Light Scattering and Suspended Particulate Matter on a Transect of the Atlantic Ocean at 11° N

Abstract

A combined investigation of the optical, physical and chemical properties was carried out on the suspended particulate matter on a transect of the North Atlantic Ocean at about 11°N. A nepheloid layer approximately 100 m thick was found at near-bottom in the western basin of the North Atlantic; no evidence of a near-bottom nepheloid layer was found in the eastern basin. This interbasin difference is evident in measurements of suspended particulate matter and absolute light scattering β (45) and is thought to be due to differences in bottom water movement between the two basins, Within the western basin, near-bottom maxima in the above parameters coincides with the maximum flux of Antarctic Bottom Water. Increases in suspended particulate matter and absolute light scattering occur between 3000 and 4000 m at 2 stations near South America. Potential temperature/salinity characteristics of this water are consistent with its being a southerly extension of the Western Boundary Undercurrent. At 2 stations over the Mid-Atlantic Ridge, large (30–40%) increases in the mass of suspended particulate matter occur 30 m above the bottom. Particulate organic carbon and particulate carbonate determinations indicate these increases are due to greater amounts of refractory oxides and/or hydroxides in the suspended matter and may represent an injection or diffusion of materials from the ridge to deep ocean waters. Samples taken in Subtropic Underwater, oxygen minimum water and Antarctic Intermediate Water show relatively low amounts of suspended particulate matter and absolute light scattering, indicating that the nepheloid character reported for these water masses in the northwestern Caribbean and Yucatan Channel must be acquired in transit through the Carribbean Sea. Near-bottom nepheloid layers were found at four stations along a 240 km stretch of the African continental rise. The source of much of the particulate matter between 1000 m and the bottom in this part of the eastern basin is thought to be terrigenous material delivered by a northwest-moving bottom current from the continental shelves of Liberia and Sierra Leone.

Chemical analyses of samples from the mixed layer at eleven stations along the transect indicate atmospheric dust is the most likely source for much of the near-surface suspended matter.