Submarine Groundwater Discharge Revealed by 228Ra Distribution in the Upper Atlantic Ocean

Submarine groundwater discharge (SGD) is defined as any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force. The flux of SGD provides an important pathway for enriching coastal waters in nutrients, metals, and carbon. Determining the total flux of SGD to the ocean is a daunting task. Here I use the decay of 228Ra (half life = 5.7 years) in the upper Atlantic Ocean to infer the total flux of 228Ra to the upper Atlantic. The fact that radioactive decay, a highly predictable process, controls the inventory of 228Ra places a strong constraint on its flux. About 12% of the 228Ra inventory disappears each year. To maintain steady-state, there must be an equivalent flux from the continents. No other isotope, element, or compound shares these attributes of widespread distribution throughout the upper ocean, a removal term that is constrained to within a few percent, and a supply term that is due almost entirely to continental input. Inputs from dust, rivers, and coastal sediments explain less than half of the total 228Ra loss. The remainder must derive from SGD. Using estimates of the concentration of 228Ra in SGD from sites throughout the Atlantic yields a total SGD flux of (0.8-2.4) x 1016 L/yr. The calculated SGD flux is 33-100% of the river flux to the Atlantic. Because concentrations of nutrients, metals, and carbon in SGD are typically much higher than in rivers, the fluxes of these materials due to SGD are likely much greater than their riverine fluxes.