[A-OS12] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS12-13] An insight into the area characteristic of seasonal change in nitrogen isotope ratio based on a marine nitrogen isotope model

*吉川 知里¹、重光 雅仁¹、山本 彬友¹、岡 顕²、大河内 直彦¹ (1.海洋研究開発機構、2.東大大気海洋研)

キーワード：海洋窒素循環、窒素同位体比、海洋生態系モデル

The condition of nitrogenous nutrient in the surface water is an important factor that determines the primary production and the carbon cycle in the ocean. Nitrogen isotope ratios (δ¹⁵N) of nitrate, sinking particles, sediments have been widely used as a proxy for the condition of nitrogenous nutrient in the surface water. However, the interpretation of δ¹⁵N values are not simple. The δ¹⁵N value of surface primary producer records δ¹⁵N value of nitrate supplied to the euphotic layer, nitrate utilization efficiency and rate of N₂ fixation. As there are multiple possibilities for the interpretation, δ¹⁵N values should be interpreted with ample consideration of the characteristics of surface nitrogen cycle by region. In this study we show the area characteristic of seasonal change in δ¹⁵N value by using a global ocean nitrogen isotope model in order to improve the interpretation of observed δ¹⁵N values. We installed a marine nitrogen isotope model (Yoshikawa et al., 2005) with denitrification (Shigemitsu et al., 2016) and N₂ fixation (Coles et al., 2007) schemes into an off-line biogeochemical model that is driven by climatological monthly mean physical fields obtained from the outputs of a preindustrial control simulation performed with MIROC 3.2 (K-1 Model Developers, 2004). The simulated δ¹⁵N of sinking particles had a global distribution similar to the observed δ¹⁵N of surface sediments (Tesdal et al., 2013). The simulated sinking particles were enriched in ¹⁵N in the eastern and western tropical North Pacific and the Indian Ocean where water column denitrification occurs and were depleted in ¹⁵N in the Sargasso Sea, the Kuroshio extension region, and the Indian Ocean where N₂ fixation occurs. The seasonal variations in δ¹⁵N of sinking particles vary considerably from one region to another. In this talk, we will report the seasonal δ¹⁵N patterns categorized by region and the factors in the determination of those seasonal patterns.