Nitrogen isotope ratios of nitrate as a clue to the origin of nitrogen on the Pacific coast of Japan |
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| Authors: | Ryo Sugimoto Akihide Kasai Toshihiro Miyajima Kouichi Fujita |
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| Institution: | 1. Graduate School of Agriculture, Kyoto University, Oiwake, Kitashirakawa, Sakyo, Kyoto 606-8502, Japan;2. Ocean Research Institute, University of Tokyo, Minimidai 1-15-1, Nakano, Tokyo 164-8639, Japan;3. Mie Prefectural Science and Technology Promotion Center, Shiroko 1-6277-4, Suzuka, Mie 510-0243, Japan |
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| Abstract: | To clarify the sources and transformation of NO3− on the Pacific coast of Japan, observations over the continental shelf were conducted during the summer in 2005 and 2006 when the Kuroshio flowed close to and away from the coastal area, respectively. Below the halocline, there are two prominent salinity peaks that originated in two different waters. In the subsurface layer, the salinity maximum (Smax) was indicative of the Kuroshio Water (KW), while the salinity minimum (Smin) in the middle layer at ∼400 m depth was indicative of the North Pacific Intermediate Water (NPIW). δ15NNO3 ranged from 4.1‰ to 5.1‰ with a mean of 4.8±0.4‰ in the deeper water around Smin. Below 50 m depth over the shelf break, δ15NNO3 values (3.1±0.8‰ in 2005 and 4.6±0.3‰ in 2006) clearly increased as contribution of NPIW increased in 2006. On the contrary, subsurface δ15N of NO3− values (−1.1±0.1‰) remained unchanged in both years, although the contribution of the KW to the subsurface water changed significantly. This suggests that the source of NO3− has little effect on the δ15N of NO3− in this layer. The negative δ15N values also coincided with the base of the chlorophyll maximum layer suggesting that these isotopic signals must be evidence of active nitrification in the upper layer. |
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| Keywords: | δ15N Kuroshio Nitrate Nitrification North Pacific Intermediate Water |
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