Global Fluxes of Recirculated Submarine Groundwater Discharge Individual Components
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Submarine groundwater discharge (SGD) consists of freshwater SGD (FSGD) and recirculated saline SGD (RSGD). FSGD contributes approximately 0.5-1% of the influx from rivers to the ocean (Liu et al., 2020; Luijendijk et al., 2020), but it has significant implications for coastal water chemistry and ecology. RSGD is several orders of magnitude higher than FSGD, and does not directly impact the ocean water budget. Due to water-rock interactions, RSGD exhibits a distinct chemical composition compared to seawater. As a result, it plays a crucial role in coastal ecology and the ocean’s solute budget (Santos et al., 2021). RSGD is driven by several mechanisms, including density-driven, tidal-driven, and wave-driven circulations, each with different spatial and temporal scales. Wave-driven fluxes are short-term, while density-driven fluxes are long-term, and tidal-driven fluxes are intermediate. Different water-rock interactions exhibit varying reaction rates; those with low reaction rates are not reflected in short-term fluxes. This study quantifies global density-driven and tidal-driven RSGD fluxes. The fluxes were calculated using numerical simulations, sensitivity analysis of geohydrological parameters, and upscaling to a global scale using a hydraulic parameter database and bilinear interpolation. Results indicate that higher hydraulic conductivity increases the density-driven circulation and reduces tidal-driven nearshore circulation. An increase in hydraulic gradient has no significant impact on density-driven circulation, while it slightly decreases short-term nearshore circulation. The estimated global fluxes are 1380 km³/yr for density-driven RSGD (accounting for heterogeneity effects), 250 km³/yr for tidal-driven circulation, and 385 km³/yr for tidal pumping. Using a typical enrichment, calcium flux to the ocean by the density-driven RSGD may be comparable to the riverine calcium input. The results enable the identification of "hotspots." For instance, the Mediterranean Sea and the Caribbean Sea are "hotspots" of density-driven RSGD, while the East Coast of Africa, the west coast of India, and Oceania are "hotspots" of tidal-driven RSGD.
