The geochemistry of Cu, Cd, Zn, Ni and Pb in sediment cores from the continental slope of the Banc d'Arguin (Mauritania)

Trace and major elements were measured in sediment cores collected at the s helf and continental slope of the Bane d'Arguin (Mauritania). Most of the s ediments have a calcium carbonate content > 50% and as a consequence have a low aluminium (< 1.5%) and iron (< 1.0%) content. The concentrations of th e latter two elements increase down slope. Based on the major elemental com position, the sediments seem to be well mixed and concentration gradients a re only observed on a spatial distribution. Concentrations of Cu, Cd, Zn, N i and Pb are low and approximately equal to "natural background" values, in dicating negligible anthropogenic influences. An exception is Cd for which very high values were found in the northern most part of the research area, close to Nouadhibou. A possible explanation for this high Cd content can b e the existence of phosphate rich sediments with a high affinity for Cd. Sm all increases in the available trace metal concentrations (exchangeable fra ction, 0.1 N HCl treated) observed near the sediment surface could be attri buted to early diagenetic processes, and not to pollution effects. The inpu ts from the Saharan dust plume with low trace metal contents, the transport of small particles with higher trace metal content down slope together wit h the escape of redissolved trace metals from the sediment, and the absence of a supply of metal rich material to the Bane d'Arguin, has created a met al poor environment. There is clear evidence of trace metal redistributions , associated with Mn recycling. Mean enrichment factors, calculated relativ e to the crustal abundance, show that Zn, Pb and Cd are equal to unity and Cu and Ni below unity. These observations suggest that the Cu and Ni conten ts in sediments of Bane d'Arguin are depleted relative to the accepted valu es in the earth crust. The very low trace metal concentrations detected in these sediments can be taken as "baseline" values for unpolluted sediments in the global ocean coastal zone. (C) 1999 Elsevier Science Ltd. All rights reserved.