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Cockles and mussels reveal all

April 01, 2004

Cockles and mussels harvested on the shores of the Irish Sea may have provided a staple diet for Molly Malone and her fellow Dubliners, but for scientists at the University of Wales, Aberystwyth they are, along with longer living species such as the clam Arctica islandica*, a detailed record of pollution extending back over two centuries.

The shells of molluscs are made up of layers of calcium carbonate which grow in regular cycles. With each cycle a layer is added causing an effect similar to the rings formed as trees grow. As layers are formed pollutants such as heavy metals become incorporated or trapped into the shell.

Led by Dr Bill Perkins of Centre for Research in Environment and Health at the Institute of Geography and Earth Sciences, the Aberystwyth team have been working on a technique that enables them to extract samples from the individual layers on a shell in minute quantities. Once analysed the extracted shards of shell reveal their chemical composition including trace metals in very low concentrations - parts per million.

The technique (Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)) involves firing a laser at the shell that leaves a hole measuring between 20 and 30 microns across. Such accuracy is essential as the layers are not unsually distinguishable to the human eye. The samples are analysed in a matter of minutes whilst other techniques can take hours to achieve such low levels of detection. .

According to Dr Perkins the sea has been the ultimate 'sink' for waste since man started mining and processing metals.
"The oceans have been used to wash away man's dirt for thousands of years and there are many techniqes that allow scientists to monitor the levels of pollution and to look at long term changes in concentrations," he said.

"However, there is an increasing need to pinpoint when a pollution incident takes place and the likely source of the contamination, as people become more aware of the effects of pollution on the environment and their lives."

"The technique we have developed at UWA, coupled with our increased understanding of how shells are formed, means that we will soon be able to pinpoint a pollution incident to a particular day and even identify the source. Longer living species such as Arctica islandica* provide information on naturally occuring levels whilst the shells of shorter living species such as cockles provide a much more detailed record of pollution levels from day to day," he added.

Environmental Forensics
The work of the Aberystwyth team has attracted the attention of scientists working in the relatively new field of Environmental Forensics.

Driven primarily by litigation in the US, where instances of ground water becoming contaminted by fuel from petrol stations and refineries is on the increase, the need to prove who is responsible and when a particular incident happened is ever more pressing.

In March 1989 the Exxon Valdez ran aground in Prince William Sound, Alaska, polluting hundreds of miles of pristine coastline. Scientists investigating the spill were able to prove that the area had a level of naturally occurring pollution as well as oil from the tanker spill. According to Dr Perkins, the work at Aberystwyth should in future enable scientists to map in much more details the presence of various pollutants in a particular marine environment, and provide important information that would be available in the event of a future spill and the ensuing legal action.

Dr Perkins presented the findings of his work to the International Society of Environmental Forensics at a meeting of the society held in Italy in May 2003

Parys Mountain
The team are currently looking at shell fish off the coast of Anglesey, north Wales. Over 200,000 cubic meters of heavily contaminated water from disused mine works on Parys Mountain are currently being dumped untreated into the Irish Sea. Working on the shells of limpets, mussels and whelks collected prior to, during, and for a period following the dumping, they will be able to build up a pollution profile.

Dr Perkins said: "The samples collected will enable us to build up a complete picture of any pollution caused by this work. Our technique works in a mater of minutes and, as the samples need very little preparation, there is little risk of contamination once they are removed from their natural habitat."

*Shell fish of Ceredigion Bay
Ceredigion Bay is home to a rich and diverse shell-fish community, amongst them one of the longest living organisms on the Earth today, the clam Arcitca islandica. Measuring up to 120mm across, Arctica islandica can typically live for anything between 150 and 225 years, developing new layer of shell every year. During its „youth" the shell can grow at a rate of several mm per year but the rate slows as the clam reaches „old age". Unsuprisingly it has also become known as the tree of the sea. At the opposite end of the longevity spectrum are welks, limpets and mussels which live for between 4 and 5 years, and cockles whose shells develop a new layer with every tidal cycle.

Aberystwyth, University of Wales