[A-OS22] Physical, chemical and biological processes and variability in the Indian Ocean

[AOS22-P08] Nitrate isotopes in the Bay of Bengal

*Chisato Yoshikawa¹, Akiko Makabe¹, Yohei Matsui¹, Sakae Toyoda², Makio Honda¹, Naohiko Ohkouchi¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Tokyo Institute of Technology)

Keywords:Marine Nitrogen Cycle, Nitrogen Isotopes, 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 and denitrification). 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. Here we determined δ¹⁵N_Nitrate and δ¹⁸O_Nitrate along 88°E in the Bay of Bengal during the cruise KH-18-6-leg2 of R/V Hakuho-Maru. The nitrate concentrations were below 1 µM in the surface water of the whole area. Especially, the nitrate depleted water spreads out below the depth of 400 m at the station near 20°S. The δ¹⁵N_Nitrate values were expected to increase toward the surface in conjunction with nitrate depletion unless ¹⁵N depletion by N₂-fixation and ¹⁵N enrichment by denitrification are dominant. The δ¹⁵N_Nitrate showed 5‰ in the bottom and deep waters and increased to 7‰ in the intermediate water. The horizontal maximum of the δ¹⁵N_Nitrate in the intermediate water was found at the northern edge site where the oxygen concentration dropped below 1 µM. The ¹⁵N enrichment in the intermediate water suggests the nitrate in the Indian Central Water is affected by benthic or water-column denitrification. The δ¹⁵N_Nitrate once decreased to 6‰ in the subsurface water and then increased to the surface. The horizontal minimum of the δ¹⁵N_Nitrate in the subsurface water was found in the nitrate depleted water near 20°S. The ¹⁵N depletion in the subsurface water suggests one of the sources of surface nitrogen is the remineralized nitrogen originated from N₂-fixation. In the presentation, we will discuss the nitrogen cycle of the Bay of Bengal in more detail by using the δ¹⁸O_Nitrate values.