GFDL - Geophysical Fluid Dynamics Laboratory

Data-based estimates of suboxia, denitrification and N2O production in the ocean, and their sensitivities to dissolved O2

Bianchi, D, John P Dunne, Jorge L Sarmiento, and E D Galbraith, in press. Global Biogeochemical Cycles. DOI:10.1029/2011GB004209. 3/12.

Key Findings

  • Open ocean suboxic zones are a factor of three larger than gridded estimates provided in the World Ocean Atlas.
  • We obtain a removal of fixed nitrogen of 70 ± 50 Tg yr-1 in the open ocean and 198 ± 64 Tg yr-1 in the sediments, and a global N2O production of 6.2 ± 3.2 Tg yr-1.
  • These results reconcile water column denitrification rates based on global oxygen distributions with previous estimates based on nitrogen isotopes.
  • We revise existing estimates of sediment denitrification down by one third through the use of spatially-explicit fluxes.
  • We provide independent evidence supporting the idea of a historically-balanced oceanic nitrogen cycle.

Goals of the research

Oxygen minimum zones (OMZs) exclude many classes of the biological community and are major sites of fixed nitrogen removal from the open ocean. Previous estimates of rate of fixed nitrogen loss through denitrification in the pelagic ocean and in sediments have been unable to match estimates of the supply of fixed nitrogen via N2 fixation, rivers, and atmospheric deposition. This has led some scientists to the conclusion that the global ocean nitrogen cycle is out of balance, and the oceans are accumulating fixed nitrogen. However, commonly-used gridded data sets such as the World Ocean Atlas (WOA) tend to overestimate the concentration of O2 compared to measurements in grids 1 where O2 falls in the suboxic range (O2 < 2 ? 10 ?M), thereby underestimating the extent of O2 depletion in OMZs. We utilize a combination of recently available satellite and field-derived, globally spatially explicit datasets – in particular correcting WOA for its bias at the O2 extremes – for a revised look at the global nitrogen cycle.

Relevance to NOAA science

Because the presence of oxygen is critical for habitat of living marine resources, this effort supports NOAA’s research into the functioning of the earth as a system to better quantify global biogeochemcial cycling and the environmental constraints on marine ecosystems. This work serves to broaden our understanding of the interplay between varieties of climatically and ecologically relevant processes at work in the present day marine environment.

Relevance to society

This work helps us to better understand and reduce uncertainty in the ocean oxygen and nitrogen cycles with implications for living marine resources, biogeochemical cycles, and earth system dynamics.

Unique aspects of this study

This research is unique in reconciling the combination of globally spatially explicit products from field observations (NOAA’s World Ocean Atlas), satellite products (ocean color, surface temperature, and photosynthetically available radiation), and biogeochemical algorithms to better understand oxygen and nitrogen cycling in the world oceans.

Description of the methodology

We evaluate the distribution of the OMZs by (1) mapping high-quality oxygen measurements from the WOCE program, and (2) by applying an empirical correction to the gridded WOA based on in situ observations. We then combine the new oxygen data sets with estimates of global export and simple models of remineralization to estimate global denitrification and production.

Known weaknesses or uncertainties

These estimates are most sensitive to uncertainties in the global export production, the oxygen threshold for suboxic processes, and the efficiency of particle respiration under suboxic conditions. Ocean deoxygenation, an expected response to anthropogenic climate change, could increase denitrification by 14 Tg yr-1 of nitrogen per ?M of oxygen reduction if uniformly distributed, while leaving N2O production relatively unchanged.

Table. Estimates
of oceanic nitrogen budgets (TgN yr-1)