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Irish bog produce yet another treasure

One of the most exciting new techniques in the dating of prehistoric events recorded in our peat bogs is tephra analysis. Tephra is literally glass. It can be extracted from peat samples by burning off the organic peat. Dr Rosaleen Dwyer has discovered seven tephra layers in the blanket bogs of Mayo. She describes the study of volcanic ash layers in blanket bogs and the significance of the find.

The Bog Treasure Trove
Over the years, Ireland's bogs have yielded up many treasures such as gold, ancient trackways and farms, and even human bodies. However, there is another range of bog treasures which can only be seen with the aid of a microscope. Included in this list are pollen grains, charcoal fragments, fungal spores, and the remains of tiny one-celled animals or testaceans. This microscope archive has allowed us to build up a detailed picture of past changes in Ireland's climate and vegetation.

Recently, another microscopic treasure has been discovered, adding to the many reasons why bogs are such unique and special habitats. Layers of volcanic ash have been detected in Irish bogs, layers so fine that they cannot be seen with the unaided eye. Where has this volcanic ash (also known a tephra) come from and how has it ended up in Irish bogs?

Micro-shards of volcanic glass from the Helka eruption preserved in blanket peat, Co. Mayo. They are up to 4,000 years old. (Photo Rosaleen Dwyer)

Explosive Events
Erupting volcanoes spew out huge volumes of gas, dust, lava, ash and rock. During particularly explosive eruptions, gas, dust and ash can travel high into the atmosphere. They can mix with the air masses which circulate the world and can thus travel great distances across continents and oceans. In severe cases, this suspended veil of dust may actually affect weather patterns by acting as a barrier to the sun's rays. The material eventually comes back down to earth in falling rain. When it falls onto peatland, the ash becomes incorporated into the growing bog, waiting to be discovered.

Volcanic Fingerprinting
Tephra is really a type of glass, composed mainly of silica, aluminium, sodium, iron, and other trace elements. Each volcano produces tephra with specific percentages of these elements, thereby producing its own chemical fingerprint by which its ash can be identified. By extracting and analysing the tephra found in our bogs, we can thus determine the parent volcano and perhaps even date the eruption.

So far, all of the tephra which has been analysed from Irish peat matches the chemical profile of several Icelandic volcanoes. So, for thousands of years, air circulation patterns over the North Atlantic Ocean have been carrying Icelandic ash across the seas to Irish bogs.
One particular Icelandic volcano, Helka, has erupted numerous times. Tephra dating to around 4,000 years ago (known as Helka 4), has been found in a number of Irish and British bogs. Another tephra, Helka 1, has been attributed to its eruption in 1104, while the eruption in 1362 of another volcano, Oraefajøkull, has also been detected here.

What Can Tephra Tell Us?
The study of tephra has produced some exciting developments. Up to now, we have relied mainly on radiocarbon dating to date our bogs. Unfortunately, this is not a precise technique. By identifying and dating the parent eruptions of tephra layers in our bogs, we can date, much more accurately, the peat in which the ash layer has been preserved.

There is another exciting aspect to tephra. We know from changes in pollen records that the climate in Ireland became cooler and wetter about 4,000 years ago. Is it just a coincidence that the ash layer Helka 4 was also deposited around this time? Or does it suggest instead that the eruption of a volcano in Iceland could have such a dramatic effect that it altered the climate here in Ireland? Scientists disagree on the effect that volcanic eruptions can have on our climate so the debate is only beginning..........

Dr Rosaleen Dwyer,
Botany Department, TCD.

Map showing the location of Ireland in relation to Helka, the site of the eruption in Iceland. Volcanic ash from this eruption has been identified in Irish bogs. It provides an important dating tool. (Photo: Rosaleen Dwyer)