Digging up evidence of metal pollution

Katharine Sanderson finds out how the truth about human influence on the environment has been dug up from the depths of a peat bog.

Eerie scenes, reminiscent of an old horror movie, will stick in the mind of anyone who followed the discovery in a Danish peat bog of the Tollund Man, a perfectly preserved snapshot of one of our ancestors. So well preserved, in fact, that you can even see the stubble growth on his chin. Similar bodies have been found in the UK, with the Lindow man in Manchester providing more of an archive of human behaviour. Even in days of yore, bogs as archives was not an outrageous idea, with Reverend Robert Rennie from Kilsyth, Scotland, claiming in 1807 that water in bogs was direct evidence for the Great flood in biblical times. It is undisputed that biological materials are well preserved, but what about other things? What if we could glean information about human influence on the environment from these archives? Maybe we can. Evidence is fast gathering that peat bogs can also act as accurate archives of global metal pollution.

The story begins with lead. Nowhere on our planet is free from anthropogenic lead - that is lead present in the atmosphere predominantly from human activity. Before blame is poured on the motor industry and 20th and 21st century machinery, it is worth looking back briefly at the history of lead.

This toxic metal has been used for centuries for a host of applications. Silver mining involves lead, because the ores that are mined are not just silver-rich, but are also lead-rich. Similarly, mining of other metals also releases significant amounts of lead to the atmosphere. As far back as Roman times, over 3000 years ago, humans were polluting the air with lead particles, and have continued to ever since as we set about burning as much coal as we could lay our hands on. How can we be so sure about this? Archaeological studies can give an insight into processes used throughout history, but bogs hold the real key, and the damning evidence.

In the heart of the Jura mountains in Switzerland sits a peat bog at Etang de la Gruyere (ETG). This peaceful place is in a national park and is arguably one of the most beautiful spots in Europe. It is also a phenomenal archive of atmospheric metal depositions over the past 15 000 years.

ETG is an ombrotrophic bog. This means that the peat has grown in a raised mound and the only metal deposits contained in it come from the air, with nothing coming from the ground. On the top layer of the bog grow sphagnum mosses, which then decay to form the main body of the bog, taking the particles from the atmosphere with them. By taking a core sample from the ombrotrophic bog’s mound, and doing detailed analysis along the core’s full 6.5m length, it is possible to get a record of lead deposits throughout the bog’s life. An obvious question would be how can you be sure that the lead deposits throughout the depth of the bog are an accurate record? Why don’t the lead particles move around in the bog? Bill Shotyk, director of the Institute of Environmental Geochemistry at the University of Heidelberg, Germany, wanted to address this question and admits that when he first began looking at peat bog cores he didn’t believe for a moment that they could be used as an archive for lead deposits.

As the results from his studies came in, Shotyk began to realise that, contrary to his first instinct, what was emerging was a perfect and accurate archive of lead over the years. The very nature of the ombrotrophic bog was keeping the lead particles trapped and immobile just where they landed. The reason why lead is immobile in the bog is still not fully understood, but Shotyk has a hypothesis: the peat bogs are full of huge organic molecules, almost colloidal in their nature, and once a dust particle, containing within it lead and other metals, lands on the bog, it complexes with these molecules. From then on it is trapped and unable to move around, even with rain, floods and evaporation. The organic molecules and resulting complexes are just too big and immobile to be shifted. Whether this hypothesis is accurate or not, the facts remain: lead is immobile and can act as an archive.

Ombrotrophic bogs are different to marshes, fenlands, swamps and other bogs because they are acidic. The bog is full of organic acids, and as nutrients to the bog come only from the air, there is no mineral matter to neutralise these acids. In a swamp, floods bring in minerals that neutralise the acids, but if a bog is acidic, it’s a sure sign that the only inputs it has are from the air. Couple this with the immobility of lead particles and the scene is set to decipher just how much lead has been present in the air for the entirety of the bog’s life.

Core analysis

Calculating metal pollution using bog-time is a precise art. A core from the bog is carefully collected and frozen at the collection site. It is then transported to a clean room and cut into exact 1cm slices, dried and then analysed. If you think that sounds easy enough, bear in mind that contamination of the sample must be avoided at all costs and the sampling involves cutting away the edges of the core with special titanium blades, discarding these edges and then using a band-saw to get precise 1cm slices, again using titanium blades that are rigorously cleaned. It’s not like slicing up a loaf of bread, although in the early years of this research, the sampling was slightly more rudimentary and would often be performed with a bread knife, cutting doorstop-sized bog slices. Unfortunately, this haphazard sampling technique muddied the results in the past, but recently, more exact sampling methods have allowed date resolution of one year gaps to be achieved along the core. Lead levels are measured directly using X-ray fluorescence (XRF) spectroscopy, and the different lead isotopes are measured using incredibly sensitive and accurate mass spectrometry, in particular inductively coupled plasma mass spectrometry (ICP-MS).

