Study of the mechanisms of mineral deposition at sea-floor by biological and mineralogical experiments

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1. Microbially-mediated metal deposition in seawaterThe mechanism of heavy-metal deposition mediated by microorganisms was studied experimentally. The bacterial slime collected at the pilot shaft of the Seikan Undersea Tunnel under the Tsugaru Straits of Japan shows strong manganese-oxidation effect. An aliquot of artificial seawater containing heavy metals (Mn, Ni, Co, Cr etc.) and a small amount of the bacterial slime were incubated at 25℃ and the changes of metal concentrations with time were analyzed. The results of the experiments showed that Mn in seawater began to decrease rapidly by addition of the bacterial slime and then disappeared completely after four days. The heavy metals other than Mn in the solution were also decreased with time, but the manners of the reduction differ in each metal. These results indicate that some kinds of marine bacteria not only catalyze manganese-oxidation but also accelerate precipitation of heavy metals other than manganese. This fact suggests that bacterial activity may play an important role on the deposition of heavy metals on the seafloor. The manganese nodules from Pacific Ocean and those from Baltic Sea were analyzed to confirm this hypothesis.2. Metal deposition by water-rock interaction at the ocean floorIn order to investigate the mechanisms of sulfide deposition at the ocean floor, seawater and volcanic rocks were interacted by using a hydrothermal reactor. The experiments showed that heavy metals (Pb, Zn, Cu and Fe) were dissolved quickly and sulfide precipitated by reducing sulfate in seawater. These results are applicable not only to understand the mechanism of sulfide mineralization but also to study the long-time stability of underground rocks as CO_2 and nuclear sludge reservoir.