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The natural ocean acidification and fertilization event caused by the submarine eruption of El Hierro

The shallow submarine eruption which took place in October 10th 2011, 1.8 km south of the island of El Hierro (Canary Islands) allowed the study of the abrupt changes in the physical-chemical properties of seawater caused by volcanic discharges. In order to monitor the evolution of these changes, seven oceanographic surveys were carried out over six months (November 2011-April 2012) from the beginning of the eruptive stage to the post-eruptive phase. Here, we present dramatic changes in the water column chemistry including large decreases in pH, striking effects on the carbonate system, decreases in the oxygen concentrations and enrichment of Fe(II) and nutrients. Our findings highlight that the same volcano which 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.


Results
The submarine eruption of the island of El Hierro was the first one to be monitored from the initial unrest and also the first submarine eruption reported in about 600 years of historical records in the Canary Islands. The eruption started in October 11 th 2011, evidence of the eruption was observed in the sea surface. A discoloration of the surface water in the area was observed, ranging from light-green, to milky-blue to a dark brown colour. These changes were related to the discharge of high temperature hydrothermal fluids as well as magmatic gases and volcanic particles 1,20 . Bubbling and degassing took place, and abundant rock fragments were found floating on the ocean surface during the first months of the eruption (Fig. 1b-f). The strongest episodes of bubbling took place between November 5 th and 8 th with large bubbles which reached 10-15 m in height (Fig. 1e).
On October 23 th , the R/V Ramón Margalef (IEO) undertook the first survey in the volcanically affected area and the base of the active volcano was found at a depth of 350 m at 27u379070N -017u599280W. The volcano was located on a rift with the lava flowing south-westwards. The volcano was 650 m wide and its peak was situated at a depth of 220 m below sea level. In January 2012, the cone had risen to a depth of 130 m and in February it reached its maximum elevation of 88 m below sea level 21,22 (Fig. 1b). While both the structure and the height of the volcanic edifice were changing, the emission plume was also being modified. Additionally, in the surface, the plume was affected by the changing meteorological conditions in the area.
The acidification of the area and the changes in the carbonate system. The lack of steady state conditions during the eruption produced highly variable chemical properties in the water column. The strongest eruptive episode occurred during Leg 3 on November 5 th 2011 (Fig. 1e) when observations of highly elevated total dissolved inorganic carbon coupled with large decreases in alkalinity and pH T were made (Table 1). At that time, when the cone was more than 200 m below sea level, the lowest values of pH T were concentrated in a layer of 75-100 m depth at most of the stations, but due to the intensity of the ejected gases, low values of pH T also reached the surface waters. Figure 2a depicts the distribution of pH T values at in situ conditions at four selected depths during the first week of November 2011, where station 1 represents a typical vertical distribution for pH T in the Canary basin 23 . The pH values throughout the text are expressed as pH T in situ conditions. During Leg 3, surface seawater chemistry around the entire southwest end of the island was highly altered, with pH T values as low as 5.1 in close proximity to the volcano increasing to 6.5 in the surrounding area. In the layer at 75-100 m depth, the effects of the emissions were more localised around the volcano with pH T values of between 5.8 and 6.4 which increased to values between  The C T in the surface waters around the volcano was as high as 7,682 mmol kg 21 (Fig. 2b). To the west, and following the direction of the volcanic plume, the C T values decreased gradually to 2,780 mmol kg 21 (Sts. 6, 11, 13 and 20). Normal surface C T values of 2,100 mmol kg 21 were detected in stations 1 and 18. At a depth of 75 m, at station 21, the closest point to the South of the volcano, the C T was 4,526 mmol kg 21 . An area of 100 km 2 to the southwest of the volcano presented average values of 2,700 mmol kg 21 . As was similarly observed for the pH T , at 400 m depth, the C T was also affected, with values ranging between 2,350 and 2,400 mmol kg 21 .
