Chemical signals from submarine fluid advection onto the continental shelf

Abstract

We report unambiguous signals of subsurface fluid input on the continental shelf 40-80 km from shore in water depths of 20-45 m. The site of this input is near the continental shelf break off the South Carolina coast between 32°-33°N and 78.5°-79.5°W. Repeated transects over this area reveal enrichments of 226Ra, 228Ra, and Ba by factors of 3-4 above ambient ocean concentrations. The short-lived 223Ra and 224Ra are enriched by over an order of magnitude. The Ra and Ba enrichments are not caused by up welling of deep water as no such 226Ra or Ba concentrations occur in the upper 2000 m of the Atlantic Ocean. Sediments in the region are largely relic sand and do not offer a viable source for 226Ra, 228Ra, and Ba enrichments. We conclude that the source is the discharge of submarine fluids enriched in 226Ra, 228Ra, and Ba. The salinity of water in the Ra-Ba-enriched zone is diluted by 0.2-0.6‰ relative to water at the same depth farther offshore. Pronounced salinity and density anomalies characterize one site of active discharge. Estimates of 226Ra and Ba concentrations in the fluids and the replacement time of water in the enriched zone suggest the fluid flux is similar to the summertime flow of major rivers in the area. The region of Ra-Ba enrichment is also characterized by a strong fluorescence signal, indicating high concentrations of chlorophyll in the water. It is probable that the leaking fluids supply nutrients that stimulate productivity in the bottom waters which are within the photic zone. Because the fluorescence signal is restricted to depths greater than 20 m, it is unlikely that the signal could be recognized by satellite or aircraft sensors.