# Strong geochemical anomalies following active
submarine eruption offshore Mayotte

Manon Mastin, Cecile Cathalot, Olivia Fandino, Thomas Giunta, Jean-Pierre Donval, Vivien Guyader, Yoan Germain, Carla Scalabrin, Sébastien Dehez, Stéphane Jouenne, et al.

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► To cite this version:

Manon Mastin, Cecile Cathalot, Olivia Fandino, Thomas Giunta, Jean-Pierre Donval, et al. Strong geochemical anomalies following active submarine eruption offshore Mayotte. Chemical Geology, 2023, 640, 121739 (25p.). 10.1016/j.chemgeo.2023.121739. hal-04304273

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# Strong geochemical anomalies following active submarine eruption offshore Mayotte

Manon Mastin<sup>a, *</sup>, Cécile Cathalot<sup>a</sup>, Olivia Fandino<sup>a</sup>, Thomas Giunta<sup>a</sup>, Jean-Pierre Donval<sup>a</sup>, Vivien Guyader<sup>a</sup>, Yoan Germain<sup>a</sup>, Carla Scalabrin<sup>a</sup>, Sébastien Dehez<sup>b</sup>, Stéphane Jouenne<sup>b</sup>, Eric C. Gaucher<sup>c</sup>, Olivier Rouxel<sup>a</sup>, Emmanuel Rinnert<sup>a</sup>

## ARTICLE INFO

Editor: Michael E. Boettcher

Keywords:

Mayotte

Submarine eruption

Volcano

Geochemistry

Dissolved gases

## ABSTRACT

Submarine volcanic activity releases large amounts of gases and metals in the water column, affecting biogeochemical cycles and ecosystems at a regional and local scale. In 2018, Fani Maoré submarine volcano erupted 50 km offshore Mayotte Island (Comoros Archipelago, Indian Ocean). Active eruptive plumes were observed in May 2019 at and around the summit with acoustic plumes rising 2 km into the water column coupled to strong geochemical anomalies. Between May 2019 and October 2020, three research cruises monitored the eruptive activity. Here, we report spatial and temporal variability of water column chemistry above the volcano, focusing on dissolved gases, trace metal concentrations, and physico-chemical parameters. In May 2019, concentrations above 800 nM in CH₄ and H₂ were measured throughout the water column, with Total Dissolvable Mn and Total Dissolvable Fe concentrations above 500 nM, and CO₂ values of 265 µM. Strong water column acidification was measured (0.6 pH unit) compared to the regional background. From May 2019 to October 2020, we observed a general decrease in gas concentrations, and an evolution of the TDMn/TDFe ratios similar to previously reported values in other submarine volcanic contexts, and consistent with a decrease of the eruptive activity at the volcano. In October 2020, a rebound of high H₂ concentrations resulted from new lava flows, which were identified by seafloor observation using deep-towed camera, 5 km further than the volcano summit. During 2 years timespan of our observations (2019–2020), He, CO₂ and CH₄ concentrations correlate highlighting a magmatic origin of dissolved gases. δ¹³C-CH₄ values of –34‰ vs. vPDB might suggest magma/sediments interaction during the magma ascent, and potential thermal cracking of organic matter, although abiotic methane generation cannot be ruled out given the volcanic context. Weak correlations between H₂ and excess of ³He suggest complex processes of H₂ from magmatic degassing, lava/seawater interaction, and oxidation processes in the water column. Strong and correlated Fe, Mn and Si water column anomalies are also consistent with fluid-rock reactions induced by acidic fluids rich in magmatic volatiles. Water column acidification appears to be associated with the release of CO₂-rich fluids. A year after the main eruptive event, the system seems to be back to steady-state highlighting the buffer capacity and resilience of the seawater column environment.

## 1. Introduction

Submarine volcanism represents about 85% of the global Earth volcanism (White et al., 2015). It is responsible for the transfer of chemicals from the crust to the water column, including volatile elements or reduced transition metals, and is also the main mechanism responsible for deep stored carbon release to the surface (Baker et al.,

2012; Rubin et al., 2012). Water column geochemical signatures of submarine eruptions are quite different from one site to another but generally exhibit increases in magmatic volatile gases (³He, CO₂), enrichments in H₂ and CH₄, decreases in pH by the addition of CO₂, SO₂ and mineral acidity (H⁺), and discharges of reduced species including H₂S (Baumberger et al., 2014; Buck et al., 2018; Resing et al., 2009; Resing et al., 2007). Each of these compounds are valuable tracers for

<footnote><sup>a</sup> Geo-Ocean UMR 6538 CNRS - Ifremer - UBO - UBS, F-29280 Plouzané, France</footnote>

<footnote><sup>b</sup> TotalEnergies, CSTJF EB 437, Avenue Larribau, F-64018 Pau Cedex, France</footnote>

<footnote><sup>c</sup> Lavoisier H2 Geoconsult, FR-74400 Chamonix, France</footnote>

<footnote>* Corresponding author at: Ifremer Centre Bretagne, ZI de la Pointe du Diable - CS 10070, 29280 Plouzane.
E-mail address: manon.mastin@ifremer.fr (M. Mastin).</footnote>
submarine eruptions and hydrothermal circulation: combining them
bring insights of their origin and generation mechanisms involved.

Studies on submarine eruptions started in the 1980's, but deep-sea eruptions are very difficult to detect and observe due to their shortness (Baker et al., 2012; Rubin et al., 2012). They have mostly been sampled after the end of the eruption, often capturing the hydrothermal activity state instead of the eruptive state (Rubin et al., 2012). Before the Mayotte submarine eruption, only three active submarine eruptions have been observed: NW Rota-1 (an explosive shallow eruption – at ~500 m deep – in the Mariana arc (Chadwick et al., 2008)), West-Mata (a deeper explosive eruption – at ~1200 m deep – in the Lau basin (Embley et al., 2014; Resing et al., 2011)), and event plumes at the NE Lau Spreading Center eruption (rising from summits between 1600 and 1300 m deep, Baker et al. (2011)). The Mayotte submarine eruption and the significant scientist mobilization that followed offer well-defined temporal and spatial constraints on the birth of a volcanic edifice, and constitutes therefore a great opportunity to study the origin and processes responsible for gas release, their impact in the water column and the carbonate system response.

After a few thousand years of aseismicity, (Zinke et al., 2003a), in May 2018, Mayotte Island started to experience a major seismic crisis. This phenomenon was associated with the birth of a volcano edifice 50 km East offshore Mayotte, with characteristics that make it the largest active eruption ever documented since the Laki eruption in 1783 (Thordarson and Self, 1993). Indeed, this volcano stands 820 m above the seafloor, with a magma released estimated to be about 6.5 km³, inducing strong geochemical anomalies through the water column (Cesca et al., 2020; Feuillet et al., 2021; Lemoine et al., 2020). We present here for the first time a comprehensive survey of gas chemistry (H₂, CO₂, CH₄ and He) in the water column and its evolution from the birth of the volcano (i.e. eruptive state) to the initiation of hydrothermal activity, covering a time period of about 30 months, from May 2018 to October 2020. We aim to discuss the concomitant evolution of water column chemistry over the course of the eruption and discuss the geochemical mechanisms involved. For instance, the isotopic composition of He provides a valuable and reliable indicator of its own origin but can be decoupled from the more reactive gases (Giggenbach et al., 1993). Indeed, the presence of primordial ³He within the water column indicates input of magmatic gases from the mantle (Craig and Lupton, 1981), whereas radiogenic ⁴He indicates gas inputs originating from the crust (Barry et al., 2020). Aside from hydrothermal reactions (e.g. serpentinization), generation of H₂ occurs during lava and seawater interactions, generally associated with gas-rich eruptions (Sansone et al., 1991). In submarine volcanic systems, H₂ is therefore the typical tracer for ongoing eruption, with a short residence time as it is rapidly consumed by microorganisms and chemical oxidation over hourly or daily timescales (Baumberger et al., 2020; Baumberger et al., 2014). On another hand, methane can be a major component of hydrothermal gases but is usually present in trace amounts in volcanic gases. In natural systems, CH₄ may originate from thermocatalytic decomposition of organic matter, microbial production or from abiotic processes that may involve various chemical reactions (Fiebig et al. (2004) and references therein). In marine hydrothermal systems for instance, methane is thought to be generated abiotically during fluid-rock interactions involving serpentinization reactions and Fischer-Tropsch-type chemical reactions (Giggenbach, 1996; Truche et al., 2020). It is a potent greenhouse gas, 28 times more than CO₂ (Pörtner et al., 2022), with a potential significant effect on past climate change with volcanic basins providing a setting for rapid disruptions in the release of carbon from sedimentary reservoirs (Raynaud et al., 1993; Svensen et al., 2004).

The carbon dioxide is generally the predominant dissolved gases in
submarine volcanic fluids, generated through lava outgassing (Craig and
Lupton, 1981). Therefore, volcanic activity contributes to local ocean
acidification, in particular by the discharge of CO2-rich fluids into the
seawater column (Resing et al., 2009; Santana-Casiano et al., 2016).
Carbone dioxide is an acid gas that reacts with water to produce

carbonic acid (H₂CO₃) which is dissociated into hydrogenocarbonate
(HCO₃⁻) and carbonate (CO₃²⁻) ions depending on pH and buffer capacity
of the solution. When CO₂ is added to a solution, it gives a mixture of
these three species that results in an increase of the quantity of total
dissolved inorganic carbon (expressed as ΣCO₂). Those dissolved com-
pounds make up the carbonate system in water. The carbonate system of
the ocean is the primary buffer for the acidity of seawater and acts as a
governor for the carbon cycle by controlling the partial pressure of CO₂
in the atmosphere, which helps to regulate the temperature of the
planet. The formation rate of the most prevalent authigenic mineral in
the environment, CaCO₃, is also the major sink for dissolved carbon in
the long-term global carbon balance (Emerson and Hedges, 2008). The
emission of CO₂ from submarine volcanoes generates local ocean acid-
ification and a change in the carbonate system equilibrium that could
affect biological communities, with especially important consequences
for organisms that use calcium carbonate in their structures (Santana-
Casiano et al., 2016). This process also favors the photosynthesis
(Jansson and Northen, 2010; Suzuki, 1998).

In this study, we present a compilation of geochemical data taken at
the Fani Maoré submarine volcano, during the main eruptive event in
May 2019 and from two cruises performed in July 2019 and October
2020, leading to monitor the continuous lava flow installation. These
data provide valuable information on geochemical mechanisms during
the setup of a volcanic edifice and on the impact of an eruptive activity
on the water column at a local scale.

## 2. Geological setting and overview on the eruptive activity offshore Mayotte

Mayotte Island is located in the Comoros Archipelago, within the Mozambique Channel (Indian Ocean) between the eastern coast of the Mozambique Channel and the northern tip of Madagascar (Fig. 1A). The four islands that compose the Comoros Archipelago are aligned along a NW-SE axis (Tzevahirtzian et al., 2021). Mayotte is the easternmost island of the archipelago, and the eldest with a maximum age of 20 Ma for the onset of subaqueous volcanic activity and 11 Ma for the onset of subaerial volcanic activity (Debeuf, 2004; Michon, 2016; Nougier et al., 1986; Pelleter et al., 2014). The last reported volcanic event occurred 4–6 thousand years ago (ka) at Petite-Terre, east of Mayotte (Zinke et al., 2003b).

The origin of the Comoros volcanism is not yet well understood. Two main hypothesis have been developed to explain its origin: (1) a mantle plume that interacts with the oceanic lithosphere (Emerick and Duncan, 1982); (2) a lithospheric deformation that reactivated transform faults and controlled the magma path (Nougier et al., 1986). Recent works suggest that the volcanism is associated with lithospheric deformation rather than the result of a deep mantle plume (Famin et al., 2020; Lemoine et al., 2020; Michon, 2016; Tzevahirtzian et al., 2021).

On May 10th, 2018, the major seismo-volcanic crisis that began on Mayotte Island has sparked the interest of scientists and led to the establishment of the Mayotte volcanological and seismological monitoring network, named REVOSIMA (2021). Over a year, 32 earthquakes of $M_w \ge 5$ were recorded, included the largest event ever recorded in the Comoros Archipelago on the 15th of May 2018 with $M_w = 5.9$ (Cesca et al., 2020; Feuillet et al., 2021; Lemoine et al., 2020). These earthquakes are distributed into two swarms, one proximal and one distal, whose epicenters are respectively located 5–15 km and about 25 km east of Petite-Terre, at 25–50 km depth (Feuillet et al., 2021; Lavayssière et al., 2022). Very long period seismic events (VLP) were frequently recorded suggesting a fluid migration from active magmatic or hydrothermal processes (Lemoine et al., 2020).

This seismo-volcanic crisis is associated with the birth of a volcanic edifice, the Fani Maoré Seamount, discovered in May 2019 during the MAYOBS1 oceanographic cruise (Feuillet, 2019), that was not present in 2014 during the survey operated by the French Naval Hydrographic and Oceanographic Service (SHOM) (Feuillet et al., 2021) (Fig. 1B and C).
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (A) Map of the site with reference CTD casts and 50 m-resolution bathymetry using WGS 84 geodetic system (Geo-Ocean (Ifremer), 2022), (B) Bathymetry (30 m-resolution) of the volcano edifice with CTD casts performed above the volcano and above lava flows, (C) 3D representation of the volcano edifice (vertical exaggeration x2).</figcaption></figure>

The edifice is located 50 km east of Mayotte (–12°54′37; 45°42′42) and rises to 820 m at about 3500 m depth. The ongoing eruption was captured in May 2019 by the ship echo sounder with an exceptional acoustic plume of about 2 km high within the water column associated with strong geochemical anomalies (Feuillet et al., 2021). Lemoine et al. (2020) and Cesca et al. (2020) proposed that the eruptive activity has been going on from June–July 2018 to May 2019, and was fed by a magma reservoir of 10 to 15 km diameter located at 25 to 35 km depth. Dofal et al. (2021) suggested that this magmatic reservoir developed beneath the interface between the mantle lithosphere and the subplateau.

The eruption was followed, from May 2019 to October 2020, by a chronological lava flow emission (REVOSIMA, 2021) associated with four new distant exit points: (1) south of the volcano (~0.2 km³ of lava from May 19th to June 17th 2019); (2) west of the volcano (~0.3 km³ of lava from June 18th to July 30th 2019); (3) north of the volcano (~0.08 km³ of lava from July 31st to August 20th 2019); (4) northwest of the volcano (~0.8 km³ of lava from August 21st 2019 to May 11th 2020); (5) northwest of the volcano (~0.1–0.2 km³ of lava from May 11th to October 11th 2020). From the beginning of the eruptive activity, the volume of erupted magma is estimated to be about 6.5 km³ (REVOSIMA, 2021). The Fani Maoré acoustic plumes ended in the beginning of June 2019 during MAYOBS2 cruise (Jorry, 2019) and the last recorded lava flow occurred between October 2020 (MAYOBS15, Rinnert et al. (2020)) and January 2021 (MAYOBS17, Thinon et al. (2021)).

### 3. Materials and methods

The water column above the volcano was monitored during three oceanographic cruises on board the R/V Marion Dufresne. The first cruise, MAYOBS1, was conducted in 2019 from May 2nd to 22nd (Feuillet, 2019); the second, MAYOBS4, in 2019 from July 19th to August 4th (Fouquet and Feuillet, 2019); and the third, MAYOBS15, in 2020 from October 1st to 27th (Rinnert et al., 2020). These MAYOBS cruises are part of the REVOSIMA survey program with the aim of

monitoring this seismic and volcanic crisis which affects Mayotte Island (Feuillet et al., 2019).

#### 3.1. *Sampling sites*

Stations for hydrocasts were determined according to the presence of acoustic signals from ship echo sounder surveys (Kongsberg EM122 1° × 1°) and to seafloor observations from the submersible interactive camera system (SCAMPI, Ifremer). During the first cruise in May 2019, one hydrocast was performed within the 2000 m-height acoustic plume imaged at the volcano summit (MAY01-HY02), and one other above the northern flank of the volcano (MAY01-HY03) (Fig. 1B). In July 2019, three CTD (Conductivity, Temperature, and Depth) casts were carried out, one in the same location as MAY01-HY03 (cast MAY04-HY04) and two at the west of the volcano (MAY04-HY07, MAY04-HY09), above the recent lava flow from June 18th to July 30th 2019, detected by ship echo sounder. The third cruise focused on the northwest new lava flow from May 11th to October 11th 2020, with a CTD cast performed above incandescent lava (MAY15-HY08) and very recent lava flow (MAY15-HY06).

CTD casts for background water column evaluation (i.e. remote from volcanic activity) were performed in July 2019 and October 2020, tens of kilometers away from the study site. During MAYOBS4, the background cast was done about 20 km southward from the volcano (MAY04-HY03), whereas during MAYOBS15 the background cast was realized about 100 km northeastward (MAY-HY09).

#### 3.2. *Sampling methods and analysis*

We studied physico-chemical parameters of the water column using a Seabird 911 Plus CTD combined with a Seapoint Turbidity Meter sensor. Turbidity measurements are reported as Nephelometric Turbidity Unit (NTU). Discrete water samples were collected using a 16-bottles carousel equipped with 8 L Niskin water sampling bottles.

Turbidity signals were homogenized by setting the zero value for all
hydrocasts studied. This correction was based on the near-zero signal values encountered around 1500 m depth (i.e. below the euphotic zone and above turbidity anomalies linked to the volcanic activity). The average of 100 turbidity values (acquired at a frequency of 1 Hz, for a CTD rosette ascent velocity of 1 m/s) from 1500 m to deeper was determined and then subtracted to the entire turbidity depth profile.

Samples for noble gases, dissolved gases, metals, pH, alkalinity and silicates analyses were directly taken from the Niskin bottles, as described below. Hydrographic parameters are presented in Supplementary Figs. 1 and 2.

### 3.2.1. Helium and Neon

Samples for noble gases analysis were collected immediately after the CTD-rosette was retrieved to conserve the integrity of the dissolved gases. Water was flushed into copper tubing carefully checking for the absence of air bubbles and sealed using special stainless clamps (Sültenfuß, 2016). Quantification of helium and neon isotopes ($^{3}$He, $^{4}$He, $^{20}$Ne, $^{22}$Ne) was performed at the Helis Laboratory (Helium isotopes studies, Bremen, Germany) using a mass spectrometric system composed of a quadrupole mass spectrometer (QMS, Balzers QMG112a®) and a sector field mass spectrometer (SMS, MAP 215–50®). This method yields a precision of ±0.4% for $^{3}$He/$^{4}$He ratios and ± 0.8% for helium and neon concentrations (Sültenfuß et al., 2009).

