Is submarine groundwater discharge a major pathway of carbon and nutrients into the coastal ocean?

During the past decades, large enrichments of ²²⁶Ra in coastal waters have been reported worldwide. By means of elimination, these ²²⁶Ra enrichments were used to infer large submarine groundwater discharge from a hypothetical “subterranean estuary”. A critical assumption thereof is that regeneration of ²²⁶Ra on marine sediments contributes little to enrichments of this nuclide in the coastal ocean. In this study, we have measured ²²⁶Ra and ²³⁰Th activities in two ~ 30-meter-long sediment cores collected from the subaqueous delta of the Pearl River, China. Using this novel ²²⁶Ra/²³⁰Th tracer approach, we show that regeneration of ²²⁶Ra from surface sediments between 0 and 5 m dominated the total ²²⁶Ra flux out of the seabed. We have further demonstrated that the replenishing rate of the subterranean estuary must be < 0.01 yr^-1. As a consequence, the total groundwater flux is at least 2 orders of magnitude lower than the river-water flux. More importantly, the fluxes of associated dissolved constituents are also orders of magnitude lower than the regenerated fluxes from the surface sediments. Thus, to acquire an unbiased understanding of coastal ocean chemistry, future studies should focus on solute exchange occurring at the sediment-water interface.