The Distribution、Composition And Environmental Evolution Of Suspended Particulate Matter In The Northwest Pacific

In order to understand the particle size distribution and composition in the spring and autumn in the northwest Pacific Ocean, east of Japan, LISST-100 X instrument was used to measure the particle size distribution of suspended particulate matter(SPM) in the top 200 m surface water in May and September, 2015, by "Nanfeng" research vessel in the Northwest Pacific funded by " West Pacific environmental monitoring" project. Major component of suspended particulate matter was analyzed by Scanning Electron Microprobe(SEM) and Energy Dispersive Spectrometer(EDS).The research result showed that SPM in the study area was mainly composed of large particle size(>133 μm) and concentrated on certain layers; volume concentration(VC) of SPM in the autumn was much higher than that in the spring, but the proportion of large size particles apparently decreased, the proportion of mediumsized particles(36~133 μm) increased relatively, medium and small-sized particles(<36 μm) distribution was more uniform in the water column <200 m, and there was an increasing trend with particle diameter increasing. The main components of SPM were single mineral particles, biological particles and flocs. Mineral particles could be divided into oxy-salt minerals, oxides or hydroxides and sulfides, mainly including quartz, feldspar, calcite, iron oxide and clay mineral. Biological particles mainly include diatom, pyrrophyta, calcareous planktons and protistas. Flocs were classified into 4 main types mainly: biological flocs, mineral flocs, mixed flocculation and organic coating. In autumn, small-sized particles were decreasing, organic detritus were relatively increasing than spring. SPM mainly deprived from marine plankton and terrestrial material transport. Concentration and particle size distribution of the suspended particulates were controlled by biological growth, the continental input of aeolian sediment and dust, and ocean currents conditions.In the spring the mineral value of particles was much higher than autumn, the concentration distribution in northwest was relatively higher than other regions, and in southeast with the minimum value. The silicate mineral distribution in autumn showed the opposite trend to spring. Grain outline of quartz comprized the angular grains, subargular grains and rounded grains on mesoscopic scale. The subangular to subrounded outline were the most common features in the studied grains. Surface textural of quartz grains could be divided into mechanical microtextures, chemical microtextures and combined mechanical and chemical microtextures. The most common mechanical microtextures were conchoidal fractures, subparallel linear fractures, arcuate and straight steps, V-shaped percussion cracks, abrasion fatigue, imbricated grinding features, fresh fracture plane and so on. On the quartz grain surface there are also some oriented etch pits, solution pits, silica pellicles and adhered particles on the quartz surface as chemical or mechanical-chemical textures could be found. In autumn the erosion degree of the quartz surfaces was much higher than spring. According to the different composition, feldspar particles could be divided into potashfledspar, albite, anorthite and plagioclase. The result showed that value of potashfledspar was the highest, followed by albite, number of anorthite and plagioclase was the lowset. In autumn the feldspar size was smaller than spring.The mineral particles of SPM mainly came from Asian eolian, river and coastal’s input. Part of them may be derived from ocean. There was less impact on the bottom of the ocean. Before depositing, the particles experienced strong currents or a long time water transport period. In summer tropical cyclone frequency, underwater environment became unstable, flocs were disentangling. In autumn, input of particles were reducing, the erosion degree of particles became higher. Overall, the factors influencing the distribution of the suspended mineral particels included land-based sources of material inputs, Asian eolian, microorganisms and surface flows.