Planet Earth Online

The carbon age

28 December 2010

When I tell people I do research in a radiocarbon laboratory, a common response is, 'Oh right, like radiocarbon dating the Turin shroud?' Radiocarbon dating is a valuable technique for dating objects of historical and archaeological importance, but it's also a powerful tool in the quest to understand our environment. In particular, because it deals with an isotope of the element carbon, radiocarbon analysis can tell us about processes that are fundamental both to life on Earth and to our climate.

Radiocarbon analysis was pioneered over 60 years ago, and the technique continues to be improved. At the NERC Radiocarbon Facility (Environment) in East Kilbride we have come up with new techniques for collecting CO₂ for radiocarbon analysis. This is the story of these new sampling systems, some of their applications and the insights they have provided.

CO₂ is important to many processes that occur on Earth, a component of our planet's atmosphere and, in terms of climate change, one of the most important greenhouse gases.

Plants use CO₂ from the atmosphere for growth, through photosynthesis. Most of the CO₂ they absorb will at some stage return to the atmosphere, but crucially, the time it spends locked away can vary from less than a day to millions of years. For example, carbon fixed by a plant during photosynthesis will cycle through it very rapidly and may be returned to the atmosphere as the plant 'breathes'. Alternatively, carbon that sits in a plant's tissues is likely to end up in the soil when the plant dies, and depending on the rate of decomposition it can stay there for decades or even millennia. In extreme cases, some carbon fixed by plants millions of years ago is only now being released, as we burn fossil fuels.

The rate that carbon cycles through these various routes before returning to the atmosphere as CO₂ has a critical influence on its concentration in the atmosphere. This is because the amount of carbon in the Earth's atmosphere (mostly as CO₂) is small compared to that in the oceans and on land.

This is where radiocarbon dating comes in. It tells us how long carbon has remained in a particular pool (soil, for example) and, therefore, the rate that it cycles through that pool. Measuring the radiocarbon in the CO₂ leaving the carbon pool can show us directly the average age of the gas entering the atmosphere.

All this is possible because carbon naturally occurs in three slightly different forms (isotopes). Two are 'stable', while the third - radiocarbon - is 'unstable', because it's radioactive and decays as it emits radiation. So its concentration declines over time relative to its stable counterparts, and measuring the relative proportions of the carbon isotopes in a material forms the basis of carbon dating.

In addition, nuclear weapon tests in the mid-20th century produced a rapid but temporary global increase - a 'spike' - of radiocarbon in the atmosphere which can be tracked throughout the carbon cycle. This spike lets us date very recent materials, which can't be done using conventional carbon dating.

Our challenge was to develop a sampling system that researchers could use in remote field sites. Although a few milligrams of carbon are enough for analysis, in most cases the concentration of CO₂ in the actual samples is extremely small - typically a suitable sample would require 5-10 litres of air. Transporting such volumes in gas sample bags or glass flasks would be impractical. Alternative methods such as cryogenic purification - where CO₂ is separated from other gases in air by cooling in liquid nitrogen at -196°C - are also impractical, not to mention potentially hazardous in the field.

Sieving the carbon

Thanks to earlier work by researchers at the East Kilbride lab, we knew the key was a zeolite molecular sieve. Zeolite is a rather unimpressive looking clay material which has remarkable properties. Firstly, it contains a uniform network of tiny pores which allow small molecules (including CO₂) to pass through but exclude larger molecules. Secondly, at room or field temperatures this molecular sieve attracts certain molecules to its surface - a process called adsorption - and the type we use strongly adsorbs CO₂. This means that, when we pump air through the molecular sieve, all the CO₂ is trapped within its pores. Crucially for a system that has to be used in the field, it has a high surface area so only a small amount of molecular sieve is needed to collect a suitable sample. When heated to several hundred degrees celsius back in the lab, the sieve releases the stored gas. These characteristics make it ideal for our purposes.

Our system also uses an infra-red gas analyser, which measures CO₂ concentration in the air being sampled so we can estimate when a big enough sample has been collected. It needs no external power supply and can be easily transported and operated by one person.

Developing the system has had huge benefits. For example, in the NERC-funded International Polar Year ABACUS project it was used to work out the age of CO₂ produced from decomposing soil in birch forest and tundra heath (where cold temperatures prevent tree growth). To collect the samples required daily hikes over many miles of tundra, and sampling chambers had to be tied down to cope with the high winds and exposed conditions (fortunately they escaped the attention of the numerous passing reindeer). Results showed that, although these soils contain carbon that is hundreds of years old, most of the CO₂ emitted from the soil surface had been fixed from the atmosphere within the last decade or so. There was also evidence for much faster carbon cycling in the forest compared with the tundra heath. This will have implications for the overall rate of carbon emissions if forest replaces heath in these regions, which may be occurring due to global warming.

The system has also helped investigate CO₂ emissions from UK peatlands, which contain vast stores of carbon. One surprise was that deep-rooted plants act as conduits for greenhouse gases dissolved deep in the peat. We know that plants like sedges help transport methane to the peat surface, but it was news to scientists that they provide a similar service for CO₂ that's hundreds of years old. And by connecting the sampling system to a floating chamber, we managed to collect and date CO₂ coming from the surface of peatland streams. Surprisingly, radiocarbon results show that this CO₂ can be ancient; derived either directly from deep bedrock weathering or, potentially, from CO₂ taken in by plants more than a thousand years ago.

As if this isn't enough, a whole new range of possible applications have emerged since we developed the technique so it could also be used as a 'passive sampler'. This means that we simply rely on the CO₂ molecules' own kinetic energy to get them to the molecular sieve - no pump required. So the sieve only needs to be exposed to the atmosphere being sampled to get sufficient CO₂ before it's returned to the lab for analysis. This is particularly helpful in remote and inaccessible locations - for example, in Arctic Sweden we managed to collect CO₂ from underneath the snow during winter for the first time - completing a whole year's sampling without a break. The soil carbon emitted during the winter (a significant proportion of the annual total) proved to be of a similar age to emissions during the growing season.

This isn't the end of the story though. There are even more possibilities for applying both sampling systems, and the study of fossil-fuel emissions could be a particularly fruitful one. Because of its extreme age there is no radiocarbon in fossil fuel, so if we can't detect any radiocarbon our samples must be very old (at least 50,000 years old). Our sampling methods could be used to quantify how much of the CO₂ in the atmosphere comes from fossil fuel, helping us understand the impact of fossil-fuel burning on global warming. It could also be used to test for CO₂ leakage from carbon capture and storage facilities, helping maximise the contribution they make to reducing our carbon emissions.

Keywords: Atmosphere, Environmental change, Pollution,

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