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Understanding the behavior of terrestrially derived dissolved organic carbon (DOC) through subterranean estuaries (STEs) is essential for determining the carbon budget in coastal waters.However, few studies exist on the interaction of organic carbon (OC) and iron (Fe) in these dynamic systems, where fresh groundwater mixes with recirculated seawater. Here, we focused on the origin and behavior of DOC, and we quantified the relative proportion of OC trapped by reactive Fe-hydroxides along a sandy beach STE. The  13 C-DOC signal in beach groundwater seems to respond rapidly to OC inputs. Our results show a terrestrial imprint from the aquifer matrix dominated by the degradation of particulate organic carbon (POC) issue from an old soil horizon composed of terrestrial plant detritus ( 14 C dating ~800 to 700 years B.P) which is buried below the Holocene sand. Even though the system can be sporadically affected by massive inputs of marine OC, this transient marine imprint seems to be rapidly evacuated from the STE. As reported in soil and in marine mud, Fe-OC trapping occurs in the sandy sediment of the STE. At the groundwater-seawater interface, newly precipitated reactive Fe-hydroxides interact with and trap terrestrial OC independently of the DOC origin in beach groundwater. The molecular fractionation of DOC along the STE and preferential trapping of terrestrial compounds favor the in situ degradation and/or export of non-Fe-stabilized marine-derived molecules to coastal waters. These findings support the idea that the sandy beach STE acts as a transient sink for terrestrial OC at the land-sea interface and contributes to the regulation of marine vs. terrestrial carbon exports to coastal waters.

Abstract. The transformations of chemical constituents in subterranean estuaries (STEs) control the delivery of nutrient loads from coastal aquifers to the ocean. It is important to determine the processes and sources that alter nutrient concentrations at a local scale in order to estimate accurate regional and global nutrient fluxes via submarine groundwater discharge (SGD), particularly in boreal environments, where data are still very scarce. Here, the biogeochemical transformations of nitrogen (N) species were examined within the STE of a boreal microtidal sandy beach located in the Magdalen Islands (Quebec, Canada). This study revealed the vertical and horizontal distribution of nitrate (NO, dissolved organic nitrogen (DON) and total dissolved nitrogen (TDN) measured in beach groundwater during four spring seasons (June 2011(June , 2012(June , 2013(June and 2015 when aquifer recharge was maximal after snowmelt. Inland groundwater supplied high concentrations of NO x and DON to the STE, whereas inputs from seawater infiltration were very limited. Non-conservative behaviour was observed along the groundwater flow path, leading to low NO x and high NH + 4 concentrations in the discharge zone. The long transit time of groundwater within the beach (∼ 166 days), coupled with oxygen-depleted conditions and high carbon concentrations, created a favourable environment for N transformations such as heterotrophic and autotrophic denitrification and ammonium production. Biogeochemical pathways led to a shift in nitrogen species along the flow path from NO x -rich to NO x -poor groundwater. An estimate of SGD fluxes of N was determined to account for biogeochemical transformations within the STE based on a N-species inventory and Darcy's flow. Fresh inland groundwater delivered 37 mol NO x yr −1 per metre of shoreline and 63 mol DON m −1 yr −1 to the STE, and NH and DON. Our study shows the importance of tidal sands in the biogeochemical transformation of the terrestrial N pool. This local export of bioavailable N probably supports benthic production and higher trophic levels leading to its rapid transformation in surface sediments and coastal waters.

The application of carbon stable isotope analysis of dissolved organic carbon (δ13C‐DOC) from natural seawater has been limited owing to the inherent difficulty of such analysis, with order of magnitude differences in interfering ions and analyte concentrations. High temperature catalytic oxidation allows for the attenuation of these interferences by precipitation of inorganic ions on quartz chips upstream from the oxidation catalyst. Using a chemical trap, the OI 1030C combustion DOC analyzer unit can be coupled to an IRMS, allowing for the analysis of low DOC content saline waters with relatively high throughput. The analytical limitations and large water volumes traditionally required for these types of analyses have prevented any large‐scale δ13C‐DOC studies. Here, we present δ13C‐DOC signatures for surface and bottom waters obtained along Canada's East Coast. Included in the study are samples from the Esquiman channel (between Newfoundland and Labrador), Lake Melville, the Saglek and Nachvak Fjords, the Hudson Strait and finally covering the salinity gradient across the St. Lawrence Estuary and Gulf. Measured δ13C‐DOC signatures ranged from predominantly marine values of −19.9 ± 0.3‰ (vs. VPDB) off the coast of Newfoundland to predominantly terrestrial signatures of −26.9 ± 0.1‰ in Lake Melville. We observed a large spread in δ13C‐DOC signatures for samples with a salinity of ≈ 35 between −19.9‰ and −23.3‰ demonstrating the difficulty associated to selecting a marine end‐member to be used in stable isotope mixing models to determine the fate of organic matter along the freshwater‐marine continuum.