Inorganic carbon and alkalinity biogeochemistry and fluxes in an intertidal beach aquifer: Implications for ocean acidification

While submarine groundwater discharge (SGD) is well known to release large amounts of dissolved nutrients and organic carbon into the ocean, the contribution of SGD to the marine inorganic carbon cycle is poorly understood. Here, the biogeochemistry of inorganic carbon in an intertidal aquifer and related SGD-derived fluxes into Tolo Harbor, Hong Kong was investigated over tidal and seasonal time scales. The results revealed the production of total alkalinity (TAlk) and dissolved inorganic carbon (DIC) in the intertidal aquifer over the entire salinity range. The locally produced TAlk and DIC in the intertidal aquifer contributed to >50% of the TAlk and DIC discharged with SGD to the ocean. The distributions of TAlk and DIC in the transition (mangrove) and high salinity (bare beach) zones were different due to the distinct hydrogeological and geochemical conditions. In the organic-rich mangrove zone, TAlk and DIC productions were driven by biotic processes such as aerobic respiration, denitrification, and sulfate reduction. In the organic-poor bare beach zone, TAlk and DIC productions were likely driven by abiotic processes such as precipitation/dissolution of carbonate minerals. Temperature, pH, physical mixing, and iron cycling in the intertidal aquifer also considerably influenced the carbonate biogeochemistry. TAlk inventory in the intertidal aquifer was seasonally stable but TAlk discharged with SGD was ~60% greater in the wet season than in the dry season (73.3 vs. 45.6 mol d per m coastline). The DIC inventory in the intertidal aquifer and DIC discharged with SGD were ~24% and 95% higher, respectively, in the wet season than dry season. Overall, through analyzing TAlk:DIC ratios and related fluxes, SGD was thought to reduce the CO buffering capacity of the receiving ocean, and act as a local driver of ocean acidification.