Trace-element mobility during early diagenesis in distal turbidites: late Quaternary of the Madeira Abyssal Plain, N Atlantic

Abstract

Summary Sediments deposited on the Madeira Abyssal Plain (MAP) during the late Quaternary consist of an alternating succession of distal turbidites and pelagites. Individual turbidites may be correlated over most of the northern MAP. The sediment geochemistry of the succession has been investigated in detail using material from a representative 10 m piston core. The turbidites have been divided into organic-rich (at least 0.5% C org ) and organic-poor groups. At the time of emplacement all turbidites were characterized by homogeneous major- and trace-element distributions, although thicker beds have thin silty bases of different compositions. Organic-rich turbidites, however, have undergone considerable early post-depositional modification as a result of oxidation of labile organic matter by sea-water oxygen diffusing downwards from the sediment-water interface. The progressive advance of oxidation fronts through the turbidites has produced distinctive two-tone colourations with light-brown tops and olive-green bottoms, and has caused significant early diagenetic relocation of iron, manganese, phosphorus, cobalt, copper, nickel, uranium, vanadium and zinc. Iron is marginally enriched in formerly oxic sediments. Manganese oxyhydroxides occur as dark-brown diagenetic laminations in recently oxidized turbidite tops, but have been dissolved by anoxic conditions in older examples. Mn 2+ pore-water profiles have been preserved by the adsorption of manganese by calcite. Phosphorus, cobalt, nickel and some copper were initially complexed by the precipitation of manganese oxyhydroxides and other processes occurring at active redox boundaries. Copper was also concentrated in the reduced sediments at this time. Later migration of these elements as reduced phases has produced thin purple diagenetic laminations in both turbidities and pelagites. Uranium, vanadium and zinc were relocated into reduced sediments during early diagenesis, but were not remobilized later. The diagenetic sequence in MAP sediments contrasts strongly with established ‘steady-state’ models based on hemipelagic and pelagic environments, but may be typical of a significant proportion of recent and ancient sequences.