[A-OS13] Physical, biogeochemical, and ecological aspects and their mutual relations in the Indian Ocean

[AOS13-P01] Nitrate isotope distributions in the eastern Indian Ocean

*Chisato Yoshikawa¹, Akiko Makabe¹, Yohei Matsui¹, Takuro Nunoura¹, Naohiko Ohkouchi¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:Marine nitrogen cycle, Nitrogen isotopes, Eastern Indian Ocean

Nitrogen isotopic composition of nitrate (δ¹⁵N_Nitrate) is widely used as a tracer of ocean-internal nitrogen cycling (consumption and regeneration) and ocean-external nitrogen inputs and losses (N₂-fixation; fixation of N₂ gas into bioavailable nitrogen such as ammonia by diazotrophs, and denitrification; microbial respiration using nitrate as an electron acceptor). When the phytoplankton assimilates nitrate, nitrogen isotopes are fractionated. A δ¹⁵N_Nitrate value increases, in conjunction with nitrate depletion, due to an isotopic effect during nitrate assimilation by phytoplankton. When denitrification occurs in the water column, a δ¹⁵N_Nitrate value extremely increases due to a strong isotopic effect. N₂-fixation produces fixed nitrogen with a δ¹⁵N value of ~0‰, as nitrogen fixers take up N₂ gas with little isotopic effect. This fixed nitrogen with low δ¹⁵N value is eventually converted into low-δ¹⁵N_Nitrate through degradation of nitrogenous organic compounds called remineralization and subsequent nitrification. Those signatures of δ¹⁵N_Nitrate in the euphotic zone are conserved in nitrogenous organic compounds and transfers to the sinking particles and deep-sea sediments. Here we determined δ¹⁵N_Nitrate and δ¹⁸O_Nitrate along 110°E in the eastern Indian Ocean during the cruise MR15-05 of R/V Mirai. The nitrate concentrations were below 0.1 µM in the surface water of the whole area. The δ¹⁵N_Nitrate values are expected to increase toward the surface, in conjunction with nitrate depletion. Furthermore, if nutrient uptake by phytoplankton and remineralization occur with Redfield proportions, and if external nitrogen inputs/output do not occur, then a N* value should be ~−2.9 μM. However, in this study area, the δ¹⁵N_Nitrate value decreased to 4.0‰ and the N* value increased up to −1.3 μM toward the surface. These features suggest that the surface water of this study area is affected by N₂-fixation. In the presentation, we will discuss the nitrogen cycle of the eastern Indian Ocean in more detail by using the δ¹⁸O_Nitrate values.