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)