Céline Charbonnier

Pierre Anschutz
email: pierre.anschutz@u-bordeaux.fr

Gwenaël Abril

Alfonso Mucci

Loris Deirmendjian

Dominique Poirier

Stéphane Bujan

Pascal Lecroart

Carbon dynamics driven by seawater recirculation and groundwater discharge along a forest-dune-beach continuum of a high-energy meso-macro-tidal sandy coast

Keywords: tidal beach, submarine groundwater discharge, subterranean estuary, aerobic benthic respiration, CO2 degassing, Aquitaine coast

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

A better understanding of the carbon cycle in coastal zones is critical as these environments are especially vulnerable to global warming and ocean acidification [START_REF] Gattuso | Carbon and carbonate metabolism in coastal aquatic ecosystems[END_REF][START_REF] Orr | Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms[END_REF][START_REF] Gattuso | Ocean acidification: background and history[END_REF]. Groundwater discharge through permeable coastal sediments is now recognized as an important phenomenon, connecting continental aquifers to the coastal ocean through subterranean estuaries (STE) [START_REF] Moore | The subterranean estuary: a reaction zone of groundwater and seawater[END_REF][START_REF] Bokuniewicz | The typological approach to submarine groundwater discharge (SGD)[END_REF][START_REF] Burnett | Groundwater and pore water inputs to the coastal zone[END_REF]Santos et al., 2012a). The behaviour of dissolved carbon in this transition zone needs to be studied in different physical settings (topography, flow, tides) in order to better constrain the role of submarine groundwater discharge (SGD) on the global, coastal carbon cycle. In intertidal sandy sediment, SGD includes both freshwater flow from a terrestrial aquifer and seawater recirculation driven by waves and tides [START_REF] Burnett | Groundwater and pore water inputs to the coastal zone[END_REF]. In high-energy sandy beaches, a well-developed intertidal seawater recirculation cell develops as a result of strong physical forcings, including tidal oscillations, swells, waves, and beach topography [START_REF] Robinson | Effect of tidal forcing on a subterranean estuary[END_REF][START_REF] Abarca | Transient groundwater dynamics in a coastal aquifer: the effects of tides, the lunar cycle, and the beach profile[END_REF]. This cell complements the structure of the subterranean estuary (STE) formed between fresh and saline groundwaters [START_REF] Moore | The subterranean estuary: a reaction zone of groundwater and seawater[END_REF].

The influence of SGD fluxes on coastal ecosystems is well documented for nutrients and trace metals, with fluxes [START_REF] Burnett | Groundwater and pore water inputs to the coastal zone[END_REF]2006;[START_REF] Slomp | Nutrients inputs to the coastal ocean through submarine groundwater discharge: controls and potential impact[END_REF][START_REF] Kim | Submarine groundwater discharge (SGD) into the Yellow Sea revealed by Ra-228 and Ra-226 isotopes: implications for global silicate fluxes[END_REF]. This can lead to pronounced ecological consequences in the coastal zone, such as eutrophication [START_REF] Lee | Estimating submarine discharge of fresh groundwater from a volcanic island using a freshwater budget of the coastal water column[END_REF][START_REF] Lee | A relationship between submarine groundwater-borne nutrients traced by Ra isotopes and the intensity of dinoflagellate redtides occurring in the southern sea of Korea[END_REF]Moore et al., 2010;[START_REF] Wang | Submarine Groundwater Discharge helps making nearshore waters heterotrophic[END_REF] and harmful algal blooms [START_REF] Lee | A relationship between submarine groundwater-borne nutrients traced by Ra isotopes and the intensity of dinoflagellate redtides occurring in the southern sea of Korea[END_REF], especially in semi-enclosed systems [START_REF] Liu | How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a riverdominated shelf system ?[END_REF][START_REF] Charette | Submarine groundwater discharge in a river-dominated Florida estuary[END_REF][START_REF] Santos | Influence of porewater exchange on nutrient dynamics in two New Zealand estuarine intertidal flats[END_REF]. Over the last decade, the carbon cycle in STEs has received growing attention, because STEs are sites of intense biogeochemical transformations.

As observed for nutrients, trace metals and/or radio-isotopic tracers [START_REF] Slomp | Nutrients inputs to the coastal ocean through submarine groundwater discharge: controls and potential impact[END_REF][START_REF] Charette | Trace element cycling in a subterranean estuary: Part 1. Geochemistry of the permeable sediments[END_REF][START_REF] Moore | The role of submarine groundwater discharge in coastal biogeochemistry[END_REF][START_REF] Swarzenski | Biogeochemical transport in the Loxahatchee River estuary, Florida: The role of submarine groundwater discharge[END_REF], the transfer of carbon from land to the sea is not conservative: significant chemical transformations linked to diagenetic reactions in the STE affect the CO2 system through carbonate dissolution and/or organic matter mineralization [START_REF] Cai | The geochemistry of dissolved inorganic carbon in a surficial groundwater aquifer in North Inlet, South Carolina, and the carbon fluxes to the coastal ocean[END_REF][START_REF] Liu | How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a riverdominated shelf system ?[END_REF][START_REF] Lee | Dependence of coastal water pH increases on submarine groundwater discharge off a volcanic island[END_REF][START_REF] Liu | Carbonate system biogeochemistry in a subterranean estuary -Waquoit Bay, USA[END_REF]. Although the relative importance of all carbon transfer processes from land to sea are not always considered, recent studies reveal that the terrestrial component of SGD is a significant carbon source to the coastal ocean [START_REF] Liu | How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a riverdominated shelf system ?[END_REF][START_REF] Atkins | Carbon dioxide dynamics driven by groundwater discharge in a coastal floodplain creek[END_REF][START_REF] Wang | Net subterranean estuarine export fluxes of dissolved inorganic C, N, P, Si, and total alkalinity into the Jiulong River estuary, China[END_REF][START_REF] Liu | Carbonate system biogeochemistry in a subterranean estuary -Waquoit Bay, USA[END_REF]. Indeed, fresh groundwaters are often enriched in CO2 due to organic matter mineralization within the surficial aquifer [START_REF] Gagan | Coral oxygen isotope evidence for recent groundwater fluxes to the Australian Great Barrier Reef[END_REF][START_REF] Savoy | Radon and CO2 as natural tracers to investigate the recharge dynamics of karst aquifers[END_REF]. They are also enriched in dissolved organic carbon (DOC), although marine-derived particulate matter may be the primary source of DOC in tidal sands [START_REF] Couturier | Microtidal subterranean estuaries as a source of fresh terrestrial dissolved organic matter to the coastal ocean[END_REF]. These waters can modify the carbon budget as they discharge in the coastal zone [START_REF] Cai | The geochemistry of dissolved inorganic carbon in a surficial groundwater aquifer in North Inlet, South Carolina, and the carbon fluxes to the coastal ocean[END_REF][START_REF] Moore | Estimates of flushing times, submarine groundwater discharge, and nutrient fluxes to Okatee Estuary, South Carolina[END_REF][START_REF] Liu | How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a riverdominated shelf system ?[END_REF]Santos et al., 2012b). In fact, a number of studies have reported that terrestrial groundwater discharge can be a significant source of CO2 and CH4 to the atmosphere in coastal areas (Cai et al., 2011;[START_REF] Atkins | Carbon dioxide dynamics driven by groundwater discharge in a coastal floodplain creek[END_REF][START_REF] Call | Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring-neap-spring timescales in a mangrove creek[END_REF][START_REF] Perkins | Groundwater seepage as a driver of CO2 evasion in a coastal lake (Lake Ainsworth, NSW, Australia)[END_REF].

Dissolved carbon fluxes from SGD have been investigated in various environments including estuaries (Santos et al., 2012a;[START_REF] Faber | Pore-water exchange driven by tidal pumping causes alkalinity export in two intertidal inlets[END_REF][START_REF] Macklin | Estuarine canal estate water : hotspots of CO2 outgassing driven by enhanced groundwater discharge?[END_REF][START_REF] Perkins | Groundwater seepage as a driver of CO2 evasion in a coastal lake (Lake Ainsworth, NSW, Australia)[END_REF][START_REF] Wang | Net subterranean estuarine export fluxes of dissolved inorganic C, N, P, Si, and total alkalinity into the Jiulong River estuary, China[END_REF], a coral reef lagoon [START_REF] Cyronak | Carbon cycling hysteresis in permeable carbonate sands over a diel cycle: Implications for ocean acidification[END_REF], mangrove creeks [START_REF] Bouillon | Importance of intertidal sediment processes and porewater exchange on the water column biogeochemistry in a pristine mangrove creek (Ras Dege, Tanzania)[END_REF][START_REF] Koné | Dissolved inorganic carbon dynamics in the waters surrounding forested mangroves of the Ca Mau Province (Vietnam)[END_REF][START_REF] Miyajima | Export of inorganic carbon from two Southeast Asian mangrove forests to adjacent estuaries as estimated by the stable isotope composition of dissolved inorganic carbon[END_REF][START_REF] Maher | Groundwaterderived dissolved inorganic and organic carbon exports from a mangrove tidal creek: The missing mangrove carbon sink ?[END_REF][START_REF] Call | Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring-neap-spring timescales in a mangrove creek[END_REF][START_REF] Chen | Submarine groundwater discharge-derived carbon fluxes in mangroves: An important component of blue carbon budgets?[END_REF], continental shelves [START_REF] Liu | How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a riverdominated shelf system ?[END_REF][START_REF] Liu | Effect of submarine groundwater discharge on the coastal ocean inorganic carbon cycle[END_REF][START_REF] Stewart | Submarine groundwater discharge and associated fluxes of alkalinity and dissolved carbon into Moreton Bay (Australia) estimated via radium isotopes[END_REF], and intertidal flats [START_REF] Moore | Radium-based pore water fluxes of silica, alkalinity, manganese, DOC, and uranium: a decade of studies in the German Wadden Sea[END_REF][START_REF] Kim | Production, degradation, and flux of dissolved organic matter in the subterranean estuary of a large tidal flat[END_REF][START_REF] Reckhardt | Carbon, nutrient and trace metal cycling in sandy sediments: A comparison of high-energy beaches and backbarrier tidal flats[END_REF][START_REF] Santos | Porewater exchange as a driver of carbon dynamics across a terrestrial-marine transect: insights from coupled 222Rn and pCO2 observations in the German Wadden Sea[END_REF], but very few studies have been carried out in high-energy beaches, even if the intertidal recirculation cell in such environments has been recognized as an intense biogeochemical reactor [START_REF] Anschutz | Tidal sands as biogeochemical reactors[END_REF][START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF][START_REF] Beck | The drivers of biogeochemistry in beach ecosystems: A cross-shore transect from the dunes to the low-water line[END_REF][START_REF] Reckhardt | Carbon, nutrient and trace metal cycling in sandy sediments: A comparison of high-energy beaches and backbarrier tidal flats[END_REF]. Recent studies show that recirculated saline pore water can deliver large amounts of recycled inorganic carbon to coastal surface waters [START_REF] Weinstein | Role of aquifer heterogeneity in fresh groundwater discharge and seawater recycling: An example from the Carmel coast, Israel[END_REF][START_REF] Gleeson | Groundwater-surface water exchange in a mangrove tidal creek: Evidence from natural geochemical tracers and implications for nutrient budgets[END_REF][START_REF] Liu | Effect of submarine groundwater discharge on the coastal ocean inorganic carbon cycle[END_REF].

The objective of this study was to identify biotic and abiotic processes that modulate/drive carbon dynamics in a very high-energy coastal system. These environments are characterized by a very strong recirculation of seawater due to tidal pumping. Since the fresh and saline SGD components are usually superimposed, it is difficult to separate their relative contributions. We first investigated whether the large volumes of water exchanged in the sandy beach aquifer resulted in a high carbon recycling. We then evaluated the carbon fluxes from the terrestrially-derived groundwater that percolated towards the beach. In doing so, we tested the hypothesis that, like metals and nutrients, DIC fluxes are not conservative when water from the continental aquifer mixed with saline water from the intertidal beach aquifer. Fluxes and processes were assessed from samples collected along the forest-dune-beach continuum.

Finally, we estimated, for the first time, carbon fluxes to the ocean and atmosphere at the scale of a 240-km long exposed coastline, allowing us to assess the importance of these systems in the carbon cycle and their impact on ocean acidification.

