Nitrogen isotopic composition of nitrate in the South China Sea: A clue to the origin of nitrogen

Zhi Yang1, Jianfang Chen1, Min Chen2, Lihua Ran3, Hongliang Li3, Yong Zhu3, Chunsheng Wang3, Zhongqiang Ji3, Jingjing Zhang3 and Dongsheng Zhang3, (1)The second institute of Ocanography, State Oceanic Adiministration, China, (2)State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China, (3)Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China
Abstract:
Nitrogen isotopic composition of water column nitrate was measured in the South China Sea to clarify the sources of nitrogen. The δ15NNO3 value in deep water (5.4±0.2‰) was higher than the average deep oceanic δ15NNO3 (~5‰), and a weak δ15NNO3 maximum (5.9±0.2‰) was observed at 500 m depth, matching the salinity minimum. These indicated the intrusion of the North Pacific Water which carried nitrate with a high δ15NNO3 and showed a similar δ15NNO3 distribution profile with the South China Sea. The high N* (1.74±0.23 μmol/L) combined with the low δ15NNO3 (4.7±0.2‰) at 100 m depth indicated that N2 fixation (and possibly Atmospheric Deposition) introduces new N to the surface ocean. The distribution of δ15N values of nitrate, sinking particles and surface sediment suggest that laterally-advected sediments may be a source of nitrogen to the deep ocean.
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