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The "Escarot" gas seep, French Massif Central: CO 2 discharge from a quiescent volcanic system -Characterization and quantification of gas emissions

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Natural CO 2 emissions from the volcanic rocks of the French Massif Central are poorly constrained. It is of interest better to assess the emission of such non-anthropogenic gases that may significantly contribute to the global carbon budget. We quantified the CO 2 emissions to the atmosphere in a small area (0.052 km²) located in the Massif Central close to Lake Pavin, the most recent volcanic edifice in metropolitan France. The specific character of this area, known as the Escarot mofette, was earlier studied for soil-gas concentrations only. In June 2017, we used the accumulation chamber method for measuring CO 2 flux and related O 2 depletion in the gases emitted at the soil/atmosphere interface, resulting in 176 data acquisitions over four days. In addition, 44 soil-gas concentration measurements were made at selected locations. CO 2 emission rates are estimated at 8,100±1,800 tons/year of deep-seated CO 2 and at 660±440 tons/year of biologically produced CO 2 . The uncertainty on these evaluations comes from the high-frequency variability of CO 2 efflux in the more emissive areas and from the occurrence of heavy precipitation events. Though unexpected, these events were used for quantifying the decreases in CO 2 efflux, which were as high as 500% over a few hours or even days in some locations. However, repeat acquisitions performed under more favourable weather conditions showed errors of commonly accepted amplitude (±15%). The area showed several degassing centres aligned along a NNW-SSE direction that correlates well with known geological structures, proving the ability of soil-gas methods to map hidden faults. The whole area is characterized by strong CO 2 enrichment and related O 2 depletion, but it is nonetheless possible to detect areas influenced by the rise of deep-seated gases and a few peripheral areas where biological processes dominate (CO 2 up to 10% vol.). This study of gas emissions in a non-urban area also provides complementary information that is of use when extrapolated to similar structures in urban Accepted Manuscript 2 34 areas, where the occurrence of such gas releases, and its potential hazard may be more 35 difficult to assess.

Introduction

The Earth's natural degassing induces the release into the atmosphere of many gas species, including some "greenhouse gases" (e.g. N 2 O, CH 4 , CO 2 or H 2 O). Volcanic structures obviously play an important role in this degassing process, either during explosive (eruptive) phases, or during quiescent (non-eruptive) periods (e.g. [START_REF] Halmer | The annual volcanic gas input into the atmosphere, in particular into the stratosphere: a global data set for the past 100 years[END_REF]. Such degassing may be diffuse, or focused through volcanic edifices, hydrothermal manifestations, or gas vents, the last two commonly being related to faults (e.g. [START_REF] Judd | Gas Emissions from Mud Volcanoes[END_REF][START_REF] Burton | Deep carbon emissions from volcanoes[END_REF][START_REF] Etiope | Natural Gas Seepage -The Earth's Hydrocarbon Degassing[END_REF]. The budgets of carbon emissions into the atmosphere have been assessed since the 1980s (e.g. [START_REF] Werner | CO 2 emissions related to the Yellowstone volcanic system. 2. Statistical sampling, total degassing and transport mechanism[END_REF][START_REF] Kerrick | Present and past non-anthropogenic CO 2 degassing from the solid earth[END_REF][START_REF] Mörner | Carbon degassing from the lithosphere[END_REF] references therein) and are regularly refined (e.g. [START_REF] Holloway | Natural emissions of CO 2 from the geosphere and their bearing on the geological storage of carbon dioxide[END_REF][START_REF] Lewicki | Natural and industrial analogues for leakage of CO 2 from storage reservoirs: identification of features, events, and processes and lessons learned[END_REF], or the ongoing investigations on French active volcanoes by [START_REF] Boudoire | Investigating the deepest part of a volcano plumbing system: Evidence for an active magma path below the western flank of Piton de la Fournaise (La Réunion Island)[END_REF][START_REF] Liuzzo | New evidence of CO 2 soil degassing anomalies on Piton de la Fournaise volcano and the link with volcano tectonic structures[END_REF]. Such carbon-emission budgets should include the release of CO 2 as a gas phase as well as the amounts of CO 2 dissolved in aquifers or lakes that may represent a nonnegligible contribution (e.g. Sorey et al., 1998).

Even during periods of quiescence, the release of gases, especially CO 2 , can be high (e.g. [START_REF] Werner | Long-term changes in quiescent degassing at Mount Baker Volcano, Washington, USA; Evidence for a stalled intrusion in 1975 and connection to a deep magma source[END_REF]. Volcanic structures are not only characterized by the presence of degassing structures, such as fumaroles or gas vents, and diffuse degassing processes, but commonly also by the existence of thermal anomalies in the soil and of hot springs or steam emissions (e.g. [START_REF] Frondini | Diffuse CO 2 degassing at Vesuvio, Italy[END_REF]. This is often the case when the quiescence period has been relatively short at a geological scale, such as around 130 years at Pantelleria Island, Sicily [START_REF] Favara | Preliminary estimate of CO 2 output from Pantellaria Island volcano (Sicily, Italy): evidence of active mantle degassing[END_REF], around 400 years at Furnas volcano, Azores [START_REF] Pedone | Total (fumarolic + diffuse soil) CO 2 output from Furnas volcano[END_REF] and around 600 years at Mammoth Mountains, California (Sorey et al., 1998). Nevertheless, some volcanic edifices that have been quiescent for a similar duration will have no thermal Accepted Manuscript anomalies or fumarolic activity (Hakkoda, Japan; [START_REF] Hernández-Perez | Carbon dioxide emissions from soils at Hakkoda, north Japan[END_REF] and even no gas emissions into the atmosphere (Mount Fuji, Japan; [START_REF] Notsu | Monitoring quiescent volcanoes by diffuse CO 2 degassing: case study of Mt. Fuji, Japan[END_REF]. The longer the duration of the quiescent period, the fewer thermal anomalies there will be, such as in the West European rift and its associated volcanic structures in France and Germany.

Here, however, the absence of high-temperature manifestations does not mean that there are no gas releases, but these are less commonly quantified. When they are, quantification may refer to the gas flux in water bodies, such as a flux of 1450±350 g.m -2 .y -1 of magmatic CO 2 in the German Laacher See [START_REF] Aeschbach-Hertig | Quantification of gas fluxes from the subcontinental mantle: The example of Laacher See, a maar lake in Germany[END_REF], or a flux of 170±60 g.m -2 .y -1 of magmatic CO 2 in the French Lake Pavin [START_REF] Aeschbach-Hertig | Accumulation of mantle gases in a permanently stratified volcanic lake (Lac Pavin, France)[END_REF]. In the French Massif Central, little is known on CO 2 emissions from the numerous carbogaseous springs that are part of the West European rift system, though several studies deal with the origin of such CO 2 (e.g. [START_REF] Batard | Isotopic identification of gases of a deep origin in French thermomineral waters[END_REF][START_REF] Blavoux | Caractérisation d'une province à CO 2 dans le bassin du Sud-Est de la France[END_REF][START_REF] Czernochowski-Lauriol | The French carbogaseous province: an illustration of natural processes of CO 2 generation, migration, accumulation and leakage[END_REF]. Only a rough estimate of the CO 2 emissions associated with quiescent volcanism in the French Massif Central is reported by [START_REF] Mörner | Carbon degassing from the lithosphere[END_REF] -citing [START_REF] Hermansson | Geogas-a carrier or a tracer? Statens Karnbransle Namnd (SKN) Report[END_REF] -as being over 6 kg.m -2 .y -1 . And only one study quantified the CO 2 emissions into the atmosphere at the Sainte-Marguerite site [START_REF] Battani | Geochemical study of the natural CO2 emissions in the French Massif Central: How to predict origin, processes and evolution of CO 2 leakage[END_REF].