For this purpose, we deployed an elaborate sampling strategy, from land to sea, which allowed us to measure DOC, pH, total alkalinity (TA), and the isotopic composition of dissolved inorganic carbon (d 13 C-DIC), as well as calculate the dissolved inorganic carbon (DIC) and CO2 partial pressure (pCO2) in pore waters of all key compartments of a high-energy sandy beach along the Aquitaine coast (SW France) over multiple years. Pore water samples were collected in the intertidal beach, in the fresh-saline transition zone of the supratidal beach, and in the freshwater aquifer upstream of the beach 2. Materials and methods

Study site

The Truc Vert Beach is located a few kilometres north of the Cap Ferret sand spit, along the 240-km long Aquitaine sandy coast (SW France; Fig. 1). The beaches along this relatively straight coastline, between the Gironde and the Adour Estuaries, are meso-macro tidal and double-bar beaches subjected to high-energy conditions and bordered by high aeolian dunes [START_REF] Castelle | Double bar beach dynamics on the high-energy meso-macrotidal French Aquitanian Coast: a review[END_REF]. The Truc Vert Beach is typical of the Aquitaine coast and its difficult access has preserved it from human disturbance.

Sediment consists of medium-sized quartz sand with a mean grain size of 435 µm, a mean CaCO3 content of 1.2 wt% and a mean organic carbon concentration of 0.028 wt%. The porosity of the sand ranges from 0.38 to 0.42 [START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF]. The average tidal range is 3.2 m, extending up to 5 m during spring tides. The high tide water line is marked by the presence of scattered debris that consist mainly of driftwood, shells, minor plastic waste, and marine plants. The mean wave amplitude is 1.5 m, but can reach up to 10 m during winter storms. Ridge and runnel systems develop depending on meteorological conditions [START_REF] Michel | Short-term morphodynamic response of a ridge and runnel system on a mesotidal sandy beach[END_REF][START_REF] Castelle | Double bar beach dynamics on the high-energy meso-macrotidal French Aquitanian Coast: a review[END_REF].

The tidal regime, swells, and meteorological conditions influence the cross-shore topography of the beach and can generate large sand movements, with observed sand accretion or erosion exceeding 1 m within a few days at any given location [START_REF] Castelle | Equilibrium shoreline modelling of a high-energy meso-macrotidal multiplebarred beach[END_REF]. The extension of the intertidal zone also varies depending on topography, tidal range, and wave regime, with a beach width ranging from 80 to 200 m cross-shore. At high tide, the beach is immersed and the sand is saturated with water. At low tide, the sand surface of the lower beach remains saturated with water, whereas the water table (i.e., the boundary between the unsaturated and saturated zones of the sediment) at the upper beach stands more than one meter below the sand surface. Pore waters seeping out at the lower beach indicate that a large volume of water is flushed seaward at each ebbing and low tide. The seepage zone is located along a 10-50 m wide cross-shore region of the lower beach. The volume of pristine seawater entering the sand during each rising tide was estimated at approximately 15 m 3 per longshore meter [START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF]Fig. 2). Integrated over the whole Aquitaine coastline, the volume of seawater recirculating through the beach aquifer is about 2.5 km 3 each year.

The beach catchment, i.e., the land area that is directly drained by the beach (no drainage through rivers or lakes), was contoured using the Geographic Information System (GIS) ArcGIS. It is a 0 to 10 km wide strip of wooded land along the 240-km long coastline and covers 822 km 2 [START_REF] Anschutz | Terrestrial groundwater and nutrient discharge along the 240-km long Aquitanian coast[END_REF]. This coastal aquifer is composed of sand dunes, marine sands, and gravels of the Plio-Quaternary period. It is very permeable [START_REF] Legigan | L'élaboration de la formation du Sable des Landes, dépôt résiduel de l'environnement sédimentaire pliocène-pléistocène centre aquitain[END_REF] and sustains a terrestrial groundwater flux from the continent to the ocean through the percolation of brackish waters through the beach [START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF][START_REF] Anschutz | Terrestrial groundwater and nutrient discharge along the 240-km long Aquitanian coast[END_REF]. The volume of these discharging fresh groundwaters, estimated from a water balance calculation of the precipitation (about 810 mm y -1 at the Truc Vert Beach) and evapotranspiration (max. 570 mm y -1 ), is approximately 0.2 km 3 per year for the whole Aquitaine coast (Fig. 2; Anschutz et al., 2016). A revised value that takes into account the distribution of mean precipitations from the northern to the southern part of the coast, and not solely the value of the Truc Vert Beach, yields a value of 0.43 km 3 yr -1 [START_REF] Anschutz | Nutrient and carbon fluxes along the sandy beaches of the French Atlantic coast: recycling and groundwater discharge[END_REF].

Sampling

A variety of waters were sampled, including seawater, intertidal and supratidal beach pore waters, waters from the unconfined surficial aquifer located at the foot of the dune, and terrestrial groundwaters (Fig. 2). This assorted sample collection allowed us to construct a complete profile of the drainage waters from the forest watershed to the intertidal zone of the beach. In all cases, temperature, salinity, dissolved oxygen saturation, and pH were recorded on-site within 1 min of pore water retrieval using WTW probes. The probes were calibrated before and after each field campaign using an oxygen-free solution and an aerated solution (100% saturation) for the oxygen saturation, with IAPSO standard seawater and deionized water for the salinity, and with NIST-traceable buffer solutions (pH =4.01 and pH = 7.00 at 25°C) for the pH. Oxygen and salinity probe readings were compensated automatically for insitu temperature. When appropriate, the influence of salinity on the oxygen saturation was recalculated.

Waters destined for DOC analysis were sampled with a clean 50-mL glass syringe, filtered through pre-combusted (550°C) GF/F filters (porosity of 0.7 µm) into detergent-washed and pre-combusted Pyrex vials (30 mL), acidified with 50 µL of a 37% HCl solution to pH 2, and stored at 4°C. Waters for TA measurements were sampled using a 50-mL plastic syringe, filtered through a 0.45 µm cellulose acetate syringe-membrane into 150 mL polypropylene bottles. To analyse d 13 C-DIC, we sampled waters with a homemade funnel with a hose attached at the nozzle and from which we overfilled 100 mL glass serum vials. Glass bottles were sealed and samples were poisoned with 0.3 mL of a saturated HgCl2 solution to avoid bacterial activity.

Samples were stored in the dark before analysis.

Seawater and intertidal beach pore waters

Seawater and pore waters of the intertidal zone were sampled every two weeks during the spring tide periods and primarily on non-rainy days from February 2011 to November 2012.

Pore water sampling was conducted at low tide by digging holes along cross-shore transects to reach the top of the water table. The use of permanently installed piezometric wells to collect pore waters was ruled out because of the high-energy conditions in the intertidal zone. In previous attempts, piezometers were broken or quickly buried in the sand, sometimes after a single tidal cycle. Thirty-seven profiles were obtained during the 22 months of monitoring.

Parallel cross-shore transects were also carried out three times in 2012 for nutrient analyses and revealed that longshore spatial patchiness was low [START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF].

Holes were dug every 10 m from the low-tide water line to the high-tide watermark (Fig. 2). They were deep enough to reach the top of the water-saturated zone in the sand: one shovelful was needed in the lower beach, whereas more than 1.5 m deep holes were required in the upper beach. Whereas this method is invasive, given time and physical constraints, it is the only one that allowed us to obtain representative samples. Other sampling protocols, such as the use of piezometer wells and push-point pore-water samplers (lance), were tried, but were not practical or had major drawbacks. Because of shifting sand during storms, piezometers were often exposed, broken by waves and filled with sand, even above the high spring tide water line At deep sites on the upper beach, the insertion, pore water extrusion, and extraction of a lance could not be completed before the next rising tide. Furthermore, as we did not have prior knowledge of the depth of the water table, we still had to dig a hole to determine this variable.

The depth of the water table in the holes was between 10 and 15 cm. To insure the integrity of the pore water recovered at the bottom of the holes, water that initially filled the bottom of the hole, once we reached the water table, was scooped up with a polypropylene beaker. Given the high permeability of the sand, the hole was refilled with new interstitial water within a few seconds and this water was sampled with the beaker. Hence, we believe that the water was freshly extracted from the interstitial medium. An oxygen sensor was placed immediately at the bottom of the renewed puddle and allowed to equilibrate for a few minutes. . Once it stabilized, the probe reading (O2 saturation) remained nearly invariant, even if the water was strongly depleted in dissolved O2. In the summer of 2011, we observed anoxic waters (0% O2 saturation) in several lower beach holes, implying that during the measurement, there was neither significant O2 contamination and nor gas exchange with the atmosphere. The digging operation, in-situ measurements and samplings took from 5 to 20 min per hole.

At selected periods during this study, autonomous Aanderaa optodes and NKE data loggers were buried directly into the sediment and the in-situ oxygen saturations at the water table at low tide over several tidal cycles (Charbonnier et al., 2016). Among these deployments, two vertical profiles composed of three probes separated from each other by 10 to 20 cm were carried out. These probes recorded O2 saturations around 60% at the time of the deployments.

These results were identical along the full profile, particularly at low tide, the period corresponding to our sampling in the dug holes. Results of the autonomous measurements were always close to those measured directly in holes with the WTW O2 sensor, implying that on the scale of the thickness of the water table sampled from holes (about 10 cm), the water mass is homogeneous and that our sampling strategy was appropriate for this type of highly permeable medium.

Supratidal wells

In February 2013, three 7 m-long piezometers were deployed at supratidal sites upstream of the intertidal zone of the Truc Vert Beach, between the high tide mark and the dune foot (Fig. 2). Measurements and samplings were carried out every day during one full lunar cycle in February and March 2013 as well as in September and October 2013. The dates were chosen according to the elevation of the terrestrial water table, which is generally shallowest at the end of winter and deepest in October.

In each piezometer, three 5-mm inner diameter Tygon tubes were firmly fixed at three different depths (from 1.5 to 6 m depth) in order to construct a vertical profile. Samples were collected at low tide by pumping with a 60-mL syringe, after flushing several void volumes to rinse the tubes. We did not purge the whole well volume because of the high permeability of the beach sand and to avoid mixing. At low tide, the upper part of the three piezometers was filled with air because the water table was deeper. It was likewise the case at high tide for the upper piezometer (i.e., closest to the dune foot). Thus, it was not always possible to recover waters from all Tygon tubes and, in most cases, waters were only collected from the deepest one. Waters recovered at three different depths within a given piezometer were identical in composition, implying that the supratidal aquifer was homogeneous over the sampled depths.

Since the piezometers were made of plastic (PVC) and we used Tygon tubing, we suspected that the DOC samples were contaminated and, thus, were discarded . Due to the high volume (100 mL) of water required for d 13 C-DIC analyses, these samplings were performed only twice during the February-March 2013 campaign, once during the spring tide and once during the neap tide. In addition to the WTW probe measurements, oxygen concentrations and temperatures were also recorded with Aanderaa 3835 optodes equipped with NKE data loggers located at each sampling point of the three piezometers. These probes had a precision of ± 5 % for oxygen and ± 0.1°C for temperature and were tested in the laboratory before and after each field deployment. As observed from the water sampling, probe data were identical at the different depths within the same piezometer, further supporting the hypothesis of a homogeneous surficial aquifer at the beach face.

The water table level in the piezometers was also monitored continuously with CeraDIVER sensors (Schlumberger®). These probes measured water pressure and converted it into a hydraulic head with a precision of ± 0.2 cm. In addition, atmospheric pressure at the study site was recorded with a BaroDIVER (Schlumberger®) placed at the top of the dune. Hydraulic head data were corrected for atmospheric pressure variations and referenced to zero sea level using the DiverOffice software (Schlumberger®).

Terrestrial groundwater

We sampled terrestrial groundwater, directly drained by the beach, from wells located behind the sand dune in the pine forest (Fig. 2). Built to fight forest fires, these wells are about 10 m deep and are always drilled within the freshwater aquifer. Depending on the position of these wells on the dunes, the piezometric height was between 0.5 and 6 m deep, whereas the annual vertical oscillation of the water height is about 1.5 m [START_REF] Buquet | Cycle des éléments biogènes dans les lacs côtiers en Gironde[END_REF]. Waters were sampled 2 m below the water table using two centrifugal submersible pumps (Xylem LVM 105) connected in line to a 1/2" nylon tubing. Water was left to overflow a beaker in which WTW probes continuously measured conductivity, temperature, pH, and dissolved oxygen.

Once the probe readings had stabilized, samples were withdrawn for chemical analyses.

