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.

The iY5N composition of nitrate and N2 gas was measured in the eastern tropical North Pacific (ETNP) and centralArabian Sea (AS) suboxic regions. The 615N of nitrate increased from 6‰ at 2,500 m to 15‰ at 250-350 m in both regions, while the 615N of N2 concurrently decreased from 0.6‰ to 0.25‰. The denitrification isotopic fractionation factor (Ed,,,,) for each region was estimated using both advection-reaction and diffusion-reaction models.Values for edernl in the ETNP ranged from 25 ± 2 (advection-reaction) to 30 ± 3 (diffusion-reaction). Values for E dcmt in the central AS varied from 22 ± 3 (advection-diffusion) to 25 ± 4 (reaction-diffusion) using a starting nitrate isotopic composition of 6‰ but were indistinguishable from calculated values from the ETNP when an initial value of 5‰ was employed. Based upon the model results, an average global edenlt of 27 ± 3 is proposed for marine suboxic water columns. Isotopic enrichment of nitrate in oxic waters beneath the active denitrification regions was observed and indicates the presence of significant cross-isopycnal ventilation at depth. The isotopic composition of nitrate decreased above 250 m to ~80 m, and this pattern is hypothesized to be caused by the input of isotopically light nitrogen from nitrogen fixation in the euphotic zone. A simple isotopic mass balance indicates that a significant percentage of primary productivity in the central AS may be fueled by nitrogen fixation.Denitrification within suboxic water columns has been estimated to be either the largest or second largest sink for fixed nitrogen in the sea (Codispoti and Christensen 1985;Middleburg et al. 1996). These waters also are a significant source of N2O to the atmosphere (Law and Owens 1990) and can impact surface primary productivity via reduction in the amount of fixed nitrogen mixed to the surface (Mantoura et al. 1993). It has been observed that denitrification leaves a strong imprint upon the isotopic composition of the remaining nitrate within these regions (Cline and Kaplan 1975;Liu and Kaplan 1989). In regions where this enriched nitrate is upwelled and taken up by primary producers, the resulting isotopic enrichment in buried organic nitrogen within underlying sediments has been used as an indicator AcknowledgmentsWe thank P. D. Quay for the use of his stable isotope laboratory facilities. D. Wilbur provided assistance in operating the mass spectrometer and in data analysis. In addition, Paul Quay, Mark Teece, and two anonymous reviewers provided commentary that greatly improved the content and structure of this paper.

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Abstract. The Arabian Sea contains one of the three major open-ocean denitrification zones in the world. In addition, pelagic denitrification also occurs over the inner and midshelf off the west coast of India. The major differences between the two environments are highlighted using the available data. The perennial open-ocean system occupies two orders of magnitude larger volume than the seasonal coastal system, however, the latter offers more extreme conditions (greater nitrate consumption leading to complete anoxia). Unlike the open-ocean system, the coastal system seems to have undergone a change (i.e., it has intensified) over the past few decades presumably due to enhanced nutrient loading from land. The two systems also differ from each other with regard to the modes of nitrous oxide (N 2 O) production: In the open-ocean suboxic zone, an accumulation of secondary nitrite (NO