The present study thus aimed at quantifying the gas emissions related to quiescent volcanism in the French Massif Central near Lake Pavin [START_REF] France | Oxygen isotopes reveal crustal contamination and a large, still partially molten magma chamber in Chaîne des Puys (French Massif Central)[END_REF]. Here, the CO 2 percolation is not associated with mineral water, thus leading to the appearance of mofette structures (CO 2 dry-gas vents), such structures commonly being related to Cenozoic rifting [START_REF] Pearce | A review of natural CO 2 accumulations in Europe as analogues for geological sequestration[END_REF]. In Europe, dry CO 2 vents occur, e.g., in the Pannonian basin [START_REF] Bräuer | Helium and carbon isotope signatures of gas exhalations in the westernmost part of the Pannonian Basin (SE Austria/NE Slovenia): Evidence for active lithospheric mantle degassing[END_REF], in the Eger rift zone [START_REF] Kämpf | CO 2 discharge in an active, non-volcanic continental rift area (Czech Republic): Characterisation (δ 13 C, 3 He/ 4 He) and quantification of diffuse and vent CO 2 emissions[END_REF], in the Eifel volcanic district (Gal et al., 2011;[START_REF] Krüger | Effects of elevated CO 2 concentrations on the vegetation and microbial populations at a terrestrial CO 2 vent at Laacher See, Germany[END_REF]. A large number is reported from Italy, though gas vents there can be related to both magmatic CO 2 emissions and emissions of CO 2 accumulated in carbonated reservoirs [START_REF] Rogie | Flux measurements of non-volcanic emission from some vents in central Italy[END_REF][START_REF] Chiodini | Carbon dioxide degassing at Latera caldera (Italy): Evidence of geothermal reservoir and evaluation of its potential energy[END_REF][START_REF] Beaubien | The impact of a naturally occurring CO 2 gas vent on the shallow ecosystem and soil chemistry of a Mediterranean pasture (Latera, Italy)[END_REF][START_REF] Roberts | Assessing the health risks of natural CO 2 seeps in Italy[END_REF]. In the French Massif Central, the only occurrence reported so far is the Escarot mofette [START_REF] Gal | Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)[END_REF]Braüer et al., 2017).

Our new investigations around this mofette used the complementary approaches of measuring soil-gas concentrations (e.g. [START_REF] Fleischer | Mapping of integrated radon emanation for detection of long-distance migration of gases within the Earth: Techniques and Principles[END_REF]Toutain and Baubron, 1999) and soil-gas flux (e.g. [START_REF] Chiodini | Soil CO 2 flux measurements in volcanic and geothermal areas[END_REF]. The data were used to quantify the range of annual CO 2 emissions, to evaluate their relationship with geological structures and to evaluate their contribution to the long-term global carbon cycle (e.g. [START_REF] Werner | CO 2 emissions from the Yellowstone volcanic system[END_REF]. Another part of the study was the assessment of changes in gas-emission rates linked to the occurrence of major precipitation events, as fieldwork cannot always be done under favourable weather conditions. Finally, we envisaged the consequences such gas emissions may have in more populated areas.

Settings

Hydro(geo)logical setting

The study area is located 2 kilometres west of Lake Pavin, a maar lake characterized by its meromictic character [START_REF] Meybeck | Pavin, the Birthplace of French Limnology (1770-2012), and Its Degassing Controversy (1986-2016[END_REF] that was formed around 7,000 years ago. The volcanic system today is non-active, though some magma may still be present at depth [START_REF] France | Oxygen isotopes reveal crustal contamination and a large, still partially molten magma chamber in Chaîne des Puys (French Massif Central)[END_REF]. Compared to the duration of volcanic rest periods in the neighbouring Chaîne des Puys (10,000 years or more), the system must be seen as quiescent (or dormant), but not extinct. The dissolved CO 2 in the lake water is attributed to organic-matter degradation (70% of total input) and to seepage of magmatic CO 2 and/or CO 2 from Accepted Manuscript methanogenesis [START_REF] Jézéquel | Carbon Cycle in a Meromictic Crater Lake: Lake Pavin, France[END_REF]. Deep-CO 2 input becomes dominant in the deeper part of the lake [START_REF] Aeschbach-Hertig | Accumulation of mantle gases in a permanently stratified volcanic lake (Lac Pavin, France)[END_REF]. The presence of magmatic CO 2 from the subcontinental lithospheric mantle was reported from the Fontaine Goyon mineral spring and the Escarot mofette [START_REF] Bräuer | Helium and carbon isotope signatures of gas exhalations in the westernmost part of the Pannonian Basin (SE Austria/NE Slovenia): Evidence for active lithospheric mantle degassing[END_REF][START_REF] Bräuer | Regional distribution pattern of carbon and helium isotopes from different volcanic fields in the French Massif Central: Evidence for active mantle degassing and water transport[END_REF]. [START_REF] Meybeck | Pavin, the Birthplace of French Limnology (1770-2012), and Its Degassing Controversy (1986-2016[END_REF] reports that other degassing spots seem to have been identified in the past.

The study area is small (about 200x300 m) close to the Escarot degassing zone (Figure 1).

The Escarot site was earlier described by [START_REF] Gal | Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)[END_REF] and by [START_REF] Bräuer | Regional distribution pattern of carbon and helium isotopes from different volcanic fields in the French Massif Central: Evidence for active mantle degassing and water transport[END_REF].

A distinction has to be made between the location of the mofette as defined in 2011 (Figure 1) and the location of the data reported by [START_REF] Bräuer | Regional distribution pattern of carbon and helium isotopes from different volcanic fields in the French Massif Central: Evidence for active mantle degassing and water transport[END_REF] that concerned samples mostly taken around the Escarot piezometer (Figure 1). This observation well is part of a network of several boreholes that were drilled in the late 2000s to resolve a controversy about potential renewal of degassing activity [START_REF] Meybeck | Pavin, the Birthplace of French Limnology (1770-2012), and Its Degassing Controversy (1986-2016[END_REF].

The gas phase released at Escarot is dominated by CO 2 (>99.2% vol.) with a minor proportion of permanent gases (N 2 <0.65%; O 2 <0.09%; Ar <0.007%) and traces of CH 4 (360 to 500 ppm) and of He (27 to 30 ppm; [START_REF] Bräuer | Regional distribution pattern of carbon and helium isotopes from different volcanic fields in the French Massif Central: Evidence for active mantle degassing and water transport[END_REF]. Higher helium concentrations in soil gas, up to 47 ppm were reported by [START_REF] Gal | Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)[END_REF]. Isotope data on CO 2 ( 13 C CO2 ), from the piezometer or the mofette (Figure 1), fall in a narrow range (-3.6 to -3.8‰ VPDB, Bräuer et al., 2017;-3.1 to -4‰, unpublished BRGM data) and so do the 3 He/ 4 He ratios (6 to 6.3 R/Ra, [START_REF] Bräuer | Regional distribution pattern of carbon and helium isotopes from different volcanic fields in the French Massif Central: Evidence for active mantle degassing and water transport[END_REF]6.09 R/Ra, BRGM unpublished data). Similar values are reported regionally for magmatic CO 2 (e.g. [START_REF] Battani | Geochemical study of the natural CO2 emissions in the French Massif Central: How to predict origin, processes and evolution of CO 2 leakage[END_REF][START_REF] Gal | What may be the consequences of a CO 2 leakage? Insights from soil gas measurements in an urban area -Clermont-Ferrand, French Massif Central[END_REF].

The new data were mostly acquired to the west of a road near the Escarot piezometer; here, the ground level is approximately 3 m below the road and flooding occurs frequently. Near the Escarot mofette, the ground level is approximately 2 m above the road. Several zones of bubbling exist along the road and can be seen each time there is some stagnant water in the road shoulders (after precipitation events). As will be described later, these areas, which are frequently flooded, were investigated at the start of the fieldwork and were not accessible after large precipitation events. To the east of the road, there is a large area of peat land, where flux measurements were made on only unflooded clumps of grass as the soil is nearly saturated all year long. To the south of the monitoring area lie some meadows, but they, too, were almost saturated with water on the day they were investigated.

Figure 1

Methods

In June 2017, 44 soil-gas concentration measurements and 176 soil-gas flux measurements were made in the 200x300 m area (Figure 1). The objective was to obtain a more detailed description of the degassing area that was highlighted by 53 earlier soil-gas concentration measurements over a 250x400 m area [START_REF] Gal | Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)[END_REF]. The new sampling aimed at lowering the uncertainty in the estimate of CO 2 emissions from the Escarot area [START_REF] Werner | CO 2 emissions related to the Yellowstone volcanic system. 2. Statistical sampling, total degassing and transport mechanism[END_REF]. As emissions of chemical species into the atmosphere from mofette structures are not restricted to just the gas phases, complementary data on soil gas and dissolved elements in snow cover, acquired in February 2009, are presented as Supplementary Material.

