Grain size analyses
(Section Sedimentary Geology and Engineering Geology)
A Reineck boxcorer is used to collect surficial sediment. About one kilogram of sediment from the top 10cm is put in plastic bags for grain size analyses.
Organic Carbon Content
The organic carbon content is determined using the re-titration method (Walkley and Black method). Organic carboncompounds are oxidised with Cr2O7-2 with addition of 2 volumes H2SO4 in 1 volume K2Cr2O7 solution. The remainder of Cr2O7-2 is determined through titration with a standard solution of FeSO4. The amount oxidisable material is directly proportional with the amount of reduced Cr2O7-2.
The volumetric method is applied to determine the amount of calcium. In a closed system, HCl is added to the sample and the amount of CO2 measured to calculate the calcium content of the sample.
Grain size analysis
Grain size analysis is preceded by a determination of the biogenic carbon content of the sediment. In case the concentration of biogenic carbon exceeds 5%, all organic material is being oxidised by hydrogenperoxide.
The fraction larger than 500µm is being separated from the sample. Only in case this fraction is of organic nature (detritus) it will be included in calculating the curve. Fractions larger than 500µm are included in a table and a graph. The remainder of the sample (<500µm) is separated into two fractions. Using a set of sieves the coarse fraction (<500µm - >53µm) is being divided into separate size classes with a quarter interval. The obtained classes are being weighed to the nearest 0.01g.
The relation between the dimensionless unit Φ and the grain diameter d (mm) is expressed as
Φ = -log2 (d)
The grain size distribution of the fine fraction (<53µm) is obtained with a Micrometrics Sedigraph. This fraction is only being determined when it relative content exceeds 5% of the total sample.
The measuring principle of this instrument is based upon the relation between the transmission of a low-energy X-ray through a suspension and the concentration of the particles on any given time. It is assumed that the particles sink according the Stokes’ law. The relative decrease in weight concentration between the start and any given moment is determined through the difference in intensity of the light ray after its passage through the suspension. Data are being transported to a PC where a data acquisition programme converts the data into a inverse cumulative curve generating data for every 1/16 Φ interval.
The measurements for both the coarse and fine fractions are pooled to construct a composite cumulative curve using the relative concentrations of material in both fractions. The cumulative weight percentages and the histogram data for each Φ interval are being represented in a table.
An important part of samples containing high silt content will be smaller than the determination limits of the sedigraph. For the calculation of the curve parameters a theoretical end value is therefore adopted below which no smaller material is available. The most current lower limit is 14 Φ (680Ǻ). Particles smaller than this size are believed not to sediment according to Stokes’ law but to follow Brownian motions.
Parameters describing the curve are estimated using the moment method and the graphical method. Both methods allow comparing the size distribution with a normal distribution. A filter allows differentiating particle sedimentation from flocculation. According to Kranck & Milligan (1991) flocs will behave like particles and will thus show a uniform distribution. The particle size analysis supplies a combination of the grain population and the size distribution of the flocs. Since the particles in the fine ‘tail fraction’ did sediment like individual particles this fine fraction is not included in the calculation of the statistical descriptive parameters. The weight percentage of this fine population indicates the importance of the particles that sediment in flocs.
Kranck K. & Milligan T.G. (1991). Grain size in oceanography: 332-345. In Syvitsky J.P. (Ed.). Principles, methods and applications of particle size analysis. Cambridge University Press
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