Nepheloid layers and bottom currents in the Arctic Ocean

Abstract

Between 1965 and 1969, fifty-one profiles of light scattering were made in the central Arctic Ocean from Fletcher's Ice Island (T-3). The profiles, taken with an in situ photographic nephelometer extend from just below the surface to the bottom. Two distinctly different types of profiles were observed. At all stations the strongest scattering occurs near the surface, decreasing with depth in the upper layers. Over the Canada abyssal plain, light scattering is almost constant below 2000 meters, decreasing slightly with depth all the way to the bottom so that the bottom water is the clearest. Over the ridges and rises surrounding the Canada basin, however, scattering increases with depth below an intermediate scattering minimum. The zone of deep light scattering on the ridges and rises is called the bottom nepheloid layer. The bottom nepheloid layer is evidently caused by fine material that is maintained in suspension by the turbulent flow. Four spot measurements of bottom currents on the Mendeleyev ridge gave speeds of 4 to 6 cm/sec. One spot measurement over the Canada abyssal plain gave a speed of less than 1 cm/sec. This indication of swifter current speeds over the ridges is supported by bottom photographs in which animal tracks are much less evident on the ridges than on the Canada abyssal plain in spite of a greater abundance of life on the ridges. This is attributed to the higher current speeds, which obliterate the tracks. The observations suggest a counterclockwise deep circulation in the Canada basin with the currents confined primarily to the sloping margins of the basin. This pattern of deep circulation is in agreement with ideas and experiments on deep circulation with a concentrated source and distributed surface sink. Deep water enters the basin over a sill and leaves by upward diffusion through the halocline into the surface water, which then flows out of the basin.