In order to reduce the uncertainty in soil-gas flux evaluations caused by the use of relatively small flux chambers [START_REF] Parkin | Sampling Protocols -Chapter 3 -Chamber-Based Trace Gas Flux Measurements[END_REF], we used a 10-m-square grid close to the mofette and a 25-m-square grid farther away from the mofette (Figure 1). Flux measurements were done first, followed by soil-gas concentration measurements if warranted by the flux results. Such soil-gas concentration measuring points were shifted 20 cm away from the flux chamber, to avoid wrong estimates on later flux acquisitions at the same location. All sampling points were marked in the field so that a repeat acquisition could be done in exactly the same place; in addition, the trace of the flux chamber often remains visible.

Soil-gas concentrations were measured with the same method as used before [START_REF] Gal | Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)[END_REF]. Small holes (1 cm diameter) were drilled down to 1 m depth, and then a sampling probe was inserted that was plugged into an Infra-Red Gas Analyser equipped with a moisture filter (LFG20, ADC Gas Analysis Ltd., UK). Pumping was at a low flow rate of 200 ml.min -1 . Steady-state conditions were reached within tens of seconds. No concentration changes over a few minutes were noted. CO 2 and O 2 concentrations were directly monitored. The monitoring of CH 4 concentrations was not technically feasible (detection limit of 200 ppm at best), but the Escarot mofette is not CH 4 -rich [START_REF] Bräuer | Regional distribution pattern of carbon and helium isotopes from different volcanic fields in the French Massif Central: Evidence for active mantle degassing and water transport[END_REF]. Analytical precision for CO 2 was ±0.5% of the reading for low concentrations (0.01-10% vol.), ±3% of the reading for higher concentrations (10 to 50% vol.) and ±5% of the reading when CO 2 was >50%. The precision for oxygen was ±0.4% of the full scale reading (0-25% vol.). Samples were stored in Tedlar® bags and analysed by the end of the day for 4 He with mass spectrometry (Adixen ASM102S leak detector, France) with reference to atmospheric content (5.24 ppm). Precision of the measurement was better than 4% at full scale.

Soil-gas flux measurements used the accumulation chamber method with external recirculation (e.g. [START_REF] Chiodini | Soil CO 2 flux measurements in volcanic and geothermal areas[END_REF][START_REF] Inguaggiato | CO 2 output discharged from Stromboli Island (Italy)[END_REF]. The flux-chamber system was a customized version of the Echo Instruments (Slovenia) soil-gas flux system. The Accepted Manuscript aluminium chamber has a semi-spherical shape with a basal area of 0.0289 m 2 and a volume of 0.0018 m 3 (soft ground). The chamber was directly put on the soil surface, which is mainly grassland with some gravel patches close to the road. No soil was removed except incase (point C9). A fan with variable speed ensured the mixing of the headspace gas as required for such sampling [START_REF] Parkin | Sampling Protocols -Chapter 3 -Chamber-Based Trace Gas Flux Measurements[END_REF]. The flow rate of the recirculation flux was set to 0.5-0.6 l.min -1 , to avoid possible underestimations of the CO 2 flux when lower recirculation flow rates are used, as well as to avoid possible CO 2 flux overestimations when using higher recirculation flow rates [START_REF] Camarda | Effects of soil gas permeability and recirculation flux on soil CO 2 flux measurements performed using a closed dynamic accumulation chamber[END_REF]. Pressure, temperature and relative humidity inside the chamber, as well as pumping rate, were continuously monitored thus constraining the possible influence of external parameters on the data. The following gas species were monitored after the gas passed through a moisture filter: CO 2 and CH 4 by Non-Dispersive Infra-Red (NDIR) detectors (0 to 5000 ppmv ±2% and 0 to 10000 ppmv ±5%, respectively), and O 2 by electrochemical cells (0 to 25% vol. ±2% of the reading). Some examples of field-data acquisitions are given in Figure 2. The flux calculation was based on the rate of CO 2 increase (positive flux; e.g. point A15, Figure 2) and potential O 2 decrease in the chamber (e.g. points BZ8, E12, Figure 2). The flux was calculated by considering the linear build-up of gas concentration in the chamber during 3 minutes and by determining the initial slope of the gas-concentration vs. time relationship (e.g. [START_REF] Chiodini | Soil CO 2 flux measurements in volcanic and geothermal areas[END_REF][START_REF] Werner | CO 2 emissions related to the Yellowstone volcanic system. 2. Statistical sampling, total degassing and transport mechanism[END_REF]. Flux was normalized to take into account the influence of pressure and temperature [START_REF] Lewicki | Comparative soil CO 2 flux measurements and geostatistical estimation methods on Masaya volcano, Nicaragua[END_REF]. Normalized CO 2 flux generally was 4.5% higher than raw flux values. Not all fluxes were calculated using the initial slope of the build-up of gas concentrations. Short time-scale variations of the CO 2 flux (gas flush) were sometimes observed (Figure 2) Accepted Manuscript and were used to evaluate the uncertainty on the gas release. Increases of 10 times of the flux over some seconds were noticed in points A8 (once) and Z16 (several times). Such multilinear increases were also reported from the Yellowstone area [START_REF] Werner | CO 2 emissions related to the Yellowstone volcanic system. 2. Statistical sampling, total degassing and transport mechanism[END_REF]. Figure 2

Weather parameters

It is well established that soil-gas concentrations and fluxes can be affected by, among others, meteorological parameters such as temperature, atmospheric pressure, wind or precipitation (e.g. [START_REF] Hinkle | Environmental conditions affecting concentrations of He, CO 2 , O 2 and N 2 in soft gases[END_REF]Toutain and Baubron, 1999;Viveiros et al., 2008Viveiros et al., , 2009;;[START_REF] Werner | Decadal-scale variability of diffuse CO 2 emissions and seismicity revealed from long-term monitoring References[END_REF]. Unexpectedly for late June, the weather was strongly variable. No precipitation was reported during the week preceding fieldwork at a nearby weather station (Figure 3). Air temperatures were quite high and pressure showed little variability. The situation changed the weekend before the fieldwork. Temperature and pressure steadily fell between 24 and 29 June, and large precipitation events occurred during late afternoon on 26 and 27 June (31 mm and 34.6 mm, respectively). June 28 th and 29 th were rainy days, with intermittent precipitation events on the 28 th (10.8 mm) and continuous precipitation events on the 29 th (14.8 mm).

The gas-concentration and -flux dataset was acquired as follows: 45 flux measurements prior the rainfall (26 June), 56 flux and 22 concentration measurements after 31 mm of rain on 27 June, 42 flux and 15 concentration measurements after >65.6 mm of precipitation on 28 June, and 33 flux and 7 concentration measurements after >76.4 mm of rain on 29 June.

Accepted Manuscript

The temperature and pressure data recorded during flux acquisitions are in good agreement with the weather station data (Figure 3). Temperature during the first two days rose during the day as a consequence of sunshine thermal forcing, the humidity of the gas at the soil/atmosphere interface thus decreasing during the day. During the last two days, the situation was reversed as there was no sunshine; the soil-gas humidity was high and further rose during the day after the rain.

Because of these large precipitation events, there was a strong correlation between temperature and pressure (Pearson's coefficient of 0.88), relative humidity and temperature of the atmosphere (coefficient of -0.89), and relative air humidity and atmospheric pressure (coefficient of -0.82) during flux measurements. As may be expected from a survey designed for highlighting the behaviour of soil-gas species near a gas vent, the data distribution of soil-gas concentrations shows a very high mean CO 2 concentration (72.6±36.1% -arithmetic mean ± standard deviation), a very low mean O 2 concentration (5.8±6.8%) and a strong enrichment in helium-4 in the soil (19.9±13 ppm as a mean). By comparison, data from the 2008 to 2012 surveys in the Escarot area showed mean CO 2 and helium enrichments (47.3±43.7% and 17.3±9.6 ppm, respectively) and O 2 depletion (11.1±8.5% as a mean), suggesting that the degassing pattern of the Escarot area has remained stable over several years.

No CH 4 -flux was measured during the survey, but this is not really surprising. Even on active volcanoes, the diffuse CH 4 flux may only represent trace releases compared to diffuse CO 2 degassing [START_REF] Hernández | Diffuse emission of carbon dioxide, methane, and helium-3 from Teide Volcano, Tenerife, Canary Islands[END_REF].

