Continuous Light-Scattering Profiles and Suspended Matter Over Nitinat Deep-Sea Fan
During September 1971 and June 1972, a total of 65 light-scattering profiles were recorded in the waters over Nitinat Fan by means of a self-contained, continuously recording nephelometer. All profiles extend from the sea surface to a point 20 m above the sea floor (maximum depth 2400 m) and as many as 5 prominent scattering layers are observed within the water column at a given station. Some scattering layers can be traced over wide areas of the fan and appear on records from both cruises. The most persistent feature of the profiles, and the only one found on every record, is a bottom nepheloid layer (BNL) of steadily increasing light scattering immediately below a layer of relatively clearer water and immediately above the sea floor. Over short lateral distances the vertical extent and scattering intensity of the BNL (normalized to the scattering levels of the overlying clearer waters) change markedly in a manner apparently related to fan topography. In general, the BNL thickens to > 300 m and intensifies to > 2.5 times normal over the topographic lows, being most prominent above Cascadia Valley (the major valley crossing the fan) as well as above the steep non-channelled northern flank of the fan. Over the levees separating the smooth northern flank from Cascadia Valley to the south and over the foot of the continental slope which forms the eastern border of the fan, the BNL thins to < 50 m and the scattering intensity is < 1.5 times normal.
Twenty-four suspended sediment samples, collected by in situ filtration from within and just above the BNL, yielded inorganic particle concentrations of 20 to 80 µg/l. These concentrations correlate well (r = 0.73) with the simultaneously recorded scattering values, indicating that the nephelometer provides a fair estimate of suspended sediment concentrations in the bottom waters over Nitinat Fan.
By integrating the scattering profile within the BNL at each station and converting the average scattering value into particle mass by means of an empirical relationship derived from the measured concentration values, the average amount of inorganic suspended matter in the BNL can be calculated. In a 1-cm2 column of water, average particulate mass in the BNL was estimated at about 0.5 mg. Assuming that the bottom deposits are derived only from particles in the BNL, the measured sedimentation rate on the fan requires a mean residence time of about 1 month for these particles within the BNL.
KeywordsSuspended Sediment Continental Slope Suspended Sediment Concentration Turbidity Current Inorganic Sediment
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