The Emissions of the Tagoro Submarine Volcano (Canary Islands, Atlantic Ocean): Effects on the Physical and Chemical Properties of the Seawater The Emissions of the Tagoro Submarine Volcano (Canary Islands, Atlantic Ocean): Effects on the Physical and Chemical Properties of the Seawater

Additional available at the end of the chapter Abstract This chapter presents the changes and evolution of the physical and chemical properties of the seawater south of the El Hierro Island (Canary archipelago, Atlantic Ocean) as a consequence of the emissions of the Tagoro submarine volcano, over a 6 year study from 2011 to 2016. Since the eruption, a series of oceanographic studies have been carried out in the area focusing on the evolution of the redox potential and the pH, two master variables that control the chemical equilibrium in sea water. The changes experienced by the carbon dioxide system, the variations in the concentration of Fe(II) and their correla- tion with the decrease in the pH during the evolution of the volcano, from the beginning of the eruptive stage to the post-eruptive phase, are discussed. The increased TDFe(II) concentrations and the low associated pH T values have controlled the occurrence of an important fertilization event in the sea water around the volcano at the Island of El Hierro, providing optimal conditions for the regeneration of the area. The sites like the Tagoro submarine volcano, in its degasification stage, provide an excellent opportunity to study the carbonate system in a high CO 2 world, the volcanic contribution to the global volcanic carbon flux and the potential environmental impact of these emissions on the surrounding ocean and the ecosystem.


Introduction
The eruption of a volcano in the ocean generates important changes in the physical and chemical properties of the sea water that also affects the marine organisms in the area. When this eruption occurs in distant and deep places, they may go unnoticed if they are not specifically monitored.
( Figure 1). The volcano was located 1.8 km south of the island of El Hierro at 27 37 0 07 00 N-017 59 0 28 00 W. Over 5 months, the volcanic edifice grew up to 88 m below sea level. During that period, six geophysical surveys were carried on board the R/V Ramón Margalef in order to monitor the changes in the bathymetry of the area [13,14]. Moreover, several hydrographic cruises were carried out to study the changes in the physical, chemical and biological properties in the water column from the beginning of the eruptive stage to the post-eruptive phase [2][3][4].
The evolution of the volcanic edifice between 2011 and 2012 [13,14] can be briefly described as follows: • The first survey in the area was carried out on October 23. The base of the active volcano was found at a depth of 300 m. The volcano was located on a rift with lava flowing southwestwards. It was 650 m wide and its peak was situated at a depth of 220 m below the sea level.
• In January 2012, the cone had risen to a depth of 130 m.
• In February 2012, it reached its maximum elevation of 88 m below sea level.
While both the structure and the height of the volcano were changing, the emission of fluids gave rise to a plume in the water column that, due to the currents, also extended to its surroundings. The emission plume was also modified during that time by the dynamics of both the volcano growth and the oceanographic conditions [3]. Additionally, in the surface, the plume was  affected by changes in the meteorological and hydrodynamic conditions. In the first months, the plume ( Figure 2) was observed from the coast and also through satellite images [15,16].

Signal of the eruption in the sea surface
In the surface waters, evident signals were observed as a consequence of the volcanic emissions in the first 5 months of the eruption. The most important were observed around the volcanic edifice ( Figure 3), but the effect of the volcano emissions reached the whole southern part of the Island of El Hierro. Some influences were also observed in the northern part, as a consequence of the currents in the area [16]. The most important observations in the surface waters were: • The discolouration of the water ranging from light-green to milky-blue to a dark brown colour.
• Abundant rock fragments floating on the ocean.
The strongest eruptive episode took place between November 5 and 8 with large bubbles rising to 10-15 m above sea level [3].
The eruption produced lava balloons that were collected while floating and degassing at the sea surface above the vent [17]. Studies from petrological and geophysical data indicated that