As the variation coefficient of CO 2 flux shows a skewed distribution, which is common in high-flux areas, we refer to the arithmetic mean as explained by [START_REF] Werner | CO 2 emissions from the Yellowstone volcanic system[END_REF] and references therein. Three evaluations are presented in Figure 4. The first corresponds to the data calculated using the linear fit prevailing during most of the sampling and considering the normalization to standard temperature and pressure (STP) conditions. The second evaluation corresponds to the flux calculated from the gas flush(es) that may occur along the 3-minute-long flux acquisition (Figure 2), called "maximum value". The third evaluation combines the previous two datasets in order to account for the short-term variability induced by the occurrence of gas flushes. The STP values are thus corrected to include these potential short-term increases, the "maximum value" being integrated as a function of the number and duration of the increases observed during the acquisition. The ponderated flux value deriving from this last calculation thus represents the best estimate of the gas emission over 3 minutes.

Such gas flushes can have a noticeable influence: they were observed for 59 of the 176 acquisitions. The minimum duration of a gas flush over 3 minutes was 5 seconds (one event)

and the maximum was 75 seconds (several events of up to 15 seconds duration each).

Maximum fluxes that may be calculated during these short events are shown at least by an increase of 20% of STP-flux, a value similar to the error reported for flux measurements (±15%; [START_REF] Chiodini | Diffuse emission of CO 2 from the Fossa Crater, Vulcano Island (Italy)[END_REF]. At most, the increase corresponds to a multiplication by a factor of 20 of the flux. As a mean, gas flushes induced a short-term increase of the flux by a factor of 7 (e.g. for point A8, Figure 2). Only few points showed a decrease in CO 2 flux during the acquisition (e.g. point Z32, Figure 2), which can lower the flux by up to 40% and is only noted at low emissive points. Consequently, the initial mean value of the uncorrected STPflux data (500±2,080 g.m -2 .d -1 ) is re-evaluated at 575±2,100 g.m -2 .d -1 (Figure 4).

Data analysis

Relations between gas species

The CO 2 -O 2 relationship for soil gas is presented in Figure 5A and interpreted using the process-based approach of Romanak et al. (2012). Three main relationships are plotted. The first is the respiration line (in blue), characteristic of aerobic respiration in soil that consumes 1 mole of O 2 to produce 1 mole of CO 2 . The second relation corresponds to the CH 4 oxidation line (in green) in near-surface environments, where 2 moles of O 2 are needed to produce CO 2 +H 2 O. The last relationship (red lines) corresponds to the dilution process occurring when a deep (magmatic) CO 2 endmember dilutes the O 2 +N 2 pool in a near-surface environment (relative abundance: 20.9% of O 2 and 78.1% of N 2 ). Different dilution lines can be plotted, depending on the CO 2 /O 2 initial ratio: Figure 5 illustrates three cases at 0.03%, 5% and 10% of biologically produced CO 2 .

Only few data lie on the respiration line and these have a CO 2 concentration below 5%. Most of the data plot close to the dilution line with a possible influence of respiration processes. This kind of data distribution confirms previous data that reported a threshold value around 10% of CO 2 in soil [START_REF] Gal | Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)[END_REF]. Below this value, CO 2 mainly originates from biological processes; above it, a supply of deep-seated CO 2 is required. The area investigated around the Escarot mofette (Figure 1) is thus largely influenced by the degassing of magmatic CO 2 , as shown by the general relationship between CO 2 and O 2 concentrations (slope of -0.20). Biological processes, though existing, have little influence on the relation between these two gas species.

Strong helium enrichment (>10 ppm) was earlier measured when the CO 2 concentrations were over 10% [START_REF] Gal | Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)[END_REF]. Figure 5B links 4 He data to CO 2 data and similar conclusions arise from the 4 He-O 2 relationship in the present study. In 2017, there still was a good correlation between 4 He and CO 2 as shown by the overlapping fits in Figure 5B, even if the helium enrichment of soil gas seems to be lower in 2017 for CO 2 concentrations below 80%. However, for a nearly pure CO 2 gas phase, 4 He concentrations were higher than previously measured. Despite a less sensitive character, the new data still suggest that helium is a good proxy for detecting the presence of deep-seated CO 2 .

The variability of the helium signal in soil gas may have at least three different origins. First, it may have been a result of the poor weather conditions during the 2017 fieldwork, with their potential impact on pressure and temperature gradients in soil that govern advective transport (e.g. [START_REF] Werner | CO 2 emissions related to the Yellowstone volcanic system. 2. Statistical sampling, total degassing and transport mechanism[END_REF][START_REF] Etiope | Migration of carrier and trace gases in the geosphere: an overview[END_REF][START_REF] Rinaldi | Effects of atmospheric conditions on surface diffuse degassing[END_REF]. Second, such variability may be caused by demixtion phenomena that are variable over time [START_REF] Gal | Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)[END_REF]. Third, such variability in the helium signal this may also be caused by longterm variability of the deep-seated gas efflux (e.g. [START_REF] Chiodini | Long-term variations of the Campi Flegrei, Italy, volcanic system as revealed by the Accepted Manuscript monitoring of hydrothermal activity[END_REF][START_REF] Werner | Decadal-scale variability of diffuse CO 2 emissions and seismicity revealed from long-term monitoring References[END_REF].

Finally, our data show a good correlation between CO 2 flux and O 2 depletion. Figure 5C presents this relation with reference to the domain of biological CO 2 efflux as recently established in France by [START_REF] Pokryszka | Valeurs de référence du flux de CO 2 et de CH 4 d'origine naturelle biogénique des sols[END_REF]. The maximum efflux mentioned by these authors is close to 84 g.m -2 .d -1 (or 30 cm 3 .min -1 .m -2 ) for summer acquisitions and close to 34 g.m -2 .d -1 (or 12 cm 3 .min -1 .m -2 ) for winter acquisitions, the last value being closer to maximum values for biogenic flux reported in other areas (e.g. [START_REF] Werner | CO 2 emissions from the Yellowstone volcanic system[END_REF] 30 g.m -2 .d -1 ). Any CO 2 flux over 84 g.m -2 .d -1 strongly suggests a relation with deep-seated processes. Figure 5C shows a fairly good relationship between the CO 2 flux and O 2 depletion, with large depletions associated with strong emission of CO 2 into the atmosphere. This is the translation on the flux parameter of the flush effect existing for soil-gas concentrations, this effect being induced by the intrusion of deep CO 2 in the soil that sweeps away the naturally existing O 2 +N 2 pool in soil. When the strength of the CO 2 efflux is high, then the sweeping of O 2 can also be monitored using the depletion of O 2 concentrations at the soil/atmosphere interface. Such monitoring may be an interesting -and complementary -tool for testing the strength of deep-seated gas emissions.

Correlation-matrix calculation fails to suggest the existence of other relations between the different gas phases. It is nonetheless important to assess if CO 2 flux is related to CO 2 or 4 He soil-gas concentrations, markers of deep-seated processes in the present case (Figure 6).

Four groups are defined in Figure 6A. Group A has typical CO 2 -flux values overlapping the ones of biological sources, though the CO 2 concentrations may be much higher than those of biologically produced CO 2 in soil gas. Group B marks a clear departure from this biological Accepted Manuscript pool, especially when considering the high CO 2 concentration values, and group C is only related to CO 2 leaks from depth. There is a graphical trend from Group A to Group C, which suggests that the higher the CO 2 concentration, the higher will be the CO 2 flux. Group D corresponds to high CO 2 concentration in the soil, but low flux at the surface. Figure 6B shows that this 'low CO 2 flux' group is He-rich too, suggesting that the soil structure has an influence on gas emission to the surface (June 26 th acquisitions) and that an additional influence of precipitation may be invoked for acquisitions made on June 27 th .

