Diffuse CO2 degassing and volcanic activity at Cape Verde islands, West Africa
- Samara M Dionis1Email author,
- Nemesio M Pérez1, 2, 3,
- Pedro A Hernández1, 2, 3,
- Gladys Melián1, 2, 3,
- Fátima Rodríguez1,
- Eleazar Padrón1, 2, 3,
- Hirochika Sumino4,
- Jose Barrrancos1, 2,
- Germán D Padilla1, 2,
- Paulo Fernandes5,
- Zuleyka Bandomo5,
- Sónia Silva6,
- Jose M Pereira6,
- Hélio Semedo7 and
- Jeremias Cabral7
© Dionis et al.; licensee Springer. 2015
Received: 23 March 2015
Accepted: 24 March 2015
Published: 10 April 2015
Diffuse CO2 emission surveys were carried out at São Vicente, Brava, and Fogo islands, Cape Verde, archipelago to investigate the relationship between diffuse CO2 degassing and volcanic activity. Total amounts of diffuse CO2 discharged through the surface environment of the islands of São Vicente, Brava, and Fogo were estimated in 226, 50, and 828 t d−1, respectively. The highest CO2 efflux values of the three volcanic islands systems were observed at the summit crater of Pico do Fogo (up to 15.7 kg m−2 d−1). Statistical graphical analysis of the data suggests two geochemical populations for the diffuse CO2 emission surveys. The geometric mean of the peak population, expressed as a multiple of the geometric mean of the background population, seems to be the best diffuse CO2 emission geochemical parameter to correlate with the volcanic activity (age of the volcanism) for these three island volcanic systems at Cape Verde. This observation is also supported by helium isotopic signature observed in the Cape Verde’s fluids, fumaroles, and ground waters. This study provides useful information about the relationship between diffuse CO2 degassing and volcanic activity at Cape Verde enhancing the use of diffuse CO2 emission as a good geochemical tool, for volcanic monitoring at Cape Verde as well as other similar volcanic systems.
Gases emitted from a volcano are usually released as visible emanations from the main crater areas (as plumes and fumaroles) as well as through the surface of the volcano as diffuse degassing. This last type of degassing can be as important as visible emissions (Baubron et al. 1990; Allard et al. 1991; Chiodini et al. 1996; Hernández et al. 1998, 2001a, 2003, 2012b; Gerlach et al. 2001; Rogie et al. 2001; Salazar et al. 2001; Pérez et al. 2004, 2013; Padrón et al. 2008). Among volcanic gases, CO2 is widely used in volcano gas studies and monitoring because it is one of the first gas species released from ascending magma, and it is considered conservative (Gerlach 1986). The study of diffuse CO2 degassing is important for gas budgets of volcanoes (Burton et al. 2013; Hards 2005; Pérez et al. 2011) and for monitoring volcanic activity, since the large emissions of CO2 into the atmosphere (Gerlach et al. 2001; Hernández et al. 2001a; Granieri et al. 2006; Arpa et al. 2013; Pérez et al. 2013; Melián et al. 2014) as well as changes in the temporal evolution of CO2 efflux (Salazar et al. 2002; Carapezza et al. 2004, 2012; Pérez et al. 2006, 2012; Liuzzo et al. 2013; De Gregorio et al. 2014; Padilla et al. 2014) can be correlated with volcanic activity.