In this study, we choose to define each component of the measured concentrations of $^{3}$He and $^{4}$He (i.e. equilibrium, air contamination and non-atmospheric concentrations). Assuming that the entire Ne concentration comes from atmospheric air, the corrected He concentration from air contamination and atmospheric air in equilibrium is expressed as $^{3}$He$_{xs}$ and $^{4}$He$_{xs}$. Considering that the tritiogenic $^{3}$He (i.e. resulting from the radioactive decay of tritium) is negligible in the Indian Ocean offshore Mayotte Island (Jenkins et al., 2019), the $^{3}$He$_{xs}$ and $^{4}$He$_{xs}$ should only corresponds to the supply of mantle and crustal helium. Therefore, the closure equations for neon and helium are:

$$ egin{cases} N_{em} = N_{eq} + N_{air} \\ ^{4}H_{m} = ^{4}He_{eq} + ^{4}He_{air} + ^{4}He_{xs} \\ ^{3}H_{m} = ^{3}He_{eq} + ^{3}He_{air} + ^{3}He_{xs}. \end{cases} $$

with m the measured concentration, eq the equilibrium concentration and air the concentration for air contamination. Equilibrium and air contamination neon concentrations were defined as $N_{eq} = N_{atm} \times \beta_{Ne}$ and $N_{air} = N_{em} - N_{eq}$, and helium isotopes concentrations as $^{3.4}He_{eq} = ^{3.4}He_{atm} \times \beta_{He}$ and $^{3.4}He_{air} = N_{air} \times \frac{^{3.4}He_{atm}}{N_{atm}}$.

We introduce solubility coefficients depending on the temperature and salinity conditions of sampling, which vary respectively from 1.44 to 10.82 °C and from 34.64 to 34.90 psu. Solubility coefficients of Ne and He in seawater ($\beta_{Ne}$ and $\beta_{He}$, respectively) were determined for each sample according to the water temperature (T, in kelvin) and salinity (S, in psu) of the sampling depth, using Weiss (1971) solubility equations:

$$ \ln \beta_{He} = -34.6261 + 43.0285 \left(\frac{100}{T}\right) + 14.1391 \ln \left(\frac{T}{100}\right) + S \left[ -0.042340 + 0.022624 \left(\frac{T}{100}\right) - 0.0033120 \left(\frac{T}{100}\right)^2 \right] $$

$$ \ln \beta_{Ne} = -39.1971 + 51.8013 \left(\frac{100}{T}\right) + 15.7699 \ln \left(\frac{T}{100}\right) + S \left[ -0.124695 + 0.078374 \left(\frac{T}{100}\right) - 0.0127972 \left(\frac{T}{100}\right)^2 \right] $$

The corrected ratio of helium (Rc/Ra) was determined by only applying the correction from atmospheric air contamination, as the equilibrium value ($He_{eq}$) is not negligible compare to $He_{xs}$. That means,

$$ Rc = \frac{^{3}He_{xs} + ^{3}He_{eq}}{^{4}He_{xs} + ^{4}He_{eq}}. $$

### 3.2.2. Dissolved gases ($CH_4$, $CO_2$, $H_2$)

Water samples for $CH_4$ analyses were collected into 125 mL two-valve glass ampoules. During MAYOBS1 and MAYOBS4, samples were poisoned with $NaN_3$ before onshore analyses, whereas during MAYOBS15, samples were analyzed onboard. Both onshore and onboard analyses were performed using the “purge and trap” method developed by Charlou et al. (1987). Dissolved gases were completely extracted by helium purging and trapped onto activated charcoal at -80 °C. The $CH_4$ was desorbed from the trap by increasing the temperature and was injected into a gas chromatograph (GC) equipped with a flame ionization detector (FID) for quantitative analysis. External calibration was performed using a standard gas of $CH_4$ at appropriate pressure and constant temperature. This method allows the determination of $CH_4$ concentrations equivalent to those of open ocean seawater (0.3 nM), with residual standard deviation below 2%.

For onboard $CO_2$ and $H_2$ analyses, water samples were carefully collected from the Niskin bottles, without injecting air bubbles, in 240 mL brown glass bottles closed by a screw cap fitted with a PTFE/silicone gas tight septum. Analyses were run by gas chromatography coupled to an helium ionization detector (GC-HID) and using the Headspace method developed by Donval and Guyader (2017) which consists of replacing ~20 mL of the seawater sample by a gaseous phase of pure helium in which dissolved gases equilibrate according to solubility coefficients at equilibrium temperature and to water salinity. External calibration was carried out using a standard gas mixture of $H_2$, $CO_2$ injected through 0.50 mL and 2 mL injection loops at appropriate pressure and constant temperature. The detection limit was 1 nM for hydrogen and 0.1 µM for $CO_2$ with a residual standard deviation of 3%.

For characteristic samples, the headspace phases were transferred in evacuated 12 mL Labco tubes, completed with helium above the atmospheric pressure, and sent to the Isolab b.v. laboratory (Stable isotopes and geochemical laboratory services, The Netherlands) for $^{13}$C isotopic composition analysis of $CH_4$ and $CO_2$, mentioned hereafter as $\delta^{13}C-CH_4$ and $\delta^{13}CO_2$. Carbon isotopes of methane were analyzed with an Agilent 6890 N GC (Agilent Technologies, Santa Clara, US) interfaced to a Finnigan Delta S IRMS (Thermo Scientific, Bremen, Germany) using a Finnigan GC-C II interface. The GC is equipped with a 12 m, 0.32 mm Molsieve column (Agilent) and an injection valve. Samples are calibrated regularly against a calibration standard and results are reported in promille vs. vPDB. The minimum concentration needed to perform the analysis is about 25–50 ppm. Carbon isotopes of $CO_2$ were analyzed on an Agilent 7890 A GC (Agilent Technologies, Santa Clara, US) interfaced to a MAT 253 IRMS (Thermo Scientific, Bremen, Germany) using a GC-Isolink or a Finigan GC-C III interface. The GC is equipped with a 25 m, 0.32 mm Porabond-Q column (Agilent) and an injection valve. Cold trapping is used to pre-concentrate the sample when necessary. Samples are run at least 3 times after which the average of the results is calculated. The system is calibrated at least once a day using an in house natural gas standard and results are reported in promille vs. vPDB. The minimum concentrations needed to perform the analysis depends on sample composition.

### 3.2.3. Total dissolvable iron and manganese (TDFe and TDMn)

Sampling for analysis of total dissolvable iron and manganese, respectively TDFe and TDMn, was carried out in 30 mL low density polyethylene bottles (LDPE, Nalgene®) previously washed with 10%$_{v/v}$ hydrochloric acid (HCl 37%, Analytical Grade, Merck) then rinsed with ultrapure water (Milli-Q Millipore element system). All raw samples for metal analysis were acidified to 0.025 mol/L hydrochloric acid (ultrapur® grade) within 6 h of sampling. Quantification of TDFe and TDMn was carried out in laboratory at the Pôle Spectrométrie Océan (PSO, Ifremer, Brest, France), using HR-ICP-MS (High Resolution Inductively Coupled Plasma Mass Spectrometry; Element XR, ThermoFisher Scientific). Samples were spiked with Indium (In) at 2 ppb and diluted by a factor of 100 with 0.28 mol/L distilled nitric acid. This method allows the quantification of metal concentrations in seawater, with detection
limits at sub-micromolar order (about 10 nmol/L for Mn and 100 nmol/L for Fe) and residual standard deviation of 3% for TDMn and 2% for TDFe.

### 3.2.4. pH, total CO₂, total alkalinity, silicates

Sampling for the study of pH, total alkalinity ($A_T$) and total $CO_2$ ($\Sigma CO_2$) was carried out in 30 mL polyethylene bottles (LDPE, Nalgene®) previously rinsed with ultrapure water (Milli-Q Millipore element system) and dried. Samples were analyzed onboard for titration of dissolved alkaline species with hydrochloric acid solution (0.01 mol/L HCl Titrinorm) using a titrimeter (Titrino 848, Metrohm®) combined with a pH electrode (Metrohm®). Total alkalinity and $\Sigma CO_2$ were determined from titration equivalence volumes. Residual standard deviations were of 1.04% for pH measurements, and below 2% for DIC and $A_T$ measurements.

Samples for the quantification of silicates ($SiO_2$) were taken in 60 mL polyethylene bottles previously rinsed with ultrapure water (Milli-Q Millipore element system) and dried. Analyses were done onboard

during MAYOBS1 and MAYOBS4 cruises, and onshore during MAYOBS15 cruise, by a segmented continuous flow analysis technique using an autoanalyzer (SEAL AutoAnalyzer 3 HR, SEAL Analytical®) based on the spectrophotometry detection (Aminot and Kérouel, 2007). Residual standard deviation are below 0.2%.

### 3.3. Combined data for background evaluation

Background composition was evaluated by combining data from background hydrocasts performed both during MAYOBS4 (MAY04-HY03) and MAYOBS15 (MAY15-HY09) (Fig. 1). We defined the background as an envelope that integrates the variations of water masses and their movements through the seasons. The background reference for turbidity was determined by first combining the turbidity data from both hydrocasts recorded at 1 Hz by the CTD. Then averaging over 40 values so that the standard deviation for depth did not exceed 10 m and that a signal frequency of 1/40 Hz was obtained. For the chemical parameters studied, the background reference was determined by first

<caption>**Table 1**</caption>

Most striking features of the different lava flows and sampling sites, including site location, sampling date, visual observations from the submersible interactive camera system (SCAMPI, Ifremer) and main physicochemical characteristics.

<table><thead><tr><th>Lava picture</th><th>CTD station &amp; sampling date</th><th>Site</th><th>Activity</th><th>Suggested lava age</th><th colspan="3">Observations</th></tr><tr><th></th><th></th><th></th><th></th><th></th><th>Visual</th><th>Turbidity, heat, density</th><th>Chemicals</th></tr></thead><tbody><tr><td>Ship echo souder<br>Figure from Feuillet et al., 2021</td><td>MAY01-HY02<br>(16/05/2019)</td><td>Summit</td><td>Eruption</td><td></td><td>Acoustic plume of ~2 km high</td><td>Turbidity up to 0.8 NTU<br>Presence of temperature and density anomaly</td><td>Maximum concentrations:<br><sup>3</sup>He<sub>xs</sub> = na<br>CO<sub>2</sub> = 114.6 µmol/L<br>CH<sub>4</sub> = 224.8 nmol/L<br>H<sub>2</sub> = 5992.9 nmol/L<br>Maximum concentrations:<br><sup>3</sup>He<sub>xs</sub> = na<br>CO<sub>2</sub> = 264.7 µmol/L</td></tr><tr><td></td><td>MAY01-HY03<br>(16/05/2019)</td><td>Northern flank</td><td>Eruption</td><td></td><td>1 km from the acoustic plume of ~2 km high</td><td>Turbidity up to 1.4 NTU from 2500 to 3000 m, and > 4.9 NTU for depths >3000 m<br>Presence of temperature and density anomaly</td><td>CH<sub>4</sub> = 830.5 nmol/L<br>H<sub>2</sub> = 547.9 nmol/L<br>Maximum concentrations:<br><sup>3</sup>He<sub>xs</sub> = 11.33 fmol/L<br>CO<sub>2</sub> = 32.7 µmol/L</td></tr><tr><td></td><td>MAY04-HY04<br>(23/07/2019)</td><td>Northern flank</td><td>None</td><td>May 2019</td><td>Already emplaced lava flow, covered by a fine sediment deposit, with evidence for incipient fluid shimmering</td><td>Turbidity from 0.3 to 1.6 NTU<br>Absence of temperature and density anomaly</td><td>CH<sub>4</sub> = 173.8 nomol/L<br>H<sub>2</sub> = 1.3<br>Maximum concentrations:<br><sup>3</sup>He<sub>xs</sub> = 15.44 fmol/L<br>CO<sub>2</sub> = 105.9 µmol/L</td></tr><tr><td></td><td>MAY04-HY07<br>(22/07/2019)<br>MAY04-HY09<br>(29/07/2019)</td><td>West lava flow</td><td>Recent</td><td>June 18th to July 30th 2019</td><td>More recent lava flow with yellowish staining and thin mats of probably microbial origin</td><td>Turbidity from 0.3 to 1.6 NTU<br>Presence of temperature and density anomaly</td><td>CH<sub>4</sub> = 777.4 nmol/L<br>H<sub>2</sub> = 39.3 nmol/L<br>Maximum concentrations:<br><sup>3</sup>He<sub>xs</sub> = 5.49 fmol/L<br>CO<sub>2</sub> = 69.6 µmol/L</td></tr><tr><td></td><td>MAY15-HY06<br>(17/10/2020)</td><td>Northwest lava flow</td><td>Very recent</td><td>May 11th to October 11th 2020</td><td>Very glossy black lava</td><td>Turbidity up to 0.6 NTU<br>Presence of temperature and density anomaly</td><td>CH<sub>4</sub> = 204.4 nmol/L<br>H<sub>2</sub> = 38.2 nmol/L</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 1 (continued)**</caption><thead><tr><th>Lava picture</th><th>CTD station &amp; sampling date</th><th>Site</th><th>Activity</th><th>Suggested lava age</th><th colspan="3">Observations</th></tr><tr><th></th><th></th><th></th><th></th><th></th><th>Visual</th><th>Turbidity, heat, density</th><th>Chemicals</th></tr></thead><tbody><tr><td></td><td>MAY15-HY08<br/>(21/10/2020)</td><td>Northwest<br/>lava flow</td><td>Active<br/>flow</td><td>May 11th to<br/>October 11th<br/>2020</td><td>Incandescent lava</td><td>Turbidity up to 0.6 NTU<br/>Presence of temperature and<br/>density anomaly</td><td>Maximum<br/>concentrations:<br/>³He<sub>xs</sub> = 7.78 fmol/L<br/>CO<sub>2</sub> = 53.2 µmol/L<br/>CH<sub>4</sub> = 283.6 nmol/L<br/>H<sub>2</sub> = 647.6 nmol/L<br/>L</td></tr></tbody></table>

<figure><img src="image_5.png" /></figure>

combining the two background hydrocasts and then averaging the data over a close depth range. Same depth ranges were used for all parameters. The envelopes were defined by the standard deviations resulting from the combination of the two profiles. (See Supplementary Fig. 3).

# 4. Results

## 4.1. Seafloor observations

### 4.1.1. Identification of various lava flows

Hydrocast sampling was concomitant to lava flow emplacement occurring throughout the volcanic structure. To understand if geochemical anomalies in the water column could be attributed to active or recent lava flows at the bottom, it is important to evaluate the nature of the volcanic seafloor (Table 1). The hydrocast on the northern flank in July 2019 (MAY04-HY04) was performed above an already emplaced lava flow, covered by a fine sediment deposit, with evidence for incipient fluid shimmering, identified during the SCAMPI exploration (Table 1). At the western lava flow, where casts MAY04-HY07 and MAY04-HY09 were performed in July 2019, SCAMPI images show a more recent lava flow with yellowish staining and thin mats of probably microbial origin (Table 1). In October 2020, the northeast lava flow captured at the hydrocast MAY15-HY06 was a very glossy black lava (Table 1). Combining this visual aspect with bathymetric surveys gives evidence of the very recent nature of these lava flows. At the MAY15-HY08 cast, an active lava flow was identified by incandescent lava observation (Table 1).

### 4.1.2. Strong turbidity layer

Monitoring of the turbidity (Fig. 2) shows a signal evolution from the regional background during the ongoing eruption and the lava flow installation. Outside the zone of influence of the volcano (i.e. background hydrocasts), turbidity does not exceed 0.06 NTU. Below 2000 m deep, maximum turbidity remains lower than 0.02 NTU. In May 2019, during the ongoing eruption, the turbidity at the volcano summit features maxima up to 0.8 NTU. However, these levels were not as significant compare to the turbidity values recorded at the northern flank of the volcano, with a maximum of 1.4 NTU between 2500 and 3000 m depth, reaching the saturation value of the sensor (> 4.9 NTU) over the last 200 m above seafloor. Two months later, in July 2019, at the same location in the northern flank, turbidity ranged from 0.3 to 1.6 NTU between 2000 m and the seafloor. Casts performed at the same time above the recent western lava flow shows the same turbidity pattern, with maximum turbidity zone starting at 2250 m deep. October 2020 monitoring highlight weaker turbidity signal measured above the new lava flow, with anomalies from 2500 m and only up to 0.6 NTU (Fig. 2).

## 4.2. Concentration of gases within the water column

### 4.2.1. Helium measurements

From July 2019 to October 2020, ³Hexs measurements for depths below 2000 m remain well distinct from the background value (~1.5 fmol/L) (Fig. 3; Table 2). The highest ³Hexs concentration of 15.4 fmol/L was measured in July above the western lava flow (cast MAY04-HY07), at 2924 m depth. Seawater samples taken during the same cruise above the northern flank of the edifice (cast MAY04-HY04) show lower ³Hexs values of 11.3 fmol/L that were still higher than samples from October

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Turbidity profiles determined by background evaluation, and measured for the studied hydrocasts from May 2019, July 2019 and October 2020 (MAYOBS1, MAYOBS4 and MAYOBS15 cruises, respectively). Please pay attention to the differences in NTU scales between the background profile and the others.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 3. Depth profiles of ³Heₓs concentration measured at the northern flank of the volcano and above the recent Western lava flow in July 2019 during MAYOBS4 cruise (in green), and above the North West very recent lava flow in October 2020 during MAYOBS15 cruise (in purple). The evaluated background for ³Heₓs concentrations is represented by the bold line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

2020 taken above the northwest lava flow (casts MAY15-HY06 and MAY15-HY08). These two last CTD casts exhibit similar profiles with maximum ³Heₓs values around 8 fmol/L and 2900 m depth. TDMn concentrations are well correlated with ³Heₓs (TDMn = 8.97 × 10⁻⁶ ³Heₓs, r² = 0.95), while CH₄ concentrations show weaker correlation with ³Heₓ (CH₄ = 20.3 × 10⁻⁶ ³Heₓs, r² = 0.78). CO₂ concentrations do not show any significant correlation with other gas concentrations (see Supplementary Fig. 4).

### 4.2.2. Concentration of gases above the volcano edifice

During the May 2019 eruption, extremely high H₂ concentrations, up to 6000 nmol/L, were measured at the volcano summit (cast MAY01-HY02), inside the acoustic plume (Fig. 4; Table 3). Samples from the northern flank exhibit lower, but still strong H₂ concentrations up to 600 nmol/L. In both cases, maxima concentrations are located at remarkably shallow depths within the water column, whereas strong CO₂ and CH₄ anomalies are found deeper, near the seafloor, with maximum concentrations of 265 µmol/L and 830 nmol/L, respectively. The same cast above the northern flank was also investigated in July 2019 (cast MAY04-HY04) where no eruptive activity was identified. Concentrations of H₂ and CO₂ remain at background level whereas CH₄ anomalies were measured up to 180 nmol/L at depth 2250 m and below 2500 m.