Eruptive phase and changes in the chemistry of sea water
During the eruptive phase that lasted 5 months (October-March), important changes in the carbon dioxide system and in the emissions of reduced species were observed south of the El Hierro Island. Over this period, intensive water column sampling was carried out. The samples were taken with a conductivity-temperature-depth sensor (CTD)-rosette with 24 Niskin bottles. The pH in total scale (pH T ), the total dissolved inorganic carbon (C T ), the total alkalinity (A T ), Fe(II) and the total sulfur reduced species were measured, together with the temperature, the salinity and the dissolved oxygen. The nutrients and the fugacity of CO 2 (fCO 2 ) were also analyzed in the most of cruises.

Changes in the pH and carbonate system
Due to the volcanic emissions of CO 2 , SO 2 and H 2 S/HS À , important changes were observed in the pH, with values lower than 6. The largest anomalies, ΔpH = 2.9, were observed in November 5, 2011, during the explosive event, where a 10 m high bubble emerged from the sea surface. The C T and the A T also changed, in the surface waters around the volcano, where anomalous values  of C T and A T of 7682 and 1338 μmol kg À1 were measured, respectively ( Figure 4). Normal surface values for the area are 2100 μmol kg À1 for C T and 2430.1 μmol kg À1 for A T .
The CO 2 exchanged between the ocean and the atmosphere after being emitted by the volcano was calculated using pH T and C T data. fCO 2(pH, CT) increased to values as high as 155,000 μatm, reaching 230,316 μatm just after the bubble emerged. The surface seawater in the south-west of the island had fCO 2(pH,CT) values in the range of 11,000-19,000 μatm. fCO 2 in the areas unaffected by the volcanic emissions was of 414 AE 2 μatm.
The flux of CO 2 , FCO 2 , exchanged during the week of the explosive event (November 4-9) was 5 Â 10 10 gd À1 . fCO 2 was computed considering an affected area of 385 km 2 and using a weekly mean wind speed of 7.5 ms À1 [3]. The studies carried out by Longpré et al. [17] with samples of Volcanoes -Geological and Geophysical Setting, Theoretical Aspects and Numerical Modeling, Applications to Industry and Their Impact on the Human Health basanite lava balloon indicated that the initially high volatile contents of this material and the intense shallow level of degassing led to submarine Strombolian-type explosivity at El Hierro [17]. The authors stated that their conclusions were consistent with the construction of a 200-300 m tall submarine cone [14], and the strong bubbling at the ocean surface. They estimated that the Tagoro eruption released at least 1.3-2.1 Tg CO 2 and 1.8-2.9 Tg S to the environment. However, they emphasize that these amounts must be considered as minima, particularly for CO 2 . Taking into account the fluxes of the CO 2 calculated in the water column during the week from November 4th to 9th, 2011 by Santana-Casiano et al. [4], coincident with the most intense bubbling and degassing at the surface water, and extrapolating over the entire course of the eruption (147 days), they estimated an upper limit of 7.4 Tg CO 2 for the CO 2 emissions [17].

Emission of reduced species and changes in E x and O 2
During the period of volcanic activity, the emission of reduced species such as iron and sulfur, contributed to decrease both the redox potential (E R ) and the concentration of dissolved oxygen (O 2 ) in the system ( Figure 5). The oxygen decreased due to the oxidation of the reduced species emitted by the volcano. The presence of these reduced species produced low redox potential which reached values of À0.03 V. Background redox potential was 0.2 V. in the stations close to the volcano.
The flux and oxidation of Fe(II) and reduced sulfur species were so high that oxygen values were often below detection limit. Patches of anoxia at 100 m depth in the affected area were observed. The low oxygen concentrations in these waters together with the low pH T values contributed to the high mortality of marine biota that was observed in the area after the first two months of the eruption [2].

