Isotopic fractionation of dissolved ammonium at the oxygen‐hydrogen sulfide interface in anoxic waters

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).