| In this study,nitrogen and oxygen isotope composition of nitrate in subtropical Western North Pacific and South China Sea were determined by denitrifying bacteria method.The characteristics of nitrogen and oxygen isotopes of nitrate were obtained and the nitrogen cycle were discussed,following conclusions had been obtained:(1)Biogeochemical processes in the euphotic zone of the Western North Pacific Oyashio-Kuroshio transition zone and the South China Sea are different from each other:nitrate in the euphotic zone of Oyashio-Kuroshio transition zone come from high latitude area and has two sources of ?15N(in-situ signals and high latitudes),while nitrate in the euphotic zone of the South China Sea comes from the subsurface water.(2)Atmospheric deposition is an important contributor to the subsurface nitrate?15N minimum both in the Western North Pacific and South China Sea.The rates of atmospheric nitrogen deposition in the Oyashio-Kuroshio transition zone and the South China Sea were 40 ?molN·m-2·d-1 and 57 ?molN·m-2·d-1(similiar to the rates of nitrogen fixation in the South China Sea),respectively.(3)The higher nitrate-?15N of NPIW in the Western North Pacific comes from Eastern North Pacific oxygen minima zone.During the transport from eastern Pacific to western Pacific,the higher signal of nitrate-?15N of NPIW were weakened by the mixing with the water above the NPIW,which has lower nitrate-?15N,and dropped to 6.50‰ in the Oyashio-Kuroshio transition area.Using the signal of higher nitrate-?15N of NPIW as a tracer of the exchange of the water mass from the South China Sea and the Philippine Sea water mass,we found that the higher 815N of NPIW were gradually weakened and narrowed after entering the South China Sea.This indicates that the mixing of intermediate water,deep water and surface water in the Southern South China Sea,which may cause the disappear of the signal of higher nitrate-?15N of NPIW.The higher ?15N of NPIW was disappear in South China Sea. |
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