This week (21st May 2014), Alastair Crawford and Mark Grosvenor ventured onto Dartmoor in search of evidence of past fire events.
Peat bogs provide a great archive of environmental history as they build up through time. Some bogs can be several meters deep with a layer of living vegetation overlying decomposed bog vegetation – known as peat. Even though a lot of the bog plants degrade over time, certain macro-fossils (small identifiable things) and micro-fossils (really small identifiable things) can be preserved. These can include plant fragments, pollen, insect remains, and charcoal. The length of the record depends on the environmental history of the location – in the UK the oldest peat bogs cover a large proportion of the time period known as the Holocene (since the last ice age). If environmental conditions are right, a continuous record of history can be preserved which is perfect for researchers like us to examine the earth system.
Unsurprisingly, in the wildFIRE Lab, we are interested in identifying the charcoal particles in the peat. Micro-charcoal is dispersed by the wind during burning events (whether natural or started by humans), with a fine covering falling on the surface of the peat. If the charcoal is not immediately washed away, it is likely to be preserved in the peat as fresh material grows above it. More and more vegetation grows above it, and compresses the degrading vegetation underneath, but slowly over thousands of years the bog grows, with the charcoal fragments in it. The ancient peat samples can then be processed (anything from sieving to using acid to digest the plant material) before being analysed under a microscope. A range of sizes may be present, which can help to tell how far the site of burning was (i.e. large, heavy particles are likely to be blown a shorter distance) and in some cases, it is possible to identify the type of vegetation being burned (such as grass, or wood).
The samples we collected will be analysed by Alastair for part of his PhD research. He will be looking at the morphological features of the charcoal. We collected the samples using what is known as a ‘Russian peat corer’ – it is a chamber on the end of some metal rods which are pushed into the peat (to whatever depth we need) and then rotated to capture the sample. We take a series of these to piece together the full record. Another method could be to dig trenches to expose the layers of peat, but the advantage of using a corer is that there is no lasting damage to the bog.
The samples were taken at Shovel Down, just above Fernworthy Forest on the north eastern edge of Dartmoor. The site has been investigated before for its environmental history (by Ralph Fyfe and others) so we know there should be charcoal at the site. Shovel Down is also notable for a series of ceremonial monuments constructed around 4000 years ago including stone circles and standing stones (megaliths). A number of ancient field boundaries are still visible as stoney ridges (known as ‘reaves’). Whilst archaeology can provide insight into human activity, the environmental record preserved in the bogs provides another layer of detail about how peoples environment was changing around them, or whether they were altering the landscape themselves.For more information on all things bog, check out bogology.org
Blogged by Mark Grosvenor