SUBMARINE HYDROTHERMAL BRINE SEEPS OFF MILOS, GREECE - OBSERVATIONS AND GEOCHEMISTRY

A shallow hydrothermal brine seep located off the creek island of Mile s in the Aegean Sea was studied. The brine fluid outcropped as a pool of water in a seabed depression and was detected in the surrounding po re-waters of sediments colonised by the sulphur bacterium Achromatium volutans. The seep fluid was highly saline and sulphidic, depleted in Mg2+ and SO42-, but enriched over seawater in Na+, Ca2+, K+, Cl-, SiO2 , reduced species and dissolved gases. The high concentrations of Na+, Ca2+ and K+ were consistent with the Miles tectonic setting. Na-K and Na-K-Ca geothermometers predicted a reservoir temperature of 300-325 degrees C for the most concentrated seep samples. The deep geothermal reservoir within the metamorphic basement of Miles island has already been located and studied and may represent the source of the seep flui d. Faunal diversity was lowest in seep-influenced sediments, but a sul phide-intolerant species was found in areas of the bacterial mat where salinity and temperature were much lower. Pressure-induced variations in the vertical depth of the brine interface may be occurring in the sediment.