Les estuaires souterrains en milieu transgressif : source d'acidification à l'océan côtier

RÉSUMÉ : Dans le bilan global du cycle du carbone, l'apport de CO2 à l'océan côtier par les rivières est relativement bien contraint alors que la contribution des eaux souterraines commence à peine à être abordée. Pourtant, il a été démontré que localement, les apports en carbone inorganique dissous (DIC pour Dissolved Inorganic Carbon) par les décharges d'eau souterraine à l'océan (SGD pour Submarine Groundwater Discharge) pouvaient excéder ceux de surfaces et contrôler la chimie des eaux côtières. Ces décharges n'agissent pourtant pas toutes comme des sources d'acidification à l'océan côtier. C'est l'alcalinité totale (TA) qui assure la stabilité du pH de l'eau et le rapport TA/DIC détermine si l'apport sera une source ou non d'acidification. C'est pourquoi mon projet s'est particulièrement intéressé à la détermination du rôle qu'ont les SGD dans le transfert du DIC à l'océan côtier. Plus spécifiquement ce mémoire vise à 1) identifier et quantifier les sources de DIC et d'alcalinité qui transitent de l'aquifère à l'océan côtier et à 2) détailler les mécanismes biogéochimiques qui produisent du DIC et de l'alcalinité dans l'estuaire souterrain. Ces paramètres impliqués dans les paramètres du système des carbonates permettront de déterminer le rôle des systèmes côtiers comme source d'acidification à l'océan côtier. Nos travaux ont été réalisés dans l'estuaire souterrain d'une plage de l'archipel des Îles-de-la-Madeleine (Qc, Canada) qui a subi une forte hausse du niveau marin relatif au cours du dernier millénaire (~1,4 mm an-1) et une transgression datée d'environ 800 ans ayant laissé un paléosol enfoui sous environ 50 cm de sable sur le site d'étude. Des échantillons d'eau souterraine ont été collectés en juin 2015 dans la zone intertidale de la plage (d'une largeur d'environ 15 m) de 0,05 à 1,85 m sous la surface du sédiment. Il a été démontré que les fortes concentrations en DIC (3,6 mmol kg-1) et en alcalinité totale (3,6 mmol kg-1) mesurées dans le gradient de salinité proviennent principalement de la minéralisation du carbone organique contenue dans le paléosol. En raison des conditions suboxiques à anoxiques, la sulfato-réduction et la méthanogénèse semblent être des processus clés dans la formation de DIC alors que le fer jouerait un rôle central dans la production d'alcalinité. Étant données la teneur élevée en DIC sous forme gazeux (±30%, pCO2>400 ppm) à l'interface sédiment/atmosphère et une vitesse d'écoulement de l'eau souterraine lente (~0,30 m j-1), il est montré que la plage est une source de CO2 à l'atmosphère avec un flux estimé à 310 mol m2 d-1. De plus, le rapport TA/DIC moyen de 0,7 permet de conclure que ce système agit également comme une source d'acidification des eaux côtières, soulignant l'importance d'intégrer ce type de systèmes dans le cycle globale du carbone. -- Mot(s) clé(s) en français : Carbone inorganique dissous, acidification, décharge d'eau souterraine, estuaire souterrain, hausse du niveau marin, alcalinité totale, minéralisation matière organique, pCO2. -- ABSTRACT : In the global carbon cycle, the fluxes of carbon derived from rivers to the coastal ocean are relatively well constrained while the contribution of groundwater-derived carbon inputs has only recently been tackled. It has been shown that locally, carbon inputs from groundwater discharge can exceed those from surface flow and then control the chemistry of coastal waters. This project focused on determining the role of groundwater discharges in the transfer of dissolved inorganic carbon (DIC) to the coastal ocean. Specifically, the objectives were to 1) identify and quantify the sources of DIC and alkalinity transiting from the aquifer to the coastal ocean and 2) detail the biogeochemical mechanisms that produce DIC and total alkalinity (TA) in the subterranean estuary. Its parameters (DIC and TA) involved in the carbon cycle will determine the role of transgressive coastal systems as a source of acidification in the receiving coastal waters. The study took place in the subterranean estuary of Martinique Beach on the Magdalen Islands (Qc, Canada). This coastal system is characterized by a strong relative sea level rise since the last millennium (~1.4 mm y-1), that buried a ~800 years aged peat soil. Groundwater samples were collected in June 2015 in the intertidal zone of the beach (~15 m width) from 0.05 to 1.85 m depth below the surface. The high DIC (3.6 mmol kg-1) and TA (3.6 mmol kg-1) concentrations mainly originated from the mineralization of organic carbon contained in the old forest soil. Under suboxic to anoxic conditions, sulphate reduction and methane production were major processes in DIC generation, whereas iron played a central role in the production of TA. Given the high DIC content in gaseous form (±30%, pCO2>400 ppm) at the sediment/atmosphere interface and a slow groundwater flow rate (~0.30 m d-1), the beach acted as a source of CO2 to the atmosphere with a degassing rate of 310 mol m2 d-1. In addition, the average TA/DIC ratio of 0.7 pointed out that this system also acted as a source of acidification to the adjacent the coastal waters. Our finding highlights the need to include transgressive STE systems as a key component of the marine carbon budget. -- Mot(s) clé(s) en anglais : Inorganic carbon, acidification, submarine groundwater discharge, sea level rise, organic alkalinity, mineralization, old peat soil, pCO2

