Early diagenetic processes in the muddy sediments of the Bay of Biscay C. Hyacinthe 1 , P. Anschutz * , P. Carbonel, J.-M. Jouanneau, F.J. Jorissen Universite  Bordeaux 1, Departement de Geologie et Oceanographie D.G.O.), CNRS UMR 5805, 33405 Talence Cedex, France Received 23 September 1999; accepted 29 December 2000 Abstract 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 pro®les of Fe, Mn, and N species. These reactions deal with the ammonia oxidation by manganese oxide, the aerobic denitri®cation and the oxidation of dissolved iron II) by nitrate or Mn-oxides. Vertical ¯ux 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 signi®cant part of the total dinitrogen production. The relative importance of these pathways on early diagenetic processes and benthic ¯uxes 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 in¯uence the carbon cycling at the ocean ¯oor. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Geochemistry; Sediment; Early diagenesis; Bay of Biscay; Manganese; Iron; Nitrogen 1. Introduction Modern oceanic sediments comprise the ®rst deci- metres below the sediment±water interface. This compartment contains organic carbon recently imported from the overlying water column. This carbon sustains biogeochemical reactions, which are important for the oceanic budget of C, N, P, and S species, and for some metals, such as Mn and Fe. Continental margin sediments play an impor- tant role, because their organic carbon content is much higher than in deep oceanic sediments. The mineralization of organic carbon alters the initial mineralogy and chemistry of deposited particles and interstitial waters, and determines the benthic cycle of redox species and the fate of sedimentary organic matter. The early diagenetic processes are mostly driven by the biological activity of the Marine Geology 177 2001) 111±128 0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0025-322701)00127-X www.elsevier.nl/locate/margeo * Corresponding author. Tel.: 133-556-848873; fax: 133-556- 840848. E-mail address: anschutz@geocean.u-bordeaux.fr P. Anschutz). 1 Present address: Faculty of Earth Sciences, Department of Geochemistry, Budapestlaan 4, 3584 CD Utrecht, The Netherlands.