### 4.2.3. Concentration of gases above new lava flow emplacements

Strong levels of H₂ were measured above the active lava flow in October 2020 (cast MAY15-HY08), with concentrations up to 650 nmol/L, between 2700 and 3260 m depth (Fig. 5; Table 3). Lower H₂

concentrations, up to 40 nmol/L and near the seafloor, were found above the recent western lava flow in July 2019 (casts MAY04-HY07 and MAY04-HY09) and the very recent western lava flow from October 2020 (cast MAY15-HY06).

The maxima concentrations in CO₂, from 95 to 106 µmol/L, were measured in July 2019 above the recent western lava flow (casts MAY04-HY07 and MAY04-HY09). Seawater samples from the very recent northwest lava flow site (cast MAY15-HY06) exhibit lower maximum concentrations, up to 70 µmol/L, while concentrations levels above the active northwest lava flow (cast MAY15-HY08) remains within the background envelope.

Methane concentrations are at their highest level of 780 nmol/L above the recent western lava flow (casts MAY04-HY07 and MAY04-HY09), with an anomaly that seems to increase progressively as it goes deeper from 2500 to 3200 m below sea level. Lower anomalies, up to 280 nmol/L, were measured in October 2020 above the actively forming lava flow (casts MAY15-HY06 and MAY15-HY08).

### 4.3. Carbon isotope composition

Carbon isotope ratios of CH₄ and CO₂ throughout the water column (referred as δ¹³C-CH₄ and δ¹³C-CO₂ versus vPDB, respectively) display limited variability through time (Table 4). Mean δ¹³C-CH₄ is -34.4 ± 0.6 ‰ (2.s.d, *n* = 2), while δ¹³C-CO₂ is -1.0 ± 0.4 ‰ (2.s.d, *n* = 6). In this study, we will not use the signature of δ¹³C-CO₂ for data interpretation because, in our water column samples, the CO₂ emitted from the eruption is strongly diluted within the surrounding seawater where the CO₂ is about 30–40 µmol/L. As we do not have endmember values, the signature of the emitted CO₂ is hardly definable. We consider that δ¹³C-CO₂ values result from the mixing between a magmatic component (-8 ‰ < δ¹³C- CO₂ < -4 ‰; Sano and Marty (1995)), a marine limestone component (0 ‰; Sano and Marty (1995)), the remineralisation of the organic matter (mean δ¹³C- CO₂ of the marine organic matter is about-20 ‰; Degens et al. (1968)), and the Indian Ocean seawater composition (0.1 ‰ < δ¹³C- CO₂ < 1.5 ‰; Liu et al. (2021); Schmittner et al. (2013)).

### 4.4. Evolution of pH, total alkalinity, ΣCO₂, silicates

During the ongoing eruption (May 2019), exceptional pH anomalies in water column up to -0.6 pH unit were measured both at the volcano summit and at the northern flank, for seawater samples taken below 2500 m (Fig. 6; Table 3). Only one comparable anomaly was detected again, in July 2019 above the recent western lava flow, and was located between 3150 m deep and the seafloor (~ 3200 m). The strong pH anomalies observed during the eruption, are associated with a net increase in ΣCO₂, A_T and SiO₂, compare to background levels and to the other studied casts. Maximum ΣCO₂ of 2489 µmol/L has been observed at 2640 m deep, above the volcano summit in May 2019. Similar anomalies up to 2434 µmol/L were also observed during the eruption, above the northern flank but at deeper depths (~ 3160 m). This last anomaly level was found again in October 2020, at the same depth, above the active northwest lava flow. The strongest alkalinity addition happens during the eruption at the volcano summit, with concentrations up to 2700 µmol/L, associate to a strong silicate addition, up to 20 µmol/L.

In May 2019, while the strong anomalies are located below 2000 m depth, the entire water column seems to be impacted by the eruption with pH, ΣCO₂ and A_T levels outside the background envelope. July 2019 and October 2020 values, except for the few anomalies mentioned above, show moderate anomalies localized below 2000 m, with an acidification that does not exceed 0.3 pH unit, and ΣCO₂, A_T and ΔSiO₂ up to 2370 µmol/L, 2490 µmol/L and 14 µmol/L, respectively. Overall, anomaly levels were stronger in May 2019 than July 2019, which were themselves stronger than October 2020 levels.
<caption>**Table 2**</caption>

Helium isotope concentrations, neon concentrations and isotopic ratio calculations for seawater samples taken from July 2019 and October 2020 cruises at the northern volcano flank and above the various lava flows.

<table><thead><tr><th>Sample ID</th><th>Depth<br>(m)</th><th><sup>3</sup>He<br>(mol/L)</th><th><sup>4</sup>He<br>(mol/L)</th><th>Ne<br>(mol/L)</th><th><sup>3</sup>He/<sup>4</sup>He<br>(mol/L)</th><th>He/<br>Ne</th><th>std<br>He/Ne</th><th><sup>4</sup>He/<sup>20</sup>Ne<br>(fmol/L)</th><th>R/<br>Ra</th><th><sup>4</sup>He<sub>xs</sub><br>(mol/L)</th><th><sup>3</sup>He<sub>xs</sub><br>(fmol/L)</th><th>std<br><sup>3</sup>He<sub>xs</sub><br>(fmol/L)</th><th>Rc/<br>Ra</th><th>std<br>Rc/<br>Ra</th><th>CO<sub>2</sub>/<sup>3</sup>He<br>(mol/L)</th></tr></thead><tbody><tr><td colspan="18" style="text-align: left; font-weight: bold;">MAY4-HY04 (July 2019, northern volcano flank)</td></tr><tr><td>MAY04-<br>HY04-<br>B01</td><td>3119.6</td><td>1.39 ×<br>10<sup>-14</sup></td><td>3.34 ×<br>10<sup>-9</sup></td><td>8.23 ×<br>10<sup>-9</sup></td><td>4.17 ×<br>10<sup>-6</sup></td><td>0.406</td><td>0.008</td><td>na</td><td>3.01</td><td>1.46 ×<br>10<sup>-9</sup></td><td>11.33</td><td>0.23</td><td>3.09</td><td>0.06</td><td>2.16 ×<br>10<sup>9</sup></td></tr><tr><td>MAY04-<br>HY04-<br>B07</td><td>2748.2</td><td>1.17 ×<br>10<sup>-14</sup></td><td>3.10 ×<br>10<sup>-9</sup></td><td>8.18 ×<br>10<sup>-9</sup></td><td>3.77 ×<br>10<sup>-6</sup></td><td>0.379</td><td>0.008</td><td>na</td><td>2.72</td><td>1.22 ×<br>10<sup>-9</sup></td><td>9.07</td><td>0.18</td><td>2.79</td><td>0.06</td><td>2.70 ×<br>10<sup>9</sup></td></tr><tr><td>MAY04-<br>HY04-<br>B14</td><td>2047.9</td><td>2.91 ×<br>10<sup>-15</sup></td><td>1.99 ×<br>10<sup>-9</sup></td><td>7.99 ×<br>10<sup>-9</sup></td><td>1.46 ×<br>10<sup>-6</sup></td><td>0.249</td><td>0.005</td><td>na</td><td>1.06</td><td>1.59 ×<br>10<sup>-10</sup></td><td>0.38</td><td>0.01</td><td>1.06</td><td>0.02</td><td>9.09 ×<br>10<sup>9</sup></td></tr><tr><td>MAY04-<br>HY04-<br>B15</td><td>1499.0</td><td>2.93 ×<br>10<sup>-15</sup></td><td>1.99 ×<br>10<sup>-9</sup></td><td>7.79 ×<br>10<sup>-9</sup></td><td>1.47 ×<br>10<sup>-6</sup></td><td>0.255</td><td>0.005</td><td>na</td><td>1.06</td><td>2.03 ×<br>10<sup>-10</sup></td><td>0.46</td><td>0.01</td><td>1.07</td><td>0.02</td><td>9.49 ×<br>10<sup>9</sup></td></tr><tr><td colspan="18" style="text-align: left; font-weight: bold;">MAY4-HY07 (July 2019, Western recent lava flow)</td></tr><tr><td>MAY04-<br>HY07-<br>B03</td><td>2924.0</td><td>1.80 ×<br>10<sup>-14</sup></td><td>3.74 ×<br>10<sup>-9</sup></td><td>8.16 ×<br>10<sup>-9</sup></td><td>4.82 ×<br>10<sup>-6</sup></td><td>0.458</td><td>0.009</td><td>na</td><td>3.48</td><td>1.87 ×<br>10<sup>-9</sup></td><td>15.44</td><td>0.31</td><td>3.56</td><td>0.07</td><td>3.66 ×<br>10<sup>9</sup></td></tr><tr><td colspan="18" style="text-align: left; font-weight: bold;">MAY15-HY06 (Oct. 2020, Northwest very recent lava flow)</td></tr><tr><td>MAY15-<br>HY06-<br>B01</td><td>3257.1</td><td>8.13 ×<br>10<sup>-15</sup></td><td>2.38 ×<br>10<sup>-9</sup></td><td>8.32 ×<br>10<sup>-9</sup></td><td>3.42 ×<br>10<sup>-6</sup></td><td>0.286</td><td>0.006</td><td>0.289</td><td>2.47</td><td>4.67 ×<br>10<sup>-10</sup></td><td>5.49</td><td>0.11</td><td>2.57</td><td>0.05</td><td>6.76 ×<br>10<sup>9</sup></td></tr><tr><td>MAY15-<br>HY06-<br>B02</td><td>3202.8</td><td>6.87 ×<br>10<sup>-15</sup></td><td>2.26 ×<br>10<sup>-9</sup></td><td>8.30 ×<br>10<sup>-9</sup></td><td>3.04 ×<br>10<sup>-6</sup></td><td>0.272</td><td>0.005</td><td>0.277</td><td>2.20</td><td>3.46 ×<br>10<sup>-10</sup></td><td>4.22</td><td>0.08</td><td>2.29</td><td>0.05</td><td>7.96 ×<br>10<sup>9</sup></td></tr><tr><td>MAY15-<br>HY06-<br>B04</td><td>3098.8</td><td>6.94 ×<br>10<sup>-15</sup></td><td>2.25 ×<br>10<sup>-9</sup></td><td>8.29 ×<br>10<sup>-9</sup></td><td>3.08 ×<br>10<sup>-6</sup></td><td>0.272</td><td>0.005</td><td>0.276</td><td>2.23</td><td>3.44 ×<br>10<sup>-10</sup></td><td>4.30</td><td>0.09</td><td>2.31</td><td>0.05</td><td>8.10 ×<br>10<sup>9</sup></td></tr><tr><td>MAY15-<br>HY06-<br>B08</td><td>2893.6</td><td>1.08 ×<br>10<sup>-14</sup></td><td>2.60 ×<br>10<sup>-9</sup></td><td>8.22 ×<br>10<sup>-9</sup></td><td>4.16 ×<br>10<sup>-6</sup></td><td>0.316</td><td>0.006</td><td>0.320</td><td>3.01</td><td>7.09 ×<br>10<sup>-10</sup></td><td>8.19</td><td>0.16</td><td>3.11</td><td>0.06</td><td>5.73 ×<br>10<sup>9</sup></td></tr><tr><td>MAY15-<br>HY06-<br>B11</td><td>2618.6</td><td>6.31 ×<br>10<sup>-15</sup></td><td>2.19 ×<br>10<sup>-9</sup></td><td>8.27 ×<br>10<sup>-9</sup></td><td>2.89 ×<br>10<sup>-6</sup></td><td>0.264</td><td>0.005</td><td>0.274</td><td>2.09</td><td>2.83 ×<br>10<sup>-10</sup></td><td>3.68</td><td>0.07</td><td>2.16</td><td>0.04</td><td>9.11 ×<br>10<sup>9</sup></td></tr><tr><td>MAY15-<br>HY06-<br>B14</td><td>2302.7</td><td>4.79 ×<br>10<sup>-15</sup></td><td>2.09 ×<br>10<sup>-9</sup></td><td>8.33 ×<br>10<sup>-9</sup></td><td>2.29 ×<br>10<sup>-6</sup></td><td>0.251</td><td>0.005</td><td>0.256</td><td>1.65</td><td>1.68 ×<br>10<sup>-10</sup></td><td>2.12</td><td>0.04</td><td>1.71</td><td>0.03</td><td>1.20 ×<br>10<sup>10</sup></td></tr><tr><td>MAY15-<br>HY06-<br>B16</td><td>1000.6</td><td>2.87 ×<br>10<sup>-15</sup></td><td>1.85 ×<br>10<sup>-9</sup></td><td>7.90 ×<br>10<sup>-9</sup></td><td>1.55 ×<br>10<sup>-6</sup></td><td>0.235</td><td>0.005</td><td>na</td><td>1.12</td><td>7.56 ×<br>10<sup>-12</sup></td><td>0.32</td><td>0.01</td><td>1.13</td><td>0.02</td><td>2.15 ×<br>10<sup>10</sup></td></tr><tr><td colspan="18" style="text-align: left; font-weight: bold;">MAY15-HY08 (Oct. 2020, Northwest active lava flow)</td></tr><tr><td>MAY15-<br>HY08-<br>B02</td><td>3202.2</td><td>3.94 ×<br>10<sup>-15</sup></td><td>2.03 ×<br>10<sup>-9</sup></td><td>8.44 ×<br>10<sup>-9</sup></td><td>1.94 ×<br>10<sup>-6</sup></td><td>0.240</td><td>0.005</td><td>0.247</td><td>1.40</td><td>7.70 ×<br>10<sup>-11</sup></td><td>1.24</td><td>0.02</td><td>1.45</td><td>0.03</td><td>9.85 ×<br>10<sup>9</sup></td></tr><tr><td>MAY15-<br>HY08-<br>B04</td><td>3098.8</td><td>9.90 ×<br>10<sup>-15</sup></td><td>2.57 ×<br>10<sup>-9</sup></td><td>8.23 ×<br>10<sup>-9</sup></td><td>3.85 ×<br>10<sup>-6</sup></td><td>0.313</td><td>0.006</td><td>na</td><td>2.78</td><td>6.84 ×<br>10<sup>-10</sup></td><td>7.29</td><td>0.15</td><td>2.88</td><td>0.06</td><td>4.61 ×<br>10<sup>9</sup></td></tr><tr><td>MAY15-<br>HY08-<br>B07</td><td>2899.6</td><td>1.05 ×<br>10<sup>-14</sup></td><td>2.54 ×<br>10<sup>-9</sup></td><td>8.39 ×<br>10<sup>-9</sup></td><td>4.11 ×<br>10<sup>-6</sup></td><td>0.303</td><td>0.006</td><td>0.306</td><td>2.97</td><td>6.07 ×<br>10<sup>-10</sup></td><td>7.78</td><td>0.16</td><td>3.12</td><td>0.06</td><td>4.78 ×<br>10<sup>9</sup></td></tr><tr><td>MAY15-<br>HY08-<br>B09</td><td>2751.6</td><td>7.23 ×<br>10<sup>-15</sup></td><td>2.28 ×<br>10<sup>-9</sup></td><td>8.21 ×<br>10<sup>-9</sup></td><td>3.17 ×<br>10<sup>-6</sup></td><td>0.278</td><td>0.006</td><td>0.282</td><td>2.29</td><td>3.97 ×<br>10<sup>-10</sup></td><td>4.62</td><td>0.09</td><td>2.36</td><td>0.05</td><td>6.48 ×<br>10<sup>9</sup></td></tr><tr><td>MAY15-<br>HY08-<br>B11</td><td>2699.0</td><td>7.63 ×<br>10<sup>-15</sup></td><td>2.30 ×<br>10<sup>-9</sup></td><td>8.20 ×<br>10<sup>-9</sup></td><td>3.31 ×<br>10<sup>-6</sup></td><td>0.281</td><td>0.006</td><td>0.284</td><td>2.39</td><td>4.21 ×<br>10<sup>-10</sup></td><td>5.02</td><td>0.10</td><td>2.47</td><td>0.05</td><td>6.18 ×<br>10<sup>9</sup></td></tr><tr><td>MAY15-<br>HY08-<br>B14</td><td>2001.4</td><td>3.69 ×<br>10<sup>-15</sup></td><td>1.97 ×<br>10<sup>-9</sup></td><td>8.37 ×<br>10<sup>-9</sup></td><td>1.87 ×<br>10<sup>-6</sup></td><td>0.236</td><td>0.005</td><td>0.245</td><td>1.35</td><td>3.09 ×<br>10<sup>-11</sup></td><td>1.01</td><td>0.02</td><td>1.39</td><td>0.03</td><td>1.29 ×<br>10<sup>10</sup></td></tr></tbody></table>

nd: not determined; na: not analyzed.
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Depth profiles of studied dissolved gases at the eruption site during the May 2019 eruption (in orange) and two months later (in green). MAY01-HY02 cast (from May 2019) was performed at the volcano summit while MAY01-HY03 (from May 2019) and MAY04-HY04 (from July 2019) were sampled at the northern flank of the volcano. (A₁) H₂ profiles; (A₂) Zoom in H₂ profiles for H₂ concentrations from 0 to 600 nmol/L; (B) CO₂ profiles; (C) CH₄ profiles. Evaluated background is represented by the bold line and the grey envelop. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

4.5. Evolution of TDFe and TDMn

Whatever the sampling date or location, depth profiles of TDMn and TDFe (Fig. 7A and B) show a similar trend with elevated concentrations below 2000 m deep compared to the local background, and with a strong enrichment near the seafloor (i.e. within the last 100 m). The upper part of the water column presents low TDMn and TDFe concentrations (TDMn <10 nmol/L; TDFe <200 nmol/L), close to background level. Depth profiles of TDMn and TDFe concentrations are indeed well correlated to those of ³HeXS and CH₄ (see supplementary material Fig. 4). Maximum TDMn and TDFe concentrations at the seafloor were of 560 nmol/L and 10,575 nmol/L, respectively, found during the May 2019 eruption, above the northern flank of the edifice (at 3160 m deep). Shallower in the water column (i.e., between 2000 m and 3100 m), maximum TDMn and TDFe concentrations above lava flows were up to 160 nmol/L and 2380 nmol/L, respectively. Concentration levels decrease from hydrocasts taken in May 2019 to those taken in October 2020. Nonetheless, maximum TDMn and TDFe concentrations measured during the last cruise were still out of the background envelope with respective values up to 80 nmol/L and 880 nmol/L at 3000 m deep, and

up to 120 nmol/L and 1880 nmol/L close to the seafloor (3260 m).

