Hydrothermal Signatures In The Sediments From The Middle Okinawa Trough By Sequential Extraction

Based on the complexity of the sources for the sediments in the Okinawa Trough, the sequential extraction procedure was applied to the hydrothermal influenced sediments in the core H9 recovered from the middle Okinawa Trough. Five fractions, including exchangeable, bound to carbonates, bound to iron and manganese oxides, bound to organic matter, and residual were acquired. Based on detailed analysis for major, minor and rare earth element compositions, the distributions of the elements in various speciations of the sediments were determined and the controlling mechanism was discussed. The variations of hydrothermal signatures in the core, partitioned from the terrigenous, volcanic and biogenic materials, have been demonstrated and the accumulation rates of some elements were evaluated. In addition, the change of hydrothermal fluid during the diffusing process was discussed according to the distribution patterns of rare earth elements compositions.The main conclusions are listed as following:(1) The residual speciation is characterized by the highest elemental concentrations in the core H9 sediments. Moreover, the concentrations of Al,Ti and K exhibit little variation with the depth in the core. These features reflect that terrigenous and volcanic inputs are relatively steady, and they are the main factors to control the elemental distribution in the sediments.(2) The fractions of exchangeable, bound to carbonates and organic matter are not the important speciations of most elements in the sediments.(3) The fraction bounded to iron and manganese oxides is another main occurrence speciation besides the residual for most elements. It is also the main factors to control the abundances of noncrystal phase. The typical feature is that the elemental concentrations in this speciation and the percentages of this speciation in samples have significantly variation down the core, especially there is a sharp change at the depth of 80cm. Hydrothermal components mainly occur in this speciation and control the downcore variations of the sediment chemical compositions. The concentrations of Mn, Cu, Ni, Co and V in the core sediments above the depth of 80cm are higher than those in the below that depth. The ratios of Mn and Co in the upper 80cm core sediments to the lower part are both nearly 14, which clearly demonstrates the enhancement of hydrothermal inputs in the core above the depth of 80cm.(4) The accumulation rates of elements derived from hydrothermal inputs were evaluated. Mn and Fe accumulation rates are similar to or slightly higher than those previously determined for sediments from Mid-ocean ridges and the Lau Basin, which reflectes the high levels of suspended particulate and sedimentation rates in the trough.(5)The distribution patterns of rare earth element (REE) compositions in the hydrothermal components are similar to that of pacific seawater. This infers the complex and significant interactions of hydrothermal fluid with surrounding seawater during its diffusing process. Dissolved trace metals (especially REEs) in the hydrothermal fluid can be removed by settling terrigenous or biogenic particulates and precipitated quickly from the hydrothermal plume round the vent. On the other hand, hydrothermal particulates would also scavenge some elements from the seawater to coprecipitate into the sediments. Thus, the hydrothermal system in the Okinawa Trough may be a sink for dissolved REEs in seawater.