The difficulty of monitoring low fluxes over soils that only have CO 2 as a soil-gas phase may be linked to several reasons. One reason is the presence of variable amounts of clay in the soil in the area (Figure 7). Because of changing weather conditions and large precipitation events, the upper part of the soil can reach near-saturation conditions (low CO 2 flux), but at 1-m depth the soil may allow efficient gas advection. Another reason may be the distance from the main CO 2 supplying areas, i.e. from highly permeable fault zones. Above a degassing structure, high flux and high CO 2 concentrations are supposed to occur conjointly (Gal et al., 2011[START_REF] Gal | What can be learned from natural analogues studies in view of CO2 leakage issues in Carbon Capture and Storage applications? Geochemical case study of Sainte-Marguerite area (French Massif Central)[END_REF], and references therein), but at some distance of such structures, the strength of advection processes may be lower, thus lowering the efflux at the soil/atmosphere interface. This reasoning can be extended to the helium concentrations in soil gas. Tendency (1) in Figure 6B is found close to or above a deep-rooted degassing structure where a strong ascent of gas causes combined strong CO 2 and 4 He enrichment (>30 ppm). Nonetheless, as highlighted by Tendency (2), the higher 4 He values in soil are not found in high CO 2 flux zones (i.e. they are found with some distance to the degassing structures), but they correspond to 100% of CO 2 in the soil. In that case, soil permeability may play a role even if some authors consider permeability to be of secondary importance [START_REF] Camarda | CO 2 flux measurements in volcanic areas using the dynamic concentration method: Influence of soil permeability[END_REF]. More precisely, the presence of water in the upper part of the soil may be the cause. If the first 10-20 cm are saturated after precipitation, then the emissivity of the soil will be low (low CO 2 flux), but farther down in the soil, the soil-gas concentrations can be high: the data shown on Figure 4 suggest that high CO 2 concentrations are widespread at the scale of the Escarot site so that having 100% CO 2 in soil gas is a common phenomenon. Helium has a larger diffusive capacity than CO 2 (e.g. [START_REF] Jähne | Measurement of the diffusion coefficients of sparingly soluble gases in water[END_REF], as well as a much lower solubility in water (e.g. [START_REF] Weiss | Solubility of helium and neon in water and seawater[END_REF][START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF]. If a saturated horizon is present in the upper part of the soil, then the barrier effect ("soil capping", [START_REF] Werner | CO 2 emissions related to the Yellowstone volcanic system. 2. Statistical sampling, total degassing and transport mechanism[END_REF] induced by this horizon may not be noticed when considering CO 2 concentrations (high background), but it may cause an increase in 4 He concentration as the leaking of helium to the atmosphere is considerably reduced because of the presence of a water-saturated horizon. Lateral diffusion of 4 He may also contribute to local enrichment.

Relationship with meteorological parameters

Figure 3 illustrates the changing weather conditions during sampling. This offered an opportunity further to evaluate the influence these parameters may have on flux quantification, even it is preferred to sample soil gases during periods of dry weather.

Correlation-matrix calculations highlight a good correspondence between the changes in temperature and relative humidity (Pearson's coefficient of correlation r of -0.887), of temperature and pressure (r of +0.876), and thus of pressure and relative humidity (r of -0.816). The amounts of precipitation also negatively correlated with the relative humidity (r of -0.830). Huge amounts of rain fell during the strong precipitation events, increasing the humidity at the soil/atmosphere interface. By the end of the sampling period, the amount of precipitation was much lower, but the cumulative amount was high and thus the rise of relative humidity continued. Nonetheless, these calculations fail to illustrate if there is a global influence of these meteorological parameters onto the soil-gas and soil-flux data. For instance, correlation coefficients are <0.1 (r) when comparing CO 2 -flux data with temperature, pressure or relative humidity. This suggests that external parameters exert little to insignificant influence on degassing processes; this is congruent with a deep-seated origin of the gases and with the low variability of the degassing strength that is expected for a monitoring period of short duration (e.g. [START_REF] Werner | Decadal-scale variability of diffuse CO 2 emissions and seismicity revealed from long-term monitoring References[END_REF]. However, it is well established that external parameters can have a strong influence on the acquisitions (e.g. [START_REF] Oertel | Greenhouse gas emissions from soils -A review[END_REF], and especially on flux data when considering the duration of our survey. A time span of three days may be too short to allow the percolation of water to 1-m depth, particularly during summer when water is easily taken up by the vegetation. For instance, [START_REF] Werner | Decadal-scale variability of diffuse CO 2 emissions and seismicity revealed from long-term monitoring References[END_REF] report a decrease of emission rates by a factor of 2-3 over several days after the passage of a weather front associated with high winds and strong precipitation: they highlight the influence of wind in this decrease as this may change the pressure gradient in soil. This may be less obvious for our dataset because the wind speed measured during the survey (maximum 3.33 m.s -1 ) and far below the 14 m.s -1 reported by [START_REF] Werner | Decadal-scale variability of diffuse CO 2 emissions and seismicity revealed from long-term monitoring References[END_REF]. As temperature, pressure, amount of precipitation and relative humidity are interdependent, it is not easy to designate the most influent parameter, though rain and its consequences on soil moisture are of primary importance (e.g. Viveiros et al., Accepted Manuscript 2014, and references therein). We will thus use the generic term "precipitation event" to describe the influence of external parameters on flux evaluation. Some examples are shown in Figure 7 to illustrate different aspects. Short-term variations under consistent weather conditions were assessed through repeat acquisitions at points Z13, Z18 and Z19 before precipitation events (grey and blue dots on Figure 7) and at point E12 after several precipitation events, in order to evaluate the bias that may exist for high CO 2 fluxes. Under dry weather conditions, the variability in CO 2 flux was between 2.5% and 12% for points showing similar degassing properties (points Z13 and Z18). This 2.5-12% variability is in reasonable agreement with the standard error of this method (e.g. [START_REF] Chiodini | Diffuse emission of CO 2 from the Fossa Crater, Vulcano Island (Italy)[END_REF][START_REF] Evans | High CO 2 emissions through porous media: transport mechanisms: and implications for flux measurement and fractionation[END_REF]. This is also similar to what is reported for the amplitude of flux variation even under stable weather conditions (10% to 20%; [START_REF] Viveiros | Periodic behavior of soil CO 2 emissions in diffuse degassing areas of the Azores archipelago: Application to seismovolcanic monitoring[END_REF]. The variability can be much higher when gas flushes exist (300% of variability for point Z19). High intrinsic variability may thus be caused by these flushes of unpredictable variability, which were common during our acquisitions and represent fluctuations of the advective component [START_REF] Werner | CO 2 emissions related to the Yellowstone volcanic system. 2. Statistical sampling, total degassing and transport mechanism[END_REF]. For point E12, the reduction of flux was around 150% and associated with increased soil moisture, but no acquisition was done before precipitation events so that the decrease between dry and wet conditions may be much larger.

The occurrence of a precipitation event may not always points to a decrease in CO 2 flux. An increase of the flux was monitored at point Z13 after the first precipitation event (Figure 7).

As shown on the three photographs above the plot, the area of point Z13 was bare and the soil was humid (31 mm precipitation, evening of 26 June). Surprisingly, this had no influence on the gas emission as the flux was 25% higher. This is certainly due to the location of this point Z13, as shown by the strong bubbling on 28 June after another large precipitation event. The monitoring point was just above a degassing centre and soil saturation had almost no influence on this process at least over a short period. Though not frequent, such an increase of gas flux after rainfall may also occur in areas where water taken up by the vegetation, such as point EZ12. General flux was low, in the range of biological CO 2 flux [START_REF] Pokryszka | Valeurs de référence du flux de CO 2 et de CH 4 d'origine naturelle biogénique des sols[END_REF], but an increase of +60% was measured after the second precipitation event. Some of this variability may also be linked to the daily variability of biological soil-gas efflux (e.g. [START_REF] Parkin | Temperature controls on diurnal carbon dioxide flux: implications for estimating soil carbon loss[END_REF].

Normally, flux (e.g. [START_REF] Werner | CO 2 emissions related to the Yellowstone volcanic system. 2. Statistical sampling, total degassing and transport mechanism[END_REF] and soil-gas concentrations (e.g. [START_REF] Hinkle | Environmental conditions affecting concentrations of He, CO 2 , O 2 and N 2 in soft gases[END_REF] decrease after precipitation events. This is illustrated for flux by repeat acquisitions at points CZ4, Z18 and Z19 (Figure 7); a decrease by a factor 5 or 10 occurred after 31 mm of rain (respectively, points Z18 and Z19). [START_REF] Werner | CO 2 emissions related to the Yellowstone volcanic system. 2. Statistical sampling, total degassing and transport mechanism[END_REF] reported a decrease by a factor 2 after 21 mm rain in the Yellowstone area. Further decrease may occur as the cumulative precipitation increases (-15% at Z18, 8-fold decrease at CZ4). Such huge decreases are the direct result of soil capping, which is maximized when acquisitions occur during precipitation events. It is thus recommended to wait for two days without any precipitation to get rid of the potential effects of water saturation in the soil pores (e.g. [START_REF] Fridriksson | CO 2 emissions and heat flow through soil, fumaroles, and steam heated mud pools at the Reykjanes geothermal area, SW Iceland[END_REF], but this was not possible during our survey.