Depending their ease of access, some wells were sampled three times in 2013-2014, whereas some others were sampled only once. These wells are made of PVC and we used nylon tubing to pump the water up, but since we were able to purge the system several times by pumping more than 100 L before sampling, we deemed that the collected water was not contaminated for DOC measurements.

Analyses

DOC concentrations were determined by the high temperature catalytic oxidation method using a Shimadzu TOC 5000 analyser after removing DIC by bubbling oxygen through the acidified samples [START_REF] Sharp | The dissolved organic carbon controversy: an update[END_REF]. The instrument was calibrated using a standard potassium phthalate solution of, diluted to different concentrations according to the estimated DOC content of the samples. The precision of these measurements was about 10 µM, based on replicate analyses of the same samples. TA was measured on 50 mL filtered samples by automated potentiometric titration (Metrohm 794 Basic Titrino ® ), with a combined pH glass electrode (Metrohm 6.0262.100 ® ) and 0.1N HCl solution as a titrant. The equivalence point of the titration was determined by the Gran linearization method [START_REF] Gran | Determination of the equivalence point in potentiometric titrations, part II[END_REF]. Replicates were performed (two per sampling station) and precision was about 2 µM. From these TA measurements and the in-situ pH, salinity, and temperature, we calculated DIC concentrations and pCO2 values using the carbonic acid dissociation constants of [START_REF] Mehrbach | Measurement of the apparent dissociation constants of carbonic acid in sea water at atmospheric pressure[END_REF] refitted by [START_REF] Dickson | A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media[END_REF], the borate dissociation constant from [START_REF] Lyman | Buffer mechanism of seawater[END_REF] with B:S value of Uppstrom (1974) and the CO2 solubility from [START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF]. Precision was about ± 10 µM for DIC and ± 10 ppm for pCO2.

The d 13 C-DIC measurements were performed according to the method described by [START_REF] Gillikin | Determination of d 18 O of water and d 13 C of dissolved inorganic carbon using a simple modification of an elemental analyzer-isotope ratio mass spectrometer EA-IRMS: an evaluation[END_REF]: a headspace was created in 100 mL sealed glass bottles by injecting a volume of helium equivalent to about 30% of the total vial volume. Samples were then acidified with 0.3 mL of 85% phosphoric acid to convert DIC species into CO2. After shaking and equilibrating for more than one night at the thermostated room temperature of the mass spectrometer laboratory, samples were analysed using a dual inlet EA-IRMS (EA: Carlo Erba NC2500; IRMS: Isoprime). About 0.5 mL of headspace was injected through an injection port that was placed before the water trap of the EA. The d 13 C values were calibrated with a homemade standard (45 mg of Na2CO3 dissolved with 3 mL of 85% phosphoric acid in a 100 mL helium flushed vial) whose isotopic composition was previously determined to be -4.5 ± 0.2 ‰ [START_REF] Polsenaere | Export and degassing of terrestrial carbon through watercourses draining a temperate podzolized catchment[END_REF]. The d 13 C of the total DIC was computed using these corrected d 13 C values and the equation of [START_REF] Miyajima | Determining the stable isotope ratio of total dissolved inorganic carbon in lake water by GC/C/IRMS[END_REF], which defines the partitioning of CO2 between the headspace and the water phase of the sample as a function of temperature and salinity. The precision of these measurements was estimated at about ± 0.1 ‰, based on replicate analyses of the same samples.

Results

Surface seawater

The practical salinity (SP) of surface seawater at the beach was 35.1 ± 0.4 (s.d., n = 37) and oxygen saturation close to 100% (98 ± 4.5% s.d.) throughout the year. The pH (on the infinite dilution convention or "NBS" scale) ranged from 8.10 to 8.30, whereas temperature displayed a seasonal variation with values ranging from 8. . DOC concentrations varied between 84 and 600 µM, with an average concentration of 260 ± 150 µM (n = 37) and maximum values observed during spring and summer (Fig. 3).

TA values ranged from 2100 to 2480 µmol kg -1 , with a mean value of 2310 ± 110 µmol kg -1 (n = 37) and minimum observed values in winter. Calculated DIC and pCO2 values ranged, respectively, from 1900 to 2300 µM (average value of 2070 ± 102 µM; n = 37) and from 300 to 526 ppm (average value of 395 ± 63 ppm; n = 37), and displayed a similar trend, with minima observed during autumn and winter. The d 13 C-DIC was relatively constant, with a mean value of +0.4 ± 0.5 ‰ (n = 37) (Fig. 3).

Intertidal beach pore waters

Irrespective of the season, pore waters collected at the surface of the intertidal beach water table had a salinity close to that of seawater, with 80% of samples having a practical salinity between 34 and 36. Nevertheless, several samples collected in the lower beach had salinities as low as 30, with a minimum value of 25.6 on April 19 th , 2011 (Fig. S1 in Supplementary material for detailed cross-shore profiles). In the upper beach (between 30 and 70 m from the dune foot), pore waters retrieved from deep holes had chemical characteristics close to those of seawater (Fig. 3). Dissolved oxygen saturation was between 100% and 80% and pCO2 values were close to atmospheric, with a mean of 455 ± 113 ppm (n = 37). DIC concentrations were below 2300 µM, with a mean value of 2120 ± 170 µM (n = 37). The d 13 C-DIC had a mean value of 0.0 ± 1.0‰ (n = 37) (Fig. 3). Pore water DOC concentrations in the upper beach were higher than those of seawater, with values up to 840 µM (April 19 th , 2011) and a mean concentration of 310 ± 180 µM (n = 37) (Fig. S2 in Supplementary material for average values).

In the lower beach, pore waters were generally depleted in oxygen and enriched in DIC, including the highest pCO2 and lowest d 13 C-DIC values along the cross-shore transects (Fig. 3). Pore waters characteristics along the cross-shore transects varied throughout the year: minimum oxygen saturations ranged from 22% to 86% in winter (average value of 55 ± 23%, n =10), from 3% to 50% in spring (average value of 36 ± 22%, n = 8), from 0% to 58% in summer (average value of 23 ± 20%, n = 13) and from 9% to 72% in autumn (average value of 44 ± 23%, n = 6) (Fig. S2 in Supplementary material for seasonal average values). Maximum DIC values also varied seasonally. For samples with salinity close to that of seawater, the DIC concentrations of the most oxygen-depleted pore waters ranged from 1950 to 2510 µM in winter (average value of 2130 ± 130 µM, n = 10), from 2210 to 2550 µM in spring (average value of 2360 ± 90 µM; n = 8), from 2190 to 2600 µM in summer (average value of 2340 ± 130 µM, n = 13) and from 2020 to 2350 µM in autumn (average value of 2180 ± 120 µM, n = 6) (Fig. 3).

The computed pCO2 displayed the same trend: the highest values were observed during summer, ranging from 610 to 1520 ppm (average value of 1040 ± 200 ppm, n = 13). During the rest of the year, pCO2 values of lower beach pore waters ranged from 350 to 960 ppm in winter (average value of 540 ± 190 ppm, n = 10), from 610 to 1420 ppm in spring (average value of 870 ± 260 ppm, n = 8), and from 520 to 1010 ppm in autumn (average value of 720 ± 230 ppm, n = 6).

Pore water d 13 C-DIC values were almost always more negative in the lower beach than in the upper beach (Fig. 3). The lower beach values ranged from -2.7 to +0.3‰ in winter (average value of -0.9 ± 0.9‰, n = 10), from -3.9 to -0.8‰ in spring (average value of -1.8 ± 1.1‰, n = 8), from -3.4 to -0.3‰ in summer (average value of -1.4 ± 0.9‰, n = 13) and from -1.7 to -0.1‰ in autumn (average value of -0.8 ± 0.6‰, n = 6).

Pore water DOC concentrations along the cross-shore profile displayed no uniform trend: 21 out of the 37 completed profiles showed DOC values higher in the upper beach than in the lower beach; 14 transects showed uniform or scattered DOC values along the cross-shore profile. An increase in DOC concentration in the lower beach was observed on 02/22/2011 and 05/19/2011 (Fig. 1 in Supplementary material). On the 21 dates when DOC concentrations were decreasing from the upper beach to the lower beach, most of the decrease was observed in the upper part of the intertidal zone and DOC concentrations were often lower than 300 µM in the lower beach pore waters (mean value of 220 ± 140 µM, n = 37), except from May to August 2012 when high DOC concentrations were observed along the whole cross-shore profile (mean value of 520 ± 100 µM, n = 50). When pore water DOC concentrations displayed no crossshore trend, they typically ranged from 150 to 200 µM (Fig. 1 in Supplementary material).

Supratidal wells

The three piezometers placed between the dune foot and the high tide mark allowed us to sample waters from the surface beach aquifer at low tide over a full lunar tidal cycle in February-March 2013 and September-October 2013. The hydraulic head measurements revealed that pore waters flowed seaward, i.e., from PZ1 (dune foot) to PZ3 (high spring tide watermark), except during the high spring tides (Fig. 4A) when waves reached the PZ3 position and surface seawater percolated through the sand. This lead to a brief inversion of the hydraulic gradient, with the water table being higher in PZ3 than in PZ2 and PZ1.

The pore water salinity changed according to the tidal cycle. The piezometer (PZ3), located at the high spring tide water line, contained saline water (SP ~ 34.5) during spring tide periods and brackish waters (SP ranging from 0 to 5) during neap tide periods (Fig. 4). The other piezometers (PZ1 and PZ2), located a few meters upstream, contained fresh to brackish waters, with SP lower than 15 (Fig. 4).

Due to the limited availability of sensors, oxygen saturation was measured only in PZ2 and PZ3. The data show that pore waters were depleted in oxygen during neap tides, with values ranging from 11 to 48% in February-March and from 0 to 10% in September-October in PZ2, and ranging from 22 to 63% in February-March and from 48 to 71% in September-October in PZ3 (Fig. 4B). Pore waters were more oxygenated during spring tides because of saline water intrusion, with values ranging from 85 to 100%, except for PZ2 in September-October as saline waters did not reach this station.

Pore water DIC concentrations, calculated from TA and pH measurements carried on samples recovered at low tide, were higher in the freshwater of PZ1 than in the fresh to saline waters of PZ2 and PZ3, with values in PZ1 ranging from 2780 to 4080 µM (average value of 3570 ± 380 µM, n =17), with no particular trend throughout the lunar tidal cycle (Fig. 4B). In contrast to PZ1, in PZ2 and PZ3, DIC concentrations evolved concomitantly with salinity, i.e., to the lunar tidal cycle. The DIC concentrations were higher in the fresh-brackish pore waters of PZ2 and PZ3 during neap tides than in saline waters during spring tides. In PZ3, freshwater DIC values ranged from 2780 to 3070 µM (average value of 2880 ± 100 µM, n =7) in February-March, and from 3020 to 3480 µM (average value of 3160 ± 150 µM, n =7) in September-October. In PZ2, DIC values ranged from 3000 to 3380 µM (average value of 3220 ± 170 µM, n =8) in February-March, and were nearly invariant at ~3800 µM in September-October when salinity remained close to 0. During spring tides, the mean DIC concentrations were 2130 ± 130 µM (n = 5) for both seasons in PZ3, and 2780 ± 160 µM (n = 4) in PZ2 for the February-March sampling (Fig. 4B).

Pore water pCO2 displayed a similar trend to DIC in all three piezometers and during both sampling campaigns, with high values during neap tides and lower values during spring tides. In February-March, pCO2 values in PZ1 ranged from 1000 to 1770 ppm (average value of 1330 ± 390 ppm, n = 3) during spring tide, whereas two higher values (1690 and 2820 ppm) were observed during neap tide. In PZ2, the mean pCO2 was 780 ± 15 ppm (n = 3) during spring tide and ranged from 520 to 2030 ppm (average value of 1460 ± 550 ppm, n = 6) during neap tide. The same trend was observed in PZ3, with pCO2 ranging from 365 to 448 ppm during spring tide (mean value of 413 ± 38 ppm, n = 4) and 354 to 974 ppm during neap tide (mean value of 510 ± 240 ppm, n = 6) (Fig. 4B). In September-October, average pCO2 values in PZ1, PZ2 and PZ3 were respectively 2780 ± 250 (n = 4), 2130 ± 800 (n = 4) and 770 ± 70 (n = 4) ppm during spring tide, and about 3510 ± 1310 (n = 4), 3140 ± 810 (n = 6) and 1010 ± 260 (n = 5) ppm during neap tide (Fig. 4B). The pore water d 13 C-DIC values changed with the salinity, ranging from -9.9 to -12.2‰ in fresh and brackish waters to -3.8‰ in saline waters (Fig. 4A).