Once a quantitative analysis of lead levels in the peat is complete, the different lead isotopes in the peat, and their ratios, can easily identify where that lead has come from, because each source of lead, for example dust from the Sahara desert, or lead particles in gasoline, has a distinct composition. So, lithogenic lead, or lead from the dust formed from the earth’s crust, has a particular ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁶Pb ratio, as does lead from anthropogenic sources, making it possible, according to Shotyk, to address the question of ’how much [lead] is mother nature contributing and how much are we contributing’.

The lowest ²⁰⁶Pb/²⁰⁷Pb lead isotope ratio corresponds to the highest influence of lead from gasoline, and the peak for this occurs in 1979, after which time unleaded gasoline began to be used and the lead isotope ratios changed accordingly.

However, it becomes clear that, although leaded petrol used in cars was a major contributor to lead levels in the atmosphere, most of the damage had already been done by the time this practice was widespread. The maximum concentration of total lead to natural lead was around 1954, just before the Clean Air Act was introduced in the UK in 1956. It has declined since, with a more marked decline seen after 1979. By looking at the lead isotope ratios, the source of the lead from 1954 has been traced to coal. Is the message therefore: blame coal, not cars? There is no doubt that leaded gasoline was, in Shotyk’s words ’a dumb idea’, but before blame can be given to a specific incident or policy a more general view of the issues is warranted. Policy makers have to understand that gasoline lead is part of the pollution story but by no means is it the whole story, nor has the story reached its end. For example, a cumulative record of anthropogenic lead shows that by the time leaded gasoline was introduced in Switzerland (1947) 75 per cent of the lead was already in the bog. Shotyk says: ’If there’s one metal that we should be studying today in the environment, it’s lead, because we have affected the geochemical cycle so much, we have emitted so much to the environment. The beauty is we can follow it using the lead isotopes. And now the question is: what are the health implications? This I think has to be re-evaluated’.

Peat bogs are in the country, where the air is relatively clean. Shotyk shudders at the thought of how high the lead levels might be in other areas: ’can you imagine the lead levels we would find if we had a bog in the city of London, or the city of New York?’

Lead pollution was discovered by Clair Patterson at Caltech, US, who was measuring meteorites using lead isotopes in an attempt to date the earth. It was during his measurements in the 1960s that he realised that from the South Pacific to Greenland, everything was contaminated with industrial lead. Patterson recommended a complete ban on lead.

Studying trends

Silver, cadmium, mercury, thallium, lead and antimony are Shotyk’s ’favourite elements’ for studying anthropogenic metal pollution trends. Scandium, titanium and zirconium are also interesting because they are good indicators of dust deposition. For example, the scandium accumulation rate can be measured and will show how much dust has been gathering on a bog surface everyday, as it grows. Scandium has nothing to do with human activity so it is useful as a reference element to see how much ’natural’ dust has been deposited, with the remainder being anthropogenic pollution. Selenium and bromine are also used as reference elements, and are useful as indicators of marine aerosol levels.

After lead, the most widely studied metal in bogs is mercury. Mercury is well known for its detrimental health effects, and Lewis Carroll’s Hatter would testify to that if he weren’t so mad.

Concern about high mercury levels in hair samples of the inhabitants of the remote Faroe islands has prompted various studies into the metal. Mercury comes to the Faroese partly from their diet, which is often based on meat and blubber from whales, where mercury accumulates. But to find out how and where mercury is entering the food chain is not easy. Theories abound as to where this mercury might originate from, with volcanoes in Iceland taking most of the blame. Peat bog data, however, suggests a different story.

Inspired to investigate mercury pollution in order to verify the source of the Faroese people’s high mercury levels, Nicolas Givelet, who works with Shotyk, looked at mercury accumulation in the local peat bog. His claim is that the amount of mercury in the environment with an anthropogenic source is greater than widely believed. Givelet also shows that the maximum concentration of anthropogenic mercury coincides historically with the maximum concentration of anthropogenic lead (1954), making coal-burning the culprit yet again. More worryingly, the new data from peat cores not only in the Faroes, but in Canada, Greenland and at ETG, estimate that the natural rates of mercury accumulation have been underestimated by a factor of five. The implication here is huge - humans may have impacted global mercury pollution much, much more than previously thought. And the implications for worldwide mercury-restricting legislation that go with that are equally grave.

By using peat bogs as an archive of metal pollution, Shotyk and his colleagues around the globe are doing a fine job of digging up dirt about human intervention into global metal pollution. Shotyk takes this one step further, claiming that ’there is no question that a bog is an archive of climate change’. By analysing bogs from all over the globe, a picture will emerge outlining where and to what extent humans have altered the planet. With any luck, we will learn valuable lessons from the bogs, and hopefully before it’s too late.