LABORATORY PROCEDURES

Fine Fraction Analysis (Silt plus Clay)

The fine-grained fraction of a sediment sample is defined as silt (particles with diameters less than 62 microns down to 4 microns) plus clay (particles with diameters less than 4 microns, with colloidal clay being less than 0.1 microns). Because of their small size, fine-fraction particles are difficult to measure by sieving. Therefore, the classical techniques to analyze the fine-grained portion of grain-size distributions are dominated by sedimentation methods, such as the hydrometer and pipette methods (Krumbein and Pettijohn, 1938; Milner, 1962; Folk, 1974). These methods involve preparing a dispersed, homogeneous suspension of the fine fraction, dilution with a dispersant solution (usually 0.5% sodium hexametaphosphate) to 1000 ml, and allowing the particles to settle in a graduated cylinder. As the settling of sediment particles continues according to Stoke's Law, samples are either withdrawn (pipette) or measurements made (hydrometer) of the suspension at preset time intervals. However, many sedimentation laboratories have discontinued or limited the use of pipette and hydrometer techniques because of inherent problems with settling (for example, Brownian motion), thermal convection, irregular particle shape, mass settling in density currents, rounding errors due to large multiplication factors, and, especially, the time necessary to extend an analysis down into the clay range. For example, over 24 hours are required to extend a pipette analysis down to 11 phi at 20^oC, room temperature (Krumbein and Pettijohn, 1938; Milner, 1962).

Although originally designed to count and size blood cells (Coulter, 1957; Berg, 1958), Electro-Resistance Multichannel Particle Size Analyzer (EMPSA) have found other applications because of the short time and small amount of material required for analysis. These devices, which are currently manufactured by Beckman Coulter (Coulter Counter) and Micromeritics (Elzone), are also used in industry, in the biological sciences (Sheldon and Parsons, 1967), and in geology, where they are used to measure the grain size of sediments by measuring differences in electrical resistivity produced by sediment particles (McCave and Jarvis, 1973; Schiedler, 1976; Kranck and Milligan, 1979; Muerdter and others, 1981; Milligan and Kranck, 1991).

Because the aperture tubes used by EMPSA can determine the size of particles of only 2-40% of the aperture diameter, at least two aperture tubes with overlapping ranges are required to determine the size distribution within the fine fraction of a marine sediment. For example, a 200-micron aperture tube would be used to resolve the size distribution between 62 to 8 microns during a standard multi-aperture analysis using a Coulter Counter, and a 30-micron aperture tube would be needed to resolve the size distribution between 12 microns down to approximately 0.5-0.7 microns. These individual analyses or passes through the respective aperture tubes are then mathematically combined to produce the fine-fraction (silt plus clay) distribution. The fine fraction is, in turn, combined with the coarse fraction data to generate a complete grain size distribution.

An obvious limitation of this method is that an EMPSA, even if calibrated to resolve down to about 0.6 microns (10.75 phi), can only resolve a portion of the clay distribution. The fine clay in the 0.6 to 0.1 micron range (some of the 11 phi and all of the 12 phi and 13 phi fractions that extend down to the colloidal clay boundary) is not detected. Although an analysis performed down to 0.6 microns is adequate for most freshwater, estuarine, and shelf environments, the sediments from many deeper water marine environments (for example, rise and abyssal plain) may contain significant material in the fine clay fraction. Details of the fine-fraction can be extended through the fine clay range by combining a double-point pipette analysis with the data from an EMPSAs analysis, but this solution is less desirable because of the problems with pipette analyses cited above.

Earlier efforts to extrapolate grain size data beyond the limits of the analysis used graphical methods (Schlee and Webster, 1967; Schlee, 1973). For example, if it was determined that more than 5% of the fine-grained sediment from a sample was less than 1 micron, additional data points were estimated for the finer sizes at whole-phi intervals by projecting the grain-size curve as plotted on probability paper and following the slope of the line. While this is a practical solution, it is much more labor intensive than the computer program utilized as part of this system and provided herein. Before 2005, grain-size data were extrapolated with the program CLAYES2K; currently, grain-size distributions can be extrapolated to 13 phi with the program GSSTAT.

The fine fraction, which has been stored in mason jars with distilled water or sodium hexametaphosphate solution, may be analyzed by pipette or EMPSAs. The EMPSA determines particle volume; the pipette method measures settling rates. The reason for performing the analyses should determine which method is used (see Comments section).

The pipette method consists of preparing a dispersed (by sonic probe), homogeneous (by vigorous stirring) suspension of the fine fraction, dilution with a 0.5% sodium hexametaphosphate solution to 1000 ml, and allowing the particles to settle in a graduated cylinder (Royse, 1970; Folk, 1974). The optimum sample is approximately 20 cm³ (about 15-g dry weight). With more sample, the grains interfere with each other during settling; with less sample, the effects of weighing errors on the analysis increase. Subsamples of 20 ml are withdrawn from the suspension at levels at 5, 10, or 20 cm below the water surface at standard intervals of time. Because all particles of a given equivalent diameter (based on calculations using Stoke's Law) will have settled below that level after a standard interval of time, the samples should contain only finer particles. These aliquots are either suction-mounted on pre-weighed filters and rinsed in distilled water or placed in pre-weighed 100-ml beakers. If pre-weighed beakers are used, the operator must remember to account for the weight of the dispersant (that is, sodium hexametaphosphate) when calculating the phi-fraction weights. The pipette is rinsed with distilled water after each extraction and the rinse water is also passed through the filter or drained into the pre-weighed beaker with the aliquot. To begin the analysis, start the timer as soon as the stirring rod emerges for the last time. At the end of 20 seconds, insert the pipette to a depth of 20 cm and withdraw the precise subsample (exactly 20 ml). Inasmuch as the subsequent analysis is based on this subsample, this is the most important single step. If excess liquid is accidentally drawn into the pipette, do not attempt to return it to the cylinder. Remove the pipette and discard the excess.