Terrestrial groundwater

Waters collected in upland, forest wells, above the Truc Vert Beach, were fresh (SP ≈ 0) and always anoxic. In all these wells, the water table was about 3.2 to 3.5 m NGF (French national reference levelling) higher than in the upper beach (1.6 to 2.5 m NGF), evidence of a groundwater hydraulic gradient from the upland to the beach. The pH of these terrestrial groundwaters was circum-neutral, with pH values ranging from 6.27 to 7.53 (Table 1). DOC concentrations ranged from 1060 to 2320 µM, except in DFCI 2 where higher DOC concentrations were observed (3170 and 5800 µM; Table 1). The average DIC concentration in the forest wells was 4700 ± 1100 µM (n = 12). DIC concentrations varied between wells but also with time, as observed in well DFCI2 in which DIC was 2120 µM in Sept. 2013 and 6180 µM in Jan. 2014 (Table 1). Despite this temporal variability, the DIC concentrations of the terrestrial groundwaters in the upland wells were generally higher than in the beach pore waters.

Likewise, the aqueous pCO2 was much higher in forest wells than in beach pore waters, and ranged from 5200 to 82000 ppm in the former (Table 1). The highest value (82000 ppm), calculated from the total alkalinity, in-situ pH and temperature, is probably an overestimate due to the acidic and organic-rich nature of the waters in well DFCI2 (pH of 6.27; DOC content of 3170 µM) [START_REF] Abril | Technical note: Large overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters[END_REF]. The computed pCO2 in the other wells, in which pH was always close to 7 and DOC concentrations always lower than 2350 µM (average value of 1700 ± 470 µM, n = 9), are more uniform (Table 1). If we exclude the anomalous high value at DFCI2, the mean pCO2 in forest wells was 16500 ± 6500 ppm (n = 11), which was two-to five-fold higher than pCO2 values calculated in the beach pore waters. The d 13 C-DIC values were always very negative in the upland wells, ranging from -22.5 to -13.9‰ (average value of -16.9 ± 2.2‰).

4. Discussion 4.1. DIC transformation below the foredune Most tidal beaches host a redox front along the salinity gradient between terrestrial fresh groundwaters and saline pore waters [START_REF] Windom | Biogeochemical processes in a freshwater-seawater mixing zone in permeable sediments along the coast of Southern Brazil[END_REF][START_REF] Slomp | Nutrients inputs to the coastal ocean through submarine groundwater discharge: controls and potential impact[END_REF][START_REF] Spiteri | Flow and nutrient dynamics in a subterranean estuary (Waquoit Bay, MA, USA): field data and reactive transport modeling[END_REF][START_REF] Santos | Tidal pumping drives nutrient and dissolved organic matter dynamics in a Gulf of Mexico subterranean estuary[END_REF]. This redox transition zone is the site of biogeochemical transformations linked to the oxidation, by oxygenated seawater, of reduced compounds dissolved in fresh groundwater. For example, the oxidative precipitation of dissolved iron at the redox front can create a trap for solutes, such as orthophosphate and trace metals, that are co-precipitated with or adsorbed to the authigenic oxides [START_REF] Cable | Phosphate uptake in coastal limestone aquifers: a fresh look at wastewater management[END_REF][START_REF] Charette | Oxidative precipitation of groundwater-derived ferrous iron in the subterranean estuary of a coastal bay[END_REF][START_REF] Slomp | Nutrients inputs to the coastal ocean through submarine groundwater discharge: controls and potential impact[END_REF][START_REF] Spiteri | Flow and nutrient dynamics in a subterranean estuary (Waquoit Bay, MA, USA): field data and reactive transport modeling[END_REF]. The terrestrial fresh groundwater sampled in the forest, upland of the Truc Vert Beach, were anoxic and enriched in Fe(II) and NH4 + (Table 2). Dissolved Fe(II) and NH4 + concentrations were measured in the upland wells (59 ± 30 µM and 17 ± 18 µM, respectively), whereas these species were nearly absent in the dune foot freshwater end-member (0.1 ± 0.2 µM and 0.5 ± 1.4 µM, respectively) (Table 2; [START_REF] Anschutz | Terrestrial groundwater and nutrient discharge along the 240-km long Aquitanian coast[END_REF]. Unlike most other STEs, the salinity gradient in the supratidal beach did not host a redox gradient. Since freshwaters of the surficial aquifer at the dune foot are oxic, the oxic-anoxic front must occur in the freshwater aquifer below the foredune. The foredune at the Truc Vert Beach is a wide (up to 200 m), sandy and organic soil-depleted area. It is believed that rainwater can percolate through the sand dune and bring oxygen to the shallow, fresh groundwaters [START_REF] Anschutz | Terrestrial groundwater and nutrient discharge along the 240-km long Aquitanian coast[END_REF]. In addition, the groundwater TA was higher in the forest than in the upper beach and this may reflect the impact of local redox reactions that serve as sources or sinks of alkalinity (Kempe, 1990;[START_REF] Stumm | Aquatic Chemistry[END_REF][START_REF] Abril | Nitrogen-alkalinity interactions in the highly-polluted Scheldt basin (Belgium)[END_REF]Thomas et al., 2009). The oxidation of reduced Fe and nitrification of ammonia by oxygen consumes alkalinity (Eqns. 1 and 2) [START_REF] Stumm | Aquatic Chemistry[END_REF][START_REF] Abril | Nitrogen-alkalinity interactions in the highly-polluted Scheldt basin (Belgium)[END_REF][START_REF] Burdige | Geochemistry of Marine Sediments[END_REF]:

4Fe 2+ + O2 + 8HCO3 -+ 2H2O à 4Fe(OH)3 + 8CO2 (1) NH4 + + 2O2 + 2HCO3 -à NO3 -+ 2CO2 + 3H2O (2)
According to the stoichiometry of these reactions, the oxidation of the anoxic groundwater iron and ammonia would have removed 152 µM of TA, very close to the difference we measured at the two sites (142 µM; Table 2). Hence, these TA values clearly support the hypothesis of the presence of a redox front located below the foredune. Hydrologic mass balance calculations yield an annual flux of terrestrial SGD of 0.43 km 3 . Accordingly, the flux of alkalinity to the coastal ocean would be 1.54 Gmol per year for the whole Aquitaine coast, based on the properties of the forest well end-member. The value is reduced to 1.47 Gmol per year because of oxidation reactions below the foredune.

Protons released by the redox reactions (Eqns. 4 and 5) should decrease TA and pH, but increase the pCO2. In contrast, we observe a lower pCO2 in the oxic dune foot aquifer than in the anoxic forest aquifer and these lower pCO2 values are accompanied by higher pHs, lower DIC concentrations and heavier d 13 C-DIC values (Table 2). These observations suggest that CO2 degasses into the interstitial air of the dune sands and finally to the atmosphere. As CO2 progressively degasses along the groundwater flow path, the d 13 C-DIC increases due to the preferential diffusion of 12 CO2 [START_REF] Mook | Carbon isotope fractionation between dissolved bicarbonate and gaseous carbon dioxide[END_REF][START_REF] Cerling | On the isotopic composition of carbon in soil carbon dioxide[END_REF][START_REF] Polsenaere | Modelling CO2 degassing from small acidic rivers using water pCO2, DIC and δ 13 C-DIC data[END_REF][START_REF] Deirmendjian | Carbon dioxide degassing at the groundwater-streamatmosphere interface: isotopic equilibration and hydrological mass balance in a sandy watershed[END_REF] while DIC decreases and the pH increases (Fig. 5).

Discharge of terrestrially-derived CO2-enriched groundwater may modify the pH of the coastal ocean and locally influence ocean acidification [START_REF] Cai | The geochemistry of dissolved inorganic carbon in a surficial groundwater aquifer in North Inlet, South Carolina, and the carbon fluxes to the coastal ocean[END_REF]. When the groundwater DIC:TA ratio is greater than 1, groundwater discharge may decrease seawater pH, whereas a DIC:TA ratio <1 will buffer seawater against ocean acidification [START_REF] Pain | Sources and sinks of CO2 and CH4 in siliciclastic subterranean estuaries[END_REF]. Terrestrial groundwater sampled in forest wells have a DIC:TA ratio above 1 (Tab. 2), mostly because of high pCO2 values. Along the groundwater flow path, TA decreases due to oxidation of reduced iron and ammonium while DIC decreases due to CO2 degassing. The latter has a larger influence on the TA:DIC ratio than the former, as the ratio evolves towards 1 in supratidal fresh groundwater and ultimately becomes lower than 1 in brackish waters (Tab. 2, Fig. 5).

Accordingly, terrestrial SGD at the Truc Vert Beach will not lower pH in receiving seawater, unlike most other coastal environments where DIC fluxes have been investigated so far (de [START_REF] De Weys | Linking groundwater discharge to severe estuarine acidification during a flood in a modified wetland[END_REF]Cyronak et al., 2014;[START_REF] Sadat-Noori | Groundwater Discharge as a Source of Dissolved Carbon and Greenhouse Gases in a Subtropical Estuary[END_REF][START_REF] Liu | Carbonate system biogeochemistry in a subterranean estuary -Waquoit Bay, USA[END_REF][START_REF] Mouret | Biogeochemistry in an intertidal pocket beach[END_REF]. Seawater pH may also increase due to enhanced biological productivity buoyed by a SGD-derived nutrient flux [START_REF] Borges | Carbonate chemistry in the coastal zone responds more strongly to eutrophication than to ocean acidification[END_REF], which consist mostly of a nitrate flux at the Truc Vert Beach [START_REF] Anschutz | Terrestrial groundwater and nutrient discharge along the 240-km long Aquitanian coast[END_REF]. Therefore, both the DIC:TA ratio and nutrient fluxes of SGD result in an increasing seawater buffering capacity against acidification in this high-energy beach. 4.2. DIC behavior in the subterranean estuary 4.2.1. Non-conservative behavior of DIC in the STE Terrestrial groundwaters are a source of nutrients, trace metals, and carbon to the coastal ocean [START_REF] Burnett | Groundwater and pore water inputs to the coastal zone[END_REF][START_REF] Bowen | A review of land-sea coupling by groundwater discharge of nitrogen to New England estuaries: mechanisms and effects[END_REF][START_REF] Santos | Tidal pumping drives nutrient and dissolved organic matter dynamics in a Gulf of Mexico subterranean estuary[END_REF]Santos et al., , 2012b)). In permeable beaches, the reaction zone between terrestrial groundwaters and saline pore waters forms the STE [START_REF] Moore | The subterranean estuary: a reaction zone of groundwater and seawater[END_REF]. As observed in surface estuarine systems, terrestrial groundwater undergoes chemical transformations in the STE before discharging to the coastal ocean.

Multiplying the average elemental concentration by the subterranean freshwater flux often provides inaccurate estimates of mass fluxes to the coastal ocean for species like nutrients and carbon that do not behave conservatively in the STE [START_REF] Charette | Oxidative precipitation of groundwater-derived ferrous iron in the subterranean estuary of a coastal bay[END_REF][START_REF] Slomp | Nutrients inputs to the coastal ocean through submarine groundwater discharge: controls and potential impact[END_REF][START_REF] Beck | Importance of geochemical transformations in determining submarine groundwater discharge-derived trace metal and nutrient fluxes[END_REF][START_REF] Chaillou | Transport and transformations of groundwater-borne carbon discharging through a sandy beach to a coastal ocean[END_REF][START_REF] Liu | Carbonate system biogeochemistry in a subterranean estuary -Waquoit Bay, USA[END_REF]. In the Truc Vert Beach, geophysical measurements coupled to hydraulic head and salinity measurements reveal that the subsurface salinity gradient is located in the supratidal beach and is a few meters wide. The fresh-saline interface is almost vertical and shifts only a few meters cross-shore during the spring-neap tidal cycle, implying that the transition zone between fresh and saline groundwaters is relatively narrow and stable [START_REF] Buquet | Shape of the shallow aquifer at the freshwater-seawater interface on a high-energy sandy beach[END_REF]. The PZ3 piezometer was located in this transition zone. Electrical resistivity tomographic measurements also show that the tidally-driven seawater recirculation cell drags parcels of brackish water from the supratidal transition zone toward the lower beach (Fig. 2) [START_REF] Buquet | Shape of the shallow aquifer at the freshwater-seawater interface on a high-energy sandy beach[END_REF], which explains why we sampled brackish pore waters in piezometers located in the supratidal beach as well as in holes of the lower beach. High DIC concentrations and pCO2 in the freshwater end-member from forest wells (Table 1) originate from autotrophic root respiration and from carbon fixed by terrestrial plants, then recycled within soils through heterotrophic respiration, and finally transferred to the groundwater [START_REF] Hanson | Separating root and soil microbial contributions to soil respiration: A review of methods and observations[END_REF][START_REF] Cole | Plumbing the global carbon cycle: integrating inland waters into the terrestrial carbon budget[END_REF]. Fresh groundwater was also sampled in piezometers at the foot of the dune, but the measured d 13 C-DIC and computed DIC and pCO2 values were dissimilar those in waters from forest wells (Table 2).