Nutrient enrichment in the area
An important input of nutrients was produced in the area. Concentration of nitrate and silicate higher than 3 and 17 μmol kg À1 , respectively, were found in the surface waters, while at 100 m depth, values increased to 8 μmol kg À1 for the nitrate and 23 μmol kg À1 for silicate. In the oligothrophic areas not affected by the volcano, the concentration of these nutrients in the first 100 m is close to the detection limit. Phosphate values increased by one order of magnitude between 100 and 125 m depths, reaching maximum values of 0.6 μmol kg À1 . In surface water, the total value of Fe(II) was 12.7 μmol kg À1 , where 3.3 μmol kg À1 was dissolved and thus bioavailable for the marine organisms. These compounds were directly introduced into the photic zone where they affected the primary production. The presence of nutrients together with the input of Fe(II) produced an enrichment in this oligotrophic area, resulting in a fertilization event caused by the shallow volcano emissions [3].

pH and E x , two variables that control the chemical equilibria
In the context of climatic change, it is known that due to anthropogenic CO 2 emissions the pH in the ocean surface waters is decreasing by about 0.02 pH units/decade and it will decrease by about 0.3-0.4 units at the end of this century [23,24]. In the El Hierro waters, the pH decreased by 3 units in less than 1 week. As a result of the ongoing magmatic activity, the submarine eruption produced an unprecedented episode of severe acidification that affected the chemical equilibria in the sea water ( Figure 6). As a consequence of the emission of reduced species, the E x drastically changed from values higher than 0.2 V, characteristic of oxygenated waters, to values lower than À0.1 V, due to the corrosive environment. In Figure 6, the chemical speciation for normal ocean conditions is represented in blue (by dots) and the changes due to the volcanic emissions in orange (by stripes). Changes in the CO 2 ( Figure 6a) and Fe(II) speciation were produced (Figure 6b).
In the oceanic regions where submarine volcanoes experience magmatic or hydrothermal activity, the physicochemical properties of sea water change, modifying the system. The changes Volcanoes -Geological and Geophysical Setting, Theoretical Aspects and Numerical Modeling, Applications to Industry and Their Impact on the Human Health observed in pH T were a consequence of the emissions of CO 2 , SO 2 and H 2 S/HS À the submarine volcano (Eqs. (1)- (5)). As soon as the volcanic fluid mixes with seawater, the dissolved reduced species, Fe(II), Mn(II) and H 2 S/HS À , produce changes in the E x . These species in contact with the oxygenated seawater are oxidized, consuming oxygen and acidifying the system (Eqs. (6)- (9)). The emission of Fe(II) also contributed to acidifying the system due to the reaction of Fe(II) with H 2 S (Eq. (10)). Part of the pH decrease is compensated by reactions that also contribute to increase the alkalinity (Eqs. (11)- (13)).
After its emission, Fe(II) is oxidized (Eq. (8)) and precipitates into various mineral forms, mainly oxy-hydroxide [25,26] forming massive deposits of iron. A fraction of the total emitted iron (4%) is stabilized against loss from solution due to complexation by dissolved organic ligands (Eq. (14)), or by incorporation into inorganic or organic colloids which reside within the dissolved size fraction [27,28]. Moreover, reduced species of Fe and S form FeS colloidal complexes (Eq. (12)), which remain suspended in the water and are modified as the pH changes [29]. These nanoparticles can remain suspended in the deep sea for years with slower settling rates [30], solubilizing and releasing the Fe(II). Mn The conclusions obtained by Longpré et al. [17] through study of volatiles and trace element concentrations from the Tagoro lava balloons agreed with the changes in the carbonate variables and redox species observed in the water column. They also indicated a strong degassing and associated redox change of the initially volatile-rich and oxidized magma as it ascended from the mantle to produce the submarine eruption.