During Leg 5 from November 16 th to 20 th , the volcanically affected area was visited twice. During the first visit, surface pH T values for stations 4 and 5 were 5.70 and 6.28, respectively, while surface waters at station 3 remained unaffected (pH T 5 8.05) due to the prevailing northwest currents. However, station 3 exhibited extraordinarily large changes in carbonate chemistry at 80 m depth with pH T and alkalinity as low as 5.20 and 1153 mmol kg 21 respectively, with C T as high as 12,015 mmol kg 21 and pCO 2 calculated from pH T and C T to be as high as 316,832 matm. Similar results were observed for stations 4 and 5 at the same depth. During the second visit, the same conditions were observed at station 3 while at station 4 the effect was only observed in the layer between 75-125 m depth (at 125 m, pH T 5 6.04, C T 5 4,057 mmol kg 21 , A T 5 2213 mmol kg 21 , pCO 2(pH,CT) 5 49,888 matm). This was a consequence of a change in the direction of the surface currents.
In December 2011 (Raprocan1211 cruise) the carbonate chemistry at station 3 in the surface waters was the most anomalous: 5.40 for the pH T , C T of 9,515 mmol kg 21 , A T of 2140 mmol kg 21 and pCO 2(pH,CT) of 232,490 matm. In the upper 300 m of the water column around the volcano, pH T was , 7, except in the layer at 125-150 m depth where the pH was 7.90. Similar effects from the emission of magmatic CO 2 were observed in January 2012 during Leg 8.
In April 2012, the eruption had stopped and the system evolved to a hydrothermal system. The pH T in the whole water column outside of the volcanically affected area was the typical for the Canary basin, with anomalous pH T values found only in an area within a radius of 500 m around the cone. Directly above the cone, the pH T measured at the surface waters was 7.3, while at 50 m depth, the value was 7.2. The seawater in proximity to the crater had pH T values of just 6.1, while the C T increased to 4,191 mmol kg 21 . However, unlike the observations of decreases in alkalinity during most of the eruptive phase, the alkalinity increased above the natural values of around 2400 mmol kg 21 to 2686 mmol kg 21 (Table 1 and Table S4, Supplementary Information). Increased A T values were also observed at the end of February close to the volcanic cone, when the activity decreased, and can be related to changes in the compounds and reactions that influence the carbonate system throughout the eruptive process.
Fluxes of CO 2 to the atmosphere during the explosive episode. During Leg 3 ( Fig. 3), the CO 2 emitted by the volcano increased the pCO 2(pH,CT) to values as high as 155,000 matm in station 3, reaching 223,400 matm just after a 10 meter high bubble emerged in November 5 th (Fig. 1e). The surface seawater to the southwest of the island had pCO 2(pH,CT) values in the range of 11,000 to 19,000 matm. Measured pCO 2 in the areas unaffected by the volcanic emissions were of 414 6 2 matm, in accordance to computed pCO 2(pH,CT) . During the week of the explosive event (November 4 th to 9 th ), a flux of CO 2 of 5 3 10 10 g d 21 was computed 24 for the 385 km 2 area covered by this Leg (shaded area in The volcanic emissions also introduced a large amount of Fe(II) into the water column (Fig. 4), which was strongly correlated with the pattern observed for the sulfur reduced species. In November 17 th , the concentration of total Fe(II) reached maximum values of 50 mmol kg 21 at 75 m depth at station 3. On December 8 th a maximum value of 20 mmol kg 21 was measured in the surface waters. Normal dissolved Fe(II) values 26 in the surface ocean are , 0.2 nM which is five orders of magnitude below the values determined for total Fe(II) in the volcanic area.
A size speciation study was carried out on December 8 th at station 23 (Fig. 5a). Total Fe(II) concentration in surface waters was 12.7 mmol kg 21 , with a secondary maximum of 4 mmol kg 21 at 100 m depth. At the bottom, the total Fe(II) concentration increased again reaching a value of 4 mmol kg 21 . In surface waters 64% of the total Fe(II) was particulate (8.1 mmol kg 21 ), while dissolved   The presence of these reduced species produced low redox potentials (Fig. 4) which reached values of 20.03 V at 100 m depth in station 3 and values of 0.15 V in surface waters at station 4, where the maximum values of Fe(II) and reduced sulfur species were found, on November 20 th . Low redox potential with values below 0.05 V were only recorded at the station above the volcano during January and February 2012. Background redox potential is 0.2 V and was observed at station 1.
