Nitrogen isotope ratios of nitrate as a clue to the origin of nitrogen on the Pacific coast of Japan

Abstract

To clarify the sources and transformation of NO 3 − 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 ( S max) was indicative of the Kuroshio Water (KW), while the salinity minimum ( S min) in the middle layer at ∼400 m depth was indicative of the North Pacific Intermediate Water (NPIW). δ 15N NO3 ranged from 4.1‰ to 5.1‰ with a mean of 4.8±0.4‰ in the deeper water around S min. Below 50 m depth over the shelf break, δ 15N NO3 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 δ 15N of NO 3 − 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 NO 3 − has little effect on the δ 15N of NO 3 − in this layer. The negative δ 15N 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.