Origins and transformations of terrigenous dissolved organic matter in a transgressive coastal system

• Published in: Estuarine, coastal and shelf science Vol. 279; p. 108137
• Main Authors: Hébert, Anne-Josée, Flamand, Aude, Chaillou, Gwénaëlle
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 05.12.2022
• Subjects: Carbon; Dissolved organic matter; EEM-PARAFAC model; Radon-222; Submarine groundwater discharge; Transgressive coastal system; Radon-222; Transgressive coastal system; Dissolved organic matter; Carbon; Submarine groundwater discharge; EEM-PARAFAC model
• ISSN: 0272-7714; 1096-0015
• DOI: 10.1016/j.ecss.2022.108137

Hydrogeological, geochemical, and optical approaches were combined to explore how a transgressive coastal system, where 2-kyr-old organic matter is preserved, contributes to dissolved organic matter (DOM) and dissolved organic carbon (DOC) flux to coastal waters. The sand spit system, located on the St. Lawrence shore (Chaleur Bay, Québec, Canada), was mainly composed of saline groundwater from seawater recirculation, and we estimated submarine groundwater discharge (SGD) velocities fluctuating from 0 during the rising tide to 106 cm d−1 during the ebb tide. The radon-222 (222Rn) activities revealed the presence of two distinct water masses in the spit: a surface recirculated saline groundwater cell with a short residence time and low or no 222Rn activity, and a deeper recirculated saline groundwater with a longer residence time, from a few hours to a few days, and thus exhibited higher 222Rn activities. At the falling tide, the upward flow of this 222Rn-rich groundwater contributes to volumetric discharges ranging from ∼6 to ∼17 m3 d−1, corresponding between 10 and 30% of the total SGD. Both DOC and DOM were produced in the subsurface. However, despite the concentrations being high in the discharge zone, the DOC export remained weak, with mean fluxes of 60.7 (±15.5) and 55.8 (±36.5) mol C yr− 1 in 2018 and 2019, respectively. While surface recirculated saline groundwater dominates the SGD, absorbance and fluorescent indices indicated a strong terrestrial character in the DOM pool likely originated from material sources with a high degree of humification. An EEM-PARAFAC model reveals the dominance of humic-like components of high molecular weights and the occurrence of degraded material, whatever the tidal regime. When the residence time of the groundwater was longer, with higher radon activities, protein-like components with lower molecular weights were microbially produced, probably enhancing the bioavailability of the DOM exported to surrounding seawater. Our results suggest that transgressive coastal systems can be a hot spot for terrigenous DOM transformation which could affect the optical and chemical properties of coastal waters. •Upward pulses of 222Rn-rich deep groundwater contribute to less than 30% of total SGD.•The residence time of deep groundwater is from 6 h to 6 days.•Absorbance and fluorescent indices indicate a strong terrestrial character of DOM.•Humic-like components of high molecular weights dominate when the residence time is short.•Microbial degradation occurs by enhancing the bioavailability of the exported DOM.