Geochemical study of hydrothermal solution using chemical elements as tracer.

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Sea water and sediment samples were collected in 1984-1986 by R/V. "Tokai-Daigaku Maru" from the Mariana and the Okinawa Troughs,where hydrothermal eruption has been confirmed by "Alvin" and "Shinkai-2000". Nutrients,dissolved oxygen,methane,major elements and manganese in sea water, and several chemical components in the sediments were analysed. The ratios of Na,K,Mg and Ca versus salinity were uniform the surface to the bottom waters and showed a slight increase just above the bottoms. Manganese concenteation in seawater columns in the Mariana Trough ranged 0.71-2.48 nmo1/Kg,and was high in the surface and low in the lower layer and then increased again in the bottom. Concenteation of manganese in the Okinawa Trough waters on March'86 ranged 0.18-3.64 nmo1/Kg, showing sharp increases from the middle,about 700m,to the bottom,about 1500 m,layers. Anomalies of manganese concentration,about 5 nmo1/Kg,were observed just above the bottoms in the Okinawa Trough on November'86. These anomalies of manganese concentration may be related to the hydrothermal activity. Methane concentrations in the Okinawa Trough ranged 30-50nl/l and showed a slight increase above the bottoms. The sediments in these two Troughs were presumed to be of the hydrothermal characteristics from the triangle diagram of Fe-Mn-(Ni+Cu+co)x10. Average concentrations of total carbonate and phosphate in the pore waters of the sediments in the Okinawa Trough were 4.06 and 9.29 mM/l resp.,These values being higher than those in seawater just above the sediments.