The Madeira Abyssal Plain was chosen by the Nuclear Energy Agency (NEA) as a study area for high-level radioactive waste disposal in 1980. Subsequently, the area has been intensively investigated, in particular by the Geological Survey of the Netherlands, the Institute of Oceanographic Sciences, and latterly by the international ESOPE expedition organized by the NEA Seabed Working Group The large data set from this area of 2 ° x 2 ° includes 16 000 km of low-frequency seismic lines, 28 000 km of high-frequency seismic lines, 70 000 km2 of GLORIA sidescan and 120 sediment cores Core coverage extends through the upper 34 m of the sediment column, representing the last 730 000 years, and shows deposition dominated by turbidite sedimentation Seismic profiles indicate that total thickness of turbidites averages 350 m, with about 200 m of pelagic sediment beneath. Thus the turbidites continue considerably deeper than proposed penetrator emplacement depths of 30–70 m Individual turbidites are separated by thin pelagic layers which can be dated, and show the turbidites to be emplaced at the beginning and end of glacial periods. The largest single turbidite is over 5 m thick with a total volume in excess of 120 km3 There are three primary types of turbidite depending on their source (1) Organic-rich, from the NW African upwelling areas, (2)‘Volcanic’, from the Atlantic oceanic islands and non-upwelling areas off NW Africa, (3)Calcareousrich, from the seamounts to the west of the abyssal plain.
The overall average accumulation rate of the sediments is about 5–10 cm ka−1, an order of magnitude greater than the pelagic average for the area. Only minimal amounts of erosion have been detected in the pelagic layers despite regular turbidite emplacement. The turbihtes have fine grain-sizes, generally with means of about 3–4 pm, giving rise to relatively low permeability. Some areas of the plan have dense networks of faults which make them unpredictable for disposal purposes, but other areas over 100 km2 can be found which appear to be unfaulted
As a consequence of the frequency of turbidite emplacement and the quantity of organic carbon introduced with the turbidites, the sediments are oxic to <1m. The oxidation which occurs at the top of each organic-rich turbidite between emplacement and burial by the next turbidite, results in a two-tone coloration which is persistent for at least 730 ka. Weakly reducing conditions exist below the surficial oxic layer, the former extending to at least 30 m. Sulphate reduction is negligible within this interval. The geochemical behaviours of several elements which migrate in pore waters in response to these redox changes in the sediment column have been studied Manganese, uranium and iodine are of particular importance, since they provide analogues for radionuclides introduced by waste disposal Models describing the distribution of these naturally occurring elements are discussed.
