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

Abstract

S U M M A RY: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 0.59/00 Corg) 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 twotone 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 turbidites 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.

Occurrence of thin, metal-rich layers in deep-sea sediments: a geochemical characterization of copper remobilization

“…These colour changes are often accompanied by black or purple diffusive fronts above and below the oxidation front. In addition, some black and purple fronts do not seem to be associated with the brown-green colour transition (JARVIS and HIGGS, 1987).…”

“…WILSON et al, 1985;DE LANGE, 1986). Elements mobile under oxidized conditions accumulate below the redox boundary (U, Vand Zn) during the development of such a progressive oxidation front (JARVIS and HIGGS, 1987;THOMSON et al, 1990;COLLEY and THOMSON, 1990). Fe and Mn, mobilized during anoxic and sub-oxic conditions, subsequently scavenge Ni and Co and to a lesser extent Cu.…”

“…Another result of the emplacement of an organic-rich turbidite is that the underlying sediment becomes reduced and redistribution of transition metals occurs. This can result in the formation of reduction haloes and local trace metal enrichments (HARTMANN, 1979;JARVIS and HIGGS, 1987;THOMSON et at., 1989).…”

Fine-grained turbidites of the Amazon Fan: facies characterization and interpretation

“…These features are interpreted as redox-controlled diagenetic features resulting from the re-establishment of a pore-water profile after rapid deposition of a turbidite. Similar features have been described from many fine-grained turbidites Damuth et al (1988) and Manley and Flood (1988). and are best known from the Madeira Abyssal Plain (Jarvis and Higgs, 1987).…”

Calcareous nannofossil biostratigraphy and sediment accumulation of turbidite sequences on the Madeira Abyssal Plain, Sites 950-952

“…During the Quaternary, these turbidites were emplaced at frequencies related to climate and sea-level change (Weaver and Kuijpers, 1983;Weaver et al, 1992), with each sea-level change (both rises and falls) generally associated with a single turbidite. Most of the turbidites can be grouped into three compositionally distinct types, each with different source areas (Jarvis and Higgs, 1987;De Lange et al, 1987): (1) volcanic turbidites, which are rich in volcaniclastic material derived from the volcanic Canary Islands and Madeira; (2) organic turbidites, which have high total organic carbon contents and originate from upwelling areas on the African shelf; and (3) calcareous turbidites derived from pelagic carbonate sediments accumulating on the volcanic seamounts of the Great Meteor Seamount Chain.…”