The behaviour of DIC and d 13 C-DIC in the STE, i.e. in brackish waters of the supratidal and lower beaches, can be assessed by comparing results of end-member, conservative mixing and field data [START_REF] Fry | Conservative mixing of stable isotopes across estuarine salinity gradients: A conceptual framework for monitoring watershed influences on downstream fisheries production[END_REF][START_REF] Bouillon | Inorganic and organic carbon biogeochemistry in the Gautami Godavari estuary (Andhra Pradesh, India) during pre-monsoon: the local impact of extensive mangrove forests[END_REF]. The equation describing conservative mixing of DIC in the salinity (S) gradient is given by:

DIC = % !"# !" $!"# #" % !" $% #" & S + DIC &' (3)
where subscripts refer to the seawater (SW) and freshwater end-members (FW). Theoretical DIC concentrations were calculated using the two freshwater end-members observed in the immediate vicinity of the Truc Vert Beach: fresh forest groundwaters (salinity of 0; average DIC of 4700 µM) and supratidal fresh pore waters (average salinity of 0.6; average DIC of 3500 µM). The seawater end-member properties correspond to the average salinity and DIC concentration of the local surface seawater (35.1 and 2070 µM, respectively). The equation describing the conservative mixing of d 13 C-DIC was derived by [START_REF] Bouillon | Inorganic and organic carbon biogeochemistry in the Gautami Godavari estuary (Andhra Pradesh, India) during pre-monsoon: the local impact of extensive mangrove forests[END_REF] and is:

δ () C = %*!"# #" + $% # #" $!"# !" + $% # !" ,-% #" !"# !" + $% # !" $% !" !"# #" + $% # #" %(!"# #" $!"# !" )-% #" !"# !" $% !" !"# #" ( 4
)
As SFW is 0, equation ( 4) simplifies to

δ () C = %*!"# #" + $% # #" $!"# !" + $% # !" ,$% !" !"# #" + $% # #" %(!"# #" $!"# !" )$% !" !"# #"
(5)

where d 13 C represents the carbon isotopic composition of the DIC. d 13 C end-members were defined as +0.4‰ for seawater, -16.9‰ for the terrestrial groundwater, and -12.1‰ for the supratidal freshwaters.

Irrespective of the freshwater end-member, results of the calculations clearly show that DIC does not behave conservatively along the salinity gradient (Fig. 6). In the supratidal beach brackish waters, DIC concentrations were below the theoretical mixing lines, indicating that DIC was depleted (Fig. 6A). The d 13 C-DIC values from these samples were markedly higher than predicted (Fig. 6B). Carbonate dissolution cannot explain this trend as it would increase the DIC beyond the mixing line. The combination of decreasing DIC and increasing d 13 C-DIC, coupled with lower pCO2 values in brackish waters (SP > 1) than in freshwaters (SP < 1), is best explained by CO2 degassing at the interstitial water -interstitial air interface in the supratidal STE.

In the brackish pore waters of the lower beach, DIC concentrations were either on or above the mixing lines between the supratidal-beach freshwaters and seawater (Fig. 6A). Most of the d 13 C-DIC data were close to the theoretical mixing line (Fig. 6B). More negative d 13 C-DIC values, coupled to enriched DIC concentrations, suggest the presence of a source of metabolic carbon from organic matter mineralization, as proposed in section 4.3. The DIC in the STE of the Truc Vert Beach was therefore controlled by two main processes: CO2 degassing along the salinity gradient in the supratidal and upper beach, and organic matter respiration within the intertidal zone. A few samples with higher than expected d 13 C-DIC values and enriched in DIC suggest that some carbonate dissolution may also occur (see detailed discussion in section 4.3.4).

Fluxes of carbon from the freshwater end-member

Mixing between fresh and saline pore waters was observed during neap tides, when low salinities (from 1 to 2) were observed in PZ2 and PZ3. pH and d 13 C-DIC increased from PZ1

(SP = 0.6) to PZ3 (SP ~2), whereas pCO2 decreased from 2340 to 770 ppmv (Table 2). The high mean pH value of 8.31 observed in these nearly freshwaters corresponded to the value computed from electroneutrality at in-situ pCO2, temperature, and measured TA. The increase in pH from the upland groundwater to the beginning of the salinity gradient of the supratidal beach STE suggests that CO2 was degassing to the atmosphere below the foredune and in the STE, and implies that the upland aquifer cannot be used as a freshwater end-member to calculate fluxes of carbon from terrestrial SGD (Fig. 5). The loss of DIC due to CO2 degassing below the dune between the forest soil aquifer and the supratidal beach is about 1550 µM. This corresponds to a yearly flux of about 8000 tons of C (or 29000 tons of CO2) to the atmosphere when extrapolated over the whole Aquitaine sandy coast. Given that the average DIC concentration of these waters after degassing below the dune is 3150 µM (Table 2), the DIC flux to the coastal ocean is estimated at 16200 tons of carbon per year. It would have been overestimated by 8000 tons per year if active CO2 degassing between the forest soil aquifer and the supratidal beach had been neglected. High-energy sandy beaches are recognized as environments in which organic matter mineralization and recycling of the associated nutrients are very active [START_REF] Avery | Nitrogen release from surface sand of a high energy beach face along the Southeastern Coast of North Carolina[END_REF][START_REF] Rauch | The effects of Phaeocystis globosa bloom on the dynamics of the mineralization processes in intertidal permeable sediment in the eastern English Channel (Wimereux, France)[END_REF][START_REF] Rocha | Sandy sediments as active biogeochemical reactors: compound cycling in the fast lane[END_REF][START_REF] Anschutz | Tidal sands as biogeochemical reactors[END_REF][START_REF] Dugan | Marine macrophyte wrack inputs and dissolved nutrients in beach sands[END_REF][START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF]. Previous studies at the Truc Vert Beach revealed that oxygen consumption through aerobic respiration takes place in the intertidal recirculation cell [START_REF] Anschutz | Tidal sands as biogeochemical reactors[END_REF][START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF]. Oxygenated seawater seeps into the sandy sediments during floods, filling the pore space. As seawater percolates from the upper beach to the lower beach, oxygen is partly consumed through aerobic respiration and, ultimately, oxygen-depleted pore waters are expelled from the sand and are flushed to the coastal ocean when the lower beach is exposed [START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF]. Thus, the pore water of the upper beach is young as it has just entered the aquifer, whereas the water in the lower beach has percolated into the aquifer and is therefore older. The residence time of pore water in the beach aquifer is between 7 and 20 tidal cycles, as inferred from dissolved silica concentrations and experimental measurements of quartz dissolution [START_REF] Anschutz | Tidal sands as biogeochemical reactors[END_REF]. This suggests that the pore water volume was renewed during our bi-weekly samplings. Hence, the pore water of a given cross-shore profile was not the same as that sampled on our previous visit 15 days before.

Partially oxygen-depleted pore waters in the lower beach are also enriched in DIC (Fig. 3). In fact, the highest DIC and pCO2 values are associated with the most O2-depleted pore waters (Fig. 3), implying that DIC is mostly produced by aerobic respiration processes within the intertidal sands [START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF]. Temperature differences, between seawater entering the aquifer during the rising tide and the pore water that seeps out the aquifer in the lower beach, were for most profiles less than 1°C (22 out of 37). The largest temperature difference was 2.2°C, which leads to a 10% differential on the computed pCO2 values (+4.23% °C-1 ; [START_REF] Takahashi | Seasonal variation of CO2 and nutrients in the high-latitude surface oceans: a comparative study[END_REF]. Temperature differences arose as we sampled pore waters close to the surface in the lower beach and, thus, they were influenced by the air temperature. The largest temperature differences were observed in summer, when the computed pCO2 is highest.

The temperature differential translates into a 100 ppm change in pCO2 when their values at in situ temperature exceed 1000 ppm (Fig. 3). Irrespective, the impact of temperature on computed pCO2 values are minor compared to those resulting from microbial respiration.

Most of cross-shore transects performed in 2011-2012 showed that oxygen depletion and the concomitant DIC enrichment in the lower beach pore waters are coupled with a decrease in DOC concentrations along the transects (Fig. 3; Fig. 1 in Supplementary material). DOC concentrations were higher in upper beach pore waters than in the local seawater on most sampling dates, but especially in the spring and summer. During these two seasons, the upper beach pore waters contained, on average and respectively, 140 and 60 µM more DOC than the seawater (Fig. 3), but they could be a few hundred µM higher on any given day (Fig. 1 in Supplementary material).

DOC is a key intermediate in benthic POC catabolic processes [START_REF] Kristensen | Decay of plant detritus in organic-poor marine sediment: Production rates and stoichiometry of dissolved C and N compounds[END_REF][START_REF] Komada | Transformations of carbon in anoxic marine sediments: Implications from D 14 C and d 13 C signatures[END_REF]. DOC can be produced through cells lysis and/or bacterial enzymatic activity, and be consumed through heterotrophic respiration [START_REF] Carlson | DOM source, sinks, reactivity, and budgets[END_REF]. The DOC concentration gradients observed along the transects likely result from a balance between these two processes. Our data suggest that DOC production dominates in the upper part of the Truc Vert Beach. The DOC in excess of the local seawater contribution most likely originates from the degradation of fresh marine POC, including scattered, drifting plant debris and phytoplankton cells (Huettel and Rusch, 2000), that are filtered and trapped by the sand as seawater percolates through it during rising tides. As pore waters percolate seaward through the beach aquifer, DOC is consumed by heterotrophic aerobic respiration and its concentration decreases to a value similar to that measured in open seawater. This background, residual DOC is probably more refractory, as the concentration remained relatively stable despite its transit in the aquifer aerobic reactor. Given the limited amount of labile organic carbon and the constant renewal of dissolved O2 through seepage, heterotrophic processes remain strictly aerobic. Accordingly, carbon and alkalinity fluxes are not affected by anaerobic processes, unlike most other coastal permeable environments studied to date (e.g. [START_REF] Cai | The geochemistry of dissolved inorganic carbon in a surficial groundwater aquifer in North Inlet, South Carolina, and the carbon fluxes to the coastal ocean[END_REF][START_REF] Liu | Carbonate system biogeochemistry in a subterranean estuary -Waquoit Bay, USA[END_REF]. Because DOC concentrations in lower beach pore water are similar to those of the local seawater, pore waters discharging from the Truc Vert Beach are apparently not a source of DOC to the coastal ocean, in contrast to less energetic beaches and tidal flats worldwide [START_REF] Goñi | Seasonal dynamics in dissolved organic carbon concentrations in a coastal-water-table aquifer at the forest-marsh interface[END_REF][START_REF] Santos | Tidal pumping drives nutrient and dissolved organic matter dynamics in a Gulf of Mexico subterranean estuary[END_REF][START_REF] Avery | Dissolved organic carbon release from surface sand of a high energy beach along the Southeastern Coast of North Carolina[END_REF][START_REF] Kim | Production, degradation, and flux of dissolved organic matter in the subterranean estuary of a large tidal flat[END_REF][START_REF] Goodridge | The influence of submarine groundwater discharge on nearshore marine dissolved organic carbon reactivity, concentration dynamics, and offshore export[END_REF]. Hence, irrespective of the season, carbon is mostly exported to the coastal ocean as DIC.

