SUMMARY: New data force us to raise previous estimates of oceanic denitrification. Our revised estimate of ~ 450 Tg N yr -1 (Tg = 10 12 g) produces an oceanic fixed N budget with a large deficit ) that can be explained only by positing an ocean that has deviated far from a steady-state, the need for a major upwards revision of fixed N inputs, particularly nitrogen fixation, or both. Oceanic denitrification can be significantly altered by small re-distributions of carbon and dissolved oxygen. Since fixed N is a limiting nutrient, uncompensated changes in denitrification affect the ocean's ability to sequester atmospheric CO 2 via the "biological pump". We have also had to modify our concepts of the oceanic N 2 O regime to take better account of the extremely high N 2 O saturations that can arise in productive, low oxygen waters. Recent results from the western Indian Shelf during a period when hypoxic, suboxic and anoxic waters were present produced a maximum surface N 2 O saturation of > 8000%, a likely consequence of "stop and go" denitrification. The sensitivity of N 2 O production and consumption to small changes in the oceanic dissolved oxygen distribution and to the "spin-up" phase of denitrification suggests that the oceanic source term for N 2 O could change rapidly.

Lignin, elemental, and stable carbon isotope compositions are reported for local plants and for coarse (>63 pm) and fine (~63 pm) suspended particulate materials collected along a 1,950-km reach of the lower Amazon River during four contrasting stages of the 1982-1983 hydrograph.

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[1] A nitrogen stable isotopic model was constructed in order to constrain the Holocene marine-fixed nitrogen budget. The primary sources and sinks considered were riverine and atmospheric sources, nitrogen fixation, sedimentary and water column denitrification, and sediment burial. The source budget was found to be insensitive to changes in nitrogen fixation rates, and thus could not be used to constrain this term. However, the isotopic value of fixed nitrogen losses was very sensitive to the amount of sedimentary denitrification. If the isotopic value of marine-fixed nitrogen has not changed during the Holocene, as supported by sedimentary records, then in order to balance the isotopic value of sinks and sources, approximately 280 Tg N yr À1 of sedimentary denitrification is required. If such a high rate of denitrification has been sustained throughout the Holocene, it implies that present-day estimates of marine nitrogen fixation are grossly underestimated. It also implies that the marine nitrogen budget has a residence time of less than 2000 years.