Which is the sound you get when you add hydrochloric acid to test tubes of soil. At least you do if there are lots of carbonates in the soil. An effervescent reaction is produced, sometimes to the point where it threatens to effervesce right out of the tube. We are easily impressed by anything that looks like B movie science and nothing says ‘but Doctor you are meddling with things you can’t possibly comprehend!‘ better than a nicely bubbling test tube.
Martina has started working on examining the soil samples from New Laund for their pollen content. The bubbling test tube thing meant that she started the work with an audience of about six staff and postgrads looking on. In this photo you can see the second set of samples having their doses of hydrochloric acid.
Plant pollen is extremely hard-wearing stuff. Given the right soil conditions it can survive for thousands of years. This means that by looking at the concentration of different kinds of preserved pollen from a soil sample, or better still from a series of samples, you can build up a picture of what was growing in any area in the remote past. These samples come from the deep sequence of silt and clay deposits inside Temple Cave, where we excavated in 2011.
We took a series of samples all the way down this 1.4 m deep section. So Martina’s work ought to be able to tell us about what was growing around the cave and how that changed through time. As well as the caves we have already dug, New Laund Farm has many sinkholes – the technical term for them is dolines – which are vertical solution-holes in the limestone. These have filled up with sediment over the centuries and we are hopeful that they will also give us good long sequences of preserved pollen.
Martina’s plan, if she finds good pollen preservation in the samples she is looking at now, is to build up a series of sequences across the landscape using caves, dolines and archaeological features like the ditches of the New Laund enclosure. This should allow her to map the changing vegetation of the whole area through the centuries. What is particularly important here is that changes in vegetation, at least after the end of the last Ice Age, are usually the result of things people did. Pollen evidence like this has the potential to let us study the past environment as another artefact, something that prehistoric people made, in the same way that we study the distributions of their flint tools or pottery.
Of course, the counting of the pollen grains is not anywhere near as simple as I have made it sound. A detailed knowledge of what different plant species pollen looks like is obviously needed but before Martina can even begin to prepare a microscope slide the samples need to go through a complicated procedure to clean them. I have only seen bits of this in action and have almost certainly missed some important nuances; so pollen scientists should look away now.
The hydrochloric acid which made such a good spectator sport is just the first stage. This removes carbonates. Then there is a dose of sodium hydroxide which gets rid of clays. The whole process also involves a lot of washing in distilled water, agitating the tubes in the whirling device in the picture above. These tubes are then spun at 3000 rpm in a centrifuge so that the suspended clays and silicates can be poured off leaving, hopefully, the pollen behind.
Our samples are very full of clay minerals, which means they are taking a very long time to clean up. By next week we should know whether we have good enough pollen survival or not, watch this space…