Total Alkalinity and Dissolved Inorganic Carbon Production in Groundwaters Discharging through a Sandy Beach

AbstractThe paper presents evidence of the complexity of describing carbon transport from submarine groundwater discharge and chemical reactions in the subterranean estuary. Vertical and horizontal profiles of total alkalinity (TA) and dissolved inorganic carbon (DIC) concentrations were analyzed in a cross-shore transect of a sandy beach and an evaluation of DIC and TA fluxes to coastal waters is proposed based on groundwater discharge velocities and beach hydrogeology. The study was conducted in the Magdalen Islands in the Gulf of St. Lawrence (QC, Canada) where an unconfined sandstone aquifer rapidly discharges to the coastal ocean with a rate of ∼3000 m3 d-1. Increases in DIC and TA observed along the discharge pathway exceed the expected conservative mixing between inland groundwaters and seawater. This local production is attributed to anaerobic respiration. Slow groundwater velocity (1 to 9cm d-1), low oxygen conditions (∼20%) and redox oscillations probably induced by tidal pumping present a suitable environment for bacterially-mediated carbon oxidation and anaerobic respiration. Depleted δ13C-DIC (from −14‰ to −28‰) and the characteristic odour of H2S during sample collection support the idea that sulphate reduction may generate high alkalinity and DIC concentrations at the seepage face, leading to potentially high discharge to the coastal ocean at this beach (2.0 to 8.2mol DIC/day and 1.9 to 7.9 mmol TA/day). To our knowledge, this study is the first attempt to estimate the transport and transformations of dissolved inorganic carbon by biogeochemical processes in the subterranean estuary of a northern sandy beach

Production and fluxes of inorganic carbon and alkalinity in a subarctic subterranean estuary

In this study, we focus on the biogeochemical processes that produce both dissolved inorganic carbon (DIC) and total alkalinity (TA) along a subarctic subterranean estuary (STE) located in the Gulf of St. Lawrence (Magdalen Island, Qc, Canada) in order to evaluate the DIC and TA fluxes as well as the buffering capacity of the exported groundwater to coastal waters. DIC and TA do not behave conservatively during mixing along the groundwater flow path and this implies the occurrence of internal redox reactions that control both their production and consumption. In addition, we show that the origin and composition of the organic carbon within the system alter the carbonate parameters by generating low pH conditions (5.9 - 7.2) and contributing to non-carbonate alkalinity (NCA) that accounts for more than 30% of TA. Whereas iron cycling plays a key role in the production of DIC in the fresh and low-salinity groundwaters, the precipitation of sulfide minerals neutralize the acidity produced by the metabolically produced CO2, in the saline groundwater where sulfate is available. The STE pCO2, computed from the DIC-pHNBS pair ranged from a few ppm to 16000 ppm that results in a CO2 evasion rate of up to 310 mol m−2d−1 to the atmosphere. Based on Darcy flow and the mean concentrations of DIC and carbonate alkalinity (Ac = TA - NCA) in the discharge zone, fluxes derived from submarine groundwater discharge were estimated at 1.43 and 0.70 mol m−2d−1 for DIC and Ac, respectively. Despite a major part of the metabolic CO2 being lost along the groundwater flow path, the SGD-derived DIC flux was still greater than the Ac flux, implying that groundwater discharge reduces the buffering capacity of the receiving coastal waters. This site-specific scale study demonstrates the importance of diagenetic reactions and organic matter remineralization processes on carbonate system parameters in STE. Our results highlight that subarctic STEs could be hot spots of CO2 evasion and a source of acidification to coastal waters that should be considered in carbon budgets

Geochemical and isotope data from shallow aquifers in the Bas-Saint-Laurent area (Qc, Canada)

In this study, we combined radon in water activity (222Rn), stable isotopes of inorganic carbon (δ13CDIC), water stable isotopes (δ18O; δ2H) as well as major and trace ions measured in 161 private wells to develop a conceptual hydrogeochemical model of the groundwater flow in this region. More specifically, the applied method consists of (1) analyses of the hydrogeochemical facies and their spatial distribution, (2) sequential multivariate analyses of the groundwater geochemistry and assessment of the evolutionary processes, (3) origin and distribution of 222Rn in the study area and (4) a conceptual model of the groundwater flowpath and mixing processes combining geochemistry and isotopes