LABORATORY PROCEDURES

Preparation

If the whole sample will not be used during analysis, it is typically split using a micro-splitter, cone and quartering, or random bulk (Krumbein and Pettijohn, 1938). Sample size, homogeneity, and precision that is required govern which method is selected for analysis. As a general rule, the larger the analyzed sample the more accurate the grain size analysis. However, unless a sample contains gravel, a 50-g split is usually sufficient. Larger samples are more time consuming to analyze and do not provide significantly more reliable results. If gravel is present, the sample must contain enough material to give adequate representation of the largest sizes present. For example, approximately 250 g would be needed to texturally analyze a sample containing -2 phi materials.

Place the split wet sample in a pre-weighed 100-ml beaker, weigh, and record the weight on a "grain-size analysis" form. The beakers should be scribed with a unique number and, therefore, should require no additional labels that would alter the weight of the beaker. Gross wet-sample weight minus the weight of the beaker gives the net wet-sample weight. Dry these samples in a convection oven at, or slightly below, 100^o C. This temperature will only drive off unbound water and should not affect the grain size. When the samples are dry (overnight is usually sufficient), place them in a desiccator to cool. Weigh the samples as they are removed from the desiccator or the sample will absorb water from the air and their weights will be less accurate. Gross dry-sample weight minus the weight of the beaker gives the net dry sample weight; net wet sample weight minus the net dry sample weight gives the water weight. The weight of salt can be calculated from the salinity and the weight water. The net dry-sample weight minus the weight salt gives the corrected sample weight. Inasmuch as the weight of fines are typically determined by subtracting the coarse weight from the net sample weight, the salt in marine samples must be determined to prevent overestimating the fines present.

Whole or fragmented calcite secreting micro- and macro-organisms can bias the grain size distribution if they occur in significantly high concentrations. Because biogenic carbonates commonly form in situ, they usually are not considered to be hydraulically representative of the depositional environment from a textural standpoint (Reineck and Singh, 1980). Their presence alters the textural data and complicates interpretation. If biogenic calcite or aragonite are pervasive, these minerals may be selectively dissolved from the bulk sample using cold, dilute (10%) hydrochloric acid (HCl) or a 5-pH NaOAc buffer (Jackson, 1956; see section on insoluble residues below). After the carbonate has been dissolved, the HCl or NaOAc can be removed with multiple decantations or centrifugations using distilled water to remove the salts and acid and (or) buffer. If limited to the gravel fraction, it is often easier to manually remove the fragments of bivalve shells and other biogenic carbonate debris (see below) rather than treating the whole sample. Care must be taken to use dissolution techniques only when detrital carbonate is absent.

The removal of organic matter may be necessary to achieve complete dispersion of the clay and, in sediments with an elevated organic content (>3%), to prevent the organics from being counted as part of the sample, which would bias the grain-size distribution. The sample is placed in a 600-ml beaker and a small volume (~10 ml) of 30% hydrogen peroxide is added. The sample is stirred and, if necessary, water is added to slow the reaction down and prevent bubbling over. More hydrogen peroxide is added until the dark color of the organic matter has largely disappeared.  The sample is then washed three times with a NaOAc buffer of pH 5 and once with methanol to remove the remaining released cations (Jackson, 1956).

If the samples have been treated with acid or hydrogen peroxide, they should be re-dried and re-weighed to recalculate the post-treatment net sample weight. Subtracting this value from the original sample weight will determine the weight of the removed constituent.

Soluble salts and exchangable polyvalent cations can be removed by decantation or centrifugation with distilled water to prevent flocculation of the sediment and to give effective dispersion during pipette analysis. However, this decantation may result in partial loss of the colloidal-clay fraction.

Wet sieve the sample through a 62-micron, American Society for Testing and Materials (A.S.T.M), number 230 sieve to divide the sample into coarse (sand and gravel which are retained on the sieve) and fine (clay and silt) fractions. Distilled water is used during wet sieving if the fine fraction comprises greater than 5% of the sample. Samples will disaggregate more easily if allowed to soak in a small amount of distilled water or electrolyte solution prior to wet sieving. An approximately 3-5% solution of sodium hexametaphosphate (800 g of purified (NaPO₃)₆, 80 ml of formaldehyde to retard biogenic growth, and 20 liters of distilled water) is recommended for wet sieving if an EMPSA (for example, Coulter Counter or Elzone) will be used in the fine-fraction analysis and only small amounts of fines are present. This solution acts as an electrolyte, which is necessary when EMPSAs are used. Seawater may also be used for sample soaking and sieving purposes and as an electrolyte for the Coulter Counter as dictated by the objectives of the specific project. A 0.5% sodium hexametaphosphate solution should be used if the fine fraction is to be analyzed by pipette; at this concentration, the sodium hexametaphosphate acts as a dispersant. The solution must be filtered to 0.2 microns; a sequential submicron filtration system combining 5, 0.45, and 0.2 micron filters works well. A rubber policeman and a squeeze bottle of distilled water or the solution provide the best sieving results. The coarser sand and gravel fractions are retained on the screen while the finer silt and clay fractions are collected in a catch pan or mason jar. Wash the coarse fraction into a pre-weighed 100-ml beaker and seal the fine fraction in the mason jar. Wet sieve, using only enough liquid to fit in a single 32-ounce mason jar to prevent any possible fractional biasing of the fine fraction.

Modifications Since 2005

The “Grain-size Analysis” form is no longer used. Weights are now entered directly into a spreadsheet (GRAINSIZE.XLS) that is the input file for the processing program GSSTAT.