The significant correlation between iron and manganese concentra-
tions allows to define a range of TDFe/TDMn ratios between ~11 and ~
27 (Fig. 7C₁ and C₂). Elevated ratios were measured at deep water
layers, close to the seafloor and near the surface of the volcano walls.
The maximum TDFe/TDMn ratio of ~27 was found above the northern
flank in May 2019 at depths between 3025 and 3110 m (MAY01-HY03),
and was measured again in July 2019 at the same location for equivalent
depths (cast MAY04-HY04). Within the layer from 2600 m to 2900 m
deep, lower values of ~22 are measured in May 2019 and ~ 20 in July
2019. Molar ratios from ~27 to ~20 were respectively measured from
right above the volcano summit (at ~2720 m) to shallower depths (~
2590 m) during the May 2019 eruption (cast MAY01-HY02). This ratio
also decreases with decreasing sampling depth, with values from ~15 to
~11 for depths between 2500–2400 m. Intermediate ratios between
~14 and ~ 18 were found above the recent western lava flow in July
2019 (MAY04-HY07, MAY04-HY09) and the very recent northwest lava
flow in October 2020 (MAY15-HY06). Although samples for the later
hydrocast spread along a ratio of ~16, depletions in TDFe are observed
giving a ratio of ~11 at 2894 and 3002 m. Samples above the active
<caption>Table 3</caption>

Summary of H₂, CH₄, CO₂, TDMn, TDFe concentrations and pH, A_T, ΣCO₂ and SiO₂ measured within the water column at the volcano summit, at the northern edifice flank and above the western and northwest lava flows, in May 2019, July 2019 and October 2020. These parameters are also presented for the both background hydrocasts used for background evaluation.

<table><thead><tr><th>Station</th><th>Bottle</th><th>Site</th><th>Activity</th><th>Sampling date<br>(dd/mm/<br>yyyy)</th><th>Latitude</th><th>Longitude</th><th>Depth<br>(m)</th><th>CH<sub>4</sub><br>(nmol/L)</th><th>H<sub>2</sub><br>(nmol/L)</th><th>CO<sub>2</sub><br>(µmol/L)</th><th>pH</th><th>ΔpH</th><th>A<sub>T</sub><br>(µmol/L)</th><th>ΣCO<sub>2</sub><br>(µmol/L)</th><th>Δ(ΣCO<sub>2</sub>)<br>(µmol/L)</th><th>SiO<sub>2</sub><br>(µmol/L)</th><th>ΔSiO<sub>2</sub><br>(µmol/L)</th><th>TdMn<br>(nmol/L)</th><th>TdFe<br>(nmol/L)</th></tr></thead><tbody><tr><td colspan="15">MAYOBS01 cruise (May 2019), ongoing eruption</td></tr><tr><td>MAY01-<br>HY02</td><td>B01</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.908872</td><td>45.71182</td><td>2717.0</td><td>224.8</td><td>39.2</td><td></td><td>7.16</td><td>-0.5</td><td>2501</td><td>2304</td><td>174.3</td><td>166.6</td><td>19.5</td><td>89.2</td><td>1841.8</td></tr><tr><td>MAY01-<br>HY02</td><td>B02</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.908831</td><td>45.71198</td><td>2640.3</td><td>56.6</td><td></td><td>19.1</td><td>7.47</td><td>-0.2</td><td>2701</td><td>2488</td><td>361.1</td><td>158.8</td><td>13.3</td><td>37.5</td><td>606.8</td></tr><tr><td>MAY01-<br>HY02</td><td>B03</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.908838</td><td>45.71196</td><td>2589.8</td><td>25.7</td><td>876.2</td><td>63.7</td><td>7.51</td><td>-0.2</td><td>2527</td><td>2379</td><td>253.9</td><td>156.5</td><td>12.1</td><td>60.6</td><td>1500.7</td></tr><tr><td>MAY01-<br>HY02</td><td>B04</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90882</td><td>45.71198</td><td>2565.8</td><td>13.5</td><td></td><td></td><td>7.5</td><td>-0.2</td><td>2636</td><td>2423</td><td>298.8</td><td>154.3</td><td>10.5</td><td>23.2</td><td>306.1</td></tr><tr><td>MAY01-<br>HY02</td><td>B05</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.908835</td><td>45.71198</td><td>2490.4</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>160.6</td><td>18.4</td><td></td><td></td></tr><tr><td>MAY01-<br>HY02</td><td>B06</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90882</td><td>45.71198</td><td>2460.5</td><td>17.9</td><td>111.7</td><td>114.6</td><td>7.45</td><td>-0.3</td><td>2527</td><td>2359</td><td>238.6</td><td>153.6</td><td>12.0</td><td>19.3</td><td>287.0</td></tr><tr><td>MAY01-<br>HY02</td><td>B07</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90883</td><td>45.71198</td><td>2410.7</td><td>27.8</td><td>2.8</td><td>104.2</td><td>7.52</td><td>-0.2</td><td>2504</td><td>2323</td><td>204.4</td><td>151.0</td><td>10.6</td><td>21.2</td><td>316.6</td></tr><tr><td>MAY01-<br>HY02</td><td>B08</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90882</td><td>45.71198</td><td>2389.7</td><td>66.2</td><td>833.7</td><td></td><td>7.5</td><td>-0.2</td><td>2438</td><td>2291</td><td>173.1</td><td>155.8</td><td>15.8</td><td>60.0</td><td>647.3</td></tr><tr><td>MAY01-<br>HY02</td><td>B09</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.908822</td><td>45.71198</td><td>2181.0</td><td>0.9</td><td>3351.5</td><td>43.6</td><td>7.65</td><td>-0.1</td><td>2436</td><td>2245</td><td>134.7</td><td>140.8</td><td>5.4</td><td>4.2</td><td>37.0</td></tr><tr><td>MAY01-<br>HY02</td><td>B10</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90882</td><td>45.71198</td><td>1779.9</td><td>0.9</td><td>2287.8</td><td>60.6</td><td>7.63</td><td>-0.1</td><td>2435</td><td>2240</td><td>144.1</td><td>129.8</td><td>3.1</td><td>0.0</td><td>18.1</td></tr><tr><td>MAY01-<br>HY02</td><td>B13</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90882</td><td>45.71198</td><td>1378.9</td><td>0.9</td><td>298.5</td><td>48.7</td><td>7.59</td><td>-0.2</td><td>2432</td><td>2249</td><td>167.6</td><td>120.1</td><td>2.1</td><td>3.0</td><td>55.7</td></tr><tr><td>MAY01-<br>HY02</td><td>B14</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90882</td><td>45.71198</td><td>917.4</td><td>1.3</td><td>5992.9</td><td>36.9</td><td>7.62</td><td>-0.2</td><td>2439</td><td>2251</td><td>186.3</td><td>82.4</td><td>-25.6</td><td>2.2</td><td>22.5</td></tr><tr><td>MAY01-<br>HY02</td><td>B15</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.908805</td><td>45.712</td><td>485.3</td><td>1.9</td><td>1054.5</td><td>31.3</td><td>7.77</td><td>-0.1</td><td>2364</td><td>2143</td><td>93.9</td><td>13.3</td><td>-85.2</td><td>0.8</td><td>7.1</td></tr><tr><td>MAY01-<br>HY02</td><td>B16</td><td>Summit</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.9088</td><td>45.712</td><td>179.4</td><td>2.6</td><td></td><td>17.0</td><td>7.89</td><td>0.0</td><td>2378</td><td>2111</td><td>72.9</td><td>6.2</td><td>-85.7</td><td>0.7</td><td>8.2</td></tr><tr><td>MAY01-<br>HY03</td><td>B01</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90154</td><td>45.7149</td><td>3163.4</td><td>830.5</td><td>10.8</td><td>263.6</td><td>7.04</td><td>-0.6</td><td>2545</td><td>2434</td><td>288.2</td><td>162.7</td><td>5.8</td><td>560.2</td><td>10,578.1</td></tr><tr><td>MAY01-<br>HY03</td><td>B02</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90156</td><td>45.7149</td><td>3110.2</td><td>757.2</td><td>56.2</td><td>237.4</td><td>7.12</td><td>-0.5</td><td>2542</td><td>2397</td><td>253.1</td><td>164.2</td><td>8.5</td><td>312.2</td><td>8513.1</td></tr><tr><td>MAY01-<br>HY03</td><td>B03</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90156</td><td>45.71494</td><td>3071.3</td><td>766.7</td><td>56.3</td><td>264.7</td><td>7.13</td><td>-0.5</td><td>2529</td><td>2409</td><td>266.5</td><td>162.3</td><td>7.5</td><td>309.1</td><td>8150.7</td></tr><tr><td>MAY01-<br>HY03</td><td>B04</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90156</td><td>45.7149</td><td>3026.5</td><td>474.1</td><td>23.5</td><td>160.7</td><td>7.15</td><td>-0.5</td><td>2477</td><td>2349</td><td>208.2</td><td>162.1</td><td>8.3</td><td>288.7</td><td>7721.9</td></tr><tr><td>MAY01-<br>HY03</td><td>B05</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90158</td><td>45.71494</td><td>2976.1</td><td>260.6</td><td>88.5</td><td>154.4</td><td>7.16</td><td>-0.5</td><td>2467</td><td>2310</td><td>171.0</td><td></td><td></td><td>84.6</td><td>1775.0</td></tr><tr><td>MAY01-<br>HY03</td><td>B06</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90156</td><td>45.71494</td><td>2891.0</td><td>441.8</td><td>21.9</td><td>264.4</td><td>7.16</td><td>-0.5</td><td>2439</td><td>2324</td><td>188.1</td><td>160.7</td><td>9.8</td><td>142.1</td><td>3128.3</td></tr><tr><td>MAY01-<br>HY03</td><td>B07</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90152</td><td>45.7149</td><td>2840.3</td><td>430.6</td><td>13.7</td><td>258.4</td><td>7.14</td><td>-0.5</td><td>2495</td><td>2381</td><td>246.9</td><td>161.4</td><td>11.6</td><td>138.4</td><td>3136.4</td></tr><tr><td>MAY01-<br>HY03</td><td>B08</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90156</td><td>45.71492</td><td>2720.9</td><td>280.3</td><td>20.3</td><td>202.2</td><td>7.16</td><td>-0.5</td><td>2448</td><td>2324</td><td>194.2</td><td>160.5</td><td>13.3</td><td>120.5</td><td>2550.0</td></tr><tr><td>MAY01-<br>HY03</td><td>B09</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90156</td><td>45.71492</td><td>2640.9</td><td>290.8</td><td>58.2</td><td>220.5</td><td>7.16</td><td>-0.5</td><td>2444</td><td>2299</td><td>172.1</td><td>160.5</td><td>15.1</td><td>109.2</td><td>2495.6</td></tr><tr><td>MAY01-<br>HY03</td><td>B10</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019<br>16/05/</td><td>-12.90156</td><td>45.71492</td><td>2600.4</td><td>198.9</td><td>34.4</td><td></td><td>7.16</td><td>-0.5</td><td>2483</td><td>2301</td><td>175.5</td><td>157.4</td><td>12.8</td><td>85.5</td><td>1846.1</td></tr></tbody></table>

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<table><caption>Table 3 (continued)</caption><thead><tr><th>Station</th><th>Bottle</th><th>Site</th><th>Activity</th><th>Sampling date<br>(dd/mm/yyyy)</th><th>Latitude</th><th>Longitude</th><th>Depth<br>(m)</th><th>CH<sub>4</sub><br>(nmol/L)</th><th>H<sub>2</sub><br>(nmol/L)</th><th>CO<sub>2</sub><br>(µmol/L)</th><th>pH</th><th>ΔpH</th><th>A<sub>T</sub><br>(µmol/L)</th><th>ΣCO<sub>2</sub><br>(µmol/L)</th><th>Δ(ΣCO<sub>2</sub>)<br>(µmol/L)</th><th>SiO<sub>2</sub><br>(µmol/L)</th><th>ΔSiO<sub>2</sub><br>(µmol/L)</th><th>TdMn<br>(nmol/L)</th><th>TdFe<br>(nmol/L)</th></tr></thead><tbody><tr><td>MAY01-<br>HY03</td><td>B11</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019</td><td>-12.90154</td><td>45.71492</td><td>2430.7</td><td>2.2</td><td>212.7</td><td>50.0</td><td>7.57</td><td>-0.1</td><td>2429</td><td>2251</td><td>131.7</td><td>142.7</td><td>1.8</td><td>7.0</td><td>232.5</td></tr><tr><td>MAY01-<br>HY03</td><td>B12</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019</td><td>-12.90156</td><td>45.71492</td><td>2180.1</td><td>1.6</td><td>1.8</td><td>40.4</td><td>7.62</td><td>-0.1</td><td>2455</td><td>2275</td><td>164.7</td><td>132.3</td><td>-3.1</td><td>2.8</td><td>74.2</td></tr><tr><td>MAY01-<br>HY03</td><td>B13</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019</td><td>-12.90156</td><td>45.7149</td><td>1929.6</td><td>2.6</td><td>519.8</td><td>42.1</td><td>7.58</td><td>-0.2</td><td>2461</td><td>2278</td><td>176.7</td><td>128.9</td><td>-1.1</td><td>0.9</td><td>73.2</td></tr><tr><td>MAY01-<br>HY03</td><td>B14</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019</td><td>-12.90156</td><td>45.7149</td><td>1779.6</td><td>1.0</td><td>4.3</td><td>41.0</td><td>7.59</td><td>-0.2</td><td>2442</td><td>2283</td><td>187.2</td><td>124.5</td><td>-2.3</td><td>0.0</td><td>79.5</td></tr><tr><td>MAY01-<br>HY03</td><td>B15</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019</td><td>-12.90156</td><td>45.71492</td><td>1614.7</td><td>3.7</td><td>547.9</td><td>44.3</td><td>7.56</td><td>-0.2</td><td>2459</td><td>2237</td><td>147.1</td><td>120.4</td><td>-2.7</td><td>3.0</td><td>49.1</td></tr><tr><td>MAY01-<br>HY03</td><td>B16</td><td>Northern<br>flank</td><td>Eruption</td><td>16/05/<br>2019</td><td>-12.90162</td><td>45.71488</td><td>1074.0</td><td>1.0</td><td>277.3</td><td>37.7</td><td>7.6</td><td>-0.2</td><td>2410</td><td>2237</td><td>166.6</td><td>85.7</td><td>-25.7</td><td>3.3</td><td>56.9</td></tr><tr><td colspan="18">MAY0BS04 cruise (July 2019)</td></tr><tr><td>MAY04-<br>HY03</td><td>B01</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.06938</td><td>45.65382</td><td>3498.7</td><td>0.6</td><td>0.0</td><td>20.0</td><td>7.79</td><td>0.15</td><td>2459.9</td><td>2251.8</td><td>93.9</td><td>159.5</td><td>-4.6</td><td>4.44</td><td>115.63</td></tr><tr><td>MAY04-<br>HY03</td><td>B03</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.06942</td><td>45.65388</td><td>3147.1</td><td>1.1</td><td>0.0</td><td>20.8</td><td>7.78</td><td>0.11</td><td>2381.2</td><td>2320.9</td><td>175.8</td><td>156.5</td><td>0.1</td><td>3.33</td><td>24.04</td></tr><tr><td>MAY04-<br>HY03</td><td>B04</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.06962</td><td>45.65376</td><td>2998.7</td><td>0.6</td><td></td><td>21.2</td><td>7.77</td><td>0.09</td><td>2346.8</td><td>2183.6</td><td>43.8</td><td>156.0</td><td>2.8</td><td>6.15</td><td>46.29</td></tr><tr><td>MAY04-<br>HY03</td><td>B05</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.06964</td><td>45.65374</td><td>2916.2</td><td>1.0</td><td>0.0</td><td>23.3</td><td>7.76</td><td>0.08</td><td>2380.3</td><td>2256.8</td><td>119.9</td><td>156.4</td><td>4.9</td><td>4.24</td><td>32.20</td></tr><tr><td>MAY04-<br>HY03</td><td>B07</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.06972</td><td>45.65356</td><td>2498.4</td><td>0.5</td><td>0.0</td><td>21.7</td><td>7.77</td><td>0.06</td><td>2383.2</td><td>2211.7</td><td>89.9</td><td>143.2</td><td>0.9</td><td>1.71</td><td>9.62</td></tr><tr><td>MAY04-<br>HY03</td><td>B08</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.06978</td><td>45.65352</td><td>1996.5</td><td>0.8</td><td>0.0</td><td>22.5</td><td>7.76</td><td>0.01</td><td>2363.5</td><td>2225.7</td><td>122.0</td><td>129.7</td><td>-1.7</td><td>8.10</td><td>126.34</td></tr><tr><td>MAY04-<br>HY03</td><td>B09</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.0698</td><td>45.65354</td><td>1948.5</td><td>0.6</td><td>0.0</td><td>23.2</td><td>7.74</td><td>-0.01</td><td>2364.5</td><td>2196.6</td><td>94.7</td><td>128.0</td><td>-2.4</td><td>2.81</td><td>3.90</td></tr><tr><td>MAY04-<br>HY03</td><td>B10</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.06982</td><td>45.65348</td><td>1496.8</td><td>0.7</td><td>0.0</td><td>24.2</td><td>7.7</td><td>-0.08</td><td>2362.6</td><td>2201.6</td><td>116.0</td><td>118.1</td><td>-2.5</td><td>4.66</td><td>318.16</td></tr><tr><td>MAY04-<br>HY03</td><td>B12</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.06978</td><td>45.6535</td><td>1347.2</td><td>0.5</td><td>0.0</td><td>23.9</td><td>7.7</td><td>-0.09</td><td>2288.8</td><td>2015.2</td><td>-65.1</td><td>113.8</td><td>-3.5</td><td>2.50</td><td>9.98</td></tr><tr><td>MAY04-<br>HY03</td><td>B14</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.06982</td><td>45.65354</td><td>998.4</td><td>1.1</td><td>0.0</td><td>27.1</td><td>7.7</td><td>-0.11</td><td>2335.0</td><td>2202.6</td><td>135.0</td><td>92.6</td><td>-17.1</td><td>1.83</td><td>13.91</td></tr><tr><td>MAY04-<br>HY03</td><td>B16</td><td>Background</td><td>None</td><td>22/10/<br>2019</td><td>-13.0698</td><td>45.6535</td><td>497.5</td><td>0.9</td><td>0.0</td><td>18.6</td><td>7.86</td><td>0.01</td><td>2324.2</td><td>2106.4</td><td>56.8</td><td>16.2</td><td>-82.6</td><td>1.83</td><td>20.61</td></tr><tr><td>MAY04-<br>HY04</td><td>B01</td><td>Northern<br>flank</td><td>None</td><td>23/10/<br>2019</td><td>-12.90178</td><td>45.71518</td><td>3119.6</td><td>163.5</td><td>0.0</td><td>30.1</td><td>7.61</td><td>-0.06</td><td>2372.0</td><td>2257.9</td><td>113.7</td><td>157.9</td><td>2.1</td><td>110.61</td><td>2956.48</td></tr><tr><td>MAY04-<br>HY04</td><td>B03</td><td>Northern<br>flank</td><td>None</td><td>23/10/<br>2019</td><td>-12.90174</td><td>45.7152</td><td>3074.0</td><td>103.7</td><td>0.0</td><td>25.4</td><td>7.69</td><td>0.02</td><td>2360.0</td><td>2270.2</td><td>127.6</td><td>155.5</td><td>0.6</td><td>70.32</td><td>1729.92</td></tr><tr><td>MAY04-<br>HY04</td><td>B04</td><td>Northern<br>flank</td><td>None</td><td>23/10/<br>2019</td><td>-12.90174</td><td>45.7152</td><td>2997.4</td><td>101.6</td><td>0.0</td><td>26.0</td><td>7.64</td><td>-0.04</td><td>2451.0</td><td>2190.4</td><td>50.6</td><td>157.0</td><td>3.8</td><td>64.89</td><td>1599.10</td></tr><tr><td>MAY04-<br>HY04</td><td>B05</td><td>Northern<br>flank</td><td>None</td><td>23/10/<br>2019</td><td>-12.90172</td><td>45.71522</td><td>2949.5</td><td>146.9</td><td>0.0</td><td>30.2</td><td>7.57</td><td>-0.11</td><td>2438.0</td><td>2350.0</td><td>211.9</td><td>158.5</td><td>6.3</td><td>92.29</td><td>2484.55</td></tr><tr><td>MAY04-<br>HY04</td><td>B06</td><td>Northern<br>flank</td><td>None</td><td>23/10/<br>2019</td><td>-12.90176</td><td>45.71514</td><td>2798.8</td><td>130.2</td><td>0.0</td><td>32.2</td><td>7.55</td><td>-0.14</td><td>2394.0</td><td>2309.0</td><td>176.4</td><td>155.1</td><td>6.2</td><td>63.71</td><td>1248.33</td></tr><tr><td>MAY04-<br>HY04</td><td>B07</td><td>Northern<br>flank</td><td>None</td><td>23/10/<br>2019</td><td>-12.90174</td><td>45.71514</td><td>2748.2</td><td>92.0</td><td>0.0</td><td>31.5</td><td>7.57</td><td>-0.12</td><td>2445.0</td><td>2371.5</td><td>240.7</td><td>154.9</td><td>7.1</td><td>49.48</td><td>956.23</td></tr><tr><td>MAY04-<br>HY04</td><td>B09</td><td>Northern<br>flank</td><td>None</td><td>23/10/<br>2019</td><td>-12.90174</td><td>45.71516</td><td>2598.2</td><td>78.5</td><td>0.0</td><td>29.3</td><td>7.66</td><td>-0.04</td><td>2413.0</td><td>2269.1</td><td>143.8</td><td>151.2</td><td>6.7</td><td>35.96</td><td>635.77</td></tr></tbody></table>