Soil properties must be considered as well. In volcanic areas, high degassing zones are generally located on or near faults, meaning that soil permeability will have only minor influence on flux values [START_REF] Camarda | CO 2 flux measurements in volcanic areas using the dynamic concentration method: Influence of soil permeability[END_REF], and a shallow soil cover may only slightly affect the shape and size of the soil degassing anomalies. However, this general statement has to be verified for each study [START_REF] Viveiros | Periodic behavior of soil CO 2 emissions in diffuse degassing areas of the Azores archipelago: Application to seismovolcanic monitoring[END_REF], and in our case the influence of the soil cover is shown by the increase in CO 2 flux at point C9 (Figure 7). Here, a first direct measurement was followed by the removal of the uppermost few centimetres; the second Accepted Manuscript measurement, 15 minutes after the first, then showed a 6-fold increase in CO 2 flux. This delay may be too short to reach stability of the gas efflux; [START_REF] Fridriksson | CO 2 emissions and heat flow through soil, fumaroles, and steam heated mud pools at the Reykjanes geothermal area, SW Iceland[END_REF] indeed mentioned a duration of 30 minutes to reach steady-state conditions. Nevertheless, the amplitude of the increase also suggests that the clayey topsoil strongly reduced the gas emission, especially when rain fell before the acquisitions. This may explain some of the discrepancies between soil-gas concentrations and soil-gas flux, the second being far more affected by the humidity and type of soil. There may be exceptions to this rule as shown by the data from point E12, which is close to a road on gravel banks that are highly permeable even under wet conditions. Some variability (40%) was noted between repeat acquisitions, but this was largely due to uncertainty on the rate of CO 2 enrichment in the chamber in the case of very high emissions.

Precipitation events were common during this survey, more or less affecting three quarters of the acquisitions. Direct use of these field acquisitions to calculate the CO 2 budget into the atmosphere would probably underestimate this budget. We thus corrected the dataset to arrive at a better assessment of the gas emissions, based on relationships established between the data points that were measured before and after precipitation events.

Based on the above-described data, this underestimate can be evaluated at up to 500% for a flux range of 96-720 g.m -2 .d -1 just after a large precipitation event, whereas higher fluxes (>1440-1920 g.m -2 .d -1 ) that are found in areas with potential bubbling manifestations are not really modified. When precipitation events are less intense but last for several hours, an additional drop of the flux by approximately 15% is seen. We thus propose the following empirical correction procedure: 1) 45 data from 26 June: no correction; 2) Data from 27 June: if CO 2 flux is <240 g.m -2 .d -1 the value is multiplied by 5 (49 data); if the flux is 240 to 1440 g.m -2 .d -1 the value is multiplied by 2 (5 data); if CO 2 flux >1440 g.m -2 .d -1 there is no correction (2 data); 3) Data from 28 and 29 June: similar correction as for 27 June data and an additional correction of +15% (multiplied by 5 +15%: 28 data; multiplied by 2 +15%: 2 data; no correction: 3 data). Data corrected only for pressure and temperature effects and those including further correction steps (gas flush and precipitation events) are compared in Figure 8 (data shown in Supplementary table ). The effect of this correction procedure is greatest for low fluxes considered as extreme. Some low CO 2 flux values after precipitation correction are still less than 40 g.m -2 .d -1 , but most are much higher and point to CO 2 flux values far above biological ones. Even if this procedure likely represents an over-correction of the data, it should still be used when evaluating CO 2 release at site scale, to give a range indication of the maximum expected gas release.

Background values and threshold levels

Soil-gas and soil-flux data are usually processed in order to determine threshold values from the data distribution. Such threshold levels help in better distinguishing between a pure biological, deep-seated, or mixed origin (e.g. [START_REF] Bloomberg | Soil CO 2 emissions as a proxy for heat and mass flow assessment, Taupo Volcanic Zone[END_REF], using the procedure proposed by Sinclair (1974) that relies on probability plot calculations. This procedure may be adapted when the dataset is skewed, as is the case here with neither a normal nor log-normal distribution, except for helium data that range under normal law with a p-value of 0.20. Some inflections in the cumulative probability plot may exist (Figure 9).

In our case, the acquisitions were oriented towards definition of the mofette structure and not towards a characterization of the background population, which may correspond to soilrespired CO 2 emissions. Nevertheless, an upper limit for biologically sourced CO 2 in the soil, close to 10% may be deduced from the 2017 CO 2 data (blue on Figure 9) and from the previous dataset (red on Figure 9). A similar result derives from the earlier described CO 2 -O 2 relationship. An upper threshold of 80-90% CO 2 may be defined, but its sole significance is to help locating potential gas vents that show strong emission into the atmosphere as a function of meteorological and soil parameters.

The helium data showed a similar distribution in 2017 (Escarot area) and in 2008-2012 (over a much larger area), suggesting some representativeness. A lower threshold of 5.5 ppm can be defined and corresponds to near-atmospheric values accounting for a measuring error of 5% (5.24 x 1.05 = 5.5). The upper threshold is around 24 ppm and any value above this threshold will indicate the presence of deep-seated gas phases.

Finally, depending on which CO 2 flux data are considered (Figure 9), the biological background value can range from <33.6 g.m -2 .d -1 (precipitation correction taken into account) to <74.4 g.m -2 .d -1 (data normalized to STP conditions). The higher value is close to the upper limit defined by [START_REF] Pokryszka | Valeurs de référence du flux de CO 2 et de CH 4 d'origine naturelle biogénique des sols[END_REF] for French soils. Higher flux values are typical of strong gas emissions with a growing influence of deep-seated CO 2 feeding. Mixed sources (biological and deep-seated) may exist until the 240 g.m -2 .d -1 threshold level is reached.

Figure 9

Accepted Manuscript

Data interpolation

As may be expected from the skewed data that were acquired near the gas seeps, all gas data show an anisotropic behaviour as pointed out by variogram calculations. Variograms for soil-gas data (data not shown) are best fitted using a preferential distribution along a N60° axis, whereas soil-gas flux data rather show a preferential distribution along a N25° axis. The lag distances were calculated between 50 and 60 m except for helium (35 m). This agrees with the size of emitting areas reported elsewhere for CO 2 or CH 4 degassing structures (e.g. [START_REF] Battani | Geochemical study of the natural CO2 emissions in the French Massif Central: How to predict origin, processes and evolution of CO 2 leakage[END_REF][START_REF] Etiope | Origin and flux of a gas seep in the Northern Alps[END_REF]. These data were used to construct interpolated maps for an area of about 310 m N-S by 170 m E-W.

The interpolated maps for CO 2 flux and correlative O 2 depletion (Figure 10A andB) show several degassing centres that seem to be aligned along a roughly NNW-SSE direction.

Several seeps of secondary degassing power occur in the north of the area, between the piezometer and the first mofette centre (as defined by [START_REF] Gal | Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)[END_REF], releasing 2,400 to 3,600 g.m -2 .d -1 of CO 2 into the atmosphere. Interestingly, this NNW-SSE direction is also revealed by O 2 depletion data at the soil/atmosphere interface, indicating the most efficient degassing centres. Two main such centres are located in the south of the area. The less emissive one releases about 6,000 g.m -2 .d -1 of CO 2 , whereas the most active can reach over 14,000 g.m -2 .d -1 of CO 2 (Point E12, Figure 7). This most active area, set close to the road over a gravel bed, shows no traces in the vegetation (grass) that might suggest the existence of such strong CO 2 release. Compared, e.g, to the Latera gas vents, of lower release strength (2,000 to 3,000 g.m -2 .d -1 ; [START_REF] Beaubien | The impact of a naturally occurring CO 2 gas vent on the shallow ecosystem and soil chemistry of a Mediterranean pasture (Latera, Italy)[END_REF] but without vegetation, the absence of visible effects at Escarot is probably related to the absence of reducing gas species such as Accepted Manuscript H 2 S or CH 4 , which are released at Latera. Based on the maps constructed from STPnormalized flux or that corrected for precipitation effects, an inner structuring of these high release seeps seems to exist. This potential structure is oriented NE-SW, but, as the map built using the flux accounting from gas flushes does not reveal the same secondary structuring, this may be an artefact from kriging calculations and thus little confidence is given to this feature.