Continue the withdrawals at the specified time intervals and depths. The aliquots are dried and weighed, the weights are recorded on an analysis form, and the size distribution is calculated from the weight of sediment. The principle behind the computation is this: if the fine sediment is uniformly distributed throughout the entire 1000-ml column by stirring, and exactly 20 ml is drawn at each of the proper times, then the amount of mud in each withdrawal is equal to 1/50 of the total amount of mud remaining suspended in the column at that given time and at that given depth (that is, the amount of mud finer than the given diameter; all particles coarser than the given diameter will have settled past the point of withdrawal). The first withdrawal is made so immediately after stirring and at such a depth that particles of all sizes are present in suspension. By example, multiply the weight of the first withdrawal by 50, will obtain the weight of the entire amount of mud in the cylinder. Then withdraw a sample at a settling time corresponding to a diameter of 6 phi, and multiply by 50, then know that the product represents the number of grams of mud still in suspension at this new time, therefore the weight of mud finer than 6 phi. Similarly, can compute the weight percent at any size and obtain an entire distribution.

Because of the length of time required to complete a whole-phi interval analysis (16 hours, 24 minutes to 10 phi at 20^oC) and because Brownian motion interferes with the settling of particles less than 10 phi, the pipette method is typically used only to determine the silt/clay boundary in percent gravel-sand-silt-clay analyses or when a sample contains a significant amount of material smaller than 0.6 microns in diameter. This latter condition is important to consider because an EMPSA determines a size distribution based solely on the grain-size range it has been calibrated to analyze (for example, 0.062 mm >x>0.6 microns) and many low-energy and deepwater environments contain significant amounts of very fine clay and colloidal clay-sized material that are undetected by EMPSAs. The time needed to perform a pipette analysis may be reduced by placing the settling cylinders in a constant temperature bath and lowering the viscosity of the settling, medium by increasing its temperature (Table 2B, C).

The data from a pipette analysis can be converted into frequency or cumulative frequency percentages. Before 2005, these percentages were subsequently combined with the coarse-fraction data using the programs ENTRY to generate a complete grain-size distribution, statistics, and database files with the program GSTATM. Currently, fine-fraction data are imported into the GRAINSIZE.XLS spreadsheet. This Microsoft Excel spreadsheet generates normalized distributions by combining data from the gravel, sand, silt, and clay fractions obtained during typical sediment grain-size analyses.

To perform a silt/clay boundary analysis by pipette, aliquots are collected at 20-cm depth after 20 seconds elapsed time to determine the concentration of sample in suspension, and at a depth of 10 cm after exactly 2 hours and 3 minutes to determine the concentration of clay in solution. Because only two aliquots are collected, the sizes of these aliquots are usually enlarged to 50 ml to minimize analytical error. The percentages of silt and clay can be determined from the weight of sediment in the aliquots.

The Coulter Counter, a widely used EMPSA, permits easy, fast, and accurate analysis of fine-fraction size distributions. The instrument's optimum precision, as with all the above analyses, is realized only if the operator is conscientious and consistently follows established procedures. The following procedure is by no means complete and is intended solely as a set of guidelines. All operators are encouraged to familiarize themselves with the complete instrument manual to achieve the best results.

Suggestions for Analyzing Fine-Fraction Size Distributions include:

a. Shake the fine-fraction portion of the sample in the mason jar vigorously, sonify the bulk sample for about 2-4 minutes (probes typically work much better than baths), and use a stirrer to completely suspend the sample. While the sample is being stirred, use a disposable pipette to transfer a subsample, drawn in a sweeping motion from the top, middle, and bottom of the mason jar, to the clean beaker of filtered electrolyte in the sample stand. Never allow the tip of the pipette to touch the side or bottom of the mason jar while subsampling because contact can cause the pipette to break.

b. Make sure bubbles are cleared initially from the aperture. This will lower system noise.

c. If the tube clogs, brush the aperture opening with a sweeping motion. If the tube is still clogged, clear the tube in a distilled-water ultrasonic bath. If this fails, replace the distilled water with 50 % nitric acid.

d. After the analyses are completed, rinse the sample stand, sample compartment, and counter top with distilled water and dry them with paper towels.

e. When the analyses are complete, remove the 30-micron aperture tube, place the 200-micron aperture tube on the sample stand, and adjust settings for the 200-micron analysis.

f. Keep the aperture tubes in a cleaning solution (such as, Coulter Clenz) when not in use.

g. With proficiency, an operator can speed up the analytical procedure by sonifying the next sample while a sample is being analyzed by the Multisizer. However, always sonify the sample for at least 2 minutes prior to analysis.

Modifications Since 2005

During 2006, the sedimentation laboratory at the Woods Hole Coastal and Marine Science Center replaced the Coulter Counter Multisizer IIe with a Beckman Coulter Multisizer 3. Commercial software provided with this new unit combines the data from the 200- and 30-micron aperture tube analyses into user-defined, whole-phi bins as relative percents. These fine-fraction data are subsequently imported into the GRAINSIZE.XLS spreadsheet. This Microsift Excel spreadsheet generates normalized distributions by combining data from the gravel, sand, silt, and clay fractions obtained during typical sediment grain-size analyses. In 2006, the laboratory also switched electrolytes, replacing sodium hexametaphosphate with a 4% solution of kosher salt. Using kosher salt is important because normal table salt contains anticaking additives that are insoluble. These additives can foul filters and add detectable particles to the grain-size analysis.