Nitrogen/argon and nitrogen isotope ratios in two anaerobic environments, the Cariaco Trench in the Caribbean Sea and Dramsfjord, Norway

Mass spectrometric measurements of nitrogen/argon and nitrogen isotope ratios have been used to examine the fate of nitrogen arising from the decomposition of organic matter in two anaerobic marine environments, the Cariaco Trench in the Caribbean Sea and the Dramsfjord in Norway.

Values of N₂/A in these waters are larger than would be expected from dissolved atmospheric gases. By assumingthat the A concentration is biologically unaffected, the excess quantities of N₂ can be calculated. These are in good agreement with the amounts of nitrogen which would be expected to arise from the decomposition of organic matter. The latter values are computed from the observed concentrations of nitrogen compounds and either the observed concentrations of inorganic phosphate or the amounts of oxygen (either free or bound in the sulphate ion) consumed from the water.

It is shown that nitrogen isotope ratios are markedly different in the nitrogen of biogenic origin from those in nitrogen dissolved from the atmosphere.

It is shown that it is probable that free nitrogen can be formed from organic matter only through the denitrification of nitrate, formed as an intermediate.