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

Abstract

Abstract Processes of early diagenesis in sediments from the Cape Verde Abyssal Plain are investigated using the complimentary techniques of environmental magnetism and solid-phase geochemistry. The sequence captured by a 2 m-long box core comprises an organic-rich turbidite derived from the NW African continental margin (emplaced ca. 330 ka BP), overlain by brown, pelagic clay. Diagenesis in the pelagic clay mainly involves slow, in-situ oxidation of the iron oxide phases present, which Mossbauer-effect data suggest are detrital haematite and magnetite (possibly maghaemitised). In the turbidite, diagenesis has proceeded in two stages: the first involving suboxic (reductive) processes, following depletion of porewater O2, and the second involving oxidative processes associated with the slow descent of an oxidation front through the bed due to diffusion of O2 from the overlying seawater. Carbonate-free magnetic parameters, interparametric quotients, and remanence acquisition, demagnetisation and coercivity measurements of the core, reveal that significant depletion of detrital ferrimagnetic iron (Fe2+/Fe3+) oxide grains like (titano)magnetite has occurred within the turbidite during suboxic diagenesis, and confirm previous suggestions that reductive dissolution of such components is a grain size-selective process. However, the higher oxidation state (Fe3+), canted-antiferromagnetic iron oxide grains like haematite and goethite are much less susceptible to reductive diagenesis. Remobilisation of transition metal ions from below the oxidation front in the turbidite has led to authigenic precipitation of ferrimagnetic Fe/Mn micronodules 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 of fine (single-domain) ferrimagnetic iron oxide grains, which are most probably associated with a population of live magnetotactic bacteria. Between this zone and the oxidation front, there is a transitional interval in which the number of live magnetotactic bacteria declines rapidly, and fossil magnetosomes in the sediment are being reductively dissolved progressively up-sequence. These results are broadly similar, though differing in some respects, to magnetic and geochemical studies of hemipelagic and high-productivity pelagic sediments in which Mn- and Fe-redox boundaries have been recognised, and further illustrate the value of environmental magnetic parameters for characterising early diagenetic processes in sediments of this kind.