Early diagenesis in an organic-rich turbidite and pelagic clay sequence from the Cape Verde Abyssal Plain, NE Atlantic: magnetic and geochemical signals

Processes of early diagenesis in sediments from the Cape Verde Abyssal Plai n are investigated using the complimentary techniques of environmental magn etism and solid-phase geochemistry. The sequence captured by a 2 m-long box core comprises an organic-rich turbidite derived from the NW African conti nental margin (emplaced ca. 330 ka BP), overlain by brown, pelagic clay. Di agenesis in the pelagic clay mainly involves slow, in-situ oxidation of the iron oxide phases present, which Mossbauer-effect data suggest are detrita l haematite and magnetite (possibly maghaemitised). In the turbidite, diage nesis has proceeded in two stages: the first involving suboxic (reductive) processes, following depletion of porewater O-2, and the second involving o xidative processes associated with the slow descent of an oxidation front t hrough the bed due to diffusion of O-2 from the overlying seawater. Carbona te-free magnetic parameters, interparametric quotients, and remanence acqui sition, demagnetisation and coercivity measurements of the core, reveal tha t significant depletion of detrital ferrimagnetic iron (Fe2+/Fe3+) oxide gr ains like (titano)magnetite has occurred within the turbidite during suboxi c diagenesis, and confirm previous suggestions that reductive dissolution o f such components is a grain size-selective process. However, the higher ox idation state (Fe3+), canted-antiferromagnetic iron oxide grains like haema tite and goethite are much less susceptible to reductive diagenesis. Remobi lisation of transition metal ions from below the oxidation front in the tur bidite has led to authigenic precipitation of ferrimagnetic Fe/Mn micronodu les in a distinct layer immediately overlying the front, and above this, a further interval of Mn-enriched, "diagenetic laminae". However, within the suboxic, organic-rich zone of the turbidite, well below the oxidation front , there is clear evidence for the presence of a significant concentration o f fine (single-domain) ferrimagnetic iron oxide grains, which are most prob ably associated with a population of live magnetotactic bacteria. Between t his zone and the oxidation front, there is a transitional interval in which the number of live magnetotactic bacteria declines rapidly, and fossil mag netosomes in the sediment are being reductively dissolved progressively up- sequence. These results are broadly similar, though differing in some respe cts, to magnetic and geochemical studies of hemipelagic and high-productivi ty pelagic sediments in which Mn- and Fe-redox boundaries have been recogni sed, and further illustrate the value of environmental magnetic parameters for characterising early diagenetic processes in sediments of this kind. (C ) 2001 Elsevier Science B.V. All rights reserved.