Early diagenetic processes in the muddy sediments of the Bay of Biscay

In order to understand the early diagenesis processes occurring in continental margin environment, modern sediments collected in six different sites from the Bay of Biscay have been studied. These sites can be separated into two groups. In the shallowest stations, where sediments are highly bioturbated, organic carbon levels are higher than 2%. In the deepest stations, sediments are much less bioturbated, and organic carbon levels are lower. In all sites, the vertical distribution of redox sensitive species can be explained by the well-established depth sequence of redox reactions, based on the bacterially mediated oxidation of organic matter. We have considered some alternative reaction pathways to explain the profiles of Fe, Mn, and N species. These reactions deal with the ammonia oxidation by manganese oxide, the aerobic denitrification and the oxidation of dissolved iron (II) by nitrate or Mn-oxides. Vertical flux calculations with a simple diffusion model indicate that these reactions could account for the reduction of all the Mn-oxides and the oxidation of all the upward diffusing Fe(II). They may also be responsible for a significant part of the total dinitrogen production. The relative importance of these pathways on early diagenetic processes and benthic fluxes has: not been determined and must be examined with additional experimental works. Our study suggests, however, that the coupling between the benthic cycles of iron, manganese and nitrogen could strongly influence the carbon cycling at the ocean floor. (C) 2001 Elsevier Science B.V. All rights reserved.