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

Highlights:

  • Presented the TAlk and DIC dynamics in intertidal aquifer over both tidal and seasonal timescale.
  • Make clear the contribution of local production to the SGD-derived DIC and TAlk fluxes to the ocean.
  • Differentiate different biogeochemical reaction responsible to the production of TAlk.
  • Specify the influences of SGD on ocean acidification.

Abstract

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 reveal 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 contributes to >50% of the TAlk and DIC discharged with SGD to the ocean. The distribution 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 production was driven by biotic processes such as aerobic respiration, denitrification, and sulfate reduction. In the organic-poor bare beach zone, TAlk and DIC production was 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-1 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 is thought to reduce the CO2 buffering capacity of the receiving ocean, and act as a local driver of ocean acidification.

Liu Y., Jiao J. J., Liang W., Santos I. R., Kuang X. & Robinson C. E., in press. Inorganic carbon and alkalinity biogeochemistry and fluxes in an intertidal beach aquifer: implications for ocean acidification. Journal of Hydrology. Article (subscription required).

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Ocean acidification in the IPCC AR5 WG II

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