[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-08] Nitrogen isotope mapping in the North Pacific using a marine nitrogen isotope model

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

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

Understanding the special distribution of nitrogen isotope composition (δ¹⁵N) of phytoplankton in the surface ocean is useful for uncovering the life histories of marine organisms including fish. The δ¹⁵N of phenylalanine (one of the “source” amino acids) of fish tissues inherits the δ¹⁵N of phytoplankton where the fish grew up. Recently, Vane et al. (2018) determined δ¹⁵N of amino acids in otoliths and showed that otoliths preserved a record of the δ¹⁵N of phytoplankton over the lifetime of individual fish. Moreover, Matsubayashi et al. (2020) determined time-series δ¹⁵N of amino acids in vertebra centra and successfully tracked a migration of chum salmon. Thus, the reconstruction of the temporal variations of δ¹⁵N of amino acids from various fish tissues has made remarkable progress. However, there is as yet no accurate δ¹⁵N map of phytoplankton due to observational heterogeneity, and it prevents improvement in the accuracy of migratory route estimation. In this study, we made a δ¹⁵N map of phytoplankton in the North Pacific by using a global ocean nitrogen isotope model. 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 phytoplankton was enriched in ¹⁵N in the northern Bering and Chukchi Seas where sedimentary denitrification is significant. The simulated phytoplankton was depleted in ¹⁵N in the Kuroshio extension region where N₂ fixation occurs, and in the subarctic North Pacific region where the phytoplankton never uses up nitrate in the surface water due to iron limitation. In this talk, we will report the validation result of the simulated δ¹⁵N map with the various δ¹⁵N data set obtained from the North Pacific.