Distribution of exchangeable, fixed, organic and total nitrogen in interbedded turbiditic/pelagic sediments of the Madeira Abyssal Plain, eastern North Atlantic

Abstract

Nitrogen compounds in the interbedded turbiditic/pelagic sediments of the Madeira Abyssal Plain (from hereon MAP) have been divided in separate extracted fractions such as ‘exchangeable nitrogen’ (N ex), ‘fixed nitrogen’ (N fix) and ‘organic nitrogen’ (N org). N ex is found to be directly proportional to the amount of organic matter (C org) in the sediment and is not related to the amount of ammonium in the porewater. Only at a relatively low C org content is the N ex determined by the mineralogy of the sediment. The N org as determined from N org = (N tot - N fix - N ex) is strongly related with C org. The N fix content and the N fix Al ratio for organic-rich sediments are high compared with other sediments and have not changed upon post-depositional oxidation of the organic matter. Post-depositional oxidation of organic matter through oxygen does lead to a decrease in the C/N ratio of the remaining organic matter in the organic-rich turbidites. The C/N ratio calculated for the organic matter that has been decomposed through oxygen is similar to that calculated through stoichiometric modelling for decomposition under reducing circumstances (C/N ratios of ≈ 17 and ≈ 18, respectively). This implies that for the sediments of the MAP, the C/N ratio is identical for organic matter decomposed under oxygenated and under reducing conditions.