(continued on next page)
<table><caption>Table 3 (continued)</caption><thead><tr><th>Station</th><th>Bottle</th><th>Site</th><th>Activity</th><th>Sampling date<br>(dd/mm/yyyy)</th><th>Latitude</th><th>Longitude</th><th>Depth<br>(m)</th><th>CH<sub>4</sub><br>(nmol/L)</th><th>H<sub>2</sub><br>(nmol/L)</th><th>CO<sub>2</sub><br>(µmol/L)</th><th>pH</th><th>ΔpH</th><th>A<sub>T</sub><br>(µmol/L)</th><th>ΣCO<sub>2</sub><br>(µmol/L)</th><th>Δ(ΣCO<sub>2</sub>)<br>(µmol/L)</th><th>SiO<sub>2</sub><br>(µmol/L)</th><th>ΔSiO<sub>2</sub><br>(µmol/L)</th><th>TdMn<br>(nmol/L)</th><th>TdFe<br>(nmol/L)</th></tr></thead><tbody><tr><td>MAY04-HY04</td><td>B10</td><td>Northern flank</td><td>None</td><td>23/10/2019</td><td>-12.90172</td><td>45.71518</td><td>2495.5</td><td>27.5</td><td>0.0</td><td>27.4</td><td>7.69</td><td>-0.02</td><td>2392.0</td><td>2265.1</td><td>143.4</td><td>147.2</td><td>4.9</td><td>19.76</td><td>311.75</td></tr><tr><td>MAY04-HY04</td><td>B11</td><td>Northern flank</td><td>None</td><td>23/10/2019</td><td>-12.90174</td><td>45.71516</td><td>2248.3</td><td>173.8</td><td>0.0</td><td>32.7</td><td>7.59</td><td>-0.14</td><td>2401.0</td><td>2242.5</td><td>129.8</td><td>145.3</td><td>8.4</td><td>58.83</td><td>1032.58</td></tr><tr><td>MAY04-HY04</td><td>B13</td><td>Northern flank</td><td>None</td><td>23/10/2019</td><td>-12.90174</td><td>45.71518</td><td>2121.7</td><td>1.9</td><td>0.0</td><td>30.4</td><td>7.76</td><td>0.02</td><td>2366.0</td><td>2203.7</td><td>95.5</td><td>133.6</td><td>-0.5</td><td>1.92</td><td>33.36</td></tr><tr><td>MAY04-HY04</td><td>B14</td><td>Northern flank</td><td>None</td><td>23/10/2019</td><td>-12.90174</td><td>45.71518</td><td>2047.9</td><td>45.1</td><td>1.3</td><td>26.4</td><td>7.73</td><td>-0.01</td><td>2388.0</td><td>2265.1</td><td>159.5</td><td>135.7</td><td>3.1</td><td>15.92</td><td>222.88</td></tr><tr><td>MAY04-HY04</td><td>B15</td><td>Northern flank</td><td>None</td><td>23/10/2019</td><td>-12.90174</td><td>45.71518</td><td>1499.0</td><td>0.8</td><td>0.0</td><td>27.8</td><td>7.73</td><td>-0.05</td><td>2370.0</td><td>2235.4</td><td>149.7</td><td>118.2</td><td>-2.4</td><td>5.53</td><td>25.26</td></tr><tr><td>MAY04-HY07</td><td>B03</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.91196</td><td>45.68738</td><td>2924.0</td><td>530.4</td><td></td><td>66.0</td><td>7.49</td><td>-0.19</td><td>2490.6</td><td>2290.7</td><td>153.5</td><td>163.7</td><td>12.1</td><td>153.81</td><td>2097.72</td></tr><tr><td>MAY04-HY07</td><td>B04</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.91196</td><td>45.68736</td><td>2848.4</td><td>543.1</td><td>29.4</td><td>94.9</td><td>7.47</td><td>-0.22</td><td>2453.5</td><td>2225.2</td><td>90.8</td><td>162.6</td><td>12.6</td><td>139.67</td><td>1870.03</td></tr><tr><td>MAY04-HY07</td><td>B05</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.91196</td><td>45.68738</td><td>2748.5</td><td>291.5</td><td>15.4</td><td>34.4</td><td>7.58</td><td>-0.11</td><td>2460.7</td><td>2238.7</td><td>107.9</td><td>155.2</td><td>7.4</td><td>83.09</td><td>1186.34</td></tr><tr><td>MAY04-HY07</td><td>B06</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.91196</td><td>45.68736</td><td>2746.8</td><td>518.0</td><td></td><td>66.7</td><td>7.46</td><td>-0.23</td><td>2491.6</td><td>2314.6</td><td>183.8</td><td>160.9</td><td>13.1</td><td>137.59</td><td>1856.08</td></tr><tr><td>MAY04-HY07</td><td>B08</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.91202</td><td>45.68736</td><td>2498.9</td><td>13.7</td><td>0.0</td><td>28.3</td><td>7.72</td><td>0.01</td><td>2428.9</td><td>2142.1</td><td>20.3</td><td>147.5</td><td>5.1</td><td>18.70</td><td>275.57</td></tr><tr><td>MAY04-HY07</td><td>B09</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.912</td><td>45.68736</td><td>2471.4</td><td>11.6</td><td>0.0</td><td>27.9</td><td>7.7</td><td>-0.01</td><td>2442.2</td><td>2235.6</td><td>114.8</td><td>146.8</td><td>5.0</td><td>21.91</td><td>334.12</td></tr><tr><td>MAY04-HY07</td><td>B10</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.912</td><td>45.68736</td><td>2313.6</td><td>112.4</td><td>0.0</td><td>31.7</td><td>7.66</td><td>-0.06</td><td>2429.9</td><td>2253.3</td><td>138.2</td><td>144.8</td><td>6.5</td><td>69.56</td><td>1295.62</td></tr><tr><td>MAY04-HY07</td><td>B12</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.912</td><td>45.68734</td><td>1997.6</td><td>1.1</td><td>0.0</td><td>25.9</td><td>7.76</td><td>0.01</td><td>2454.6</td><td>2267.8</td><td>164.1</td><td>128.4</td><td>-3.0</td><td>1.28</td><td>25.99</td></tr><tr><td>MAY04-HY07</td><td>B13</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.912</td><td>45.68732</td><td>1497.7</td><td>1.9</td><td>0.0</td><td>26.4</td><td>7.74</td><td>-0.04</td><td>2352.8</td><td>2169.1</td><td>83.4</td><td>117.5</td><td>-3.1</td><td>15.45</td><td>38.75</td></tr><tr><td>MAY04-HY07</td><td>B14</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.91202</td><td>45.68732</td><td>967.5</td><td>1.1</td><td>0.0</td><td>26.6</td><td>7.73</td><td>-0.09</td><td>2401.1</td><td>2247.0</td><td>180.5</td><td>82.7</td><td>-26.3</td><td>1.81</td><td>66.60</td></tr><tr><td>MAY04-HY07</td><td>B15</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.91196</td><td>45.6873</td><td>572.2</td><td>1.5</td><td>0.0</td><td>21.5</td><td>7.86</td><td>0.02</td><td>2343.5</td><td>2179.5</td><td>127.2</td><td>23.7</td><td>-76.8</td><td>38.92</td><td>723.30</td></tr><tr><td>MAY04-HY09</td><td>B01</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.9183</td><td>45.69218</td><td>3200.4</td><td>726.6</td><td>30.0</td><td>105.9</td><td>7.23</td><td>-0.43</td><td>2448.7</td><td>1875.2</td><td>-271.9</td><td>165.0</td><td>7.4</td><td>237.37</td><td>4244.69</td></tr><tr><td>MAY04-HY09</td><td>B03</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.9183</td><td>45.69218</td><td>3154.5</td><td>777.4</td><td>39.3</td><td>101.8</td><td>7.16</td><td>-0.51</td><td>2461.7</td><td>2155.5</td><td>10.0</td><td>167.0</td><td>10.4</td><td>268.03</td><td>4472.61</td></tr><tr><td>MAY04-HY09</td><td>B04</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.91828</td><td>45.69218</td><td>3112.7</td><td>648.6</td><td>3.7</td><td>50.9</td><td>7.56</td><td>-0.11</td><td>2434.8</td><td>2180.4</td><td>36.5</td><td>162.8</td><td>7.0</td><td>189.82</td><td>3682.72</td></tr><tr><td>MAY04-HY09</td><td>B05</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.9183</td><td>45.69218</td><td>2978.1</td><td>645.2</td><td>4.9</td><td>53.6</td><td>7.43</td><td>-0.25</td><td>2348.3</td><td>2127.6</td><td>-11.5</td><td>164.0</td><td>11.2</td><td>193.67</td><td>3222.76</td></tr><tr><td>MAY04-HY09</td><td>B06</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.91828</td><td>45.6922</td><td>2864.9</td><td>511.8</td><td>11.9</td><td>51.5</td><td>7.49</td><td>-0.20</td><td>2357.2</td><td>2218.3</td><td>83.3</td><td>162.5</td><td>12.2</td><td>144.78</td><td>2420.97</td></tr><tr><td>MAY04-HY09</td><td>B07</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.9183</td><td>45.69222</td><td>2700.4</td><td>218.3</td><td>1.8</td><td>44.6</td><td>7.54</td><td>-0.16</td><td>2369.2</td><td>2207.3</td><td>78.3</td><td>153.6</td><td>6.8</td><td>64.05</td><td>1608.02</td></tr><tr><td>MAY04-HY09</td><td>B09</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.9183</td><td>45.6922</td><td>2641.2</td><td>348.2</td><td>1.1</td><td>57.7</td><td>7.48</td><td>-0.22</td><td>2389.1</td><td>2288.1</td><td>161.2</td><td>154.9</td><td>9.4</td><td>99.87</td><td>2379.11</td></tr><tr><td>MAY04-HY09</td><td>B11</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.9183</td><td>45.6922</td><td>2478.0</td><td>9.5</td><td>0.0</td><td>34.5</td><td>7.7</td><td>-0.01</td><td>2372.2</td><td>2219.3</td><td>98.3</td><td>147.9</td><td>6.0</td><td>31.12</td><td>561.85</td></tr><tr><td>MAY04-HY09</td><td>B12</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.9183</td><td>45.6922</td><td>2348.1</td><td>90.7</td><td>0.0</td><td>40.0</td><td>7.68</td><td>-0.04</td><td>2324.4</td><td>2152.5</td><td>36.1</td><td>144.3</td><td>5.2</td><td>33.68</td><td>681.86</td></tr><tr><td>MAY04-HY09</td><td>B14</td><td>West lava flow</td><td>Recent</td><td>2019</td><td>-12.9183</td><td>45.69222</td><td>2199.9</td><td>1.1</td><td>0.0</td><td>25.0</td><td>7.8</td><td>0.07</td><td>2441.8</td><td>2236.3</td><td>125.3</td><td>134.5</td><td>-1.4</td><td>0.00</td><td>44.74</td></tr></tbody></table>

