GEOCHEMICAL DYNAMICS OF THE ATLANTIS-II DEEP (RED-SEA) - SILICA BEHAVIOR

Previous chemical analyses of Atlantis II Deep sediments and waters have shown that silica concentrations are abnormally high in comparison with those from other marine hydrothermal sites. New physical and chemical data have been obtained during recent cruises to the Atlantis II area. Seawater, free and interstitial brines have been analyzed for chloride and silica. The hydrographic measurements show that two additional. brine layers have been created since 1977. These new observations allow us to re-evaluate the hydrodynamics and the consequences for mass transfer. Silica concentration in the lower brine pool is 913+/-181 mu mol/l. Within the upper brine layers, silica concentrations are higher than those expected from simple mixing between the lower brine and seawater. The chemical mass transfers through the brine interfaces are explained by double-diffusion phenomena. Silica concentrations in the interstitial waters (<200 mu mol/l) are much lower than those in the lower brine. The high silica concentration gradients observed at the brine-sediment interface suggest that silica precipitates at this boundary layer. Thermodynamic calculation indicates that the interstitial brines are undersaturated with respect to quartz and amorphous silica. These species act probably as major sources for silica, whereas authigenic clay minerals could be the major sink.