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

The δ 15 N 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 δ 15 N‐NH 4 + was greatest near the 0 2 /H 2 S interface, with δ 15 N as high as +21‰. The depth distributions of NH 4 + and δ 15 N‐NH 4 + 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 NH 4 + 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 NH 4 + . Biosynthetic uptake of NH 4 + rather than nitrification was responsible for the fractionation. The δ 15 N‐NH 4 + in Saanich Inlet appears related to in ‐ situ regeneration of NH 4 + with little isotopic fractionation between dissolved and particulate nitrogen (PN).