The oceanic fixed nitrogen and nitrous oxide budgets: Moving targets as weenter the anthropocene?

New data force us to raise previous estimates of oceanic denitrification. O ur revised estimate of similar to 450 Tg N yr(-1) (Tg = 10(12) g) produces an oceanic fixed N budget with a large, deficit (similar to 200 Tg N yr(-1) ) that can be explained only by positing an ocean that has deviated far fro m a steady-state, the need for a major upwards revision of fixed N inputs, particularly nitrogen fixation, or both. Oceanic denitrification can be sig nificantly altered by small re-distributions of carbon and dissolved oxygen . Since fixed N is a limiting nutrient, uncompensated changes in denitrific ation affect the ocean's ability to sequester atmospheric CO via the "biolo gical pump". We have also had to modify our concepts of the oceanic N2O reg ime to take better account of the extremely high N2O saturations that can a rise in productive, low oxygen waters. Recent results from the western Indi an Shelf during a period when hypoxic, suboxic and anoxic waters were prese nt produced a maximum surface N2O saturation of > 8000%, a likely consequen ce of "stop and go" denitrification. The sensitivity of N2O production and consumption to small changes in the oceanic dissolved oxygen distribution a nd to the "spin-up" phase of denitrification suggests that the oceanic sour ce term for N2O could change rapidly.