Objectives

• To separate the fine, characteristically cohesive fraction of the sediment from the non-cohesive sand and coarser fractions, by wet sieving through a 63m m mesh

• To generate a series of identical sub-samples of the <63m m fraction of the sediments, permitting a range of analysis types to be carried out on comparable materials.

Equipment and Reagents

0.5litre Pyrex beakers

63m m and 2mm mesh sieves of nylon or stainless steel construction

Shallow stainless steel bowl just containing the sieve

Soft sieve brush of non-contaminating materials

A kitchen, hand-held, small, high-speed liquidiser/stirrer plus accompanying receptacle, minimum 0.7litre minimum capacity, graduated for volume measurement.

3mm internal diameter flexible tubing + stainless-steel wire for clipping the tube to the foot of the stirrer

Balance capable of weighing to 0.0001g

60 ml syringes

Large number of 150-200ml plastic containers with lids

Washbottle/spray with tap water

Temperature-controlled oven, preset to 75^oC + 5

Paper plates

Log sheets

A supply of tap water

SIEVING: (see Figure)

1. About 250g (dry) 350g (wet) of sediment is the maximum that can be processed. Bagged samples of larger amounts should be tipped onto a large plate and mixed thoroughly using a stainless steel spoon (sediment should be wet but not have free water). The correct sized subsample should be removed from the whole with the spoon.
2. Place the (defrosted) sediment sample in a 0.5 l glass beaker. Add tap water to the 0.5litre level. Allow to stand to soften. Mix well with a spatula.
3. Place sieve in the stainless steel bowl. Pour sediment slurry onto the sieve, washing beaker clear of sediment with washbottle. (NOTE if sediment contains large stones/shells, place the 2mm sieve over the 63m m sieve and wash the sample through this too. Spray the 2mm sieve clean, and put to one side. The contents should be added to the contents of the 63m m sieve after wet sieving has been completed. This action is to prevent damage to the fine sieve mesh.)
4. Using the sieve brush, work the material on the 63m m sieve. After a while start a gentle lifting and lowering motion (mesh not breaking the surface) to encourage the water and fine sediment to pass through. After about 5 minutes of this most of the fine sediment should be in the bowl, and water should pass quite readily through the sieve when it is lifted.
5. Lift the 63m m sieve above the level of muddy water in the bowl, and with water from the washbottle rinse the final traces of mud from the sand on the sieve, tilting the sieve in various directions. Finally tilt the sieve and wash the sand to one side of the sieve. NOTE do not use more than 200ml of wash-water in total during steps 3 to 7.
6. With the washbottle rinse the sand from the 63m m sieve onto a labelled paper plate. Add the contents of the 2mm sieve if necessary. Place in the oven to dry. When dry place in a labelled plastic bag.
7. Transfer the contents of the stainless steel bowl (the fines) to the stirrer receptacle, using the washbottle to flush across all traces of sediment.

FINES SUB-SAMPLING:

8. Record the total volume of slurry in the stirrer receptacle, to the nearest 10ml.
9. Place the stirrer into the slurry, with the foot firmly pressed onto the base of the receptacle. Stir for 15s to assure an even suspension. Stop stirring.
10. Attach a syringe to the tubing coming up from the foot of the stirrer. Hold this in the same hand as the stirrer (see photo). Stir again for 5 seconds then, whilst stirring continues, withdraw exactly 60ml of slurry. Stop stirring. Quickly check and adjust if necessary the level in the syringe (+ 2ml is acceptable). Detach the syringe.
11. Discharge the syringe into a labelled 150-200ml plastic container. Repeat the process so the container contains 120ml of slurry.
12. Fill up to five 150-200ml containers in this fashion, then discard the remaining slurry. If there is not much fine sediment in the sample, pour/washbottle the remaining slurry into a sixth container. NOTE: One of the containers needs to be pre-weighed to 0.01g [(ii) – see next section].
13. Clean thoroughly the sieving and stirring apparatus. With the syringe and tube flush through clean tapwater.

NOTE: Total dry sediment weights in the 150-200ml containers (determined as part of carbonate/organic carbon analyses – see below), together with the total volume of slurry recorded (8. Above), are used to determine the total dry weight of sediment finer than 63mm.

The five sub-samples of the fine fraction need to be handled as follows. Each subsample should provide a minimum of 10g of dry sediment.

(i) Trace metals. The container should be left to settle, covered up and in a cool place, until the supernatant water is clear. This can then be sucked (with syringe) or siphoned off and the clearly labelled sample sealed and kept frozen for dispatch to a  laboratory.

(ii) Carbonate analysis and Organic Carbon analysis. The container should be placed in the oven at 75^oC and evaporated to constant dry weight. The weight of sample plus container should be recorded (to 0.01g). The sample should then be sealed, labelled and stored.

(iii) Particle-size (pipette) analysis. The container should be left to settle, covered up and in a cool place, until the supernatant water is clear. This can then be sucked or siphoned off and the clearly labelled wet sample sealed and stored in a cool place.

(iv)&(v) Spare (frozen). The containers should be left to settle, covered up and in a cool place, until the supernatant water is clear. This can then be sucked or siphoned off . The contents of the two containers can then be amalgamated and the clearly labelled sample sealed and kept frozen.

If it is obvious that there will be a shortage of fine material for any of the above analyses the spare sample can be used as supplements immediately.