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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.

An Alvin push core collected near the base of the TAG hydrothermal mound, 26°08′N, Mid‐Atlantic Ridge, sampled a 7 cm‐thick layer of Fe‐rich red‐brown mud (20–40% Fe) overlying a 4 cm‐thick layer of carbonate ooze (5–70% CaCO3) which also contains up to 32%Fe. On the basis of chemical compositions, two separate layers can be identified within the red‐brown mud. The core position, X ray diffractometry, Pb isotope analyses, and elemental abundances all indicate that these two layers were derived from mass wasting of oxidized, originally sulphidic material from the slopes of the TAG hydrothermal mound. High concentrations (11–19 ppm) of seawater‐derived U occur in both Fe‐rich layers. These enrichments may derive from uptake of U linked to the oxidation of sulphide phases. Metal concentrations in the rich layer are lower than in the upper part of the core but higher than in open ocean pelagic sediments. The 230Thxs/Fe, 231Paxs/Fe, and rare earth element/Fe distributions indicate that the hydrothermal input to the carbonate‐rich layer is dominated by settling of suspended particulate material from the overlying hydrothermal plume. Such CaCO3‐rich sediments may record an important component of the flux of hydrothermal material to sediments of the Mid‐Atlantic Ridge rift valley.