The soil-gas concentration measurements shown on Figure 10C include all the available (2008-2012 and 2017) data. The resulting interpolated maps show a good spatial agreement between these data, as was expected from probability-plot results (Figure 9). The data define several structures in the soil where CO 2 and 4 He enrichment and the correlative O 2 depletion may be very high. The mofette and the piezometer are located on two of these structures that are also aligned along a NNW-SSE direction. Helium enrichment appears to be less important to the southeast, but this effect may be caused by a lack of data is this area.

At site scale, the soil-gas and soil flux measurements correlate. Areas where CO 2 flux is high are rich in CO 2 in the soil, but not all areas rich in CO 2 in the soil show high CO 2 release into the atmosphere. Similar spatial correlations were reported by [START_REF] Battani | Geochemical study of the natural CO2 emissions in the French Massif Central: How to predict origin, processes and evolution of CO 2 leakage[END_REF] and highlight the role of soil in the shape and size of degassing [START_REF] Camarda | CO 2 flux measurements in volcanic areas using the dynamic concentration method: Influence of soil permeability[END_REF].

The structure and surface arrangement of degassing centres perfectly match the known regional tectonic structures. [START_REF] Brousse | Explanatory Notes to the 1:50,000-scale geological map of La Tourd'Auvergne[END_REF] reported the existence of NE-SW to NNE-SSW faults whose directions were inherited from pre-existing fractures in the Hercynian basement. Locally, these authors also mention the existence of a recent NNW-SSE fault (Figure 10B) that separates two series of the Mont-Dore volcanism (3 to 1.2 Ma-old basalt to the west and 1.2 to 0.3 Ma-old benmoreite to the east). Hydrothermal traces exist along this fault over some tens of metres across the fault trace. This fault becomes invisible about 300 m north of the Escarot piezometer after offsetting the surficial formations filling the valley. Farther south this fault has no surface expression, but our data show that its trace can be extended as it still has some permeability for gas phases.

Figure 10

Quantifying gas release to the atmosphere

The strength of degassing and thus the permeability of the fault to gas flow can be assessed using the interpolated maps described above (Figure 10). Such evaluation is based on volume calculations functionalities that are found in e.g. Surfer software (Goldensoftware), provided the coordinates are in metric scale (e.g. [START_REF] Werner | CO 2 emissions related to the Yellowstone volcanic system. 2. Statistical sampling, total degassing and transport mechanism[END_REF]. Table 1 shows some results.

The CO 2 that is released into the atmosphere may not all be of deep-seated origin (Figure 5).

Three different levels of background biological flux are considered: 30 g.m -2 .d -1 , the common upper limit for biological flux; 48 g.m -2 .d -1 , the upper limit for grassland and agricultural soil; and 84 g.m -2 .d -1 , the upper limit reported for French soils. Each increase of the biological CO 2 flux induces a decrease of the deep-seated CO 2 component.

The first evaluation refers to the STP-normalized CO 2 flux, without considering any further correction. The second evaluation includes a correction to take gas flush into account. The third evaluation adds a correction to consider the effect of precipitation events, as earlier mentioned. Finally, a basic estimate is given, simply using the surface of the monitored area (52,000 m 2 ) and the mean CO 2 flux (576 g.m -2 .d -1 ). The end result is an emission of 30 tons per day (tpd) of CO 2 into the atmosphere.

The first two evaluations gave quite similar results: the area affected by degassing is smaller than 32,000 m 2 (60% of the investigated area) and larger than 17,000 m 2 (33% of the investigated area). The amount of biologically produced CO 2 for a 52,000 m 2 area is between 1.6 and 4.4 tpd (depending on soil productivity). Interpolation calculations show that some areas may not be as productive as expected, so the biological CO 2 flux was re-estimated at between 0.7 and 2 tpd. This correction could only be a calculation bias linked to the procedure itself, but it may also be representative: biological flux can naturally vary and may be low in areas where deep CO 2 escapes. The resulting quantity of such deep-seated CO 2 is evaluated between 17.3 and 20.8 tpd. The third evaluation, including the flux decrease related to precipitation events, leads to quantities of emitted CO 2 very close to that of the basic estimate (27 to 30 tpd), suggesting that even the most basic assumption can lead to a realistic result, where the role of biological CO 2 flux is higher but nearly all the area appears to be affected by diffuse degassing. This may be an optimistic appraisal, but we nevertheless used it for proposing an overall estimate of the CO 2 budget, based on the mean value of all estimates presented in Table 1. The CO 2 degassing over 0.052 km 2 at Escarot is thus estimated at between 8100±1800 tons per year with an additional biological contribution of 660±400 tons per year, or respectively 22.2±4.9 and 1.8±1.1 tpd. This amount of deep-seated CO 2 is three times larger than that reported regionally from Sainte-Marguerite, 35 km to the north-east, where a CO 2 release of 8.5 tpd over 0.06 km 2 was estimated [START_REF] Battani | Geochemical study of the natural CO2 emissions in the French Massif Central: How to predict origin, processes and evolution of CO 2 leakage[END_REF]. This quantity is similar to that emitted by diffuse degassing in volcanic environments, such as in Greece (25 tpd, 0.346 km 2 ; Brombach et al., Accepted Manuscript 2001), in Italy (Stromboli Island: 20 tpd on the flanks; [START_REF] Inguaggiato | CO 2 output discharged from Stromboli Island (Italy)[END_REF], in Japan (74 tpd of hydrothermal CO 2 reported over 0.58 km 2 by [START_REF] Hernández-Perez | Carbon dioxide emissions from soils at Hakkoda, north Japan[END_REF], or in New-Zealand (Taupo Volcanic Zone: from 8±3 tpd over 0.3 km 2 to 72±13 tpd over 0.19 km 2 ; [START_REF] Bloomberg | Soil CO 2 emissions as a proxy for heat and mass flow assessment, Taupo Volcanic Zone[END_REF]. Far higher emissions are nevertheless known from other volcanic areas (e.g. Viveiros et al., 2010;[START_REF] Burton | Deep carbon emissions from volcanoes[END_REF]and references therein) and also for other mofettes in Europe (1566 tpd over 0.075 km 2 in the Czech Republic; [START_REF] Kämpf | CO 2 discharge in an active, non-volcanic continental rift area (Czech Republic): Characterisation (δ 13 C, 3 He/ 4 He) and quantification of diffuse and vent CO 2 emissions[END_REF]. Table 1

Links to risk assessment

The second aspect of this study relates to risk assessment, of interest to the populated areas that exist a few dozen kilometres to the north-east. The Escarot area is not inhabited, but cattle grazing close to, or over, the seep apparently is unaffected by the strong gas emissions. However, this situation can be different when transposed to urban areas. Gasemission occurrences are known from the French Massif Central, especially in its capital city Clermont-Ferrand, where some degassing spots release as much as 18 to 100 tons CO 2 per year over 1 m² [START_REF] Gal | What may be the consequences of a CO 2 leakage? Insights from soil gas measurements in an urban area -Clermont-Ferrand, French Massif Central[END_REF]. In an urban context, buildings and other constructions often hamper and modify natural degassing pathways, thus focusing and/or enhancing the release, possibly causing gas accumulations in foundations or cellars. For comparison, the highest emission rate measured at Escarot leads to a release of 'only' 7.5 tons CO 2 per year over 1 m². As a result, the population could be exposed to a gas hazard, but there may be no warning signal, such as H 2 S, in the gas phase or damage to the vegetation. The Escarot case study can be seen as an analogy to the situation prevailing before human occupation. It Accepted Manuscript strongly suggests that further investigations are needed in (future) urban areas, to evaluate the potential hazard and, if yes, to assess the composition and volume of gas releases, as is done in Italy (e.g. [START_REF] Carapezza | Hazardous gas emissions from the flanks of the quiescent Colli Albani volcano (Rome, Italy)[END_REF].

