Keywords:Marine nitrogen cycle, Nitrogen isotopes, Eastern Indian Ocean
Nitrogen isotopic composition of nitrate (δ15NNitrate) 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 δ15NNitrate 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 δ15NNitrate 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-δ15NNitrate through degradation of nitrogenous organic compounds called remineralization and subsequent nitrification. Those signatures of δ15NNitrate in the euphotic zone are conserved in nitrogenous organic compounds and transfers to the sinking particles and deep-sea sediments. Here we determined δ15NNitrate and δ18ONitrate 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 δ15NNitrate 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 δ15NNitrate 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 δ18ONitrate values.