Composition and characteristics of particles in the ocean : evidence for present day resuspension

Richardson, Mary Jo

doi:10.1575/1912/2074

Cited by 4 publications

(4 citation statements)

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“…Bader [1970] observed that particle size distributions in water tended to follow a power law relationship, and several workers [Brun-Cottan, 1971Sheldon et al, 1972;McCave, 1975;Letman et al, 1977;Baker et al, 1979;Pak et al, 1980;Richardson, 1980] showed that the exponent in the distribution N = a d-'" was about m = 3 (i.e., a Junge distribution), where d is particle diameter and N is the number of particles bigger than d, for the region above intense bottom nepheloid layers and below the surface hundred meters or so. In [1970], and McCave [1975] and for nepheloid layers on the shelf and in deeper water [Richardson, 1980; this paper]. The m = 3 distribution is equivalent to a flat volume distribution, (i.e., equal volumes of particles in logarithmic size grades [Sheldon et al, 1972]) in clear water, while the two-segment distribution is a peaked volume distribution with the peak in the region 4-8/•m.…”

Section: Introductionmentioning

confidence: 77%

“…In the later transect the regions of maximum suspended sediment occur on the margins of the cold cores. In particular, The values of apparent density well below unity found here and by others [Brun-Cottan, 1971;Peterson, 1977;Richardson, 1980] result from either floc structure or organic membrane preventing passage of electric current through particle space mainly occupied by water. This is recorded as particle volume by the Coulter Counter but does not appear in particle dry weight.…”

Section: Quasi-synoptic Transectsmentioning

confidence: 97%

“…The particles are from different sources (largely resuspended but with some settled from the surface) [Biscaye and Eittreirn, 1977;Richardson, 1980] and are mainly aggregated by some means into units larger than the primary mineral and organic particles that comprise them. The principal terms in the equation for the rate of deposition of fine suspended material involve the settling velocity of the particles Ws and the limiting shear stress for deposition • which is a function of Ws [McCave and Swift, 1976].…”

Section: Introductionmentioning

confidence: 96%

“…Peaks were not detected by Plank et al [1972] or Lerman et al [1977] from nepheloid water, nor did they find any significant difference in the shape of distributions from nepheloid and clear water. However, most of these authors' observations were made in regions with relatively weak nepheloid layers (C < 12 mg m-3 for [Lerman et al, 1977]) compared to the continental rise off the eastern United States and Nova Scotia of Richardson [1980] and this paper (C > 500 mg m-3).…”

Section: Introductionmentioning

confidence: 99%

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Particulate size spectra, behavior, and origin of nepheloid layers over the Nova Scotian Continental Rise

McCave¹

1983

J. Geophys. Res.

141

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Samples of suspended sediment and light transmissometer observations were taken on two occupations of a transect of the Nova Scotian continental rise in 1980. Initial low suspended sediment concentrations were followed by high values 10 days later. Patches of high concentration were found on the edges of zones of cold (θ<1.78°C) water. Particle volume concentrations in the diameter range 1.26–32 μm measured by Coulter Counter on 10 cm3 samples correlate linearly with light beam attenuation coefficient. Correlation between particle volume and weight concentration indicates lower apparent particle density for low‐concentration suspensions, a feature that is attributed to aggregation during aging of the suspension. Particle size distributions show unimodal form with a peak at ∼4 μm in high concentrations near the bed changing to a bimodal form with an additional mode at ∼16 μm in low concentrations high above the bed (500 and 1000 m.a.b. (meters above bottom)). The presence of the coarse mode at 1000 m.a.b. coupled with its increase in the lower 250 m of the water column suggests that it both settles from the surface and is resuspended from the bed. Ultrasonic disaggregation experiments show that much of the fine mode is of aggregates, but the coarse mode is single solid particles. Distributions with a low volume‐concentration increase in volume when disaggregated and are interpreted as having a relatively large part of their total distribution in unmeasured sizes >32 μm. Distributions with a high concentration decrease in counted volume on disaggregation. Inversions in the light attenuation profiles are taken to indicate the presence of former bottom mixed layers, and their occurrence suggests thickening of the nepheloid layer by detachment and lateral injection of these turbid mixed layers. They occur at levels sufficiently high as to provide a nepheloid layer over 1500 m thick over the adjacent Sohm Abyssal Plain. The coarse mode probably comprises solid particles with a calculated settling velocity of ∼12 m day−1. Its absolute concentration increases toward the bed, but as a proportion of the distribution it increases upward. It is thus thought to be particles of surface origin both settling to the bottom and, in the lower 250 m of the water column, resuspended from the bed. The fine mode at 4 μm has a spatially constant form thought to be controlled by physical coagulation mechanisms. Rates of coagulation estimated from shear mechanisms suggest that the postulated change of peaked into flat size distribution by particle aggregation and settling has a time scale of a few years rather than months, which is probably long when compared with the lateral injection process lower in the nepheloid layer. For that reason, distribution with a fine mode peak are found well above the bottom mixed layer.

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