Keywords:Marine Nitrogen Cycle, Nitrogen Isotopes, 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 and denitrification). 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. Here we determined δ15NNitrate and δ18ONitrate 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 δ15NNitrate 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 δ15NNitrate showed 5‰ in the bottom and deep waters and increased to 7‰ in the intermediate water. The horizontal maximum of the δ15NNitrate 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 δ15NNitrate once decreased to 6‰ in the subsurface water and then increased to the surface. The horizontal minimum of the δ15NNitrate 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 δ18ONitrate values.