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.