Resolving the hydrothermal signature by sequential leaching studies of sediments from the middle of the Okinawa Trough

A sediment core H9 collected from the middle of the Okinawa Trough was studied by sequential leaching to indentify the hydrothermal component from the various other components such as detrital, biogenic, and so on. The elements were partitioned into five fractions: exchangeable, bound to carbonates, bound to iron and manganese oxides, bound to organic matter, and residual. The contents of elements Al, Ti, K, Fe, Mn, Pb, Co, Cu, Ni, and V in each fraction were determined and their percentages were calculated. Residual fraction is a dominant speciation for most studied elements except for Mn and Pb in the core sediments. For the elements Al, Ti, and K, residual fraction accounts for more than 95% of the total metal concentration. The concentrations of all studied elements in the exchangeable fraction, carbonate fraction, and organic fraction are relatively low. The metal levels in the iron and manganese oxide fraction are relatively high and this fraction represents another prevalent speciation. Also in this fraction, the metal concentrations and percentages are higher in the core section above 80 cm than those below 80 cm. Especially for Mn and Pb, the fractions bound to iron and manganese oxides account for more than 50% of the total metal concentration for the upper 80 cm section. Hydrothermal components mainly occur in this fraction. And the downcore variations of metal contents and percentages reflect the variations of hydrothermal inputs to the sediments. The high metal levels in the upper core indicate the enhancing influence of hydrothermal activities around on the core H9 during its late stage of sedimentation. The accumulation rates of hydrothermally derived Fe, Mn, Pb, Co, Cu, Ni, and V were evaluated based on their concentrations in the iron and manganese oxide fractions in this study. The results show that the accumulation rate of hydrothermally derived Mn is similar to those obtained from the Mid-ocean Ridges and Lau Basin. However, hydrothermally derived Fe and Cu present a relatively lower level.