How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a river-dominated shelf system?

[摘要] In order to assess the role of submarine groundwater discharge (SGD) and itsimpact on the carbonate system on the northern South China Sea (NSCS) shelf,we measured seawater concentrations of four radium isotopes^{223,224,226,228}Ra along with carbonate system parameters in June–July,2008. Complementary groundwater sampling was conducted in coastal areas inDecember 2008 and October 2010 to constrain the groundwater end-members. Thedistribution of Ra isotopes in the NSCS was largely controlled by the PearlRiver plume and coastal upwelling. Long-lived Ra isotopes (²²⁸Ra and²²⁶Ra) were enriched in the river plume but low in the offshore surfacewater and subsurface water/upwelling zone. In contrast, short-lived Raisotopes (²²⁴Ra and ²²³Ra) were elevated in the subsurfacewater/upwelling zone as well as in the river plume but depleted in theoffshore surface water. In order to quantify SGD, we adopted two independentmathematical approaches. Using a three end-member mixing model with totalalkalinity (TAlk) and Ra isotopes, we derived a SGD flux into the NSCS shelfof 2.3–3.7 × 10⁸ m³ day⁻¹. Our second approachinvolved a simple mass balance of ²²⁸Ra and ²²⁶Ra and resulted ina first order but consistent SGD flux estimate of2.2–3.7 × 10⁸ m³ day⁻¹. These fluxes were equivalent to 12–21 % of the PearlRiver discharge, but the source of the SGD was mostly recirculated seawater.Despite the relatively small SGD volume flow compared to the river, theassociated material fluxes were substantial given their elevatedconcentrations of dissolved inorganic solutes. In this case, dissolvedinorganic carbon (DIC) flux through SGD was153–347 × 10⁹ mol yr⁻¹, or ~23–53 % of the riverine DIC export flux. Ourestimates of the groundwater-derived phosphate flux ranged 3–68 × 10⁷ mol yr⁻¹, which may be responsible for new production on theshelf up to 0.3–6.3 mmol C m⁻² d⁻¹. This rate of new productionwould at most consume 11 % of the DIC contribution delivered by SGD.Hence, SGD may play an important role in the carbon balance over the NSCSshelf.

[学科分类] 地球化学与岩石