Efforts have been made to obtain a CO2 flux baseline for a given volcanic system (Salazar et al. 2001). However, very few studies have been focused on investigating the relationship between the magnitude of diffuse CO2 emissions and volcanic activity (eruptive recurrence) at different volcanic systems in similar geological setting (i.e., volcanic islands that belong to the same archipelago). Notsu et al. (2006) proposed a five-stage evolutionary model for the release of volcanic gas based on the relationship between level of volcanic activity and degassing pattern (diffuse vs. plume CO2 emission). This model represents an important approach to estimate both visible and non-visible emissions from different volcanoes in a similar state of activity. Williams-Jones et al. (2000) investigated the diffuse degassing (radon and CO2) at three subduction-related volcanoes: Poás and Arenal in Costa Rica and Galeras in Colombia. Although they found that fracturing, hydrothermal development and regional structure are the main variables that may affect flank degassing; this study was carried out at volcanoes with similar level of volcanic activity (all of them exhibiting plume emission and with frequent eruption episodes). The main purpose of this study is to investigate first the relationship between diffuse CO2 degassing and volcanoes of the Cape Verde, a hot spot archipelago (Holm et al. 2006), with different levels of volcanic activity in the same geological setting. To do this, we have performed a soil gas study at three different volcanic systems: São Vicente, Brava, and Fogo islands, which belong to different geological epochs, aimed at (1) quantifying the rate at which CO2 is diffusely degassed from the three volcanic islands, (2) identifying and defining the structures controlling the degassing process, and (3) investigating of the relationship between volcanic activity and diffuse CO2 degassing.
Cape Verde is consists of an intra-plate volcanic chain, Cape Verde Rise, the world’s highest intra-plate elevation in the ocean crust (Lodge and Helffrich 2006) and occupies a position on the passive margin of the African plate, coincident with important residual geoid and gravimetric and heat flow anomalies related to the well-characterized Cape Verde mantle plume (Courtney and White, 1986).The volcanic activity is originated by a hot spot located under a broad lithospheric swell that reflects the low velocity of the plate relative to the mantle plume (Holm et al. 2006). The onset of volcanism on the easternmost islands, which are deeply eroded and older, is in the middle Miocene (approximately 15 million years (Ma); Vinnik et al. 2012). Volcanic activity continued in Pliocene but on a reduced scale and migrated from east to west about approximately 6 Ma ago (Mitchell et al. 1983; Plesner et al. 2002; Duprat et al. 2007), which continues until today (Holm et al. 2008). During Holocene, volcanic activity occurred at Brava, Santo Antão, and Fogo islands (Foeken et al. 2009), but only Fogo Island has registered numerous historical eruptions since the first Portuguese colonization in the fifteenth century (Ribeiro 1960). The most recent eruptions occurred in 1951, 1995, and 2014 to 2015 (Ribeiro 1960; Torres et al. 1997; Heleno da Silva et al. 1999; Silva et al. 2015).
São Vicente island
Brava (67 km2) is the westernmost island and it is located 18 km west of Fogo. The morphology of the island is characterized by a broadly circular shape and steep coastal slopes, cut by deep erosional valleys and extremely irregular plateau with a few major fluvial valleys and several closed (or formerly closed) depressions that correspond to modern phreatomagmatic craters and phonolite volcanic domes. Three volcano-stratigraphic units are identified: (i) Brava seamount stage or lower unit composed of alternating pillow lavas, hyaloclastites, and pillow breccia accumulations (approximately 3 to 2 Ma); (ii) alkaline-carbonatite complex (middle unit) produced by intrusions from shallow magma chambers (approximately 1.9 to 1.3 Ma) which have contributed to the sub-aerial development of the island; and (iii) a major erosive event and uplift pre-dating the younger volcanism (approximately 1.3 to 0.3 Ma) exposed the plutonic rocks (Madeira et al. 2010). During the last 300 ka, a sub-aerial, post-erosional volcanic phase covered the surface of the island with phonolite lava flows, domes, and pyroclasts which present a very fresh morphology, indicating a probable Holocene age (Madeira et al. 2010). Although no historical eruptions have occurred at Brava, the island is seismically active suggesting the occurrence of shallow intrusive activity. Recent seismic data (Faria and Fonseca, 2014) indicate that seismic activity is dominated by volcano-tectonic events with a local magnitude between 0.7 and 3.2, and most have epicenters offshore. Finally, it is important to point out that at Brava there are two areas (Baleia and Vinagre; Figure 2b) characterized by high -soil CO2 contents which have produced several lethal accidents to animals as doves and goats (field observations).
