by very weak biological

Conclusions

The chemical and isotopic composition of fumaroles, as well as the positive correlation between the soil CO2 flux and soil temperature, indicates a physical process of combined gas and energy release from the hydrothermal system of Pantelleria. Deep fluids, mainly consisting of steam and CO2, rise preferentially along faults/fractures that act as drainage conduits; the isotopic composition of inert gases (essentially He) and CO2 reveals the primary origin of these gas species from deep mantle

Acknowledgment

The authors would like to sincerely thank D. Bergfeld and W. D'Alessandro for their careful reviews, which significantly improved the manuscript.

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