A critical review of the use stable N-isotopes to assess nitrogen elimination in the global ocean

The mean nitrogen (N) isotopic composition of oceanic nitrate is set by the balance of the main N sinks and inputs to the ocean, and can thus be used to constrain global N fluxes. The δ¹⁵N of organic matter preserved in marine sediments provides insight into past changes of N-inventory altering processes such as denitrification and N₂ fixation. The use of δ¹⁵N as tracer of sedimentary versus water-column nitrate reduction, today and in the past, requires the knowledge of the N isotope effects of these processes on the ocean nitrate pool. Estimates on the partitioning between the two fixed N elimination processes are particularly sensitive to the N isotope effect of benthic denitrification. In this presentation, I will provide a critical view on the use of canonical N isotope effects for water column and benthic denitrification in N isotope budgets and biogeochemical models. I will focus on the benthic environment, providing observational and model data, which suggest that the N isotope effect of N₂ loss from ocean sediments may be as high as 5 ‰, significantly larger than assumed by earlier work. I will also address the potential impact of suboxic N₂ producing processes other than denitrification (such as anammox) on the δ¹⁵N of the oceanic fixed N inventory, and I will discuss the implications for the global N-isotope balance, questioning current ideas with regards to the state of balance of the modern N budget.