Manganese in the eastern Atlantic Ocean: processes influencing deep and surface water distributions

Samples collected at four vertical water column stations during the 1990 IOC baseline survey cruise in the eastern Atlantic Ocean, were analysed for dissolved and total dissolvable manganese. The distributions show the elevated surface (0.90–2.12 nM) and low deep water (down to ca. 0.2 nM) concentrations typical of the oceanic distribution of this element. This new data in combination with literature data from elsewhere in the Atlantic, Indian and Pacific Oceans, has allowed an examination of the large time and space scale removal of dissolved manganese from the water column. A first order scavenging removal model with a steady state deep water concentration of 0.15 nM, and a removal rate constant of 0.015 yr−1 leads to a consistent background Mn concentration after about 400 years. Suspended particulate matter in the water column has a higher Mn to Al ratio than crustal material except where atmospheric inputs of crustal material are very high or significant benthic sources exist. In the surface water column, the role of atmospheric dust inputs from the Sahara desert appear very important, and highest Mn concentrations are generally associated with areas of highest dust input. However, closer to the African coast at about 12°N, another source of manganese appears to be important, and may reflect benthic sources and upwelling processes in the area.