Archaeology

Pollen - vegetation and climate history

Peat face

The surface of a peat bog is not only home to the plants and animals that live there, it also acts as a giant collecting tray for any small particles that fall on it. To the untrained eye, a section through a peat bog may all look much the same - some bits darker or wetter than others, but really just a dark brown mess with bits of moss or twigs poking out. But if a scientist prepares a vertical column of peat in a special way and examines it under a microscope they can identify important historical events from the particles in the peat. The most important of these particles are pollen and volcanic ash.

Scientists can date the remains of the plants that once grew on the peat bog using a special technique called carbon -14 dating. This makes it possible to say not only what sort of plants were in the countryside (from pollen analysis) at any time, or that a volcano erupted in a distant country (from tephra analysis), but also what year it happened in.

Pollen

Pollen from oats, a non-native plant, showing that cereal cultivation was being practised.

As part of their reproduction, most plants produce microscopic pollen or spores that the wind blows through the air and deposits on the ground. Pollen and spores have special characteristics that make them important when scientists examine a cross-section through peat:

• pollen often has a distinctive shape, so it is easy to identify the parent plant
• pollen particles are very small and can travel long distances
• pollen does not disintegrate very easily, so survives unaltered for many thousands of years

So, if the countryside around it is covered by an oak wood, a peat bog will collect lots of oak pollen. As the peat bog grows, the old surface gets buried, along with the oak pollen and any other pollen and spores present in the air. If scientists examine that layer of peat in the future, it will still have the oak pollen preserved in it, pointing to the fact that there was a lot of oak in the countryside at that time, even if the wood has now completely gone.

The pollen in the uppermost parts of lowland raised bogs, often just the top metre, contains the history of the climate and the development of the recent landscape of Ireland. This surface peat is the most vulnerable to damage from drainage and removal by mechanised cutting.

The pollen story of life in Ireland after the last ice age

Click on a time period above, or here, to read how pollen preseved in peat reveals the vegetation of the age.

Volcanic ash

As well as pollen and spores, dust also settles on the peat bog. This may not seem very exciting, unless the dust is very distinctive such as ash from erupting volcanoes, otherwise known as tephra. Volcanic ash was frequently blown over Ireland from volcanic eruptions in Iceland and preserved within the peat bogs. Scientists can identify the minerals within the ash like a volcanic fingerprint, and can link them to the volcano they came from. For eruptions that have occurred since the advent of historical records, this often allows scientists to pinpoint the actual year of the eruption, and therefore a precise date for the ash layer in the peat bog.

Eruption of Stromboli volcano in Italy		Volcanic shard from the Hekla 4 eruption, which took place in Iceland 4300 years ago

It has previously been difficult to date the most recent peat, but new tephra studies have made this easier. Volcanic ash from historic eruptions in Iceland has been detected at lowland sites. Ash in some peat bogs is known to have originated from the eruptions of Hekla in 1104 and 1510 with further finds of ash from the eruption of Oraefajokull in 1362. These volcanic events often led to cooler, wetter periods of weather in Europe, resulting in bad growing seasons and even famine, all of which is recorded in the peat. As with all such studies on peat, it is only possible to get this amount of information from it if the bog surfaces remain intact.

Using the peat record to understand the future

We have seen that the peat contains the history of climate change and the impact of people in Ireland. We know that people will bring about further changes to the world in the future. What we do not know is how quickly or how dramatically those changes will happen, nor how quickly we will be able to adapt to new environmental conditions. However, as we have seen we already have this sort of historical information in the peat record. Not only is it important to conserve peatlands for the plants and animals that live there, it is also important to keep them as we would any library containing valuable knowledge. Continuing to study peat as a record of the past will help us understand what the future may have in store for us.