Fluxes of carbon from tidally-driven seawater recirculation

The mean DIC enrichment in saline pore waters (SP > 34.5) was 144 µM, as derived from the difference between concentrations in the lower beach pore waters and seawater (Fig. 3). Considering the volume of pore water exchanged between the beach and the coastal ocean during each tide (15.2 m 3 per longshore meter; [START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF], the DIC export is 1500 moles of carbon per longshore meter per year. If we extrapolate over the 240-km long Aquitaine coast, the DIC flux, from aerobic respiration of marine organic matter in intertidal sands, to the coastal ocean is about 360 Mmol (4400 tons) of carbon per year. This value is of the same order of magnitude as the 3500 tons of mineralized carbon calculated by [START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF] from Redfield ratios and the pore water oxygen deficit.

Part of the metabolic DIC is delivered by SGD in the form of dissolved CO2, as pH decreases and pCO2 increases from the upper to the lower beach, with a mean enrichment of about 300 ppm relative to the atmosphere and discrete values reaching as high as 1500 ppm (Fig. 3). Consequently, CO2 degassing to the atmosphere may occur. In fact, we observed bubbles being expelled from the sands in the swash zone when the first waves of the flooding tide filled the intertidal sands with seawater. These bubbles originated from air trapped in the sediment during sediment desaturation at low tide. This mechanism would favour CO2 degassing, as CO2 can diffuse from the pore waters to interstitial air in the unsaturated sand at low tide and the interstitial air is expulsed to the atmosphere during rising tides. 3). Nevertheless, it stands in the high range of f DIC fluxes calculated for advective pore water fluxes in marine permeable sediments [START_REF] Cook | Benthic solute exchange and carbon mineralization in two shallow subtidal sandy sediments: Effect of advective pore-water exchange[END_REF][START_REF] Cyronak | Carbon cycling hysteresis in permeable carbonate sands over a diel cycle: Implications for ocean acidification[END_REF][START_REF] Faber | Pore-water exchange driven by tidal pumping causes alkalinity export in two intertidal inlets[END_REF].

DIC isotopic signatures in intertidal pore waters

The isotopic signature of the DIC can be used to distinguish qualitatively and quantitatively its origin [START_REF] Bouillon | Importance of intertidal sediment processes and porewater exchange on the water column biogeochemistry in a pristine mangrove creek (Ras Dege, Tanzania)[END_REF][START_REF] Miyajima | Export of inorganic carbon from two Southeast Asian mangrove forests to adjacent estuaries as estimated by the stable isotope composition of dissolved inorganic carbon[END_REF]. For saline pore waters of the lower beach (SP > 34.5), the d 13 C-DIC was linearly correlated with the consumption of dissolved oxygen (r 2 =0.37, N=265) as a result of the predominant aerobic respiration within the sands (Fig. 7A). A similar trend was observed for the brackish pore waters of the lower beach (SP < 34.5), but some samples clearly fall outside the general trend (Fig. 7A). In fact, four groups of intertidal pore waters could be distinguished on the basis of the d 13 C-DIC behaviour along the salinity gradient (SP ~25-36) of the lower beach (Fig. 7B). Combined to their DIC and oxygen contents, these four groups of pore waters highlight the different processes at play in the intertidal sands of the Truc Vert Beach: conservative mixing of fresh and saline waters, aerobic respiration, and carbonate dissolution (Fig. 7). For each group of pore waters, a Keeling plot (d 13 C-DIC versus 1/DIC) was drawn to determine the d 13 C signature of the predominant DIC source [START_REF] Keeling | The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas[END_REF][START_REF] Pataki | Seasonal cycle of carbon dioxide and its isotopic composition in an urban atmosphere: Anthropogenic and biogenic effects[END_REF].

The first group of water (group A) corresponds to saline (SP > 34.5), oxygen-depleted and DIC-enriched pore waters with d 13 C-DIC values between +0.5 and -1.22‰, waters whose variable properties are attributed to aerobic respiration of organic matter within the intertidal saline plume [START_REF] Anschutz | Tidal sands as biogeochemical reactors[END_REF][START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF]. The slope of the linear regressions is similar for every season (Fig. 7C), but the range of 1/DIC values evolves with time as the DIC concentration of the surface seawater (and TA; Fig. S3 in Supplementary material) changed seasonally (Fig. 3 and7C). The slope of the TA vs DIC for the surface seawater in all seasons is 1.02 with r 2 = 0.865 (Fig. S4 in Supplementary material), suggesting that biogenic carbonate precipitation in coastal waters explains seasonal changes of these parameters. Irrespective of the season, the isotopic composition of the DIC added to seawater, as derived from the y-intercept of the regression line, ranges from -19.1 to -15.8‰ (Fig. 7C), consistent with an addition of metabolic CO2 from the mineralization of marine organic matter [START_REF] Mook | Environmental isotopes in the hydrological cycle -Principles and applications[END_REF].

The second group of waters (group B) corresponds to few DIC-enriched brackish pore waters characterized by d 13 C-DIC values higher than predicted from conservative mixing. The combination of high d 13 C-DIC values with a DIC (and TA, Fig. S3 in Supplementary material) enrichment is indicative of carbonate dissolution. The y-intercept of the Keeling plot is about +1.8‰ (Fig. 7D), in agreement with the proposed marine calcium carbonate origin (0 to +2‰; [START_REF] Mook | Environmental isotopes in the hydrological cycle -Principles and applications[END_REF]. The CaCO3 content of the sands is up to 3.7 wt% in the lower part of the beach [START_REF] Charbonnier | Aerobic respiration in a high-energy sandy beach[END_REF]. It consists of calcite and aragonite shell debris. The accumulation of metabolic CO2 in this system, resulting from catabolic aerobic processes, could lower the saturation stage of the pore water and trigger the dissolution of the most soluble CaCO3 polymorph, aragonite. This hypothesis, inferred from d 13 C-DIC values, could be tested by measurements of dissolved calcium concentrations (e.g. [START_REF] Cai | The geochemistry of dissolved inorganic carbon in a surficial groundwater aquifer in North Inlet, South Carolina, and the carbon fluxes to the coastal ocean[END_REF].

The third group of waters (group C) corresponds to the brackish pore waters. These are characterized by conservative d 13 C-DIC values and little oxygen depletion (O2 saturation > 85%). The y-intercept of the Keeling plot for these waters yields an isotope signature of -12.0‰

for the DIC source (Fig. 7E), which corresponds to the d 13 C-DIC of the fresh groundwater endmembers collected in piezometers of the supratidal beach (Table 2). These samples most likely correspond to a simple mixture of freshwaters with seawater, unaltered by other processes like respiration or carbonate dissolution.

The last group of pore waters (group D) consists of DIC-enriched, O2-depleted (consumed O2 > 100 µM) brackish pore waters with low d 13 C-DIC. These waters are affected by mixing between upper-beach freshwaters and seawater as well as respiration processes. In these brackish waters, the respired organic matter can be either marine (entering sands during high tide with seawater) or terrestrial (brought as DOC in fresh groundwaters). The Keeling plot indicates that the isotope carbon composition of this last DIC source is about -16.5‰ (Fig. 7F), akin to organic matter of marine origin [START_REF] Mook | Environmental isotopes in the hydrological cycle -Principles and applications[END_REF]. Given the possible temporal variability of DIC sources, we estimated the d 13 C signature of the respired carbon source for each pore water of group D using the following equation [START_REF] Bouillon | Inorganic and organic carbon biogeochemistry in the Gautami Godavari estuary (Andhra Pradesh, India) during pre-monsoon: the local impact of extensive mangrove forests[END_REF]:

δ () C 01121 = !"# &'(! + $% # &'(! $!"# &) + $% # &) !"# &'(! $ !"# &) (6)
where d 13 Cadded is the carbon isotope composition of added DIC, considering that DIC enrichment originated solely from organic matter respiration. This approach yields a wider range of DIC sources, with d 13 Cadded values ranging from -19.0 to -4.8‰ (Table 4). This range of values is far from those of land sources, such as terrestrial higher plants that characterize the forest upland of the Truc Vert Beach (-28‰± 1.3; [START_REF] Dubois | Origin and composition of sediment organic matter in a coastal semi-enclosed ecosystem: An elemental and isotopic study at the ecosystem space scale[END_REF]. The heavier d 13 Cadded values can originate from C4 plants [START_REF] Ehleringer | Stable Isotopes in Physiological Ecology and Food Web Research[END_REF][START_REF] Mook | Environmental isotopes in the hydrological cycle -Principles and applications[END_REF] such as dune grasses. Most (7 out of 9) of the samples yielded d 13 Cadded values ranging from -19.0 to -9.3‰, corresponding to marine organic matter [START_REF] Mook | Environmental isotopes in the hydrological cycle -Principles and applications[END_REF] and C3 seagrasses [START_REF] Andrews | Photosynthetic carbon metabolism in seagrasses 14Clabeling evidence for the C3 pathway[END_REF]. In the study area, plants with this isotopic signature can be marine phytoplankton and drifting plant debris (also known as wracks) such as Zostera marina or Zostera noltei (-11‰±2; [START_REF] Dubois | Origin and composition of sediment organic matter in a coastal semi-enclosed ecosystem: An elemental and isotopic study at the ecosystem space scale[END_REF], dispersed on the beach at the high tide water line. Aerobic respiration in the intertidal aquifer of the Truc Vert Beach is thus mainly fuelled by marine organic matter. Degradation of terrestrial organic matter is a relatively weak oxygen sink and source of metabolic DIC.

Conclusion

The Truc Vert Beach is a high-energy beach affected by strong physical forcings (tides, waves) that drive an active seawater recirculation cell within the intertidal sands. This tidallydriven SGD is complemented by a terrestrial, fresh groundwater input from the coupled, upland pine forest aquifer. Our results show that aerobic respiration processes are active in the seawater circulation cell, particularly in spring and summer, leading to a substantial DIC flux to the coastal ocean. Up-scaled over the 240 km-long Aquitaine coast, the carbon flux is estimated at 4400 tons of carbon per year to the Bay of Biscay coastal zone. The carbon isotopic signature of the metabolic DIC, when coupled to pore water DIC and DOC concentrations, reveals that aerobic respiration of marine organic matter trapped by the sand, as seawater brought up the beach by tides and waves percolates through, is the main source of metabolic DIC within the intertidal sands. Therefore, the efficient oxic degradation of marine organic carbon in high energy beach aquifer prevents carbon storage within the sand and from being a source of DOC.

Analyses of pore waters sampled in the subterranean estuary of the supratidal beach show that carbonate system parameters (TA, DIC, pH, pCO2) do not behave conservatively along the salinity gradient. Degassing of CO2 to the atmosphere in the upper part of the beach best explains the evolution of the measured parameters. Fresh groundwaters sampled on the forest-side and beach-side of the bare foredune reveal that terrestrial groundwater is chemically altered upstream of the beach. Unlike most STEs, the redox front at the Truc Vert Beach occurs below the fore dune and does not coincide with the salinity gradient. Most of the CO2 dissolved in the forest groundwater that flows seaward though the connected aquifer, degasses in this transition zone. We calculated that CO2 evasion accounted for about 33% of the forest groundwater DIC (8000 tons out of 24200 tons of carbon), leaving a residual DIC flux to the coastal zone that was about four-fold higher than the metabolic DIC produced and released in the beach recirculation cell. Extrapolating the C-CO2 evasion to the 240 km of beach and assuming an average width of 200 m for the fore dunes, the annual flux of C-CO2 is estimated at 1.6 mmol m -2 h -1 . This flux could possibly be measured by incubating a volume of air under a chamber and following the evolution of the pCO2 (1 m 3 of air contains about 16 mmol of CO2) for a few hours.

CO2 degassing leads to a groundwater DIC:TA ratio <1, resulting in SGD buffering receiving seawater against ocean acidification. This is another feature that distinguishes this coastal environment from others where carbon fluxes through SGD have been studied and shown to amplifying the impact of ocean acidification in the coastal zone [START_REF] Robinson | Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers : Controls on submarine groundwater discharge and chemical inputs to the ocean[END_REF]. This study demonstrates that the foredune and the supratidal zone of coastal environments, with typologies similar to the Truc Vert Beach, are not inert vectors of fluids from the land to the coastal ocean, but significant sources of CO2 to the atmosphere and, must therefore be taken into consideration in SGD carbon budgets. Figure 2: Schematic W-E cross-section of the beach-dune system and the different waters sampled (1 -seawater; 2 -beach pore waters; 3 -piezometers in the supratidal zone; 4 -forest wells), with a conceptual diagram of the subterranean estuary of the Truc Vert Beach. Grey arrows represent major nearshore flow processes: (A) seawater circulation induced by waves and tides, called "intertidal saline plume", and (B) parcels of brackish water discharge, as revealed by geophysical data [START_REF] Buquet | Shape of the shallow aquifer at the freshwater-seawater interface on a high-energy sandy beach[END_REF].

