Abstract
The δ15N of dissolved ammonium was determined in three anoxic marine basins: Black Sea, Saanich Inlet, B.C., Canada, and Framvaren Fjord, Norway. In each basin, the δ15N‐NH4+ was greatest near the 02/H2S interface, with δ15N as high as +21‰. The depth distributions of NH4+ and δ15N‐NH4+ for Black Sea and Framvaren Fjord were examined with a one‐dimensional, steady‐state, vertical advection‐diffusion model to calculate the isotope fractionation during the consumption of NH4+ by bacteria. Isotope enrichments, ϵ, for Black Sea were between 5 and 15 ‰, whereas in Framvaren Fjord ϵ ranged from 20 to 30 ‰. These differences are related mainly to the ambient concentration of NH4+. Biosynthetic uptake of NH4+ rather than nitrification was responsible for the fractionation. The δ15N‐NH4+ in Saanich Inlet appears related to in‐situ regeneration of NH4+ with little isotopic fractionation between dissolved and particulate nitrogen (PN).
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