Early diagenesis in North Atlantic abyssal plain sediments characterized by rock-magnetic and geochemical indices

Processes of early diagenesis are investigated in a sequence of late Quater nary turbiditic sediments from the Madeira Abyssal Plain (MAP), west of the NW African continental margin. In three sediment cores from this region, s olid-phase geochemical indices of redox-sensitive ion mobilization are comp ared with rock-magnetic parameters as proxies for diagenesis characterizati on. The mud-dominated, distal turbidites are emplaced mainly as a result of glacio-eustatic changes in sea-level, and derive both from different sites on the NW African margin, and from the flanks of the Canary Islands. Thus, the turbidites are of variable composition, especially in terms of carbona te, detrital magnetic mineral, and organic carbon content. Within organic-r ich (> 0.5% C-org) turbidite horizons, remineralization of organic detritus proceeds in two stages. The initial stage involves suboxic (reductive) dia genesis, following depletion of pore-water O-2, while the later stage invol ves oxidative diagenesis associated with the slow descent of an oxidation f ront through the sediment, due to diffusion of O-2 from the overlying seawa ter. Rock-magnetic parameters of these sediments, when expressed on a carbo nate-free basis, reveal that significant depletion of detrital ferrimagneti c iron (Fe2+/Fe3+) oxide grains has occurred within the organic-rich turbid ites during suboxic diagenesis. Normalized quotients of magnetic parameters , and remanence-coercivity profiles of selected samples, also show that red uctive diagenesis is a ferrimagnetic grain size-selective process, but it h as a minimal effect on the canted-antiferromagnetic (Fe3+) oxides in the se diment. Such components, if present, therefore become relatively enriched i n magnetic assemblages as the ferrimagnetic grains are progressively dissol ved. In all three cores studied, however, there is clear evidence for the p resence of ultrafine ferrimagnetic iron oxide grains at depth within the su boxic zones of organic-rich turbidites, beneath both active and fossil oxid ation fronts. These grains are probably associated with populations of live magnetotactic bacteria which inhabit such organic-rich horizons, and which form a link in the chain of bacterially mediated reactions normally associ ated with suboxic diagenesis. These results comply with previous observatio ns made in studies of suboxic diagenesis in hemipelagic sediments, and demo nstrate the value of rapid and non-destructive rock-magnetic measurements f or illuminating redoxomorphic processes of diagenesis involving transformat ion of iron phases in deep-sea sediments. (C) 2000 Elsevier Science B.V. Al l rights reserved.