Ancient oak rings can tell what happened long ago


Irish scientists are tracing Ireland's environmental history using dendrochronology, or tree-ring dating. The technique has the ability to show what were the most catastrophic environmental events to affect Ireland over the last 7,000 years.

"We can see the first glimpse of an environmental history which is independent of human history and which, most of the time, is better dated than human history," stated Prof Michael Baillie of the School of Archaeology and Palaeoecology, Queen's University Belfast. He was speaking at a talk given last week at the RDS in Dublin.

"The level of dating precision offered by this method is unrivalled," he said. Dendrochronolgy differed from other areas of geology and archaeology because it could date extremely precisely.

"While other dating methods could make good estimates of a time period, tree-ring dating was the only technique that could narrow events down to a specific year and in some instances even to a specific month, Prof Baillie explained.

By looking at the cross section of a tree trunk, Prof Baillie can understand the growth history of the tree. As a tree grows, it deposits concentric rings of material which represent yearly growth.

Large distances between these rings represent good tree growth. Conversely, narrow rings indicate poor tree growth, usually as a result of unfavourable environmental conditions.

Studying these tree rings, Prof Baillie was able to detect the response of Irish oak trees to their environmental conditions over the last 7,400 years. The tree rings provided "a year by year record of what the oak trees thought of their environment. They were recording information from the environment," he explained.

He identified two major environmental events from the results of his studies of bog oak samples from Northern Ireland, one in the period between 2,354 and 2,345 BC and the other about 540 AD. Both of these events were predicted on the basis of severely stunted growth in oak trees at these times.

Based on studies in Scandinavia and America, the 540 AD event is now known to be global in its effect and not just confined to Ireland, Prof Baillie said. "In tree-ring terms, this is the worst event in the last 2,000 years," he added.

Prof Baillie said that effects of the 540 AD catastrophe were not confined to trees. Widespread failure of food crops and crashes in human population caused by epidemics of the plague also occurred at that time. These disasters were sparked off by "some sort of horrible environmental event", he said.

The cause of these two events was not entirely clear, he said, but he has come up with a few possible explanations for the devastation. Floods triggered by "tectonic activity", movements of earth's crust which cause earthquakes, could have caused the change.

Volcanoes were a second possible cause. Ice deposits in Greenland indicate that there may have been a major volcanic eruption about the same time as the 2354-2345 BC event.

The eruption could have altered climatic conditions causing a huge increase in rainfall, according to Prof Baillie. This extremely wet climate would have been unfavourable to tree growth.

There was also a possibility that "the narrowest ring events in the Irish oaks are in some way related to comet activity" he said.

This is not the first time the idea of comet activity has been considered as the cause of environmental disasters. Isaac Newton and Edmond Halley both believed that the Biblical flood was caused by the arrival of a comet.

An asteroid impact or a related high altitude meteor explosion was another possible cause. If an asteroid reached Earth's atmosphere at a very high speed it could explode causing a shower of debris, Prof Baillie stated. This cosmic swarm would be an "invisible hazard" leaving no visible trace of the asteroid.

Further evidence was still needed to prove either of these theories, he said.

"The overall question reduces to this: have the scientific community been missing evidence for significant tectonic activity affecting Ireland in the recent past?"