Concluding remarks

With the aim of better evaluating emissions of non-anthropogenic gas and their influence on the global carbon cycle, the 2017 fieldwork on the Escarot site revealed that CO 2 emissions into the atmosphere are not of minor importance compared those reported from volcanically more active environments. At ground level, a NNW-SSE elongated structure was revealed, marked by several degassing centres that are located as a function of soil properties and their relative permeability to gas transport. In the soil, the presence of CO 2 is even more common, and a clear relationship with local and regional fault directions was established. Local biological CO 2 production can be high (up to 10% of soil-gas), thus contributing to a total mean CO 2 release into the atmosphere of 660 tons per year (tpy) for the 52,000 m² area. This agrees with the magnitude of CO 2 emissions reported by [START_REF] Hermansson | Geogas-a carrier or a tracer? Statens Karnbransle Namnd (SKN) Report[END_REF], when extrapolated to a similar scale (310 tpy). The present case highlights the need for better understanding the CO 2 emissions at the scale of the French Massif Central, because existing evaluations may have missed far higher CO 2 emission rates associated to mineral springs and/or gas vents. The existence of structures emitting CO 2 in populated areas should also be taken into account in risk scenarios.

The deep-seated contribution is evaluated at over 8,000 tpy of CO 2 and compares well with gas emissions reported for diffuse degassing in some active volcanic areas. However, gas emissions may vary throughout the year, depending on the season and on meteorological Accepted Manuscript conditions. Very strong, though temporary, decreases of flux into the atmosphere (up to a factor 5) are caused by precipitation that saturates the upper soil horizons, and the presence of snow cover during winter may further reduce the yearly CO 2 budget. In addition, the rise of deep-seated CO 2 is not a linear process and high-frequency changes in gas-emission rates have been noted. The result is a quite high uncertainty as to the quantification of such gas emissions (±1,800 tpy), or about one quarter of the overall estimated amount of CO 2 . 908 (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) Top: Location of soil-gas sampling sites (red squares) performed around Lake Pavin [START_REF] Gal | Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)[END_REF].

Figure captions

Bottom: Detailed view of the Escarot site: yellow arrows indicate the precise position of the mofette as defined in the 2011 paper and the piezometer defined as a "wet mofette" by [START_REF] Bräuer | Regional distribution pattern of carbon and helium isotopes from different volcanic fields in the French Massif Central: Evidence for active mantle degassing and water transport[END_REF]. Sampling sites of soil-gas flux and soil-gas concentration in June 2017 are indicated by blacked and concentric circles, respectively. Blue pictograms show areas where gas bubbles are observed in stagnant surface puddles.

Background satellite view from Google Earth. Gal and Galalia (2011) plus some unpublished data from 2012.

Other emissions to the atmosphere

The rise of deep-seated CO 2 to the surface and its final emission into the atmosphere is not only accompanied by the degassing of other gases, such as helium, but also by the release of chemical elements such as trace metals. This aspect of degassing is less studied than the emission of gases itself, but, e.g. [START_REF] Mehlhorn | Carbon dioxide triggered metal(loid) mobilisation in a mofette[END_REF] described the consequences of trace-element release/remobilization associated with mofettes, and [START_REF] Bagnato | First combined flux chamber survey of mercury and CO 2 emissions from soil diffuse degassing at Solfatara of Pozzuoli crater, Campi Flegrei (Italy): Mapping and quantification of gas release[END_REF] discussed mercury releases associated with volcanic centres. Other work was done for mineral exploration purposes [START_REF] Pauwels | Sorption of metallic compounds on activated carbon: application to exploration for concealed deposits in southern Spain[END_REF][START_REF] Castillo | Soil gas geochemical exploration in covered terrains of northern Chile: data processing techniques and interpretation of contrast anomalies[END_REF][START_REF] Noble | Soilgas and weak partial soil extractions for nickel exploration through transported cover in Western Australia[END_REF].

Here we present some data showing that the Escarot mofette is also prone to such emission of elements. These data were not collected using the approach referring to the use of passive samplers and the adsorption of aerosols; they are the result of a one-day sampling session in February 2009 when a snow cover was present (Figure A). We used this snow cover as a natural passive sampler to test if some gases-and thus some chemical elements-can percolate and accumulate in the snow.

On this February day, three locations were investigated. First, the snow cover was removed and the soil-gas atmosphere at 1-m depth was analysed. This allowed defining a location characteristic of the mofette (referred to as Pt200; 100% CO 2 concentration in the soil gas), a location set some metres away, in a less active environment (Pt2000; 22.5% CO 2 ) and finally a remote location (Pt2001; 0.4% CO 2 ). Once the soil gas was characterized, we installed PVC pipes at 2 to 3 different levels in the snow cover, to collect snow for subsequent laboratory analyses and to measure the CO 2 concentration once the pipe was removed.

The most interesting results came from Point 200 (Figure A-A). The gas phase in the snow cover was influenced by CO 2 release from the mofette: 5 cm above ground level, the concentration was as high as 10.7%. The concentration then decreaseds to 2.5%, 30 cm above the ground level and to 0.1%, 60 cm above the ground level (or 10 cm below the snow/atmosphere interface). The other two points did not show any enrichment in CO 2 in the snow thus confirming that only areas with high CO 2 fluxes can produce a gas flux that is strong enough to percolate upward into the snow cover.

The concentrations in dissolved elements were also informative (Figure AB and-C). Points 2000 and 2001 have similar element concentrations and show little variability with emplacement in the snow cover. Na-Cl contents appear are homogeneous at site scale with a slight tendency of higher contents close to the snow/atmosphere boundary. This may be a consequence of salt spreading on roads, remobilization as dust when the salt has dried and is dispersed by wind. This also highlights that there is no input of Na or Cl from the gas vent. However, enrichment in Ca and K exists for Pt200 5 cm above the soil (enrichment factor: 4 to 5). The concentrations decrease progressively as the distance with soil increases. Similar statements can be made for trace elements such as Pb, Zn, Sr, Cd, Mn, Ni (enrichment factors of 3 at least) and also for Al or As (not shown in Figure A1). Some species are not detected at Pt2000 or Pt2001 and are only occur close to the soil surface at Pt200, such as Fe and Cr and Li (not shown in Figure A). But these data are too scarce to pretend at a general rule on the release of elements associated to gas seeps, even if they highlight a common behaviour between these two endmembers near the surface.

  Figure3

Figure 4

 4 Figure 4

Figure 5 Figure 6

 56 Figure 5

  Figure 7

Figure 1 :

 1 Figure 1: Location of the study area.

Figure 2 :

 2 Figure 2: Typology of the flux emissions (CO 2 flux and O 2 depletion).

Figure 3 :

 3 Figure 3: Top: Atmospheric meteorological conditions (air temperature, atmospheric

Figure 5 :

 5 Figure 5: A: O 2 vs. CO 2 binary plot; dilution lines correspond to the intrusion of CO 2 as a pure

Figure 6 :

 6 Figure 6: A: CO 2 concentration vs. CO 2 flux binary plot.

Figure 7 :

 7 Figure 7: Influence of precipitation events on flux acquisition -see text for explanation. The

Figure 8 :

 8 Figure 8: Relation between STP-corrected CO 2 flux and CO 2 flux corrected from gas flushes

Figure 9 :

 9 Figure 9: Probability plots for CO 2 and helium concentrations, and CO 2 flux. CO 2 -flux data are

Figure 10

 10 Figure 10: A: Various interpolated maps for CO 2 flux: from left to right: using STP normalized

Figure

  Figure A1: A: CO 2 concentrations in soil (1 m depth) and in the snow cover at 5, 30 and 50 or 60 cm above the ground level as measured on 29 February 2009; temperatures are also indicated -location of the points is plotted on the satellite picture; an overview of the procedure is given in the bottom picture. B: Some major-element (except Pb) concentrations measured in the snow 5, 30 and 50 or 60 cm above the soil surface. C: Some trace-element concentrations measured in the snow 5, 30 and 50 or 60 cm above the soil surface.

  at Mammoth Mountain, California, USA, Journal of Volcanology and 909 Geothermal Research, 289, 51-63.

	Accepted Manuscript

Table caption 970

 caption Table 1: evaluation of CO 2 emissions in the atmosphere; see text for details.

	Accepted Manuscript

Acknowledgments

The authors would like to thank C. Werner and an anonymous reviewer for their thoughtful comments, which greatly improved this manuscript. The 2017 work was carried out with the financial assistance of the BRGM Laboratory Division (IN-SITU project). The previous work was part of a project funded by the Natural Risks Department of the Puy-de-Dôme district and by the BRGM Research Division (PREVOIR project). Dr H.M. Kluijver revised the final English version of this MS.

Accepted Manuscript