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<table><caption>Table 3 (continued)</caption><thead><tr><th>Station</th><th>Bottle</th><th>Site</th><th>Activity</th><th>Sampling date<br>(dd/mm/yyyy)</th><th>Latitude</th><th>Longitude</th><th>Depth<br>(m)</th><th>CH<sub>4</sub><br>(nmol/L)</th><th>H<sub>2</sub><br>(nmol/L)</th><th>CO<sub>2</sub><br>(µmol/L)</th><th>pH</th><th>ΔpH</th><th>A<sub>T</sub><br>(µmol/L)</th><th>ΣCO<sub>2</sub><br>(µmol/L)</th><th>Δ(ΣCO<sub>2</sub>)<br>(µmol/L)</th><th>SiO<sub>2</sub><br>(µmol/L)</th><th>ΔSiO<sub>2</sub><br>(µmol/L)</th><th>TdMn<br>(nmol/L)</th><th>TdFe<br>(nmol/L)</th></tr></thead><tbody><tr><td>MAY04-<br>HY09</td><td>B15</td><td>West lava<br>flow</td><td>Recent</td><td>29/07/<br>2019</td><td>-12.9183</td><td>45.69222</td><td>992.8</td><td>1.0</td><td>0.0</td><td>27.7</td><td>7.73</td><td>-0.09</td><td>2330.4</td><td>2179.4</td><td>112.0</td><td>90.9</td><td>-18.7</td><td>0.00</td><td>0.20</td></tr><tr><td>MAY04-<br>HY09</td><td>B16</td><td>West lava<br>flow</td><td>Recent</td><td>29/07/<br>2019</td><td>-12.91832</td><td>45.69222</td><td>460.4</td><td>1.9</td><td>0.0</td><td>18.8</td><td>7.91</td><td>0.06</td><td>2217.0</td><td>2009.9</td><td>-38.4</td><td>15.5</td><td>-82.6</td><td>0.00</td><td>27.57</td></tr><tr><td colspan="18">MAYOBS15 cruise (October 2020)</td></tr><tr><td>MAY15-<br>HY06</td><td>B01</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.8616</td><td>45.6771</td><td>3257.1</td><td>124.8</td><td>9.0</td><td>55.0</td><td>7.56</td><td>-0.1</td><td>2356.1</td><td>2083.8</td><td>-65.3</td><td>156.1</td><td>-2.8</td><td>78.71</td><td>1165.50</td></tr><tr><td>MAY15-<br>HY06</td><td>B02</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.8616</td><td>45.67712</td><td>3202.8</td><td>42.8</td><td>2.8</td><td>54.7</td><td>7.6</td><td>-0.1</td><td>2387.8</td><td>2136.2</td><td>-11.0</td><td>154.0</td><td>-3.7</td><td>45.30</td><td>614.48</td></tr><tr><td>MAY15-<br>HY06</td><td>B04</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.86162</td><td>45.6771</td><td>3098.8</td><td>78.5</td><td>13.0</td><td>56.2</td><td>7.58</td><td>-0.1</td><td>2366.7</td><td>2124.8</td><td>-18.7</td><td>155.4</td><td>-0.1</td><td>44.89</td><td>707.64</td></tr><tr><td>MAY15-<br>HY06</td><td>B06</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.8616</td><td>45.6771</td><td>3001.6</td><td>204.4</td><td>38.2</td><td>69.6</td><td>7.49</td><td>-0.2</td><td>2390.7</td><td>2152.4</td><td>12.4</td><td>158.1</td><td>4.8</td><td>76.89</td><td>880.66</td></tr><tr><td>MAY15-<br>HY06</td><td>B08</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.8616</td><td>45.6771</td><td>2893.6</td><td>95.1</td><td>7.1</td><td>61.9</td><td>7.54</td><td>-0.1</td><td>2377.2</td><td>2134.3</td><td>-1.7</td><td>153.8</td><td>2.8</td><td>58.75</td><td>657.89</td></tr><tr><td>MAY15-<br>HY06</td><td>B09</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.86162</td><td>45.6771</td><td>2796.8</td><td>43.5</td><td>0.0</td><td>58.5</td><td>7.59</td><td>-0.1</td><td>2386.8</td><td>2143.8</td><td>11.3</td><td>150.8</td><td>1.9</td><td>34.07</td><td>501.63</td></tr><tr><td>MAY15-<br>HY06</td><td>B10</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.8616</td><td>45.6771</td><td>2762.0</td><td>12.3</td><td>1.5</td><td>55.7</td><td>7.62</td><td>-0.1</td><td>2358.0</td><td>2153.3</td><td>22.0</td><td>146.9</td><td>-1.2</td><td>19.06</td><td>294.75</td></tr><tr><td>MAY15-<br>HY06</td><td>B11</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.86162</td><td>45.6771</td><td>2618.6</td><td>25.0</td><td>1.0</td><td>57.5</td><td>7.64</td><td>-0.1</td><td>2363.8</td><td>2141.9</td><td>15.8</td><td>147.5</td><td>2.5</td><td>24.05</td><td>409.64</td></tr><tr><td>MAY15-<br>HY06</td><td>B12</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.86164</td><td>45.6771</td><td>2542.8</td><td>21.3</td><td>1.7</td><td>57.7</td><td>7.63</td><td>-0.1</td><td>2361.9</td><td>2156.2</td><td>32.8</td><td>146.1</td><td>2.7</td><td>19.68</td><td>294.61</td></tr><tr><td>MAY15-<br>HY06</td><td>B13</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.8616</td><td>45.67712</td><td>2400.3</td><td>1.3</td><td>1.1</td><td>57.2</td><td>7.67</td><td>0.0</td><td>2357.1</td><td>2131.4</td><td>13.2</td><td>138.3</td><td>-1.9</td><td>5.11</td><td>115.23</td></tr><tr><td>MAY15-<br>HY06</td><td>B14</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.86158</td><td>45.67712</td><td>2302.7</td><td>0.7</td><td>0.0</td><td>57.6</td><td>7.65</td><td>-0.1</td><td>2363.8</td><td>2142.9</td><td>28.1</td><td>135.7</td><td>-2.4</td><td>6.14</td><td>145.05</td></tr><tr><td>MAY15-<br>HY06</td><td>B16</td><td>Northwest<br>lava flow</td><td>Very<br>recent</td><td>17/10/<br>2020</td><td>-12.86162</td><td>45.6771</td><td>1000.6</td><td>1.2</td><td>0.0</td><td>61.7</td><td>7.62</td><td>-0.2</td><td>2279.3</td><td>2053.4</td><td>-14.4</td><td>82.1</td><td>-27.7</td><td>0.80</td><td>160.72</td></tr><tr><td>MAY15-<br>HY08</td><td>B01</td><td>lava flow</td><td>Active<br>flow</td><td>21/10/<br>2020</td><td>-12.87088</td><td>45.6855</td><td>3259.2</td><td>283.6</td><td>171.3</td><td>46.9</td><td>7.58</td><td>-0.1</td><td>2451.6</td><td>2423.5</td><td>274.2</td><td>158.6</td><td>-0.3</td><td>117.26</td><td>1878.76</td></tr><tr><td>MAY15-<br>HY08</td><td>B02</td><td>Northwest<br>lava flow</td><td>Active<br>flow</td><td>21/10/<br>2020</td><td>-12.87088</td><td>45.6855</td><td>3202.2</td><td>5.0</td><td>12.4</td><td>38.8</td><td>7.61</td><td>-0.1</td><td>2393.4</td><td>2165.7</td><td>18.5</td><td>154.0</td><td>-3.7</td><td>10.91</td><td>194.94</td></tr><tr><td>MAY15-<br>HY08</td><td>B04</td><td>Northwest<br>lava flow</td><td>Active<br>flow</td><td>21/10/<br>2020</td><td>-12.87086</td><td>45.6855</td><td>3098.8</td><td>44.5</td><td>14.1</td><td>45.7</td><td>7.57</td><td>-0.1</td><td>2212.1</td><td>2202.1</td><td>58.7</td><td>156.2</td><td>0.8</td><td>49.67</td><td>600.08</td></tr><tr><td>MAY15-<br>HY08</td><td>B06</td><td>Northwest<br>lava flow</td><td>Active<br>flow</td><td>21/10/<br>2020</td><td>-12.87088</td><td>45.6855</td><td>2999.9</td><td>131.3</td><td>308.2</td><td>45.5</td><td>7.61</td><td>-0.1</td><td>2413.8</td><td>2166.7</td><td>26.8</td><td>157.8</td><td>4.5</td><td>55.13</td><td>673.76</td></tr><tr><td>MAY15-<br>HY08</td><td>B07</td><td>Northwest<br>lava flow</td><td>Active<br>flow</td><td>21/10/<br>2020</td><td>-12.87088</td><td>45.6855</td><td>2899.6</td><td>122.4</td><td>290.8</td><td>50.0</td><td>7.58</td><td>-0.1</td><td>2410.8</td><td>2203.1</td><td>66.8</td><td>157.0</td><td>5.9</td><td>61.99</td><td>682.35</td></tr><tr><td>MAY15-<br>HY08</td><td>B08</td><td>Northwest<br>lava flow</td><td>Active<br>flow</td><td>21/10/<br>2020</td><td>-12.87088</td><td>45.6855</td><td>2827.7</td><td>107.3</td><td>525.4</td><td>48.3</td><td>7.7</td><td>0.0</td><td>2418.6</td><td>2184.9</td><td>51.2</td><td>156.1</td><td>6.6</td><td>49.56</td><td>570.35</td></tr><tr><td>MAY15-<br>HY08</td><td>B09</td><td>Northwest<br>lava flow</td><td>Active<br>flow</td><td>21/10/<br>2020</td><td>-12.87088</td><td>45.6855</td><td>2751.6</td><td>110.6</td><td>537.9</td><td>46.9</td><td>7.63</td><td>-0.1</td><td>2404.1</td><td>2172.4</td><td>41.5</td><td>157.0</td><td>9.1</td><td>51.21</td><td>614.98</td></tr><tr><td>MAY15-<br>HY08</td><td>B11</td><td>Northwest<br>lava flow</td><td>Active<br>flow</td><td>21/10/<br>2020</td><td>-12.8709</td><td>45.68548</td><td>2699.0</td><td>140.6</td><td>647.6</td><td>47.1</td><td>7.67</td><td>0.0</td><td>2422.5</td><td>2173.4</td><td>44.3</td><td>158.2</td><td>11.5</td><td>54.13</td><td>666.90</td></tr><tr><td>MAY15-<br>HY08</td><td>B12</td><td>Northwest<br>lava flow</td><td>Active<br>flow</td><td>21/10/<br>2020</td><td>-12.8709</td><td>45.6855</td><td>2600.6</td><td>0.8</td><td>6.7</td><td>50.3</td><td>7.68</td><td>0.0</td><td>2393.4</td><td>2140.8</td><td>15.3</td><td>149.3</td><td>4.8</td><td>68.75</td><td>146.75</td></tr><tr><td>MAY15-<br>HY08</td><td>B13</td><td>Northwest<br>lava flow</td><td>Active<br>flow</td><td>21/10/<br>2020</td><td>-12.87088</td><td>45.6855</td><td>2499.2</td><td>2.3</td><td>6.8</td><td>49.2</td><td>7.66</td><td>-0.1</td><td>2410.8</td><td>2181.0</td><td>59.2</td><td>144.5</td><td>2.2</td><td>9.23</td><td>97.67</td></tr></tbody></table>

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<table><caption>Table 3 (continued)</caption><thead><tr><th>Station</th><th>Bottle</th><th>Site</th><th>Activity</th><th>Sampling date<br>(dd/mm/yyyy)</th><th>Latitude</th><th>Longitude</th><th>Depth<br>(m)</th><th>CH<sub>4</sub><br>(nmol/L)</th><th>H<sub>2</sub><br>(nmol/L)</th><th>CO<sub>2</sub><br>(µmol/L)</th><th>pH</th><th>ΔpH</th><th>A<sub>T</sub><br>(µmol/L)</th><th>ΣCO<sub>2</sub><br>(µmol/L)</th><th>Δ(ΣCO<sub>2</sub>)<br>(µmol/L)</th><th>SiO<sub>2</sub><br>(µmol/L)</th><th>ΔSiO<sub>2</sub><br>(µmol/L)</th><th>TdMn<br>(nmol/L)</th><th>TdFe<br>(nmol/L)</th></tr></thead><tbody><tr><td>MAY15-HY08</td><td>B14</td><td>Northwest lava flow</td><td>Active flow</td><td>21/10/2020</td><td>-12.87086</td><td>45.6855</td><td>2001.4</td><td>0.6</td><td>2.5</td><td>47.7</td><td>7.76</td><td>0.0</td><td>2326.5</td><td>2112.0</td><td>8.2</td><td>127.4</td><td>-4.1</td><td>3.21</td><td>43.54</td></tr><tr><td>MAY15-HY08</td><td>B16</td><td>Northwest lava flow</td><td>Active flow</td><td>21/10/2020</td><td>-12.87088</td><td>45.6855</td><td>987.7</td><td>1.2</td><td>1.8</td><td>53.2</td><td>7.71</td><td>-0.1</td><td>2334.3</td><td>2124.5</td><td>57.2</td><td>83.5</td><td>-26.0</td><td></td><td></td></tr><tr><td>MAY15-HY09</td><td>B01</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.08746</td><td>46.0565</td><td>3507.0</td><td>0.6</td><td>0.0</td><td>44.4</td><td>7.61</td><td>0.0</td><td>2376.2</td><td>2179.9</td><td>21.7</td><td>156.8</td><td>-7.5</td><td>0.00</td><td>17.45</td></tr><tr><td>MAY15-HY09</td><td>B03</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.08748</td><td>46.05648</td><td>3299.4</td><td>0.4</td><td>0.0</td><td>45.4</td><td>7.64</td><td>0.0</td><td>2417.1</td><td>2154.6</td><td>3.9</td><td>157.8</td><td>-2.0</td><td>4.51</td><td>43.77</td></tr><tr><td>MAY15-HY09</td><td>B04</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.08746</td><td>46.0565</td><td>2992.2</td><td>0.4</td><td>0.0</td><td>46.6</td><td>7.64</td><td>0.0</td><td>2399.6</td><td>2079.8</td><td>-59.8</td><td>155.9</td><td>2.8</td><td>5.98</td><td>130.66</td></tr><tr><td>MAY15-HY09</td><td>B06</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.08748</td><td>46.05648</td><td>2901.9</td><td>0.2</td><td>0.0</td><td>47.7</td><td>7.65</td><td>0.0</td><td>2394.7</td><td>2145.9</td><td>9.5</td><td>152.3</td><td>1.1</td><td>5.18</td><td>62.92</td></tr><tr><td>MAY15-HY09</td><td>B07</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.08748</td><td>46.0565</td><td>2797.1</td><td>0.2</td><td>0.0</td><td>50.6</td><td>7.66</td><td>0.0</td><td>2408.3</td><td>2153.7</td><td>21.1</td><td>153.2</td><td>4.3</td><td>22.98</td><td>108.60</td></tr><tr><td>MAY15-HY09</td><td>B08</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.08746</td><td>46.0565</td><td>2587.5</td><td>0.2</td><td>0.0</td><td>47.3</td><td>7.68</td><td>0.0</td><td>2443.4</td><td>2224.6</td><td>99.6</td><td>149.7</td><td>5.4</td><td>5.99</td><td>119.07</td></tr><tr><td>MAY15-HY09</td><td>B09</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.0875</td><td>46.0565</td><td>2294.4</td><td>0.1</td><td>0.0</td><td>49.7</td><td>7.67</td><td>-0.1</td><td>2407.4</td><td>2155.6</td><td>41.2</td><td>141.1</td><td>3.2</td><td>1.74</td><td>15.64</td></tr><tr><td>MAY15-HY09</td><td>B11</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.08748</td><td>46.05648</td><td>1585.5</td><td>0.7</td><td>0.0</td><td>53.9</td><td>7.65</td><td>-0.1</td><td>2363.5</td><td>2171.2</td><td>82.3</td><td>119.2</td><td>-3.3</td><td>0.22</td><td>14.08</td></tr><tr><td>MAY15-HY09</td><td>B12</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.0875</td><td>46.05648</td><td>1000.1</td><td>1.4</td><td>0.0</td><td>56.6</td><td>7.64</td><td>-0.2</td><td>2352.8</td><td>2138.1</td><td>70.4</td><td>86.6</td><td>-23.1</td><td>15.86</td><td>188.82</td></tr><tr><td>MAY15-HY09</td><td>B14</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.0875</td><td>46.0565</td><td>498.9</td><td>1.2</td><td>1.1</td><td>52.8</td><td>7.84</td><td>0.0</td><td>2313.9</td><td>2050.6</td><td>1.0</td><td>13.2</td><td>-85.7</td><td>7.34</td><td>50.05</td></tr><tr><td>MAY15-HY09</td><td>B15</td><td>Background</td><td>None</td><td>2020/22/10/</td><td>-12.0875</td><td>46.0565</td><td>94.8</td><td>2.2</td><td>0.0</td><td>17.0</td><td>8.14</td><td>0.3</td><td>2310.0</td><td>1978.7</td><td>-56.3</td><td>1.5</td><td>-88.6</td><td>9.84</td><td>204.35</td></tr></tbody></table>
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Depth profiles of dissolved gases measured above recent (in green) and very recent lava flow (in purple). MAY04-HY07 (from July 2019) and MAY04-HY09 (from July 2019) casts were performed above the recent Western lava flow while MAY15-HY06 (from October 2020) was sampled above the very recent lava flow from and MAY15-HY08 (from October 2020) above the incandescent lava flow both from the North West site. (A₁) H₂ profiles; (A₂) Zoom in H₂ profiles for H₂ concentrations from 0 to 60 nmol/L; (B) CO₂ profiles; (C) CH₄ profiles. Evaluated background is represented by the bold line and the grey envelop. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

northwest lava flow (cast MAY15-HY08) also exhibit a ratio of ~11, with the deepest sample (3260 m) that highlight a different pattern with a ratio of ~16.

# 5. Discussion

## 5.1. Rapid shift in magmatic fluid contribution

Fluid emissions at the seafloor are a source of helium to the water column, being either primordial (³He) or radiogenic (⁴He). Mantle and crustal contributions of these fluids are defined by the corrected isotopic helium ratio relative to the atmospheric ratio (Rc/Ra). Mantle MORB-like fluids are usually described by $R_c = 8.0 \pm 1.5$ Ra and $⁴He/Ne = 1000$ (Sano and Fischer, 2013), while fluids originating from the crust present $R_c$ values of 0.02 Ra and $⁴He/Ne$ values of 1000 (Wang et al., 2020). Air saturated seawater (ASW) is defined by $R_c = 1$ Ra and $⁴He/Ne = 0.226$ (Hilton, 1996; Weiss, 1971). Therefore, a corrected ratio (Rc/Ra) close to 8 in seawater would imply that He source is dominated by a mantle-derived contribution with negligible assimilation of crust-

derived materials which may occur during magma aging and/or interactions with ⁴He-rich crustal rock, also referred as country rock (Hilton et al., 1993).

During the main eruptive event in May 2019, the magmatic contribution to the water column remains unknown as no helium isotope measurement were made at this time. However, a recent geochemical study of volcanic rocks from the volcano shows no evidence for crustal material assimilation, which could be explained by a direct and fast ascent of magma from the deep reservoir to the surface (Berthod et al., 2021). We however observe a change in magmatic fluid contribution (Fig. 8) between seawater samples taken above the recent western lava flow in July 2019 (casts MAY04-HY04 and MAY04-HY07) and those taken above the northwest active and very recent lava flow from October 2020 (casts MAY15-HY06 and MAY15-HY08). Extrapolating $R_c$ values in our samples using the linear regressions in Fig. 8A, down to a null $Ne/He$ ratio, we approximate $R_{c,fluid}$ values. Such extrapolated $R_{c,fluid}$ values of 6.46 Ra in July 2019 suggest a higher crustal (i.e. radiogenic) contribution compared to October 2020 (extrapolated $R_{c,fluid} = 8.52$ Ra). Evidence for significant crustal material assimilation in the magmatic
<caption>**Table 4**</caption>

Concentrations of CH₄ and CO₂ and their isotopic carbon composition, δ¹³C-CH₄ and δ¹³C-CO₂, in seawater samples taken above recent, very recent and active lava flows from July 2019 and October 2020 cruises.

<table><thead><tr><th>Sample ID</th><th>Depth<br>(m)</th><th>CH<sub>4</sub><br>(nmol/L)</th><th>CO<sub>2</sub><br>(µmol/L)</th><th>δ<sup>13</sup>C-CH<sub>4</sub><br>(‰, vs. vPDB)</th><th>δ<sup>13</sup>C-CO<sub>2</sub><br>(‰, vs. vPDB)</th></tr></thead><tbody><tr><td colspan="6">MAY4-HY07 (July 2019, Western recent lava flow)</td></tr><tr><td>MAY4-HY07-B3</td><td>2924.0</td><td>530.4</td><td>66.0</td><td>-34.8</td><td>na</td></tr><tr><td>MAY4-HY07-B6</td><td>2746.8</td><td>518.0</td><td>66.7</td><td>na</td><td>-0.6</td></tr><tr><td colspan="6">MAY15-HY06 (Oct. 2020, Northwest very recent lava flow)</td></tr><tr><td>MAY15-HY06-B1</td><td>3257.1</td><td>124.8</td><td>55.0</td><td>na</td><td>-0.5</td></tr><tr><td>MAY15-HY06-B2</td><td>3202.8</td><td>42.8</td><td>54.7</td><td>na</td><td>-1.1</td></tr><tr><td colspan="6">MAY15-HY08 (Oct. 2020, Northwest active lava flow)</td></tr><tr><td>MAY15-HY08-B1</td><td>3259.2</td><td>283.6</td><td>46.9</td><td>-34</td><td>-1.4</td></tr><tr><td>MAY15-HY08-B4</td><td>3098.8</td><td>44.5</td><td>45.7</td><td>na</td><td>-1.3</td></tr><tr><td>MAY15-HY08-B7</td><td>2899.6</td><td>122.4</td><td>50.0</td><td>na</td><td>-1.3</td></tr></tbody></table>

na: not analyzed (due to low gas concentrations). δ¹³C-CO₂ values are corrected for carbon isotope fractionation between CO₂(g) and CO₂(aq), at 25 °C and seawater salinity, following Eq. 4 from Assayag et al. (2006) and using fractionation factors from Zhang et al. (1995) and CO₂ solubilities from Weiss (1974).

source in July 2019 is supported by petrogeochemical studies that have also shown a more crustal signature in those lavas. Authors explain this signature by the sampling of an evolved magma batch (i.e. that has assimilated crustal material during its aging), located on the magma path, during the magma ascent from the main reservoir to the seafloor (Berthod et al., 2021). In October 2020, the return to less radiogenic (more primordial) helium signatures might be linked either to (i) a new magma path with direct ascent from the main reservoir to the seafloor, as seen in May 2019, or (ii) the passage of the magma through the same conduit as in July 2019 but with the secondary magma reservoir empty or newly recharged, and therefore no crustal assimilation (Berthod et al., 2022).

The measured helium water column composition yield the same range of Rc/Ra as Mayotte gaseous emissions in Petite-Terre (Rc/Ra = 6.4–7.5; (Liuzzo et al., 2021)) and Indian Ridge hydrothermal fluids (Rc/Ra = 7.8–8.0; (Gamo et al., 2001; Kawagucci et al., 2008)). Assuming that dissolved gas sources from the volcano exhibit a mantle MORB-like signature of Rc = 8.0 ± 1.5 Ra (Sano and Fischer, 2013), we estimate from the isotopic helium ratios that the level of crustal contamination could be between 20–35% in July 2019 (Fig. 8). However, this interpretation does not take into account the possible influence of the La Réunion hotspot where magma sources present Rc/Ra with a range from 12 to 14.5 (Boudoire et al., 2020; Marty et al., 1993). At this stage, as it was suggested by the recent studies (Famin et al., 2020; Lemoine et al., 2020; Michon, 2016; Tzevahirtzian et al., 2021), we hypothesize that there is no influence of this hotspot on Mayotte sub-marine eruption.

## 5.2. Origin of contrasted behavior of gases

Using seafloor observations (SCAMPI pictures, Table 1) and turbidity signals (Fig. 2), we interpret the geochemical anomalies in the water column according to two parameters: the freshness of lava, i.e., indi-cating the timing between lava flow emplacement and water column sampling; and the potential seawater-rock-lava interaction and their associated fluid emissions.

