Desert plants yield climatic secrets

Aisling Irwin and Juliet Vickery report from the British Association for the Advancement of Science in Birmingham.

Secrets about ancient desert farming, which will be a new source of information about climate change through history, can now be unlocked because of a new technique developed by a British scientist. The information will be particularly useful because, up till now, climate change has been measured, for technical reasons, mainly in temperate and arctic parts of the globe.

Alix Powers-Jones, of the McDonald Institute for Archaeological Research in Cambridge, has found a way of tracing the practices of Mesopotamian farmers from a signature left behind by the plants they grew after they had decayed.

Using a technique that was developed simply to identify ancient plants she can now "read" the remains to find out how much they were watered and therefore irrigated. Building a picture of the irrigation will reveal how much water was around. By drilling cores through the soil, scientists plan to work out the story of the availability of water in the desert over thousands of years. From this they will have a wealth of new information about climate change in arid areas over the centuries.

The technique hinges on the fact that when plants die and decay they leave tiny silica casts of their cells in the soil. This is because the water they imbibed while living contained silica which is laid down on the plant cell walls and then, after the plant dies, remains in the soil, resistant for millions of years to the agents of destruction.

The crucial step has been to realise that the shape, size and structure of the silica deposits, known as phytoliths, is related to the amount of water that the plants took in - and is therefore governed by how wet the surrounding soil was.