Fogo (476 km2) is the fourth largest island in the archipelago of Cape Verde with a roughly circular shape (approximately 25 km of a diameter). The most prominent geological feature is Chã das Caldeiras, consisting of a 9-km north- to- south wide caldera opened towards the east and bounded on its northern, western, and southern sides by continuous and extremely steep cliff known as the Bordeira (approximately 20 km along). Pico do Fogo (2,829 m.a.l.s.) grew up inside of this caldera and is one of the world’s most active volcanoes with about 30 eruptions since its discovery, the last occurring in 2014 (Figure 2c). The geological evolution of Fogo Island has been well reported by a few authors (Machado, 1965; Day et al. 1999). Day et al. (1999) and Foeken et al. (2009) proposed four phases for the geological evolution of Fogo: (i) uplift seamount series (approximately 4.5 Ma) composed of carbonatites and alkaline basalts; (ii) the Monte Barro group which includes the first sub-aerial lavas; (iii) the Monte Amarelo group which represents a period of intense volcanism which ended with the giant lateral summit collapse (approximately 123 to 62 ka; Foeken et al. 2009); and (iv) the post-collapse Chã das Caldeiras group (62 ka to present), where most of volcanic activity has occurred at the Chã das Caldeiras plain and a homogeneous distribution of numerous cinder cones outside the caldera along the three broadly radial volcanic rift zones (W, NNE, and SSE) of the island. Most of early historical eruptions (during the seventh and eighteen centuries) have occurred at the summit area of Pico do Fogo, whereas recent eruptions in Chã das Caldeiras have occurred mainly along WSW-ENE- oriented fissures (Day et al. 1999).
Pico do Fogo volcano is characterized by the existence of a fumarolic field situated NW inside the summit crater (Figure 2d) and composed by low- and high-temperature gas discharges (90°C to 100°C and above 200°C, respectively) with widespread sulfur precipitates at the surface, typical of hydrothermal alteration (Melián et al. 2008; Silva et al. 2011; Dionis et al. 2014). Faria and Fonseca (2014) reported that the most frequent seismic events at Fogo Islands are cigar-shaped, hybrid, long-period, volcano-tectonic events and spasmodic tremors. Volcano-tectonic events recorded on Fogo usually have a local magnitude ranging between 0.1 and 3.5, with epicenters located mostly inside Chã das Caldeiras and focal depths between 7 and 0 km (relative to the sea level), but most frequently near sea level.
In 2008, 2009, and 2010, three soil diffuse CO2 emission surveys were carried out at São Vicente, Fogo, and Brava islands, respectively. To obtain a homogeneous distribution of sampling sites at each island, 400, 468, and 228 sites were selected for São Vicente, Fogo, and Brava islands, respectively (Figure 2), depending of the local geology, the main volcano-tectonic features and the accessibility. Within 468 sites selected for Fogo Island, 51 measurements were performed inside the summit crater of Pico do Fogo (0.142 km2) (Figure 2d). All field works at Cape Verde archipelago were carried out during dry periods to minimize the influence of precipitation.
Soil CO2 flux and chemical and isotopic composition
Measurements of soil CO2 efflux were performed following the accumulation chamber method (Parkinson 1981) using two portable soil CO2 flux instruments (West Systems, Italy). One of them was equipped with a non-dispersive infrared (NDIR) CO2 analyzer LICOR-820, with a measurement range of 0 to 2,000 ppmV, an accuracy of concentration reading of 2% and a repeatability of ±5%. Due to the high-diffuse CO2 emissions that are present in the summit crater of Pico do Fogo (Fogo Island) and Baleia and Vinagre areas (Brava Island), the soil CO2 flux meter used at these areas was equipped with a Dräger Polytron IR CO2 detector. It was composed by a double beam infrared CO2 sensor compensated for temperature and atmospheric pressure. The accuracy on the flux measurement ranges between ±25% (for 0.5 to 5 mol m−2 d−1) and ±10% (for 350 to 1,500 mol m−2 d−1) and the detection limit is 1.5 g m−2 d−1. Both analyzers were interfaced to a hand-held computer running data acquisition software.
