THE EFFECT OF BIOIRRIGATION AND REDOX CONDITIONS ON NITROGEN ISOTOPE FLUXES IN MARINE SEDIMENT

The reported extent of N isotope fractionation during benthic N₂ production has differed substantially between studies. To assess the range and identify mechanisms underlying such observations, we developed a reactive transport model and ran simulations evaluating the impact of nitrification, denitrification, and anaerobic ammonium oxidation on the isotopic composition of in-situ N­₂ production. Different hydrodynamic regimes ranging from bioirrigation-dominated to purely diffusive transport were simulated, and the effects of the benthic mineralization rate and the composition of the overlying water on the isotopic signature of benthic N exchange fluxes were also quantified.

Sediment redox conditions were found to control the N isotope effect, which under reducing conditions is driven by fractionation during nitrification and anaerobic ammonium oxidation and under oxidizing conditions by fractionation during denitrification. The mineralization rate, the bioirrigation intensity, and chemical composition of the overlying water affect the benthic redox zonation and therefore also the benthic N isotope effect.

In shallow water environments, high mineralization rates leading to reducing conditions are often counteracted by active bioirrigating macrofauna injecting oxic water into the sediment. With increasing water-depth both sediment metabolism and bioirrigation intensity tend to decrease. Therefore, our simulations commonly lead to a relatively constant N isotope effect of approximately -3‰. This suggests that benthic N isotope effect of N₂ production is relatively small compared to water-column N₂ production, but due to the significant amount of benthic N₂ production nevertheless can have a large impact on the ¹⁵N:¹⁴N ratio of oceanic nitrate.