Monday, 7 January 2013

Fun with phytoliths, part 1

Weighing out: the container is zeroed and the exact weight of the sample recorded
New year, new samples! Well, newish, they were collected in the summer of 2012 but only made their way to me a few weeks ago. Today is my first day on site at the University of Edinburgh, where I am hoping to set up laboratory facilities for microfossil processing. In the meantime I managed to speedily process this last batch at York. Following drying the samples out in the oven, the next few stages involve removal of different fractions of the soil/sediment. Soil is made up of a range of organic and inorganic components - so to get at the phytoliths, we have to remove any non-phytolith material. It is actually quite an easy lab method, but rather time consuming as there is lots of drying and transferring into different sized containers! As with all sample preparation, the samples are weighed after drying so we can quantify how much material has been processed. The amount of sample that we choose to process depends on how phytolith-rich we expect them to be. Some of the material I have worked on from Catalhoyuk is packed with phytoliths, so we don't need to process too much to get a large number of cells. For these samples, I suspect the concentration will be much lower, and so have weighed out around 1g of each.

Minor/no fizzing with HCl - not much carbonate
Stage 1 - removal of carbonates. This stage is especially important in ashy samples which contain a lot of calcitic material, or with samples from areas with carbonate-rich soils. We add 10% hydrochloric acid to the tubes, and this dissolves any carbonates. Fun chemistry equation, rusting in the back of my brain from chemistry at school:
CaCO3(s) + 2 HCl(aq) --> CaCl2(aq) + CO2(g) + H2O(l)

These samples are from cultural layers in the Baltic, and so the amount of carbonate in them is fairly small. You can see a few bubbles on the sample second from the bottom - if the samples were rich in carbonates, they would all be very bubbly as the acid reacts with the carbonates to produce carbon dioxide. Sometimes they fizz so much the bubbles come all the way up the tube. After the fizzing has stopped, the acid is diluted by adding some distilled water, then removed by centrifuging the sample. It's a good idea to rinse the samples a couple more times to make sure all the acid is gone.

Centrifuge - don't forget to balance
For those of you who might not know what a centrifuge is or how it works, the idea is very much like a washing machine (but without the washing bit). The samples are placed in a holder, which then spins at a very high speed, so that all of the dense/solid material is seperated from the less dense/liquid, and forced into a solid pellet at the bottom of the tube. The liquid can then be poured off, and the sample stays in the bottom of the tube. Centrifuges come in all sizes - from tiny benchtop size to ones that are as big as a washing machine and sit on the floor.

Check back soon for the next thrilling installment: Stage 2 - removal of clays!

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