With the aim to analyze the chemical and isotopic composition of soil gases, at each site samples were collected in 10-cc glass vials with a hypodermic syringe by inserting a 50 cm stainless probe at 40 cm depth in the ground following the method described by Hinkle and Kilburn (1979). Residual gas inside the probe was always purged before sampling. Content of CO2 in the soil gas samples was analyzed by micro-chromatography with a VARIAN model 4900 (Agilent Technologies, USA) using a thermal conductivity detector and a 20-m PoraPLOT Q column using argon (Ar) as carrier gas. The temperature of the column and injector were 40°C and 60°C, respectively, and the injection time was 20 ms. The detection limit for CO2 was estimated to be about 10 ppmV and the accuracy of the measurements about 2.5% on the basis of standard sample measurements. Soil temperature was also determined by inserting a thermocouple at each sampling site at a depth of 15 to 40 cm.
The 13C/12C ratios in CO2 (expressed as δ13C-CO2‰ vs. Vienna Pee Dee Belemnite (VPDB)) from soil gas samples were determined with a Thermo Finnigan MAT 253 isotopic ratio mass spectrometer (Thermo Fisher Scientific Inc., USA) with a continuous flow injection from a Finnigan Gas Bench II (Thermo Fisher Scientific Inc., USA) at the Geochemistry Laboratory of ITER-INVOLCAN (Canary Islands, Spain). The analytical error for δ13C values is ±0.1‰. To investigate the carbon isotopic composition of the soil CO2, a certain number of samples was selected for each survey (10%, 15%, and 29 % of total samples sites for São Vicente, Brava, and Fogo, respectively (Figure 2). Samples for 3He/4He analyses were collected from fumarolic discharges at summit crater of Pico do Fogo (Figure 2d) and from a groundwater cold-spring (Agua Vinagre, Figure 2b) in Brava Island and were analyzed at the Geochemical Research Center (University of Tokyo, Japan) following the methodology described by Sumino et al. (2001). 3He/4He data was corrected for air-derived contributions following the method described by Craig et al. 1978. About 50-cm3 lead-glass bottles fitted with high-vacuum stopcocks at both ends were totally filled with gas (summit crater of Pico do Fogo) and water (Agua Vinagre). Dissolved helium and neon in the groundwater samples were extracted following the method described by Padrón et al. (2013). After extraction, helium isotopic ratios and helium and neon concentrations were measured following the method described by Sumino et al. 2001. The correction factor for helium isotope ratios was determined by measurement of an inter-laboratory helium standard named HESJ with recommended 3He/4He ratio of 20.63 ± 0.10 Ra (Matsuda et al. 2002).
Statistical and graphical treatment of the data
In order of distinguish the existence of different geochemical populations among acquired data, a statistical-graphical analysis (Sinclair 1974) was applied to the soil CO2 efflux data from each survey. Probability plots are a useful practical tool in the analysis of soil geochemical data because of the common normal or log-normal character of such data. The normal or log-normal populations are usually interpreted as background and peak populations with different mean values. Soil gas contour maps were constructed using sequential Gaussian simulation (sGs), provided by the sgsim program (Deutsch and Journel, 1998; Cardellini et al. 2003), allowing us to estimate the total diffuse CO2 output for each soil gas survey. The sGs procedure allows us to both interpolate the measured variable at not-sampled sites and assess the uncertainty of the total diffuse emission of carbon dioxide estimated for the entire studied area. The total emission rate of CO2 was expressed as the mean value of 100 equiprobable sGs realizations, and it uncertainty was considered as one standard deviation of the 100 emission rates obtained after the sGs procedure. Spatial distribution maps of diffuse CO2 emission were constructed using the average of the simulated values at each cell.