The flux and oxidation of Fe(II) and reduced sulfur species 27-29 were so high that oxygen values were often below detection limit. Patches of anoxia at 100 m depth in the volcanically affected area were determined at station 3 on November 5 th (data not shown) and at stations 4 and 5 on November 16 th (Fig. 4, Table 1). Minimum values of around 100 mmol kg 21 of oxygen were also observed during other visits to those stations in November 2011. The low oxygen concentrations in these waters together with the low pH T values contributed to the high mortality of marine biota which was observed in the area after the first two months of the eruption.
During the eruption the same pattern observed for C T and reduced species was also found in the vertical profiles for nutrients in all of the cruises (Fig. 5). In November, the maximum values were obtained around 100 m depths while in December, they were found in surface waters. Nitrate concentrations as high as 3 mmol kg 21 were found in the surface waters, while at 100 m depth values increased to 8 mmol kg 21 . Phosphate values increased one order of magnitude in the layer of 100-125 m depth, reaching maximum values of 0.6 mmol kg 21 . Silicate concentration in surface waters showed values as high as 17 mmol kg 21 in December and 15 mmol kg 21 in February. In November, a value of 23 mmol kg 21 was measured at 100 m depth.

Discussion
In the oceanic regions where submarine volcanoes experience magmatic or hydrothermal activity, the water properties change 3,17 . The reactions which mainly explain the variations in pH T values 27,30 are shown in Table 2. The changes observed in pH T are a consequence of the emissions of CO 2 , SO 2 and H 2 S/HS 2 from the submarine volcano (reactions 1-5, Table 2). As soon as the volcanic fluid mixes with seawater, the dissolved reduced species, Fe(II), Mn(II) and H 2 S/HS 2 , are oxidised, consuming oxygen and acidifying the system (reactions 5 and 6-8). The emission of Fe(II) also contributed to acidifying the system due to the reaction of Fe(II) with H 2 S (reaction 9). Part of this decrease is compensated by reactions (10)(11)(12) that also contribute to increase the alkalinity. The CO 2 -weathering reaction (12) has been described in gas-rich hydrothermal systems 31 after H 1 reacts with the host rock.
The pH T of the seawater column in the volcanically affected area during the explosive episode changed drastically, reaching values , 6, with the largest anomaly, DpH 5 2.9, detected close to the volcano (Table 1). This decrease in pH represents 81,370% increase in the proton concentration. Six months later, during the April cruise, after the eruption had stopped, the composition of the gases close to the summit was primarily CO 2 with a complete absence of dissolved reduced species, which caused a decrease of pH T values to 6.1, 1.9 units below the normal values.
An attempt was made to quantify the effect of each contributor to the carbonate system change in the area affected by the volcano (Supplementary Information). The model indicated that the emitted CO 2 ( Table 2, reactions 1 and 2) contributed to up to 95% of the observed decrease in pH during the first 3 months. The remaining unexplained contribution is related to the role of reactions (3) to (12). From the end of February onwards, when the eruptive activity had concluded, anomalous values of alkalinity were determined, with A T,exp . A T,ref and pH exp .pH ATref,CT , that indicated a major role was played by mineral alkalinity species 3 and reactions 9 to 12 ( Table 2 and Supplementary information).
The potential of volcanic emissions to fertilize the ocean has been subject to speculation over the years 1 . The volcanic activity releases nutrients into the seawater 32 , including Fe, Si, P and N. The deposition of volcanic ash onto the surface ocean has been found to increase Fe concentrations 33 to 0.4 to 2.4 nmol L 21 . During the submarine eruption of the island of El Hierro, values of total Fe(II) were as high as 50 mmol kg 21 at 75 m depth. In surface waters (Fig. 4a), from a value of total Fe(II) of 12.7 mmol kg 21 , 3.3 mmol kg 21 were dissolved and thus bio-available for the marine organisms 34 . The low pH T conditions would contribute both to favour the presence of higher amounts of iron in its reduced state, due to a decrease in Fe(II) oxidation rates 29 , and to an increase in the Fe(III) solubility 35 , and consequently, augment the amount of bio-available iron for the biological community. However, the Canary Islands are found in an oligotrophic area 23,36 . Nutrient concentrations to the south of the island of El Hierro are undetectable in the first 80-100 m of depth. During the volcanic eruption an important input of nutrients was observed. The increased amount of nutrients in the euphotic zone, together with the high dissolved iron concentrations, may contribute to the regeneration of productivity in the area. The results emerging from this study show how the same volcano responsible for eradicating most of the marine life in the waters south of the island of El Hierro also provides the ingredients bringing the affected area back to life.

