Shells tell tales on toxic sins of emission

Technology 17 December 1994

CLAMS, cockles, limpets and other shellfish have the history of heavy metal pollution in the sea recorded in their shells. Now Bill Perkins and Nick Pearce at the University of Wales in Aberystwyth have found a way to “read” this history, year by year.

“Never before have people picked out this fine-scale variation,” says Perkins. “This method has great potential that’s not being exploited.”

Shellfish are widely used in monitoring pollution, says Perkins, because they accumulate heavy metals in their shells and organic pollutants in their soft tissues. But the shells are normally analysed by crushing them up and measuring the total amount of pollution absorbed over the creature’s lifetime. Instead, Perkins, Pearce and colleagues at the university’s Institute of Earth Studies can analyse consecutive layers in the shell.

In the same way that tree rings form, shellfish lay down consecutive layers of shell every year of their life. However, they simultaneously lay down new layers on the outside and the inside of the shell – so the middle of the shell is the oldest. Perkins hopes to devise ways of pinpointing the oldest layer, and thus the exact age of the organism.


Each layer of shell is made of calcium carbonate. But the precise chemical composition of each layer reflects the varying concentrations of elements in the sea when it was formed.

The magnesium content of the sea varies temperature, rising during summer and falling in winter. The analysts can therefore identify every summer and winter in the life of the shellfish from the magnesium content in the layers of the shell.

It takes Perkins just two minutes to analyse each layer. Recent refinements to the technique enable him to measure variations in pollution in cockles over a matter of weeks, so the shellfish could be stationed downstream of industrial discharges to police pollution. During the analysis, Perkins first cuts out a cross-section of shell which reveals the different layers. He then blasts off layers of shell as shallow as 5 micrometres by vaporising them with lasers. The small particles in the vapour are whisked off in argon to be analysed by a mass spectrometer, which measures the relative abundance of each ion.

The same process is applied successively to each layer of shell until a profile of metal concentrations is built up over the entire thickness of the shell. Different species yield different types of pollution profiles, depending on how fast they grow and how long they live. Some shallow-water bivalves, for example, add as much as 2 micrometres of shell a day, giving daily discernable growth bands. Others, including clams such as Arctica islandica, live for 80 to 100 years, and provide detailed seasonal records of pollution.

One of Perkins’s analyses shows the lead pollution accumulated over six to eight decades by A. islandica clams in Cardigan Bay. The profile shows a spike in lead concentrations caused by surges of highly contaminated water in 1969 from a disused mine 15 kilometres inland.

Perkins says that old shells and fossils could be used to monitor climate change. The concentrations of magnesium in shells of known age could help to measure variations in the temperature of the sea (see Diagram).

Lead pollution in Cardigan Bay