Statistical parameters in soil CO 2 flux data and estimated diffuse CO 2 emission rates
São Vicente (228 km 2 )
Brava (67 km 2 )
Fogo (476 km 2 )
Number of sampling sites
Range soil CO2 efflux (g m−2 d−1)
≤l.d to 10.1
≤l.d to 1,343
≤l.d to 15,685
Mean background population CO2 efflux (g m−2 d−1)
Mean peak population CO2 efflux (g m−2 d−1)
Mean intermediate population CO2 efflux (g m−2 d−1)
Peak/Background ratio CO2 efflux
Total soil CO2 emission (t d−1)
226 ± 14
50 ± 10
828 ± 5
Total soil CO2 efflux (t km−2 d−1)
Range soil δ13C-CO2 (‰ vs. VPDB)
−18.8 to −3.6
−20.8 to −1.3
−27.1 to −0.2
Range soil CO2 conc. (ppmV)
354.8 to 1,985
935 to 521,300
743 to 405,933
Range temperature soil (°C)
23.8.0 to 39.5
19.1 to 43.4
18.8 to 293.8
At Brava Island, soil CO2 efflux values ranged from detection limit (approximately 0.5 g m−2 d−1) up to 1,343 g m−2d−1, with an average value of 19.2 g m−2d−1. The statistical-graphical analysis of soil CO2 efflux data indicates the presence of two log-normal geochemical populations (Figure 3b): background and peak. Background population represented 98.9% of the total data with a mean of 2.2 g m−2d−1 and a peak population represented 0.6% with a mean of 709.3 g m−2d−1. An intermediate ‘threshold’ population which represents a mixing between background and peak values had a mean of 8.7 g m−2d−1 of CO2 with 0.5% of the total soil CO2 efflux data. Peak population may be considered as representative of CO2 effluxes fed by an endogenous source as has been reported for other volcanic systems (Chiodini et al. 1998, 2001, 2008; Cardellini et al. 2003; Hernández et al. 2012a). A similar behavior is observed for São Vicente Island, with background values representing CO2 efflux produced by biological activity in the soil although significantly higher than background values of São Vicente Island. The observed difference could be explained by development of vegetation since at São Vicente soils are less vegetated than in Brava. Even if the peak population for Brava Island represents a relatively low percentage of total data, its mean value (709.3 g m−2d−1) strongly supports the contribution of a deeper source (a volcanic-hydrothermal system).
At Fogo Island soil CO2 efflux values ranged from approximately 0.5 (detection limit for LICOR) to 15,685 g m−2d−1, with an average value of 161.8 g m−2d−1. Maximum values were measured at the summit crater of Pico do Fogo, area of most intense surface geothermal activity in Cape Verde. As was done in the two previous cases, a statistical-graphical analysis was applied to the soil CO2 efflux data to distinguish the existence of different geochemical populations. Two log-normal geochemical populations were identified: background and peak (Figure 3c). Background population represented 94.6% of the total data with a mean of 1.02 g m−2 d−1 and peak population represented 3.0% with a mean of 1,704 g m−2d−1. An intermediate ‘threshold’ population which represents a mixing between background and peak values had a mean of 85.2 g m−2d−1 of CO2 with 2.4% of the total soil CO2 efflux data. Peak population values were measured at the summit crater of Pico do Fogo, where intense fumarolic degassing occurs.
δ13C (CO2) isotopic composition
3He/4He isotopic composition
The air-corrected 3He/4He ratios measured in the fumarolic discharges of Pico do Fogo summit crater (Fogo Island), varied from 7.73 to 8.53 Ra (where Ra is the atmospheric 3He/4He ratio = 1.393 × 10−6; Sano et al. 2013a) in samples collected in 2009 (7.81 and 7.73 Ra; this study) and 2010 (8.53 Ra; Dionis et al. 2014). In the case of Brava, air-corrected 3He/4He ratios measured in Agua Vinagre groundwater spring in 2007 ranged between 4.86 and 6.51 Ra.
