Preparing Sediment Samples for Diatom Analysis

A. Mass Calculations

The calculation on the mass of the sample will enable you to evaluate diatom valve density.

1. Weigh about 0.5 g of sample by deducting the weight of a petri dish from the weight of petri dish and sample.

2. Subtract the proportion of water in the sample as determined from % moisture determinations on another portion of the sample. Drying the sample for diatoms may unnecessarily fracture the valves.

B. Acid Treatment

1. Add ~ 30 ml of 10% HCl to the sample in a small beaker. If the sample is from an area where the sediments are known to be carbonate-rich, use a larger beaker and be careful as a violent reaction may cause the evulsion of the sample from the beaker with HCl.

2. Simmer the beakers for 1 - 2 hours depending on the carbonate content making sure that the water level is maintained and the HCl does not become overly concentrated.

3. Allow the sample to cool and top up with distilled water.

4. Thoroughly wash 3 times. Each time allow the sediment to settle for 6-12 hours and decant 50-70 % of the volume, topping up with distilled water.

C. Peroxide Treatment

1. Repeat 1 - 4 above with 10% H2O2. As you ought to be particularly careful with HCl in carbonate-rich sediments, you ought to be particularly careful with peroxide in organic-rich sediments.

D. Slide Preparation

Make the washed solution up to a volume which will provide an adequate density of diatom valves on the slide. This will generally come from a solution which could be termed 'slightly muddy'.

Stir the solution so that the sediments are evenly spread. Add 400 l of each sample to 2 pre-cleaned coverslips and place them in a protected place to allow the solution to dry onto the coverslip slowly.

Once completely dry, warm the frying pan to a low heat, place microscope slides on the heated tray and smear two drops of NAPHRAX mountant on the slide - one in the middle and one at one end. Your sample label should be at the other end. Place the coverslips on the NAPHRAX, the side with the sample face down.

Press down gently on the coverslip with a stirring rod or forceps to remove any air bubbles and to make the diatom valves sit in valve view.

Allow the slide to cool for 1-2 hours before examining it under the microscope.

THE PREPARATION OF MODERN SAMPLES IS AS FOR B - D ABOVE. LESS TREATMENT TIME IS NECESSARY UNLESS THERE ARE LARGE AMOUNTS OF SEDIMENT IN THE SAMPLES