Nepheloid Layer in the North Pacific

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Abstract
A distinct nepheloid layer with a vertical gradient of light scattering is observed on the continental rise of western North America. Throughout the deep North Pacific Basin the nepheloid layer extends from the bottom to the temperature minimum. The intensity of light scattering in the nepheloid layer of the North Pacific is substantially less than that of the Argentine Basin, except on the Alaskan and Tufts Abyssal Plains where it is comparable. Light scattering measured in this nepheloid layer with a photographic nephelometer is due to particulate matter; a model is proposed according to which this matter is derived principally from continents bordering the North Pacific and transported to the sea largely by rivers. Suspended particles are carried from the continental margin to the deep basin by lateral mixing and transport within the nepheloid layer. They are distributed throughout the large basins of the North Pacific by counterclockwise gyres and replace particles lost by sedimentation. Marginal trenches contain a strong nepheloid layer of trapped particles which ultimately settle to the trench floor. The particulate matter in the nepheloid layer enters the water in the North Pacific because the homogeneous bottom water moving northward through narrow equatorial channels along about 175° W. has only a weak nepheloid layer. Rapid increase in temperature and maintenance of an adiabatic gradient during transit of the channels from about 17° S. to 15° N. are attributed to strong vertical turbulence associated with vigorous flow and admixture of warmer water from above the deep temperature minimum. The vertical gradient of light scattering in the nepheloid layer is the result of a balance between particle settling and turbulent mixing in the homogeneous water below the temperature minimum. When data on settling rate of the particulate matter become available, this gradient may be used to estimate the intensity of vertical turbulence in the homogeneous water. Evidence supports the hypothesis that the nepheloid layer is a steady-state phenomenon.

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