Even when many studies have demonstrated that there is a significant relationship between diffuse CO2 degassing and volcanic activity (see references herein), this relationship has been studied at the local scale for individual volcanoes. At the regional scale for selected areas like oceanic volcanic islands in a similar geological setting, a systematic study taking into account these facts has not yet been performed. To achieve find some parameter that links both concepts, we analyzed the relationship between diffuse CO2 degassing and linked volcanic activity at Cape Verde volcanoes by applying different geochemical approaches; the results show a priori that magnitude of diffuse CO2 degassing is significantly related. However, other factors may have a more important role on defining such a relationship. The first approach we used to investigate the relationship between both parameters was comparing the mean values of the background and peak populations obtained for each of the three studied volcanic systems. Results from the statistical-graphical analysis of soil CO2 efflux data showed two overlapping log-normal geochemical populations (background (B) and peak (P)) for São Vicente, Brava, and Fogo islands (Figure 3). The estimated mean values of the background population were 0.5, 2.2, and 1.0 g m−2 d−1 for São Vicente, Brava, and Fogo, respectively, similar to the background values reported for others oceanic volcanic islands with similar soil, vegetation, and climate conditions, e.g., as the Canaries (Cumbre Vieja volcano: 1.8 g m−2 d−1, Padrón et al. 2015; El Hierro Island: 1.4 g m−2 d−1, Melián et al. 2014; and Timanfaya volcano: 0.4 g m−2 d−1, Hernández et al. 2012a). On the other hand, mean values estimated for the peak populations were 7.6, 709, and 1,704 g m−2 d−1 for São Vicente, Brava, and Fogo, respectively. These peaks are from 1 to 3 orders of magnitude higher than the estimated background values, as has been also observed in other volcanic systems like Timanfaya volcano, Canary Islands (i.e., B = 0.25 g m−2 d−1 and P = 5.9 g m−2 d−1, September 2010; Hernández et al. 2012a), Vesuvio summit cone, Italy (B = 1.03 g m−2 d−1 and P = 29.5 g m−2 d−1; Cardellini et al. 2003); and Sierra Negra caldera in Galapagos (B = 3.7 g m−2 d−1 and P = 22,000 g m−2 d−1; Padrón et al. 2012). The mean peak populations of Brava and Fogo islands indicate an important deep-seated contribution characterized by high CO2 effluxes generated most likely by degassing processes of magmatic derived CO2 as has been reported for other volcanic systems (Chiodini et al. 2001, 2008; Cardellini et al. 2003). However, direct comparison of mean background and peak values does not take into account other important factors such as the characteristics of soil and climate (different levels of development of soil horizons, organic matter content, vegetation, etc.), as well as the existence of diffuse degassing structures (Chiodini et al. 2001) which can greatly contribute to peak population values. These parameters can contribute directly to the background and peak populations at any volcanic area independently of the level of volcanic activity and geological age.
Another potential useful parameter that might link diffuse CO2 degassing and volcanic activity is to compare the total diffuse CO2 emissions between the different volcanoes under study. These values can be obtained from the spatial distribution maps of soil CO2 efflux constructed by sGs, as described by Cardellini et al. (2003). The areas that contribute most to the total diffuse CO2 degassing are those characterized by anomalous soil CO2 efflux values (Figures 4, 5, and 6 for São Vicente, Brava, and Fogo islands, respectively). Inspection of the three maps shows that main the DDSs are located in Brava Island (Baleia DDS, possible be linked to Minhoto Fault and Vinagre DDS a hidden fault or fracture) and summit crater of Pico do Fogo, which is the main diffuse CO2 degassing area not only in Fogo Island but also in Cape Verde archipelago. To eliminate the effect caused by the size of the surveyed area, CO2 emission needs to be normalized by the area (km2) of study. The normalized values of diffuse CO2 released from São Vicente, Brava, and Fogo Islands (approximately 0.9, approximately 0.7, and approximately 1.7 t km−2 d−1, respectively) were found to be similar to other volcanic systems as Timanfaya volcano (Lanzarote), with a normalized value of 0.16 to 2.05 t km−2 d−1 (252 km2; Hernández et al. 2012a) and El Hierro Island approximately 0.7 t km−2 d−1 (278 km2; Melián et al. 2014), both located in the neighboring archipelago of Canary Islands. For proper intercomparison using normalized values, selected areas of study should have similar geological and volcanic characteristics (existence of DDS). In our study, the DDS showing the most intense diffuse CO2 degassing was the summit crater of Pico do Fogo volcano. To compare its normalized value (1,035 t km−2 d−1 in 0.142 km2) with other volcanic craters in Cape Verde with surface volcano-hydrothermal gas discharges at Brava and São Vicente Islands is not possible, since they do not exist.