Methods
Cruises. Two weeks after the start of the eruption, an oceanographic research program was initiated by the Instituto Español de Oceanografía (IEO) to localise the exact position of the submarine eruption, to perform a periodic bathymetric surveys of the area, and to study the evolution of the physical-chemical anomalies in the ocean produced by magmatic activity. Over six months, an unprecedented oceanographic investigation was carried out. The name given to this study was Bimbache, the name of the original inhabitants of the island of El Hierro. The study was divided into 12 legs where geophysical and physical-chemical oceanographic researches were alternatively organised (Table S1, Supplementary Information). In addition, two more hydrographical cruises were carried out in the area (Raprocan and Cetobaph).
Hydrography. Vertical profiles of conductivity, temperature, pressure and oxygen were collected using a SeaBird 9/11-plus CTD equipped with dual conductivity and temperature sensors. CTD sensors were calibrated at the SeaBird laboratory before the cruise. Water samples were obtained using a rosette of 24-10-liter Niskin bottles.  Acoustics. Vessel-mounted Simrad EK60 (38 and 70 kHz) split-beam echosounders were used to collect acoustic data.
pH. The pH in the total scale at a constant temperature of 25uC (pH T,25 ) was measured spectrophotometrically 37 with m-cresol purple as an indicator 24 (the dye effect was removed for each pH reading) with an uncertainty of 0.002 units. In the stations which were strongly affected by the volcanic emissions, the pH T,25 was also measured by potentiometry with an Orion TM combinate glass electrode calibrated with TRIS buffer solutions 38 .
A T , C T and pCO 2 . A VINDTA 3C system (Marianda company) was used for both total alkalinity and total dissolved inorganic carbon concentration determination. The titration of certified reference material for oceanic CO 2 , CRMs (#105), was used to test the performance of the equipment. Measurements of CRMs were within 61.0 mmol kg 21 of the certified value for both parameters. The computation of other carbonate variables (pCO 2 ) and values at in situ conditions (pH T,is ) was done by using the CO2sys.xls v12 programme 39 and the set of constants of Mehrbach et al. 40 as in Dickson and Millero 41 . During the Leg 3 the partial pressure of CO 2 , pCO 2 , was computed using the pair pH T , C T . In all other cruises, pCO 2 was also computed and  measured by using a continuous PRO-Oceanus pCO 2 system. Due to sensor calibration limitations, the values of pCO 2 over 1000 matm were determined from C T and pH. The nutrient data, determined as indicated in González-Dávila et al. 23 was also considered in the computation. Oxygen concentrations were measured on board by Winkler titration. ) by the iodometric method 42,43 using an automated potentiometric titration monitored by a platinum electrode. Prior to the start of the titration the same platinum electrode was used to measure the redox potential, E(V).
Fe(II). The concentration of Fe(II) was determined by the modified version of the Ferrozine method 29,44 . The detection limit of the method was 1nM of Fe(II). Seawater samples (15 ml) were fixed with 100 ml of trace metals grade pure HCl (Aldrich) for the determination of total Fe(II). Total Fe(II) was defined as the ferrous iron dissolved in the sample at pH 1. For the analysis, samples were diluted with iron-free seawater at pH 1 when necessary in order to achieve a concentration in the nanomolar range. For the size speciation study, prior to acidification, seawater was filtered by 0.45 mm and 0.1 mm filters and collected in a container with 100 ml of HCl. The particulate Fe was defined as the difference between the measured total dissolved Fe(II) in the pure acidified sample and the Fe(II) obtained from samples filtered by 0.45 mm filters. Dissolved Fe(II) was measured in samples that had been filtered by 0.1 mm.