Effect of the chemical emissions on the biota
The volcanic emissions had important consequences on the biota. Organisms were observed to modify their distribution and survival and there was an alteration of the diurnal vertical migration of the pelagic communities [2]. Small picophytoplankton, such as Prochlorococcus and Synechococcus, showed a significant decline in abundance in the volcanic area at depths greater than 75 m if compared to that one in unaffected stations. However, heterotrophic prokaryotes increased with depth [2]. With respect to the bacterioplankton, there was an important change and groups often associated with deep-sea hydrothermal vents or sulfurrich springs were observed [31]. The increase in temperature and acidification, the decrease in oxygen and the precipitation of chemical compounds resulted in a reduction of the epipelagic stocks and a disruption of the nocturnal ascent of mesopelagic organisms [32]. Once the eruption ceased, the distribution and abundances of the pelagic biota returned to baseline levels.
The increased amount of nutrients in the euphotic zone, together with the high dissolved iron concentrations, may contribute to the regeneration of the productivity in the area. The findings highlight that the same volcano that was responsible for the creation of a highly corrosive environment, affecting marine biota, has also provided the nutrients required for the rapid recuperation of the marine ecosystem.

The post eruptive phase and effects on the water chemistry
In March 2012, the magmatic activity stopped and the system evolved into a degasification phase. In April, the composition of the gases close to the summit was primarily CO 2 with dissolved reduced S species below 1 μmol kg À1 in the water column. The affected area was Volcanoes -Geological and Geophysical Setting, Theoretical Aspects and Numerical Modeling, Applications to Industry and Their Impact on the Human Health reduced to 0.5 km around the main crater of the volcano. In this area, a decrease of pH T values to 6.1 (1.9 units below the normal values) was observed and the C T was 4191 μmol kg À1 [3]. The evolution of the chemical properties during the post-eruptive phase was studied from 2013 to 2016. The physical and chemical anomalies were still being observed close to the volcanic area. During this period, the study was concentrated in the volcanic edifice and a transect following the main cone was repeated twice a year. In order to detect anomalies, different strategies over the volcanic edifice, high-resolution study, yo-yo ( Figure 7a) and tow-yo (Figure 7b) studies, were also followed.

High-resolution study over the volcanic edifice
During the degasification phase, the oceanographic study of the area was mainly concentrated in the volcanic edifice. High-resolution sampling along the volcano was repeated twice every year from 2013 to 2016. In these studies, 10 stations following the alignment of the main and secondary cones of the submarine volcano were repeated (Figure 1c). In these stations, water samples were taken at different depths and the signal of CTD, pH and oxidation-reduction potential (ORP) sensors were also recorded.
The vertical distributions of pH and C T , A T and Fe(II) along the high-resolution section across the volcano were measured. Negative anomalies in pH values and positive anomalies in C T , A T and TDFe(II) were observed close to the main crater [4,33]. In Figure 8, the anomalies in pH and C T are shown between stations 53 and 56 for the cruises on March and November 2013 ( Figure 8). The results from these cruises also confirmed important positive anomalies in TDFe (II), coincident with negative anomalies in pH located in the proximity of the main cone [33].

Yo-yo studies
The yo-yo studies consist of raising and lowering the rosette with the CTD sensors at one location and taking samples at different depths. In the Tagoro volcano, the stations were selected from the results obtained in the high-resolution study ( Figure 8).  In order to obtain continuous records of pH and ORP during the yo-yo, together with temperature and salinity measurements, a pH sensor was added to the normal rosette-CTD package in November 2013, and an ORP sensor in March 2014. In this way, CTD-pH-ORP yo-yo studies were carried out at the selected stations, from the surface to 2-3 m above the seabed. Water column sampling for discrete pH, A T , C T and TDFe(II) was carried out in the cast where maximum anomalies were detected with the sensors. The time between each cast was 10 min and the total duration of the yo-yo study was between 6 and 10 h. Anomalous values of pH, C T , A T and TDFe(II) were observed close to the bottom around the main crater area during the yo-yo study. The pH values were lower than those expected at that depth, and C T and A T values higher than usual. There was a high correlation between the anomalies observed for the different variables measured in all the cruises from 2013 to 2016 (Figure 9). The yo-yos carried out at station 56 (with 38 casts) and station 61 (with 61 casts) in March 2014 also showed an important fluctuation in pH and δ(ORP)/δt signals related mostly with direction and velocity changes in the local currents, associated with an M2 tidal mode with a period of 12 h. The anomalous values found again in the variables sampled (pH, C T and A T ) were internally consistent. Therefore, those anomalies were related to CO 2 additions from the hydrothermal vents without other acid gasses acting in a significant way [4]. The fluid emissions producing these changes were~57% CO 2 and 43% carbonate alkalinity (HCO 3 À , CO 3 2À ). Positive anomalies for TDFe(II) were also observed and were well correlated with the decrease in pH [33]. A decrease in pH contributed to decrease the oxidation rate of Fe(II) [34] and favoured the persistence of this metal in the medium.

