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 Quaternary turbiditic sediments from the Madeira Abyssal Plain (MAP), west of the NW African continental margin. In three sediment cores from this region, solid-phase geochemical indices of redox-sensitive ion mobilization are compared with rock-magnetic parameters as proxies for diagenesis characterization. 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 carbonate, detrital magnetic mineral, and organic carbon content. Within organic-rich (0.5\% C_org) turbidite horizons, remineralization of organic detritus proceeds in two stages. The initial stage involves suboxic (reductive) diagenesis, following depletion of pore-water O₂, while the later stage involves oxidative diagenesis associated with the slow descent of an oxidation front through the sediment, due to diffusion of O₂ from the overlying seawater. Rock-magnetic parameters of these sediments, when expressed on a carbonate-free basis, reveal that significant depletion of detrital ferrimagnetic iron (Fe²⁺/Fe³⁺) oxide grains has occurred within the organic-rich turbidites during suboxic diagenesis. Normalized quotients of magnetic parameters, and remanence-coercivity profiles of selected samples, also show that reductive diagenesis is a ferrimagnetic grain size-selective process, but it has a minimal effect on the canted-antiferromagnetic (Fe³⁺) oxides in the sediment. Such components, if present, therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are progressively dissolved. In all three cores studied, however, there is clear evidence for the presence of ultrafine ferrimagnetic iron oxide grains at depth within the suboxic zones of organic-rich turbidites, beneath both active and fossil oxidation 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 associated with suboxic diagenesis. These results comply with previous observations made in studies of suboxic diagenesis in hemipelagic sediments, and demonstrate the value of rapid and non-destructive rock-magnetic measurements for illuminating redoxomorphic processes of diagenesis involving transformation of iron phases in deep-sea sediments.