Abstract
Nitrogen isotope compositions of particulate organic matter and nitrate were analyzed for seawater sampled at five stations at the Alaskan Gyre, Western Subarctic Gyre and East China Sea, focusing on the samples from the surface to 5000 m water to characterize the nitrogen cycling in the subarctic North Pacific Ocean and its marginal sea. The δ15N of particulate organic matter showed little agreement with a conceptual closed model that interprets isotopic variation as being caused by isotope discrimination on nitrate utilization. The δ15N and δ13C of particulate organic matter varied with the water depth. A correlation between isotope compositions and C/N elemental ratio was found generally at all stations, although some irregular data were also found in deep layers. We developed a hypothetical nitrogen balance model based on N2 fixation and denitrification in seawater and attempted to apply it to distinguish nutrient cycling using both δ15N-NO3 − and N* variation in seawater. This model was applied to the observed data set of δ15N-NO3 − and N* in the North Pacific water and estimated the δ15N-NO3 − of primordial nitrate in the North Pacific deep water as 4.8‰. The North Pacific intermediate water for all stations showed similar δ15N-NO3 − and N* values of 6‰ and −3 µmol/kg, respectively, suggesting a similar nitrogen biogeochemistry. In the East China Sea, analysis showed evidence of water exchange with the North Pacific intermediate water but a significant influence of nitrogen from the river runoff was found in depths shallower than 400 m.
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Minagawa, M., Ohashi, M., Kuramoto, T. et al. δ15N of PON and Nitrate as a Clue to the Origin and Transformation of Nitrogen in the Subarctic North Pacific and Its Marginal Sea. Journal of Oceanography 57, 285–300 (2001). https://doi.org/10.1023/A:1012430512137
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DOI: https://doi.org/10.1023/A:1012430512137