Abstract
We present a compilation and analysis of N sub(2) O data from the deep-water zone of the oceans below 2000 m. The N sub(2) O values show an increasing trend from low concentrations in the North Atlantic Ocean to high concentrations in the North Pacific Ocean, indicating an accumulation of N sub(2) O in deep waters with time. We conclude that the observed N sub(2) O accumulation is mainly caused by nitrification in the global deep-water circulation system (i.e., the ``conveyor belt''). Hydrothermal and sedimentary N sub(2) O fluxes are negligible. We estimate the annual N sub(2) O deep-water production to be 0.3 plus or minus 0.1 Tg. Despite the fact that the deep sea below 2000 m represents about 95% of the total ocean volume, it contributes only about 3-16% to the global open-ocean N sub(2) O production. A rough estimate of the oceanic N sub(2) O budget suggests that the loss to the atmosphere is not balanced by the deep-sea nitrification and pelagic denitrification. Therefore an additional source of 3.8 Tg N sub(2) O yr super(-) super(1) attributed to nitrification in the upper water column (0-2000 m) might exist. With a simple model we estimated the effect of changes in the North Atlantic Deep Water (NADW) formation for deep-water N sub(2) O. The upper water N sub(2) O budget is not significantly influenced by variations in the N sub(2) O deep-water formation. However, the predicted decrease in the NADW formation rate in the near future might lead to an additional source of atmospheric N sub(2) O in the range of about 0.02-0.4 Tg yr super(-) super(1) . This (anthropogenically induced) source is small, and it will be difficult to detect its signal against the natural variations in the annual growth rates of tropospheric N sub(2) O.