Assessment of human activity thanks to pollen trapped into peat (Chivres-en-Laonnois, Aisne, France, 2007)
How peat bogs can serve as records to study our history
Pollen grain of an Asteraceae
Peat bogs are constituted of dead and living organic matter that has accumulated over several centuries, even millenaries! Because of anoxic and damp conditions of peat, organic matter doesn't decompose and piles up, trapping and mummifying plants, vegetation decay, animals and even sometimes human beings (Mummies found in Northern Europe)!
Among all things caught in peat, pollen are found in high quantities and very well conserved. As peat accumulates with time, the pollen found at a certain peat depth indicates what the local vegetation composition was at the time of its burial. As a result peat bogs constitute genuine, trustworthy and rather accurate history archives through pollen grains. Nowadays we are able to dig peat carrots, identify pollens and reconstitute the vegetation history, and thus assess human activities that have an impact on this vegetation (deforestation, introduction of some non-endemic species...).
I was willing to reproduce this method to assess human activities over time in a peat bog near Laon (Aisne, France) as the year project of my sophomore year at Lycée Clémenceau.
Among all things caught in peat, pollen are found in high quantities and very well conserved. As peat accumulates with time, the pollen found at a certain peat depth indicates what the local vegetation composition was at the time of its burial. As a result peat bogs constitute genuine, trustworthy and rather accurate history archives through pollen grains. Nowadays we are able to dig peat carrots, identify pollens and reconstitute the vegetation history, and thus assess human activities that have an impact on this vegetation (deforestation, introduction of some non-endemic species...).
I was willing to reproduce this method to assess human activities over time in a peat bog near Laon (Aisne, France) as the year project of my sophomore year at Lycée Clémenceau.
Peat sampling
My group went to Chivres-en-Laonnois in November. We picked a spot not to close to the water and that was not subject to any human activity that could have affected the way peat accumulates (plowing, cattle grazing, swathing...) or that could have mixed pollen grains between levels. Rush and other grass plants were cut off, and peat was sampled using a hand core drill. Each peat carrot (50 cm) was immediately wrapped in foil and labeled to prevent desiccation or decomposition. We sampled peat smoothly over 110 cm, but water resurgence stopped us from digging further, because peat was mixing.
Processing samples
Every 10 cm on each carrot, a small amount of peat was sampled at the core and put inside a vial. Soap, KOH were added to the mixture to separate pollen grains from other materials. The solution was stirred, heated (80°C), and filtrated. Then, the mix is centrifuged twice to only keep solids. Then, they were transferred onto a microscope slide with glycerin and fuschine for conservation and coloration.
Pollen identification
Pollen of pine tree (Pinophytes)
The 10 slides of each peat level were thoroughly examined and pollen grains were identified with determination keys.
Results and discussion
We were able to build pollinic charts like in the right picture. Thanks to literature, peat levels were associated to dates (in years BP= before present), knowing the peat accumulation rate of the bog. We assumed that there was a dramatic change in the composition of the vegetation over nearly 2000 years in the region: trees (in purple) have decreased by 30%, giving way to more herbaceous species nowadays (50%). In spite of possible peat disturbance, material and method errors or far-traveling pollen contaminations that were low, it shows that unprecedented vegetation and landscape vegetation occurred. Literature confirms that this location has been under human activity for a long time. It corroborates that Man deforested for many purposes (heating, building, but mainly to make way for arable land).
A more detailed diagram displaying several plant species also showed that local populations modified the ecosystems by introducing new species (yew, lilies...) for ornament or reforestation (more ash or williow trees, rather than oaks and beeches).
A more detailed diagram displaying several plant species also showed that local populations modified the ecosystems by introducing new species (yew, lilies...) for ornament or reforestation (more ash or williow trees, rather than oaks and beeches).