### 5.2.1. Ongoing eruption and active lava flow

#### 5.2.1.1. Elevated H₂ levels.
Exceptionally high H₂ concentrations were measured during the eruption (inside the acoustic plume, in May 2019) and above active lava flow (in October 2020), up to 5993 nmol/L and 648 nmol/L respectively.

Hydrogen is usually described as an immediate tracer of the eruptive activity and provides strong evidence for on-going or very recent

reactions between seawater or magmatic water and molten or extremely hot rocks (Baker et al., 2011; Baumberger et al., 2020; Baumberger et al., 2014; Resing et al., 2011). Indeed, in marine environments, H₂ is produced from various sources, involving abiotic and biotic processes (Wang et al., 2023; Worman et al., 2020). In volcanic environments, with large magma inputs such as fast-spreading mid-ocean ridges or intraplate or hotspot volcanoes, hydrogen may originate from magmatic degassing as a result of magma crystallization, through the oxidation of ferrous iron contained within the magma by reduction of dissolved water (Hekinian et al., 1973; Klein et al., 2020; Symonds et al., 1994). H₂ may also be produced through the interaction between molten lava and seawater (Sansone and Resing, 1995), and more generally through the reduction of water during the oxidation of Fe(II)-bearing rocks (Truche et al., 2020). Indeed, some of the H₂ we observe might come from high temperature basalt alteration. During the high-temperature (~350 to 400 °C) alteration of oceanic crust by seawater, the majority of ferrous silicates alter to ferrous iron minerals (e.g., chlorite, amphibole), however, a small number, alter to ferric-bearing minerals and produce H₂. Other major sources of hydrogen in the ocean include the radiolysis of water due to radioactive decay of U, Th and K; crustal weathering or reaction of water with surface radicals during fracturing of silicate-bearing rocks (Klein et al. 202, Wang et al., 2023), all occurring in very distinct geological settings than ours. Decomposition of organic matter in sediment-hosted systems has also been reported, but is very unlikely as sediment both in the lagoon and offshore Mayotte are mainly carbonateous with very low organic carbon contents (Dullo et al., 1998; Zinke et al., 2003b).

Production of hydrogen through serpentinization of ultramafic materials (i.e. hydration reactions of olivine and pyroxene, mostly) is very common in submarine settings. This mechanism is generally considered to be the main H₂ production route during hydrothermal circulation at slow and ultra-slow spreading ridges (e.g. Charlou et al. (2010)). The geological context here is of course very distinct and we did not find evidence, neither from dredge operations nor from direct seafloor observations, that would support exhumation of ultramafic materials. Although we cannot exclude a mechanism in which seawater is interacting at greater depth with ultramafic rocks, it should be noted that H₂ generation through serpentinization likely requires a mature hydrothermal system, which seems to be in contradiction with the young age (< 1 years) and the transient nature of the H₂ anomalies observed in the water column.

We therefore propose that H₂ is produced both from magmatic degassing and from water reduction by iron-bearing compounds in the extruding high temperature lava during its interaction with seawater, following the reaction (Baker et al., 2011; Perfit et al., 2003; Worman et al., 2020): 2(FeO)ₘagma + (H₂O)ₘ seawater → (Fe₂O₃)ₘ rock + H₂. Following
<figure><img src="image_10.png" /><figcaption>Fig. 6. Depth profiles of (A) pH; (B) Total alkalinity; (C) Total CO₂; (D₁) SiO₂ and (D₂) ΔSiO₂ that is the change in SiO₂ from the regional background. Seawater samples from all three cruises from May 2019 (MAYOBS1 in orange), July 2019 (MAYOBS4 in green) and October 2020 (MAYOBS15 in purple) are plotted. Background is represented by the bold line and the grey envelop, determined from two background hydrocasts. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

its production, H₂ is likely to be consumed by oxidation through abiotic reactions or microbial processes, usually within the timespan of a few hours to a few days (McLaughlin-West et al., 1999; Worman et al., 2020).

Clague et al. (2009) documented submarine eruptions throughout the Pacific Ocean at depths from 1400 to 3800 m, and reported that an almost unfailing characteristic of submarine eruptions is the production of glassy pyroclastic fragments that are remnants of bubbles of magmatic gas. Also, Baker et al. (2011) showed that the production of pyroclastic fragments promotes extensive lava-seawater interaction during the eruption. At Mayotte submarine volcano, very fresh basanitic pillow lavas were dredged on the northeastern flank of the edifice and contain significant amounts of vesicle-trapped volatiles (Berthod et al., 2021; Feuillet et al., 2021). Such vesicular and gas-rich rocks, referred as ‘popping rocks’, may therefore provide larger effective surface area for seawater-lava interactions.

Additionally to the gas-rich characteristic of the rocks, the extremely high concentrations of H₂ probably testify for the explosive character of the Mayotte submarine eruption, as previously proposed by Baumberger et al. (2014) during the West Mata eruption and by Baumberger et al. (2020) during the Puipui eruption. Submarine eruptions may generate massive megaplumes, of several kilometers height, as a result of extremely high rates of energy discharge, including massive heat and magmatic and/or crustal fluids transfer (Baker et al., 2012; Pegler and Ferguson, 2021). Large fluxes of H₂ may have therefore been discharged into the water column as a result of a gas jet formation.

5.2.1.2. *Elevated CO₂ and CH₄ levels.* During the eruption, elevated

concentrations of CO₂ and CH₄ were released, up to 265 µmol/L and 821 nmol/L, respectively. This positive correlation between CO₂ and CH₄ values might suggest a magmatic origin for both these gases. Carbon dioxide is most likely sourced from lava outgassing (Craig and Lupton, 1981), while methane may be present as a magma volatile or could reflect a production through Fischer-Tropsch-type reactions (CO₂ + 4H₂ ⇌ CH₄ + 2H₂O) associated with water-rock interactions (Craig, 1953; McCollom, 2013; Zolotov and Shock, 2000).

The carbon dioxide is one of the main volatiles dissolved in magma along with H₂O and SO₂. During the magma ascent, gas bubbles can segregate from the melt at different depths, so the volatile components fractionate according to their solubilities. The composition of the exsolved fluids will thus change as a function of the pressure-related solubility of each volatile species. Being the CO₂ less soluble in the melt than water, it will be exsolved at deeper levels (Métrich and Wal-lace, 2008). Because the exsolved fluids are less dense than the melt they ascent faster to the surface, leading to fluid emissions with a very high concentration in a specific volatile, that may be varying in time and space. Thus, CO₂ may be generated through a separate CO₂-rich gas phase directly degassing from the magma chamber, as it was observed at NW Rota-1, where CO₂-rich gas bubbles exsolve from slowly erupting lava (Chadwick et al., 2008). When the CO₂-rich gas phase encounters circulating seawater during its ascent through the volcano edifice, a two-phase mixture of CO₂-rich gas and an aqueous phase undersaturated with CO₂ can coexist as buoyant gas bubbles ascend through and exchange with the aqueous phase (Lupton et al., 2008). The behavior of CO₂ released into seawater will be governed primarily by the pressure (i. e., depth) and temperature of the water entity into which it is released,
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Depth profiles of (A) Total Dissolvable Mn (TDMn) and (B) Total Dissolvable Fe (TDFe). (C) Plots of TDFe vs. TDMn. All hydrocasts performed during the three cruises are represented: May 2019 (MAYOBS1 in orange), July 2019 (MAYOBS4 in green), and October 2020 (MAYOBS15 in purple). (C₁) shows the entire data set; (C₂) focusses on smaller levels of TDFe and TDMn. Ratios of TDFe/TDMn evolve between 11 and 27 (in mol/mol). TDFe/TDMn ratios were determined for each cast: MAY01-HY02 (17.8, r² = 0.94); MAY01-HY03 (22.5, r² = 0.97); MAY04-HY04 (23.9, r² = 0.98); MAY04-HY07 (13.9, r² = 0.99); MAY04-HY09 (17.8, r² = 0.99); MAY15-HY06 (13.4, r² = 0.97); MAY15-HY08 (12.2, r² = 0.89). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

and to a lesser extent its salinity. Taking into account the salinity and temperature profiles of seawater obtained in the area with our CTD casts, CO₂ will become liquid at pressures of 45–50 bar, corresponding to water depths of ~450–500 m (Fig. 9). Above ~2700 m depth, density of liquid CO₂ is lower than the seawater density. When released into the water column at this depth or shallower, CO₂ will hence rise towards the surface. On the contrary, the CO₂ emitted below ~2700 m depth will likely sink to the ocean floor. This threshold of 2700 m corresponding to the summit depth of the Fani Maore volcano, we suggest that a significant amount of CO₂ emitted during the eruption sank and generated the high concentrations we observe at the bottom and the flanks of the volcano (Fig. 9).

Eruptive megaplumes may involve the rapid evacuation of existing intracrustal fluid reservoirs (Baker et al., 2012; Lupton et al., 1999; Pegler and Ferguson, 2021). Such syn-eruptive release of mature hydrothermal fluids enriched in methane are usually observed in extensional tectonic settings, which promotes seafloor hydrothermal circulation and may be significantly enhanced by dyke intrusions (Curewitz and Karson, 1998; Delaney et al., 1998; Pegler and Ferguson, 2021). However, we did not find any signs of pre-eruptive hydrothermal active venting on the seafloor images we collected (no vent fauna, no sulfide deposits, no smokers, only surficial oxy-hydroxydes iron deposits on some of the freshly extruded lava flows). Although we cannot rule it out, it seems therefore unlikely that the concurrent release of significant crustal fluids as the generation process for methane was solely responsible for the elevated anomalies we observed during the eruption. Given the low organic matter content of the sediments in the area (Manoux

et al., 2023), thermogenic cracking of organic matter during the magma ascent might not act as a significant source of methane released during the eruption.

5.2.1.3. A common pathway invoked for the formation of hydrocarbons and other organic compounds in geologic environments is the Fischer-Tropsch synthesis (McCollom, 2013). As originally described, Fischer-Tropsch type chemical reactions refer to the surface-catalyzed reduction of CO by H₂ in gas mixtures. In geological systems, dissolved CO₂ may be considered the primary carbon source for such abiotic organic synthesis (McCollom, 2013; Zolotov and Shock, 2000). Yet, despite a thermodynamic thrive for CO₂ reduction into CH₄ in conditions of hydrothermal circulation, it is now recognized that the process is likely slow and inhibited by strong kinetic barriers. For the process to overpass these kinetic barriers and produce significant amount of reduced carbon compounds such as methane (McCollom, 2016; McDermott et al., 2015), it may requires the presence of metal catalysts or of a CO-existing H₂-rich gas phase (McCollom, 2016). Although speculative at this stage, we note that one or both of these conditions could be met in the case of the Fani Maoré eruption. First, the presence of large TDFe anomalies in the water-column supports sustained interactions of seawater with Fe-bearing phases that could act as catalyst for a Fischer-Tropsch type reaction. On the other hand, given the depth of Fani Maoré eruption, liquid and gaseous phases probably coexisted. Taken together, we thus suggest that Fischer-Tropsch type reaction is a potential mechanism for the formation of methane observed in the water column. Why such a dichotomy between H₂ versus CO₂, CH₄ profiles at the edifice?. During the
<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Rc/Ra ratio versus Ne/He (A) and He/Ne (B) for seawater samples from July 2019 (MAYOBS4 in green) and October 2020 (MAYOBS15 in purple). ASW dot is the Air Saturated Water value. (A) The blue arrow shows helium enrichment due to air contamination, the green arrow would represent the addition of radiogenic $^{4}$He, the gold arrow the addition of tritiogenic $^{3}$He (from tritium radioactive decay). Our seawater samples are mainly driven by the addition of magmatic $^{3}$He. (B) ASW, mantle and crust are isotopic ratios for corresponding end-members. ASW: Rc/Ra = 1, $^{4}$He/Ne = 0.226 (Hilton, 1996; Weiss, 1971); upper mantle (MORB-like): Rc/Ra = 8 ± 1.5, $^{4}$He/Ne = 1000 (Sano and Fischer, 2013); crust: Rc/Ra = 0.02, $^{4}$He/Ne = 1000 (Wang et al., 2020). Dashed plots are the calculated binary mixing curves between the different endmembers. The uncertainty on Rc/Ra and He/Ne measurements is lower the size of the symbols. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

eruption, concentration profiles of CH₄, CO₂ and H₂ were studied at two locations in the vicinity of the volcano: at the summit and above the northern flank (Fig. 4). From the water surface down to 2500 m depth, CH₄ and CO₂ concentrations are similar to those found in the local background. Below 2500 m, concentrations of both CH₄ and CO₂ increase, with maximum values at the seafloor. However, H₂ displays the opposite trend; very high anomalies (up to 6 µmol/L) are seen at depths above 2500 m while H₂ concentrations oscillate between 11 and 56 nmol/L below 2500 m. To our knowledge, such dichotomy in gas behavior between H₂ on one hand and CO₂ and CH₄ on the other hand, has never been reported before in a context of deep submarine eruption. A plausible explanation is related both to different processes of gas generation and to the change in density equilibrium between liquid CO₂ and seawater around 2700 m depth (Fig. 9):

- On one hand, H₂ is generated as a free gas during lava quenching, i.e. at the exit point of the magma at the summit (2600–2700 m). H₂ dissolves in seawater and rises the water column, probably encouraged by the formation of a gas jet (Cahalan and Dufek, 2021). We observe no anomalies in potential temperature, potential densities (Supplementary Fig. 2) nor turbidity anomalies (Fig. 2) between 2500 and 1300 m although we do see some high dissolved H₂ concentrations randomly dispersed. Such features suggest that we did not capture the full tridimensional structure of the eruptive plume at these depths on this one cast (Pegler and Ferguson, 2021). The lower H₂ concentrations measured between 2500 and 1300 m compared to the very elevated one at 1000 m may therefore simply result from the CTD-rosette emplacement outside the plume due to local currents and the turbulent 3D plume behavior.

- On the other hand, CO₂ is discharged through magma degassing, either from the exit point at the summit, from lavas flowing down the volcano flanks, or from fractures on the volcano flanks. Since the seafloor around the Fani Maoré new volcanic edifice is around 3400–3600 m depth and the top of the volcano is around 2600–2700 m, CO₂ emitted will likely sink to the ocean floor instead of rising up to the surface. At the same time, we expect that H₂ is generated by

lava-seawater interactions from the lavas flowing down the volcano flanks. As its concentration does not exceed 56 nmol/L below 2500 m, we suggest that H₂ is consumed by the Fischer-Tropsch-type chemical reaction to generate CH₄ through the reduction of CO₂ (1 mol of dissolved CO₂ react with 4 mol of H₂). This reaction seems the most appropriate to explain why H₂ is lower at the bottom of the water column than at shallower depths. However, in this study, we are not able to estimate the amount of methane generated through this reaction. We discard microbial consumption of hydrogen as a significant process as the rates at stake and their distribution in seawater cannot explain the discrepancy of hydrogen concentration over depth (Lappan et al., 2023).

Therefore, we suggest that the dichotomy in the generation processes of the various gases and the very specific settings of the Fani Maore volcano create a unique environment. At this stage of the eruption, past the initial eruptive outburst, lava are being emitted effusively from the summit down to the flanks (no explosive activity registered as turbidity anomalies are limited to 2500 m; Fig. 2): CH₄ will likely be associated with the liquid CO₂ released by the volcano whereas H₂ will be released as a free gas in the water column. This would explain the high concentrations in methane found below 2500 m, while H₂ tends to stay in the gas phase and rises to the surface giving place to the H₂ anomalies aforementioned.

### 5.2.2. Continued lava flow

Above the northern flank, two months after the eruption, CO₂ and H₂ concentrations are back to background levels while CH₄ concentrations remains elevated (100–200 nmol/L, Fig. 5). Water column above the recent western lava flow (MAY04-HY07, MAY04-HY09) exhibit CH₄ anomalies up to 777 nmol/L (Fig. 5), which is more than twice the concentrations found above the very recent and active northwest lava flow for which maximum concentration was 284 nmol/L (Table 1).

In submarine volcanic systems, CH₄ is generally produced through hydrothermal circulation that is initiated after an eruptive event. Buck et al. (2018) have compiled maximum CH₄ concentrations measured in
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Composite representation of gas concentration profiles, water column physical parameters and bathymetry of the Fani Maoré Seamount during the May 2019 eruption (MAYOBS1 cruise). Profiles of H₂, CO₂ and CH₄ were measured at the summit and above the northern flank of the volcano. Pure-CO₂ density changes with the water column depth at the volcano was calculated using Span and Wagner's equation of state (Span and Wagner, 1996). Standard-seawater density profile was calculated from CTD data recorded at both sites and Archer's thermodynamic model for NaCl (Archer, 1992). Temperature profile was extracted from CTD data recorded at both sites. 3D representation of the volcano edifice was made using 30 m-resolution bathymetry (vertical exaggeration x2), with counter lines at 50 m intervals (Geo-Ocean, Ifremer, 2022).</figcaption></figure>

hydrothermal plumes found above Ahyi, NW Rota, West Mata, Nikko, Daikoku and Kasuga-2 and concentrations do not exceed 22 nmol/L (Baumberger et al., 2014; Buck et al., 2018; Resing et al., 2009; Resing et al., 2011). The low H₂ levels (i.e. only up to 40 nmol/L) measured above the lava flows compared to those measured right during the ongoing eruption may be due to the weakening of the lava-seawater interaction processes and to the rapid oxidation and consumption of H₂ produced through these processes (McLaughlin-West et al., 1999; Worman et al., 2020). Moreover, the elevated concentrations of CH₄ coupled with low concentrations of H₂ are consistent with a sudden release of subsurface fluids in which microbial processes could account for elevated CH₄ and low H₂. An alternative source for CH₄ is thermogenic decomposition of buried organic matter following the eruption and/or subsurface methanogenesis (McLaughlin-West et al., 1999).