Tow-yo studies
In the tow-yo studies, the SeaBird 911+ CTD-pH-ORP sensors were towed by the ship, moving at a velocity of 0.4 knots, following a saw tooth pattern by a continuous lowering and raising of the instruments between 2-3 and 40 m above the seafloor, from the base to the top of the submarine volcano. At least seven tow-yos were carried out in the area (Figure 1c). Four parallel tow-yos followed the topography of the principal and secondary cones and another two tow-yos were perpendicular to these.
The characteristic reduced and acid emissions of the fluids from the submarine volcano Tagoro allowed us to detect anomalies related with changes in the chemical potential and the proton concentration using ORP and pH sensors, respectively [4]. The combination of ORP and pH sensors with the CTD was used since 2014. The ORP sensor responded very quickly to the presence of reduced chemicals in the water column and the magnitude of this change was examined by the time derivative of ORP, δ(ORP)/δt. For pH changes, ΔpH was defined as the mean pH for each depth at a reference station, in an area not affected by the vent emission, subtracted from each point measured near the volcanic edifice. When δ(ORP)/δt and ΔpH versus latitude were plotted, the anomalies in pH and ORP were observed to follow the same pattern in latitude and longitude ( Figure 10). This indicated that the emissions of both acid and reduced components must be produced in the same area.
ΔpH values of À0.25 at 100 m depths, as obtained in 2014, cannot be explained by the arrival of water from the surrounding depths [4]. This ΔpH corresponds with a pH value of 7.89, and is only found at a depth of 900 m in the unaffected surrounding water. The same behaviour was observed in November 2015 and 2016 cruises for pH, with ΔpH lower than À0.25 (Figure 10), clearly indicating that the degasification process was still taking place. Anomalous ORP values were also observed ( Figure 10) at the same latitude as the pH anomalies.
Contour maps of the affected surface over the volcano can be obtained from the anomalies of ORP and pH observed in the tow-yo studies. The area of emission during VULCANO0314 was 13,000 AE 800 m 2 and the volume affected 206,000 AE 50,000 m 3 . Figure 11 shows the locations of the anomalous ORP values and the position of the emitting vents for the 2014 and 2016 cruises. CO 2 fluxes from the volcanic edifice can also be estimated from the calculated anomaly area for every contour, the current speed and the average anomalies of C T . In the Tagoro volcano, the  Volcanoes -Geological and Geophysical Setting, Theoretical Aspects and Numerical Modeling, Applications to Industry and Their Impact on the Human Health input of CO 2 along multiple sections combined with measurements of oceanic currents produced an estimated volcanic CO 2 flux of 6.0 10 5 AE 1.1 10 5 kg d À1 principally transported to the south-west. This emission of CO 2 reduced pH by~0.25-0.30, altering the balance of protons in the surrounding ocean and increasing the seawater acidity by~20 %.

