[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

*Chisato Yoshikawa¹, Masahito Shigemitsu¹, Akitomo Yamamoto¹, Akira Oka², Naohiko Ohkouchi¹ (1.JAMSTEC, 2.AORI)

Keywords:Marine nitrogen cycle, Nitrogen isotope ratio, Marine ecosystem model

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.