CO₂ was the dominant dissolved gas measured above the western recent lava flow (MAY04-HY07 and MAY04-HY09, July 2019), and the northwest very recent (MAY15-HY06, October 2020) and active (MAY15-HY08, October 2020) lava flows (Fig. 5). Such emissions of CO₂ may originate from different sources such as mantle, oceanic crust, sedimentary carbonates and organic matter (de Hoog et al., 2001; Marty et al., 1989; Resing et al., 2007). The mean molar ratio of CO₂ to ³He measured at the vicinity above recent (MAY04-HY07), very recent (MAY15-HY06) and active (MAY15-HY08) lava flows is at 6.82 ± 2.32

× 10⁹, which is elevated compared to the ratio found at the NW Rota-1 submarine volcano (3.25 ± 0.07 × 10⁹, Resing et al. (2007)) or more generally at global magma outgassing at mid-ocean ridges (2 × 10⁹, Marty and Jambon (1987)). As it is also outside the range of mid-ocean ridge hydrothermal fluids (0.7 to 4.6 × 10⁹, Resing et al. (2004)), we argue that a hydrothermal fluid circulation is unlikely, or hasn't been emplaced yet, consistently with the very young age of the volcano. Our value is however within the range of submarine arc volcanoes (12 × 10⁹ at Suiyo seamount, Tsunogai et al. (1994); 10 × 10⁹ at NW Eifuku seamount, Lupton et al. (2006)) and subaerial arc volcanoes (about 6 to 34 × 10⁹, Sano and Marty (1995)). Our value is also very similar to vent fluid value at Loihi seamount, where CO₂/³He ratio changed rapidly (i.e. increased) following a seismic crisis (Hilton et al., 1998). As it was demonstrated by Marty et al. (1989) the apparent excess of CO₂ relative to ³He cannot be entirely due to the physical or chemical fractionation occurring during the transfer of gases from the magmatic source to the surface. Hilton et al. (1998) have shown that the degassing of an alkalic magma may explain CO₂/³He ratios (measured in fluids) higher than the initial magma ratio due to the solubility behavior of CO₂ and He in magmas of different chemistry. In low-SiO₂ alkali basalts, the solubility of CO₂ is significantly higher than in tholeiites, while helium solubility decreases with decreasing SiO₂. Thus, volatiles residual after a degassing event will have higher CO₂/³He. A possible origin for excess carbon is
the contribution (i.e. contamination) of carbonaceous sediments being a prominent source of CO₂ under the appropriate T-P-fO₂ conditions (Marty et al., 1989). However, even if the contribution of crustal gases was more elevated for July 2019 samples than for the October 2020 samples, we do not observe a significant change in CO₂/³He values between those both periods. Therefore we attribute the excess in CO₂ relative to ³He to an enrichment of CO₂ originating likely from carbonates and organic matter of the thick sediment cover between the main volcanic layer, below the new volcano, and the top of the crust (2.2–2.5 km, Masquelet et al. (2022a))

The δ¹³C-CH₄ signatures of –34 ‰ in the water column are rather ambiguous and can point to both abiotic or thermogenic signature of methane (Etiope and Sherwood Lollar, 2013). Indeed, methane isotope signature observed in hydrothermal fluids from mid-ocean ridge regions, like the East Pacific Rise or Mid-Atlantic Ridge, generally display δ¹³C-CH₄ values between –8.6–23 ‰ and ((Kawagucci et al., 2008; Sano and Fischer, 2013; Wen et al., 2016), and references therein). Our values are however more similar to warm vents and gas bubbles from back arc submarine volcano where δ¹³C-CH₄ ranging from –24.8‰ to –41.4‰ have been reported and suggest a primarily thermogenic origin for CH₄ (Michel et al., 2018).

With values of δ¹³C-CH₄ = –34.8 ‰ and –34.0 ‰ measured in July 2019, and October 2020 respectively, the stable carbon isotopic composition of methane measured in the water column above the Fani Maoré volcano falls in the range of –75‰ to –15‰ corresponding to early mature and very late mature thermogenic gases, but also within the –50‰ to +10‰ abiotic generation range (Milesi et al., 2020; Milkov and Etiope, 2018). Milkov and Etiope (2018) demonstrated that thermogenic gases are often mixed with microbial gases (in sediments at diagenetic to early catagenetic stages) and abiotic gases (e.g., in geothermal volcanic plumbing systems involving organic-rich rocks). The low organic carbon content of the sediment does not support extensive thermogenic generation of methane through magma/sediment interaction during the ascent. However, the volcano does lie above ~2.2–2.5 km thick sediment cover (Masquelet et al., 2022b), which depending on the magma ascent rate and conditions, may release enriched methane mature fluids of thermogenic signatures (Etiope and Sherwood Lollar, 2013; Pegler and Ferguson, 2021). With the current dataset, it is not possible to favor an abiotic over a thermogenic origin of CH₄.

### 5.3. Carbonate system response to high CO₂ discharges

Submarine volcanoes discharge acid-rich fluids (e.g. sulfuric acid originating from SO₂ disproportionation), CO₂ rich-fluids (e.g. magma CO₂ volatile), and fluids rich in CO₂ and alkalinity (e.g. mixture of magmatic volatiles and products of water/rock interactions). The addition of CO₂ within the water column results in acidification of the seawater, whereas the addition of alkalinity increase the pH. When both CO₂ and carbonate alkalinity are added, pH decrease but less than when only CO₂ is added (Resing et al., 2009). Representation of the pH-CO₂ relationship as ΔΣCO₂ against ΔpH (delta values being the difference from the regional background) (Fig. 10) displays the three trends that might explain the significant water column acidification (Fig. 6): (1) CO₂ addition, (2) CO₂ and carbonate alkalinity addition, (3) mineral acid addition.

Samples taken above the northern flank during the eruption, between 2250 m and 2750 m depth (cast MAY01-HY03), exhibit a strong decrease in pH (–0.6 pH unit) that seems to be explained by an addition of CO₂ (showed by the red line, Fig. 10). According to the gas behavior study, at these pressure and temperature conditions (5.2.1.), we suppose that the CO₂ was mainly in the form of liquid CO₂ rising up through the water column. The absence of carbonate alkalinity addition suggests a limited fluid-rock interaction. Indeed, ΣCO₂ and A_T anomalies are just above the background level and the TDFe/TDMn ratio is about 2. Rather than originating from fluid-rock interaction, the significant anomalies in silicates we observe at the volcano northern flank (ΔSiO₂ up to 15 µmol/L) are likely due to basalt alteration at elevated temperature conditions. Indeed, as demonstrated by Resing et al. (2009), the presence of CO₂ without alkalinity suggests that the CO₂ does not react extensively with the host volcanic rocks to form alkalinity. Limited fluid-rock interaction is also consistent with the presence of pure CO₂ in liquid and clathrate forms that will limit CO₂ dissolution in seawater and facilitate CO₂ ascend in the water column due to density differences above 2700 m (Fig. 9), thus preventing extensive surface reaction with the volcanic rocks.

Deeper than 2750 m above the northern flank, for the same cast (MAY01-HY03), pH is mainly controlled by CO₂ addition but shows a potential carbonate alkalinity addition that suggests rock weathering processes. This hypothesis is supported by the observation of moderate ΣCO₂ and A_T anomalies, the high TDFe/TDMn ratio of 27, and the significant correlation between ΣCO₂ and TDMn for this cast (r² = 0.82, p < 0.0001). This fluid-rock interaction might be explained by the liquid

<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Plot of ΔΣCO₂ vs. ΔpH for all seawater samples taken during the three cruises: May 2019 (MAYOBS1 in orange), July 2019 (MAYOBS4 in green) and October 2020 (MAYOBS15 in purple). ΔpH values are the change in ΣCO₂ and pH from the regional background. The blue arrow shows the decrease in pH caused by mineral acidity addition (H⁺). The brown arrow represents the increase in pH due to carbonate alkalinity addition resulting from the rock weathering. The red dash line (–360 µmol/L per pH) shows the theoretical decrease in pH in the case of only CO₂ is added. When both carbonate alkalinity and CO₂ are added, pH decreases. (Theoretical trends from Resing et al. (2009)). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
CO₂ that flows down the volcano flanks at these depths. Here, silicate anomalies are however lower, and can be explained by a lower temperature anomaly that does not enhance silicate alteration. The TDFe/TDMn ratio is similar to the one found at Ruby submarine volcano, at the Mariana arc (Fe/Mn = 26, Resing et al. (2009)), for which authors have attributed the elevated Fe levels to the carbonic acid dissolution of the host rocks as Ruby volcano shows evidence of being high alkalinity/high CO₂ systems. This ratio is also consistent with the one of ~30 found in hydrothermal fluid from Pele's Vent (Loihi Seamount) where fluids were enriched in total dissolvable CO₂ and was attributed to chemical weathering of wall rock by magmatic volatiles entrained in the circulating hydrothermal fluids (Glazer and Rouxel, 2009).

At the volcano summit, between 2250 and 2750 m deep, the ongoing eruption led to a moderate acidification of about -0.3 pH unit. This pH anomaly is associated with a significant increase in ΣCO₂ (ΣCO₂ = 2644.2 µmol/L), which seems to have enhanced rocks weathering processes as indicated by a strong silicate anomaly (ΔSiO₂ = 20 µmol/L) and elevated alkalinity values (A_T = 2650.6 µmol/L). These maximum values of ΣCO₂, ΔSiO₂ and A_T are found at 2703 m, just above the summit and are associated with elevated TDFe/TDMn ratios of about 27, which is consistent with the hydrothermal fluid ratio value (i.e. ~30) from Pele's Vent (Loihi Seamount) where fluids were enriched in total dissolvable CO₂, as previously describe. At 2250 m, this ratio decreases down to 11, as well as ΣCO₂, ΔSiO₂ and A_T anomalies that decrease towards the background level. These observations seems to highlight the presence of strong water-rock interactions close to the summit of the volcano, which may have been enhanced by high temperature conditions during the ongoing eruption.

Higher than 2250 m in the water column, at the summit and above the northern flank during the ongoing eruption, pH anomalies are lower with maximum values of -0.26 pH unit, and seems to result both from CO₂ addition and carbonate alkalinity addition (brown arrow in Fig. 10). Actually, the majority of our samples (all cruises and casts) displays a pH that seems to be controlled by additions of both CO₂ and carbonate alkalinity. We suggest here, that acidification originates from the discharge through the water column of fluids rich in liquid CO₂ that slowly dissolves in seawater. This seawater, enriched in CO₂, then reacts with volcanic rocks to generate carbonate alkalinity and reach the carbonate equilibrium. This process might have occurred in contact with the seafloor, lava flows and volcano flanks, and got diluted when moving up into the water column. Such hypothesis is consistent with the low concentrations of TDFe and TDMn, and low anomalies of ΣCO₂, ΔSiO₂ and A_T observed above 2250 m above the volcano.

Samples taken above the western recent, and the northwest very recent and active lava flows display anomalies of pH, A<sub>T</sub>, SiO<sub>2</sub> and ΣCO<sub>2</sub> located below 2250 m deep that are moderate to low, and that even reach the background envelope (as the ΔSiO<sub>2</sub> value for the northwest very recent lava, cast MAY15-HY06). The TDFe/TDMn ratios range between 17 and 11, and are lower than during the eruption. This ratio value corresponds to the NW Rota-1 ratio of 17 from Resing et al. (2009), where high acidity of fluids venting from the volcano likely dissolved the host rock, producing fluids with elevated TDFe/TDMn. The depletion of TDFe from hydrocasts sampled above the recent lava flows compared to hydrocasts performed at the edifice during May 2019 eruption is either due to a decrease of the emitted TDFe or to the iron precipitation over time.

Some samples taken above the western recent (MAY04-HY09) and the northwest very recent (MAY15-HY06) lava flows align themselves along the blue arrow on Fig. 10. In particular, the sample taken at 3154 m above the western recent lava flow (MAY04-HY09) presents an acidification of -0.5 pH unit that is not linked with the total alkalinity and silicates. This behavior suggests that the acidification is controlled by the addition of mineral acid (H⁺) likely generated by the emission of sulfur dioxide (SO₂) that produces sulfuric acid (H₂SO₄). In previous studies on submarine volcanoes at Mariana arc, authors determine that the mineral acidity (H⁺) originates from the SO₂ + H₂O = HSO₃⁻ + H⁺

reaction, when magmatic gases rich in SO₂ come in contact with water (Butterfield et al., 2011; Resing et al., 2009; Resing et al., 2007). In our study, we suggest that this reaction might have occurred right above the emitted lava flows, implying the presence of magmatic gas rich in SO₂. H₂S which is also likely to be emitted during magma degassing may also generate acidity during its oxidation in the water column (e.g. Cline and Richards (1969)).

The major part of the samples taken above the lava flows, display pH and total dissolved inorganic carbon anomalies resulting from CO₂ and carbonate alkalinity addition. Even if effusive activity was detected at the northwest active lava flow (cast MAY15-HY08), acidification does not exceed -0.1 pH unit and the addition of CO₂ is also weak (< 70 µmol/L). Only one deeper sample (3259 m) appears to be strongly enriched in CO₂ (ΣCO₂ = 274.2 µmol/L) but displays a weak pH acidification (-0.08 pH unit). This is likely due to a possible rock weathering occurring during the contact between fluid and seafloor. This anomaly is coupled with a relatively high TDFe/TDMn ratio (~16) and a slight increase of ΔSiO₂ even if this value stays within the background envelope. In gas-rich hydrothermal systems, H⁺ reacts first with the host rock followed by a CO₂-weathering reaction that produces alkalinity as HCO₃⁻. The presence of alkalinity in deep samples collected above lava flows is consistent with long reaction paths where H⁺ is completely consumed and CO₂ reacts with the host rocks (Resing et al., 2009).

Looking at the entire data set, the decrease of the overall eruptive activity intensity offshore Mayotte is clear during the investigated period. Indeed, even if lava flows from October 2020 are very recent and active, anomalies of pH, A<sub>T</sub>, SiO<sub>2</sub> and ΣCO<sub>2</sub> are smaller than those measured in July 2019 above recent lava, which are themselves lower than those from May 2019. Concentrations of TDFe and TDMn are also decreasing over time. Therefore, after the eruption, the carbonate system seems to be back to steady-state highlighting either (i) that the initial eruptive impact has been exported/flushed away due to deep current, (ii) or that the impact has been indeed buffered within the seawater column environment. To answer this, models of the dissolution of CaCO<sub>3</sub> using water column geochemistry and/or correlation with local sediments analyses could be made. Although discharges of CO<sub>2</sub> at the deep Fani Maoré submarine volcano have generated a locally confined ocean acidification (< 27,000 km<sup>2</sup>) that decrease over time (i. e. returning to a steady-state) over a few months, biological impacts on organisms incorporating calcium carbonate into their structures cannot be excluded.

## 6. Conclusion

The compilation of geochemical data from Fani Maoré submarine volcano has provided valuable information about the setup of a volcanic edifice and the impact of an eruptive activity on the water column at a local scale. This study leads to the following conclusions:

(1) The turbidity signals translate the decrease of the eruptive activity intensity from May 2018 to October 2020 (about 30 months), starting from a strong eruptive event at the volcano summit to continued lava flow emplacement.

(2) A magma signature shift happens from crustal contamination to (pure) mantle signature between the western recent lava flow seen in July 2019, and the very recent and active northwest lava flow seen in October 2020. This highlight a change to a direct magma ascent from the main reservoir to the seafloor, or a rapid recharge of the secondary magma reservoir.

(3) The May 2019 eruption has generated, by seawater-lava interaction and probably encouraged by the formation of a gas jet, a strong H₂ discharge nearly 1000 m high within the water column above the volcano summit. Due to temperature and pressure conditions above the volcano flanks, CO₂ was emitted in liquid phase and sank for depths less than about 2680 m. A strong acidification of the water column was linked to the high
discharge of inorganic carbon to the water column. Fluids, including liquid CO₂, has interacted with the volcano walls producing high levels of metals, and elevated TDFe/TDMn ratios.

(4) The lava flows around the volcano edifice has generated significant gas inputs near the seafloor. H₂ levels highlighted the recent eruptive activity, produced through lava quenching, and was then rapidly found at low levels due to oxidation and microbial consumption. CO₂ was released by magma outgassing through a longer time scale. Origin of methane released in more ambiguous. Indeed, although magma outgassing as a methane source is obvious in this context, release of a more mature crustal fluid, originating from the thermogenic cracking of sedimentary organic matter during the magma ascent may also occur given the thick sediment cover that the volcano penetrated. Fluids emissions to the water column have led to a moderate acidification of the seawater mainly resulting from the addition of dissolved inorganic carbon and the addition of carbonate alkalinity released by rock weathering processes. Significant acidification for a part of the deep-water samples above lava flows appears to be controlled by the addition of mineral acid (H⁺) from fluids rich in SO₂ and/or H₂S.

(5) About a year after the main eruptive event, with a decrease of the eruptive activity, the system appears to be approaching a steady-state where pH and total carbon values are heading towards the evaluated local background.

(6) What is still unclear is whether this phenomenon is due to the decrease of the activity coupled with a renewing a water by the currents or to the buffering capacity and the resilience of the water column. To resolve this, future work could therefore focus on models of geochemical behavior of the water column under different scenarios and compare to sediment analyses for CaCO₃ investigation.

Besides the geological context that obviously drives the type of volcanism and emissions of associated magmatic volatiles, our findings clearly highlights how environmental settings (e.g. depth, morphobathymetry) might shape the manifestations of submarine eruptions and its impact on the water column. In addition to play a significant part in the eruption mode (Cas and Simmons, 2018), the hydrostatic pressure clearly constrains the behavior of magmatic or eruption-driven volatiles such as H₂, CO₂ or CH₄, and, therefore, how they affect the adjacent marine pelagic and benthic ecosystems.

## Funding sources

MAYOBS1 campaign was funded by the CNRS-INSU TELLUS Mayotte program (SISMAYOTTE project). MAYOBS4 and MAYOBS15 campaigns were conducted by several French research institutions and laboratories (IPGP/CNRS/BRGM/IFREMER) and were funded by the Mayotte volcanological and seismological monitoring network (REVOSIMA, doi: 10.18715/MAYOTTE.REVOSIMA), a partnership between the Institut de Physique du Globe de Paris (IPGP), the Bureau de Recherches Géologiques et Minières (BRGM), the Observatoire Volcanologique du Piton de la Fournaise (OVPF-IPGP), the Centre National de la Recherche Scientifique (CNRS), and the Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER). This study was funded by the REVOSIMA, IFREMER and TotalEnergies as part of the MAYFLUID PhD project (TOTAL ENERGIES FR000063751 // IFREMER 20/1001730).

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

All data used in this study are presented in the tables, and/or will be deposited in SEANOE and SISMER online data repository.

## Acknowledgments

We thank the crew members of the R/V Marion Dufresne (TAAF/IFREMER/LDA) and its Captain A. Eysautier. We thank N. Feuillet, Y. Fouquet, E. Rinnert and I. Thinon the chief scientists of the MAYOBS01 cruise (May 2019, doi: 10.17600/18001217), the MAYOBS04 cruise (July 2019, doi: 10.17600/18001238), and the MAYOBS15 cruise (July 2019, doi: 10.17600/18001745). We also thank all the scientists involved. We thank Jürgen Sültenfuß from the Institute of Environmental Physics/Section of Oceanography at the University of Bremen who performed all helium and neon isotopic analyses. We thank Bart van der Haven from Isolab b.v. who carried out the carbon isotope analyses.

## Appendix A. Supplementary data

Supplementary material presents vertical profiles of hydrographic parameters, and figure highlighting anomalies of potential temperature during the eruption and above the studied lava flows. Evaluation of background geochemistry is also presented through depth profiles. The relationship between TDMn, CO₂ and CH₄ with ³Heₓs are provided in this file.

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