Diving by the volcano
In February 2016, the Tagoro volcano was visited during the POS494-2 with the submersible JAGO (GEOMAR) for direct observation of the eruption site. The JAGO can take two people, a pilot and a scientific observer, to water depths of maximum 400 m. The objectives of POS494-2 were to take a high-resolution video documentation, mapping the eruption site, to sample the fluids and other volcanic and hydrothermal products that formed at the seafloor and to collect chemical and physical sensor data around the active sites (CTD, ORP and pH) 4 years after the eruption [35]. Two dives were done around the volcano, starting not far from the area where strong chemical anomalies had been measured 4 months before. The first visual inspection carried out in the JAGO showed that the anomalous area was an oval-shaped depression of at least several meters in depth and about 10-15 m in diameter. It was situated at the southeastern flank of the volcano at a depth of 126 m, about 38 m lower than the summit of the volcano. The volcanic rocks at the edges and inside the depression were coated in orangecoloured Fe-oxides. Several pieces of volcanic rock and crusts were collected. Warm water was also seeping diffusely through sediment pores at the bottom of the crater, and occasionally more focused through small vent holes and chimneys that were up to 5 cm high. These chimneys and the orange-coloured Fe-oxides ( Figure 12) were also observed in the HD images from the ROV Liropus 2000 during VULCANA-0417-ROV cruise. Temperature measurements 20 cm inside the sediment close to a vent site revealed 38 C while the temperature of the ambient seawater was 20 C [35]. Fluid samples were taken with a Teflon bottle directly from the emission vents and compared with the samples obtained at 1 m depth with a Niskin bottle for pH and Fe(II). pH values as low as 7.05 were found, and Fe(II) concentrations higher than  With regard to the ecology in the area, it was observed that the volcanic black rocks were already partly colonized by bivalves (small oysters), brachiopods, sponges and whip corals and a thin coat of whitish bacteria covers most of the surfaces around the vents [35]. A detailed microscopic and molecular study carried out near the summit of the volcanic cone, at about 130 m depth, revealed that the bacteria covers were composed of filaments made of bacterial trichomes enveloped within a sheath and colonized by epibiotic bacteria [36]. This system showed an unprecedented array of metabolic pathways, spanning from the exploitation of organic and inorganic carbon released by volcanic degassing to the uptake of sulfur and nitrogen compounds, providing key competitive advantages for the colonization of the new habitat created by the submarine eruption [36].

Conclusions
Sites like the submarine volcano Tagoro, in its degasification stage, provide an excellent opportunity to study the carbonate system, the volcanic contribution to the global volcanic carbon flux, the emissions of trace metals such as TDFe(II) and the potential environmental impact of these emissions on the marine ecosystem.
The inputs of CO 2 along multiple tow-yo sections, combined with measurements of oceanic currents, produced an estimated volcanic CO 2 flux of 6.0 10 5 AE 1.1 10 5 kg d À1 , mainly transported to the south-west. These emissions also alter the balance of protons in the surrounding ocean, increasing the seawater acidity by~20 %. The flux of CO 2 from the Tagoro volcano is comparable to that at other submarine volcanoes like NW Rota-1 (~1 AE 0.25 10 6 kg d À1 , an erupting submarine volcano in the Mariana arc) but relatively small compared to both the global volcanic flux of CO 2 (~0.1%) and the anthropogenic flux (~0.002%). The global volcanic flux of CO 2 is estimated to be~7.2 10 8 kg d À1 considering mid-ocean ridges, arcs and plumes.
The emission of CO 2 from submarine volcanoes generates local ocean acidification that affects the chemical equilibrium and speciation and could impact biological communities, with especially important consequences for organisms that use calcium carbonate in their structures. These emissions also alter the balance of protons in the surrounding ocean, increasing the sea water acidity by~20 %.
From the yo-yo studies, important anomalies in both pH T and TDFe(II) were observed. The increased TDFe(II) concentrations and the low associated pH T values may be causing an important fertilization event in the seawater around the volcano near the Island of El Hierro, providing optimal conditions for the regeneration of the area.