A different approach could be to compare the contribution of different geochemical reservoirs to the CO2 emissions. To do so, it is necessary to investigate the different origins of CO2 in the diffuse emissions by analyzing the isotopic composition of the carbon in the soil CO2. The addition of deep-seated CO2 (which includes mantle-derived CO2 and metamorphism of marine carbonate rocks) causes a graphical trend of samples along the arrows shown in Figure 7, towards δ13C (CO2) > −8‰ vs. VPDB (Javoy et al. 1978; Barnes et al. 1988) and [CO2] ~ 100%. São Vicente, Brava, and Fogo islands showed different degrees of atmospheric and biogenic CO2 contributions. Only Brava and Fogo islands showed an endogenous CO2 contribution with δ13C (CO2) values higher than those representative of a volcanic-hydrothermal source (Camarda et al. 2007). According to D’Alessandro and Parello (1997), δ13C (CO2) values between −1‰ and 1‰ may be subjected to isotopic fractionation due to diffusion processes and/or interaction with thermal aquifers. Since the carbon isotopic composition of the CO2 discharged from fumaroles at the summit crater of Pico do Fogo is well constrained, this isotopic composition can be assumed as representative of deep-seated CO2 (range from −4.5‰ to −4.1‰ with a mean of −4.2‰; ±0.1‰; Dionis et al. 2014). However, just comparing the behavior depicted by δ13C (CO2) vs. 1/[CO2] diagram at the three areas of study, it does not seem to be a conclusive model to establish a relationship with the level of volcanic activity, because despite both Fogo and Brava volcanic systems have different geological ages and eruptive records (Fogo is the only island with historical eruptions) they show important addition of deep-seated CO2 (Figure 7).
This study present the first diffuse CO2 degassing results in Cape Verde islands and its relationship with volcanic activity. The purely endogenous emission values (peak populations) measured in the three studied volcanic systems, São Vicente, Brava, and Fogo Islands, expressed as times the background values, seem to be directly related to the magmatic helium emission as 3He/4He isotopic ratios and therefore, to the present magmatic degassing and volcanic activity in Cape Verde. This statement is also supported by the relative geological ages of the eruptive activities occurred at the three islands. The summit crater of Pico do Fogo showed the maximum diffuse CO2 efflux and the highest 3He/4He ratios of Cape Verde and is the only area showing fumarolic discharges in the archipelago. In addition, Fogo is the only Cape Verdean island with eruptive activity in historical times (<500 years). Although Brava island has shown an important recent seismic activity (Heleno da Silva et al. 2006) and obvious endogenous CO2 emissions in specific areas (Baleia and Vinagre), it has not experienced historical eruptive activity and shows lower magmatic helium degassing values than Fogo. Finally, São Vicente, without Holocene eruptions, has shown the lower endogenous emission values.
Therefore, the above observations strongly indicate that diffuse CO2 degassing is clearly linked to the level of volcanic activity and can be used as a useful geochemical tool for volcano monitoring programs in Cape Verde. To perform discrete CO2 efflux surveys with a periodicity equivalent to the volcanic activity degree will provide important information about future episodes of volcanic unrest: higher at Fogo (mainly at the summit crater of Pico do Fogo), lesser at Brava and the lower at São Vicente islands.
This work has been partially funded by the projects (i) MAKAVOL (MAC/3/C161) of the European Union Transnational Cooperation Programme MAC 2007-2013; (ii) CABOVERDE (08-CAP2-1202) of the Spanish Agency for International Cooperation and Development, AECID; and (iii) UNICV of the Servicio de Acción Exterior del Cabildo Insular de Tenerife (Canary Island, Spain). We are also grateful to José Antonio Fernandes ‘Madjer’, António Gonçalves, Inocencio Barros, Dácil Nolasco, Isabel Gómez, Silvia Monteiro, Felipe Brito, Marcos Freitas Santos (SNPC-Mindelo), Joao Feliberto Semedo, and Judite Nascimento for their logistical support during the field work related to this research, as well as to Giovanni Chiodini and the anonymous reviewer whose useful comments and constructive suggestions greatly improved the manuscript.
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