4. 3 .

 3 Carbon behaviour in the beach aquifer 4.3.1. Aerobic respiration within the intertidal recirculation cell

  4.3.3. Fluxes of carbon compared to other coastal systemsGiven a mean intertidal beach width of 150 m, the calculated DIC flux from seawater recirculation is 27 mmolC m -2 d -1 . The total DIC flux, including the terrestrial groundwater flux is 130 mmolC m -2 d -1 . The calculated flux is not large enough to significantly affect the DIC inventory of the open ocean coastal waters because it is incremental with respect to the flow of seawater at the coast driven by coastal currents that characterize the Bay of Biscay continental shelf. The calculated DIC flux is much lower than fluxes from SGD affected by a strong input of fresh groundwaters from organic-rich environments, such as mangroves, estuaries, lagoons, or salt marshes(Sadat-Noori et al., 2016 and references therein; Table

  d 13 CMEAS and DICMEAS are, respectively, the DIC concentration and isotopic composition of each sample; d 13 CMX and DICMX correspond to the computed d 13 C-DIC and DIC values for conservative mixing between the supratidal beach fresh groundwater and seawater.

Figure 1 :

 1 Figure 1: Location of Truc Vert Beach, on the SW coast of France.Figure2: Schematic W-E cross-section of the beach-dune system and the different waters sampled (1 -seawater; 2 -beach pore waters; 3 -piezometers in the supratidal zone; 4 -forest wells), with a conceptual diagram of the subterranean estuary of the Truc Vert Beach. Grey arrows represent major nearshore flow processes: (A) seawater circulation induced by waves and tides, called "intertidal saline plume", and (B) parcels of brackish water discharge, as revealed by geophysical data[START_REF] Buquet | Shape of the shallow aquifer at the freshwater-seawater interface on a high-energy sandy beach[END_REF]. Figure3: Evolution with time of DIC, O2, DOC, pCO2 and δ 13 C-DIC in interstitial waters from tidally-driven recirculation of seawater in Truc Vert Beach. The pore water values are derived from samples with salinity close to that of seawater. Samples mixed with freshwater were discarded. The values for the lower beach come from the most O2 depleted sample of each cross-shore profile; the values for the upper beach come from the sample taken closest to the dune of each cross-shore profile. Figure 4: (A) Altitude at the top of the water table and location of probes and the three supratidal piezometers. Practical salinity (SP) and d 13 C-DIC (grey characters) are indicated for each piezometer for both the neap and spring tides in February-March 2013. (B) DIC, pCO2, oxygen saturation and salinity values of the pore waters at the three supratidal piezometers during a spring-neap tidal cycle in February-March 2013 and September-October 2013. Note that there is no data at PZ1 for a few days during the neap tide because of the emersion of the instrument.Figure 5: Conceptual diagram showing an overview the the end-members and processes affecting carbon in fresh groundwaters from the forest aquifer to the supratidal aquifer, and in the tidally-driven seawater circulation cell. For the latter, annual averages are shown. The seasonal evolution of the parameters is shown in Figure S2 of the supplementary material. Water discharge values are in m 3 per meter longshore per tide for the Truc Vert Beach site.Figure 6: Assessment of the behaviour of DIC along the subterranean estuary of the Truc Vert Beach: (A) DIC concentrations versus practical salinity; (B) d 13 C-DIC versus practical salinity; (C) pCO2 versus practical salinity; (D) d 13 C-DIC versus DIC concentrations. The grey lines denote the expected DIC (A) and d 13 C-DIC (B) values for conservative mixing between supratidal beach freshwaters and seawater. The black lines denote the expected DIC (A) and d 13 C-DIC (B) values for conservative mixing between continental freshwaters (forest wells) and seawater.Figure 7: The d 13 C-DIC of pore waters from the lower Truc Vert Beach: (A) d 13 C-DIC versus consumed oxygen in saline (SP > 34.5; light grey diamonds) and brackish (SP < 34.5; dark grey diamonds) water; (B) d 13 C-DIC versus practical salinity and definition of four groups of pore waters depending on the mixing behaviour between supratidal beach fresh groundwaters and seawater. TML = theoretical mixing line; (C) Keeling plot of DIC-enriched, saline (SP > 34.5) pore waters with low d 13 C-DIC for each of the four seasons in 2011; (D) Keeling plot of DICenriched brackish pore waters characterized by high d 13 C-DIC; (E) Keeling plot of brackish pore waters placed close to the theoretical mixing line and little affected by respiration processes (O2 > 85%); (F) Keeling plot of DIC-enriched brackish pore waters with low d 13 C-DIC values and affected by respiration processes (consumed O2 > 100 µM).

Figure 3 :

 3 Figure 1: Location of Truc Vert Beach, on the SW coast of France.Figure2: Schematic W-E cross-section of the beach-dune system and the different waters sampled (1 -seawater; 2 -beach pore waters; 3 -piezometers in the supratidal zone; 4 -forest wells), with a conceptual diagram of the subterranean estuary of the Truc Vert Beach. Grey arrows represent major nearshore flow processes: (A) seawater circulation induced by waves and tides, called "intertidal saline plume", and (B) parcels of brackish water discharge, as revealed by geophysical data[START_REF] Buquet | Shape of the shallow aquifer at the freshwater-seawater interface on a high-energy sandy beach[END_REF]. Figure3: Evolution with time of DIC, O2, DOC, pCO2 and δ 13 C-DIC in interstitial waters from tidally-driven recirculation of seawater in Truc Vert Beach. The pore water values are derived from samples with salinity close to that of seawater. Samples mixed with freshwater were discarded. The values for the lower beach come from the most O2 depleted sample of each cross-shore profile; the values for the upper beach come from the sample taken closest to the dune of each cross-shore profile. Figure 4: (A) Altitude at the top of the water table and location of probes and the three supratidal piezometers. Practical salinity (SP) and d 13 C-DIC (grey characters) are indicated for each piezometer for both the neap and spring tides in February-March 2013. (B) DIC, pCO2, oxygen saturation and salinity values of the pore waters at the three supratidal piezometers during a spring-neap tidal cycle in February-March 2013 and September-October 2013. Note that there is no data at PZ1 for a few days during the neap tide because of the emersion of the instrument.Figure 5: Conceptual diagram showing an overview the the end-members and processes affecting carbon in fresh groundwaters from the forest aquifer to the supratidal aquifer, and in the tidally-driven seawater circulation cell. For the latter, annual averages are shown. The seasonal evolution of the parameters is shown in Figure S2 of the supplementary material. Water discharge values are in m 3 per meter longshore per tide for the Truc Vert Beach site.Figure 6: Assessment of the behaviour of DIC along the subterranean estuary of the Truc Vert Beach: (A) DIC concentrations versus practical salinity; (B) d 13 C-DIC versus practical salinity; (C) pCO2 versus practical salinity; (D) d 13 C-DIC versus DIC concentrations. The grey lines denote the expected DIC (A) and d 13 C-DIC (B) values for conservative mixing between supratidal beach freshwaters and seawater. The black lines denote the expected DIC (A) and d 13 C-DIC (B) values for conservative mixing between continental freshwaters (forest wells) and seawater.Figure 7: The d 13 C-DIC of pore waters from the lower Truc Vert Beach: (A) d 13 C-DIC versus consumed oxygen in saline (SP > 34.5; light grey diamonds) and brackish (SP < 34.5; dark grey diamonds) water; (B) d 13 C-DIC versus practical salinity and definition of four groups of pore waters depending on the mixing behaviour between supratidal beach fresh groundwaters and seawater. TML = theoretical mixing line; (C) Keeling plot of DIC-enriched, saline (SP > 34.5) pore waters with low d 13 C-DIC for each of the four seasons in 2011; (D) Keeling plot of DICenriched brackish pore waters characterized by high d 13 C-DIC; (E) Keeling plot of brackish pore waters placed close to the theoretical mixing line and little affected by respiration processes (O2 > 85%); (F) Keeling plot of DIC-enriched brackish pore waters with low d 13 C-DIC values and affected by respiration processes (consumed O2 > 100 µM).

Figure 5 :

 5 Figure 1: Location of Truc Vert Beach, on the SW coast of France.Figure2: Schematic W-E cross-section of the beach-dune system and the different waters sampled (1 -seawater; 2 -beach pore waters; 3 -piezometers in the supratidal zone; 4 -forest wells), with a conceptual diagram of the subterranean estuary of the Truc Vert Beach. Grey arrows represent major nearshore flow processes: (A) seawater circulation induced by waves and tides, called "intertidal saline plume", and (B) parcels of brackish water discharge, as revealed by geophysical data[START_REF] Buquet | Shape of the shallow aquifer at the freshwater-seawater interface on a high-energy sandy beach[END_REF]. Figure3: Evolution with time of DIC, O2, DOC, pCO2 and δ 13 C-DIC in interstitial waters from tidally-driven recirculation of seawater in Truc Vert Beach. The pore water values are derived from samples with salinity close to that of seawater. Samples mixed with freshwater were discarded. The values for the lower beach come from the most O2 depleted sample of each cross-shore profile; the values for the upper beach come from the sample taken closest to the dune of each cross-shore profile. Figure 4: (A) Altitude at the top of the water table and location of probes and the three supratidal piezometers. Practical salinity (SP) and d 13 C-DIC (grey characters) are indicated for each piezometer for both the neap and spring tides in February-March 2013. (B) DIC, pCO2, oxygen saturation and salinity values of the pore waters at the three supratidal piezometers during a spring-neap tidal cycle in February-March 2013 and September-October 2013. Note that there is no data at PZ1 for a few days during the neap tide because of the emersion of the instrument.Figure 5: Conceptual diagram showing an overview the the end-members and processes affecting carbon in fresh groundwaters from the forest aquifer to the supratidal aquifer, and in the tidally-driven seawater circulation cell. For the latter, annual averages are shown. The seasonal evolution of the parameters is shown in Figure S2 of the supplementary material. Water discharge values are in m 3 per meter longshore per tide for the Truc Vert Beach site.Figure 6: Assessment of the behaviour of DIC along the subterranean estuary of the Truc Vert Beach: (A) DIC concentrations versus practical salinity; (B) d 13 C-DIC versus practical salinity; (C) pCO2 versus practical salinity; (D) d 13 C-DIC versus DIC concentrations. The grey lines denote the expected DIC (A) and d 13 C-DIC (B) values for conservative mixing between supratidal beach freshwaters and seawater. The black lines denote the expected DIC (A) and d 13 C-DIC (B) values for conservative mixing between continental freshwaters (forest wells) and seawater.Figure 7: The d 13 C-DIC of pore waters from the lower Truc Vert Beach: (A) d 13 C-DIC versus consumed oxygen in saline (SP > 34.5; light grey diamonds) and brackish (SP < 34.5; dark grey diamonds) water; (B) d 13 C-DIC versus practical salinity and definition of four groups of pore waters depending on the mixing behaviour between supratidal beach fresh groundwaters and seawater. TML = theoretical mixing line; (C) Keeling plot of DIC-enriched, saline (SP > 34.5) pore waters with low d 13 C-DIC for each of the four seasons in 2011; (D) Keeling plot of DICenriched brackish pore waters characterized by high d 13 C-DIC; (E) Keeling plot of brackish pore waters placed close to the theoretical mixing line and little affected by respiration processes (O2 > 85%); (F) Keeling plot of DIC-enriched brackish pore waters with low d 13 C-DIC values and affected by respiration processes (consumed O2 > 100 µM).

