As some of the MOLAR laboratories may not be suffiently equipped with appropriate instruments, two methods for grain size determinations are recommended within the following GrainSizeManual.

Grain sizes of unconsolidated sediments can very basically be divided into three different fractions (all grain size values in this manual are given in [m], which describes the largest diameter of a given particle):

a. Very fine particles, showing particle diameters of

<2 m = clay fraction

b. medium particles, showing particle diameters of

2 to 63 m = silt fraction

The silt fraction may be further divided into three sub-fractions:

fine silt = 2 - 10 m

medium silt = 10 - 20 m

coarse silt = 20 - 63 m

c. Coarse particles, showing particle diameters of

>63 m = sand fraction

For any technique of grain size determination sediment samples have to be either wet or freeze dried. Using freeze dried samples makes it a bit easier at the end, to calculate percentages of different size fractions.

There is one paramount rule in the grain size business: Never use oven-dried or air-dried samples for grain size analyses. Grain size results will then always be arbitrary and wrong.

For most lake sediments it is appropriate to use bulk sediment samples for grain size determination, e.g. without eliminating the organic material from the samples prior to analysis.

Commonly sample preparation is simple for most of the different methods: An aliquot (1 to 5 g) of the wet or freeze dried sample has to be dispersed into a Calgon solution. This is to avoid koagulation and the formation of larger flocs of smaller particles. To enhance homogeneous dispersion of particles, the suspension should subsequently be placed into an ultrasonic bath or be homogenized with an ultrasonic stirrer.

3.1 Sieving technique (particle fractions of <10 m to >63 m)

This basic, low cost grain size determination technique requires three to four measuring sieves, a centrifuge and an ultrasonic bath or an ultrasonic stirrer. If samples are very coarse, one may use additional sieves for sand fractions (e.g. mesh sizes of 125 m, 250 m etc.). This 'quick and dirty' technique, does not allow to determine the amount of clay fraction (<2 m) in a sample; additionally, it is rather inaccurate determining particle fractions between 10 to 30 m (more accurate determination of small size fractions without automated instrumentation could be performed by time- and labor-intensive methods such as pipette- or Atterberg-cylinders; quicker analyses with an automated sizing method see below).

This sizing technique, based on the principles of Stokes's equation on vertical particle velocities in aqueous media, uses a Sedigraph, which is an automated, microprocessor controlled particle size analysator, to determine continuous particle distributions between 0.5 m and 100 m. Required time for an analysis of size fractions between 1 m and 63 m is about 20 minutes. Results can be directly compared to pipette- and Atterberg-techniques. To compare results with Laser particle sizer, values have to be transformed from mass % (Sedigraph) to volume % (Laser instruments).

Results from Sedigraph (or similar automated techniques) should be graphically presented both as histogram and as sum curve of the grain size distribution.

Moreover, the results from Sedigraph (or similar automated techniques) should be used to calculate the following size fractions and some statistical values of the grain sizes of an individual sample:

size fractions: <1 m, 1-2 m, 2-4 m, 4-8 m, 8-16 m, 16-32 m, 32-63 m, >63 m (or additional sand fractions if available)

statistical values: median and mean diameter, sorting, skweness.

It is essential that the samples taken for grain size analyses are kept wet or freeze-dried until grain size analyzes is performed. Again (see above): air-dried or oven-dried material cannot be used for grain size analyses. Just to add: grinded samples can also not be used.

In order to obtain homogenized and comparable data of grain size analyses from the different MOLAR laboratories I recommend that a list of grain size measuring techniques is compiled, where each Institute states, which technique is available and/or can be used for the project. Keep in mind that grain sizes should be measured for all the samples, which are taken of cores from the different MOLAR-sites.

Table 1 The most common size fractions

Sizes of sediment particle fractions (diameter values)