[M-IS21_28AM1] 生物地球化学

[MIS21-06] 海洋有光層の窒素循環解析ツールとしてのクロロフィル窒素同位体比

*小川 奈々子¹、吉川 知里¹、菅 寿美¹、大河内 直彦¹ (1.独立行政法人海洋研究開発機構)

キーワード：海洋窒素循環, 光合成色素, 窒素安定同位体比, 硝酸態窒素

Nitrogen isotope record of chlorophylls has a large potential as a tool for reconstructing the nitrogen cycle and its dynamics in the photic zone. In this study, we determined the nitrogen isotopic compositions of chlorophyll a (δ¹⁵N_chl) and pheophytin a (δ¹⁵N_Phe) as well as nitrate (δ¹⁵N_NO3) collected from two sites (S1 and K2) in the northwest Pacific as a case study. Both chlorophyll a and pheophytin a were extracted from the particulate organic matter (POM) and purified by the fraction collector of high-performance liquid chromatography. The nitrogen isotopic composition of the isolated chlorophylls was determined by our ultra-sensitive elemental analyzer / isotope ratio mass spectrometry. The estimated isotopic fractionation associated with the chlorophyll synthesis is -7.9‰ to -13.1‰, confirming the previous studies. However, the δ¹⁵N of POM is not consistent with those of chlorophylls, suggesting that the POM from both sites is a mixture of phytoplankton and other materials like detritus of zooplankton. The δ¹⁵N_chl value provides pure δ¹⁵N signature of phytoplankton, which is crucial for better understanding of nitrogen dynamics in the surface ocean. Chlorophylls are also buried and preserved in the sediments for long, and thus useful for the reconstruction of nitrogen cycle in the surface ocean in the geological past. In this presentation, we will summarize the evidence and discuss advantages and pitfalls of this tool for the future use in the oceanography and paleoceanography.