Figure 6 :

 6 Figure 1: Location of Truc Vert Beach, on the SW coast of France.Figure2: Schematic W-E cross-section of the beach-dune system and the different waters sampled (1 -seawater; 2 -beach pore waters; 3 -piezometers in the supratidal zone; 4 -forest wells), with a conceptual diagram of the subterranean estuary of the Truc Vert Beach. Grey arrows represent major nearshore flow processes: (A) seawater circulation induced by waves and tides, called "intertidal saline plume", and (B) parcels of brackish water discharge, as revealed by geophysical data[START_REF] Buquet | Shape of the shallow aquifer at the freshwater-seawater interface on a high-energy sandy beach[END_REF]. Figure3: Evolution with time of DIC, O2, DOC, pCO2 and δ 13 C-DIC in interstitial waters from tidally-driven recirculation of seawater in Truc Vert Beach. The pore water values are derived from samples with salinity close to that of seawater. Samples mixed with freshwater were discarded. The values for the lower beach come from the most O2 depleted sample of each cross-shore profile; the values for the upper beach come from the sample taken closest to the dune of each cross-shore profile. Figure 4: (A) Altitude at the top of the water table and location of probes and the three supratidal piezometers. Practical salinity (SP) and d 13 C-DIC (grey characters) are indicated for each piezometer for both the neap and spring tides in February-March 2013. (B) DIC, pCO2, oxygen saturation and salinity values of the pore waters at the three supratidal piezometers during a spring-neap tidal cycle in February-March 2013 and September-October 2013. Note that there is no data at PZ1 for a few days during the neap tide because of the emersion of the instrument.Figure 5: Conceptual diagram showing an overview the the end-members and processes affecting carbon in fresh groundwaters from the forest aquifer to the supratidal aquifer, and in the tidally-driven seawater circulation cell. For the latter, annual averages are shown. The seasonal evolution of the parameters is shown in Figure S2 of the supplementary material. Water discharge values are in m 3 per meter longshore per tide for the Truc Vert Beach site.Figure 6: Assessment of the behaviour of DIC along the subterranean estuary of the Truc Vert Beach: (A) DIC concentrations versus practical salinity; (B) d 13 C-DIC versus practical salinity; (C) pCO2 versus practical salinity; (D) d 13 C-DIC versus DIC concentrations. The grey lines denote the expected DIC (A) and d 13 C-DIC (B) values for conservative mixing between supratidal beach freshwaters and seawater. The black lines denote the expected DIC (A) and d 13 C-DIC (B) values for conservative mixing between continental freshwaters (forest wells) and seawater.Figure 7: The d 13 C-DIC of pore waters from the lower Truc Vert Beach: (A) d 13 C-DIC versus consumed oxygen in saline (SP > 34.5; light grey diamonds) and brackish (SP < 34.5; dark grey diamonds) water; (B) d 13 C-DIC versus practical salinity and definition of four groups of pore waters depending on the mixing behaviour between supratidal beach fresh groundwaters and seawater. TML = theoretical mixing line; (C) Keeling plot of DIC-enriched, saline (SP > 34.5) pore waters with low d 13 C-DIC for each of the four seasons in 2011; (D) Keeling plot of DICenriched brackish pore waters characterized by high d 13 C-DIC; (E) Keeling plot of brackish pore waters placed close to the theoretical mixing line and little affected by respiration processes (O2 > 85%); (F) Keeling plot of DIC-enriched brackish pore waters with low d 13 C-DIC values and affected by respiration processes (consumed O2 > 100 µM).

Figure 7 :

 7 Figure 1: Location of Truc Vert Beach, on the SW coast of France.Figure2: Schematic W-E cross-section of the beach-dune system and the different waters sampled (1 -seawater; 2 -beach pore waters; 3 -piezometers in the supratidal zone; 4 -forest wells), with a conceptual diagram of the subterranean estuary of the Truc Vert Beach. Grey arrows represent major nearshore flow processes: (A) seawater circulation induced by waves and tides, called "intertidal saline plume", and (B) parcels of brackish water discharge, as revealed by geophysical data[START_REF] Buquet | Shape of the shallow aquifer at the freshwater-seawater interface on a high-energy sandy beach[END_REF]. Figure3: Evolution with time of DIC, O2, DOC, pCO2 and δ 13 C-DIC in interstitial waters from tidally-driven recirculation of seawater in Truc Vert Beach. The pore water values are derived from samples with salinity close to that of seawater. Samples mixed with freshwater were discarded. The values for the lower beach come from the most O2 depleted sample of each cross-shore profile; the values for the upper beach come from the sample taken closest to the dune of each cross-shore profile. Figure 4: (A) Altitude at the top of the water table and location of probes and the three supratidal piezometers. Practical salinity (SP) and d 13 C-DIC (grey characters) are indicated for each piezometer for both the neap and spring tides in February-March 2013. (B) DIC, pCO2, oxygen saturation and salinity values of the pore waters at the three supratidal piezometers during a spring-neap tidal cycle in February-March 2013 and September-October 2013. Note that there is no data at PZ1 for a few days during the neap tide because of the emersion of the instrument.Figure 5: Conceptual diagram showing an overview the the end-members and processes affecting carbon in fresh groundwaters from the forest aquifer to the supratidal aquifer, and in the tidally-driven seawater circulation cell. For the latter, annual averages are shown. The seasonal evolution of the parameters is shown in Figure S2 of the supplementary material. Water discharge values are in m 3 per meter longshore per tide for the Truc Vert Beach site.Figure 6: Assessment of the behaviour of DIC along the subterranean estuary of the Truc Vert Beach: (A) DIC concentrations versus practical salinity; (B) d 13 C-DIC versus practical salinity; (C) pCO2 versus practical salinity; (D) d 13 C-DIC versus DIC concentrations. The grey lines denote the expected DIC (A) and d 13 C-DIC (B) values for conservative mixing between supratidal beach freshwaters and seawater. The black lines denote the expected DIC (A) and d 13 C-DIC (B) values for conservative mixing between continental freshwaters (forest wells) and seawater.Figure 7: The d 13 C-DIC of pore waters from the lower Truc Vert Beach: (A) d 13 C-DIC versus consumed oxygen in saline (SP > 34.5; light grey diamonds) and brackish (SP < 34.5; dark grey diamonds) water; (B) d 13 C-DIC versus practical salinity and definition of four groups of pore waters depending on the mixing behaviour between supratidal beach fresh groundwaters and seawater. TML = theoretical mixing line; (C) Keeling plot of DIC-enriched, saline (SP > 34.5) pore waters with low d 13 C-DIC for each of the four seasons in 2011; (D) Keeling plot of DICenriched brackish pore waters characterized by high d 13 C-DIC; (E) Keeling plot of brackish pore waters placed close to the theoretical mixing line and little affected by respiration processes (O2 > 85%); (F) Keeling plot of DIC-enriched brackish pore waters with low d 13 C-DIC values and affected by respiration processes (consumed O2 > 100 µM).

Table 1 :

 1 pH, DOC and DIC concentrations, pCO2 and d 13 C-DIC values in continental 1248 groundwater collected in forest wells above the Truc Vert Beach in 2013 -2014.

	1249
	1250
	Name of well	Sampling date	pH	DOC (µM)	DIC (µM)	pCO2 (ppm)

Table 2 :

 2 Average ( ± standard deviation) values of practical salinity (SP), dissolved oxygen saturation, Fe(II), NH4 + , pH, TA, DIC, pCO2 and d 13 C-DIC in continental, fresh groundwaters (forest wells), pore waters in the salinity gradient of the STE (supratidal beach piezometers), brackish pore waters of the lower beach and surface seawater.

		Continental freshwaters	Upper beach piezometers SP < 1 1 < SP < 2 2 < SP < 25	Brackish PW of the lower beach	Seawater
		(n = 12)	(n = 36)	(n = 10)	(n = 9)	(n = 28)	(n = 37)
	SP	0.0 ± 0.0	0.6 ± 0.1	1.2 ± 0.3	10.2 ± 6.1	31.4 ± 1.6	35.1 ± 0.4
	O2 (%)	0 ± 0	40 ± 32	37 ± 20	76 ± 14	51 ± 24	97 ± 6
	Fe(II) (µM)*	59 ± 30	0.1 ± 0.2	0.2 ± 0.2	0.1 ± 0.2	-	< DL
	NH4 + (µM)*	17 ± 18	0.5 ± 1.4	0.3 ± 0.3	0.2 ± 0.4	0.2 ± 0.4	< DL
	pH	7.06 ± 0.37	7.94 ± 0.19	8.31 ± 0.20 8.20 ± 0.13	7.97 ± 0.12	8.20 ± 0.05
	TA (µM)	3583 ± 1312	3441 ± 346	3171 ± 309 2903 ± 238	2455 ± 132	2311 ± 110
	DIC (µM)	4700 ± 1130	3500 ± 380	3150 ± 330 2870 ± 260	2300 ± 160	2070 ± 100
	pCO2 (ppm)	16470 ± 6510**	2340 ± 1070	770 ± 380	810 ± 310	770 ± 250	400 ± 60
	d 13 C-DIC (‰)	-16.9 ± 2.2	-12.1 ± 0.1***	-9.9***	-5.2***	-1.3 ± 1.0	+0.4 ± 0.5
	*after Anschutz et al. (2016)
	**n = 11, as an overestimated value was deleted for well DFCI2
	***n = 5 for SP < 1; n = 1 for 1 < SP <2; n =2 for 2 < SP < 25

Table 3 :

 3 Benthic DIC fluxes in permeable sediments with various fresh, groundwater inputs. Modified from[START_REF] Sadat-Noori | Groundwater Discharge as a Source of Dissolved Carbon and Greenhouse Gases in a Subtropical Estuary[END_REF].

	Location	System description	DIC flux
			(mmolC.

m -2 .d -1 ) Reference Systems with high fresh groundwater input

	Waquoit Bay, USA	Semi-enclosed bay	1151	Liu et al., 2017
	Hat Head, Australia	Groundwater discharge in a	687	Sadat-Noori et al., 2016
		subtropical estuary
	Jiulong River estuary,	Groundwater discharge in a	121-897	Wang et al., 2015
	China	river estuary
	Moreton Bay, Australia	Embayment	153	Stewart et al., 2015
	Okatee Estuary, USA	Salt marsh/estuary	1079	Porubsky et al., 2014
	North Creek, Australia	Freshwater tidal creek	1810	Atkins et al., 2013
	Moreton Bay, Australia	Mangrove tidal creek	183-342	Maher et al., 2013
	Yarra River, Australia	Salt wedge estuary	349	Santos et al., 2012
	Indian River Lagoon,	Coastal lagoon	120-340	Dorsett et al., 2011
	USA
	Okatee Estuary, USA	Salt marsh/estuary	1963	Moore et al., 2006
	North Inlet, USA	Salt marsh/estuary	171	Cai et al., 2003
	Watson Inlet, Australia Intertidal inlet with freshwater	440	Faber et al., 2014
		input
	Systems with no or low freshwater input
	Heron Island, Australia	Permeable carbonate	1.6-18.8	Cyronak et al., 2013
		sediments
	Chinaman Inlet, Western	Intertidal inlet without	120	Faber et al., 2014
	Port, Australia	freshwater input
	Southwest Florida Shelf,	Estuarine SGD to a	28-45	Liu et al., 2014
	USA	subtropical carbonate platform
	South China Sea	River-dominated continental	16-36	Liu et al., 2012
		shelf
	Sylt Island, Germany	Advective saline pore water	31-120	Cook et al. 2007
		exchange in nearshore
		permeable sediments
	Hel Peninsula, Poland	Advective saline pore water	5-42	Cook et al., 2007
		exchange in nearshore
		permeable sediments
	Truc Vert Beach, SW	seawater recirculation + fresh	27
	France	groundwater	130	This study

Table 4 :

 4 Practical salinity, DIC concentration and d 13 C-DIC measured in brackish, DICenriched and O2-depleted pore waters of the lower beach. The theoretical DIC and d 13 C-DIC values are derived from conservative mixing between upper-beach fresh, groundwater and seawater. For each sample, the d 13 Cadded is calculated using equation (6) from[START_REF] Bouillon | Inorganic and organic carbon biogeochemistry in the Gautami Godavari estuary (Andhra Pradesh, India) during pre-monsoon: the local impact of extensive mangrove forests[END_REF].

	Sample ID	Measured values	Theoretical values	d 13 Cadded (‰)

Acknowledgements

The authors wish to thank colleagues and students who helped us in the field and in the laboratory (Bruno Deflandre, Sophie Ferreira, Sylvain Capo, Romain Naturel, Sylvain Rigaud, Orianne Jolly, Jonathan Deborde) as well as K. Charlier for her precious help with the d 13 C-DIC measurements. This work was funded by the French national projects EC2CO-MOBISEA and ESTAFET (CNRS-INSU), and PROTIDAL (ANR-06-BLANC-0040) and carried out in the framework of the Cluster of Excellence COTE (ANR-10-LABX-45). A.M. thanks the Institut France-Québec Maritime and IdEx (initiative d'excellence)-Bordeaux for financial support during his visit and sabbatical leave at the Université de Bordeaux.