Microbially-mediated precipitation of manganese oxide at the Seikan Undersea Tunnel, Japan

The present study is an attempt to investigate the geochemical and biologic al processes of marine manganese oxide deposition at the Seikan Undersea Tu nnel, where a black manganese-rich slime has been precipitating at a rate o f 25-30 tons per year. The manganese-rich slime shows a characteristic morp hology under SEM observation, aggregates of micro-spheres of 10-20 mu m in diameter. It contains Mn (41.7%) as major element and Ca, Na, Fe, Mg, Si an d Cl as minor constituents, and is identified to 10 Angstrom manganite of b userite structure. An orange-yellow Fe-rich microbial mat is always accompanied with the manga nese-rich slime in the tunnel. The microbial mat is composed of 0.5-1.5 mu m wide filamentous-sheaths with about 0.1 mu m wide fibrils, of which lengt hs are 20-100 mu m It contains Fe (19.1%) and Si (7.4%) as major element an d Ca, Cl, S, Na and Mn as minor constituents. Observations under LV-SEM sug gested that amorphous manganese oxide may be emitted from the tips of the m icrobial sheaths. Experiments on seawater-rock interaction demonstrated that considerable amo unt of Mn and Fe were leached from the tuff-rich Miocene rocks of the tunne l. Biochemical experiments showed that 10 ppm of Mn was almost completely r emoved from seawater by an addition of the fresh microbial mat, while the r emoval of Mn was inhibited by the presence of sodium azide in solution. Fro m these studies, we concluded that the precipitation of manganese oxides at the Seikan Undersea Tunnel has been mediated by catalytic action of a Mn-o xidizing bacteria.