Since the discovery of billowing black smokers on the ocean floor in the 1970s, oceanographers have assumed that these hot vents held the answer to a long-standing mystery: how to explain the salt concentrations in the world's oceans. But a study in this week's Science (30 July, p. 721 ) shows that much cooler springs may play a far bigger role in giving seawater its salty tang.
Seawater isn't simply concentrated river water; rivers carry in so much sodium, magnesium, and potassium that the ocean should be far richer in these elements than it is, while calcium presents the opposite problem. Black smokers, perched along the crests of volcanic mid-ocean ridges, looked like they might balance these disparities. Seawater sinks into the fractured ridge crest, picking up heat, calcium, and other elements, leaves behind its own magnesium, and rises back into the sea. But recent studies have suggested that the hot vents cannot entirely account for the mineral balance.
To look for other possible influences on seawater's salts, oceanographer Stephanie de Villiers of the University of Cape Town in South Africa caught a ride on a research ship that crossed the East Pacific Rise, where she collected seawater from the surface to near the active mid-ocean ridge. When de Villiers and Bruce Nelson of the University of Washington, Seattle, analyzed the samples, they found a plume of water trailing off to the west of the ridge in which magnesium was depleted by as much as 1% and calcium was enriched.
To work out the proportions of plume water from black smokers and tamer warm springs along the ridge, they checked helium isotope measurements made near their sites by other researchers. Helium-3, the lighter isotope and a signature of black smokers, was scarce in the plume, given the amount of missing magnesium, leading the researchers to estimate that up to 10 times more chemical processing takes place in the warm rock within 2 to 10 kilometers of the central ridge axis than at the ridge axis itself. Experts are impressed. "This is a very exciting discovery," says oceanographer Michael Mottl of the University of Hawaii, Manoa. "If proved to be correct, it will solve a lot of problems."