Comments on Methodology
The reasons for performing the analyses must be considered when
selecting methods will be used. For example, the Beckman Coulter Multisizer 3 EMPSA
determines particle volume as opposed to the pipette method that
measures settling rates. Therefore, if a researcher is studying flow
regimes and hydraulic equivalents, the pipette method might produce more
applicable data. Other settling-velocity analysis methods used for
fine-grain sizes are the hydrometer (Buoyocoz, 1928) and decantation
methods. However, these techniques are more difficult and less accurate
(Folk, 1974) and thus are generally not recommended. Numerous journal
articles have compared the various techniques used for size analyses of
fine-grained suspended sediments. All operators contemplating the use of
any of these methods are strongly encouraged to familiarize themselves
with this literature. For example, Shideler (1976) and Behrens (1978)
compared the Coulter Counter with pipette techniques. Hydrophotometers
have been compared with the pipette method (Jordan and others, 1971) and
with the Coulter Counter (Swift and others, 1972). Similar
considerations also apply to the coarse fraction as well. The coarse
fraction is usually determined either by sieving or by settling-tube analysis. However, if
this fraction weighs less than 10
g or contains greater than 2-5%
foraminifera which will not settle properly in a sedimentation column,
the operator must utilize sieves, which will measure nominal diameter.
These and other particle-size methods and considerations are discussed
in Barth (1984) and Syvitski and others (1991).
Aggregates of clay-sized particles, which are difficult to break up, commonly form when samples are dried. Because these aggregates or “bricks” may cause the size distribution to appear slightly (about 2-4%) coarser, care should be exercised to sonify samples with appreciable clay-sized material for at least 4 minutes prior to EMPSA analysis to minimize this effect. Other techniques to determine the weight percentages of the coarse and fine fractions that do not involve drying the fine fraction include pipette analyses and splitting the sample. In the latter technique where the sample is split, half is dried to determine the percentages of the coarse and fine fractions and the fine fraction from the other undried half is analyzed by EMPSA. Unfortunately, getting representative splits is difficult because of intra-sample heterogeneity and the splitting process can commonly introduce an error much greater than the original 2-4 percent. For those interested in determining the entire clay fraction with the greatest possible accuracy, pipette analyses or the use of a computer program that will extrapolate the fine end of the sediment distribution, such as GSSTAT, are strongly recommended
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OFR 2005-1001
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