By Euan Nisbet.
While most of the MAMM team toil in the heavens above, fighting wayward coffee mugs as the pilots roll and pitch, some of us are down below in the murk, feeding poor undernourished boreal mosquitoes and moose flies.
What we are trying to do is discover the carbon signature of the wetlands. Northern wetlands make a great deal of methane, but so do other northern sources like gas leaks (some of the world’s biggest gasfields are north of the Arctic circle) and also the methane hydrates that have recently been in the news.
Each individual methane source has its characteristic carbon signature. Carbon has two stable forms – Carbon-12, and the slightly heavier Carbon-13. There is also the rare radioactive for Carbon-14, used by archaeologists to date old relicts. Biological processes favour the 12 form, which is slightly easier to incorporate into biological molecules. Each time biology cycles carbon, the organic matter becomes progressively richer in carbon-12 and poorer in C-13. When finally the biological carbon is returned to the air as methane bubbles, it is highly enriched in C-12. Conversely, carbon-13 is richer in methane made by inorganic processes like fires, or in gas released by geological heating or from coalfields.
Thus if we sniff the air, we can tell where the methane comes from. Is the methane biological? Then it is relatively rich in C-12. Or is it geological or from a fire? Then it is slightly heavier in carbon-13.
Moreover, we can back-track the winds, by running the weather forecast computer model backwards. For example, if we sniff C-13 rich air in Ireland, we can track the wind back to a fire in Quebec. Some years ago, ‘heavy’ methane arrived in New Zealand. The air had passed over the Indian Ocean, and had last left land as far away as Mozambique. That’s where plumes of smoke carrying C-13-rich methane had been picked up. So the Methane C-13 sniffing near Wellington had actually smelled grass fires in southern Africa.
But to do this we need to measure exactly what the typical C-12 and C-13 fingerprints of each major methane source are. That’s difficult, because they vary. The sources are quoted in differences in per mil numbers compared to a relatively C-13 rich standard. “Per mil” is like per cent, but per thousand, and is written as ‰, where negative values are light, and very negative values are very light. Thus northern wetland and swamp methane can be very light, ranging from about -60 to around -70‰, while methane from fires or coal mines can be much heavier (less negative), around -25‰. Cow breath and landfill methane are between these extremes, often between -50 and -60‰, as is gas from the big Siberian gasfields.
For example, if we sniff unusually high methane in the wind, and work out the signature of the increment, we can figure out what type of source it came from. Then we can back-track the wind, and try to pinpoint the source. For example, if in summer an easterly wind arrives over Spitsbergen that is loaded with methane whose carbon is -70‰ we can often backtrack the wind to the swamps and wetlands of western Siberia. But in winter, that same easterly from Siberia could have methane whose carbon is -55‰, showing it came from the gasfields around the Ob River estuary. This methane ‘telescope’ – maybe ‘tele-rhino’ would be a better term (far-nose) – is excellent for figuring out who made the gas.
But to do this, we need to measure the sources. And the biggest northern sources are the wetlands in the boreal forest and bogs. How do we measure them? –We get down low and murky in the mosquito-infested swamps, dodge the bears, wolves and moose, and come back with plastic bags and steel cylinders of air. We take long poles, carrying hoses connected to a little pump, and collect cushion-size bags of air. Some samples come from low down over the swamp, while other samples come from holding the pole high into the wind, so we get a spread of values. The best sampling is done over 24-hour periods, at 2-hour intervals, including warm afternoons when the air mixes, and cold dawns when the still air is rich in local emissions. And at 1am is when you are likely to meet a wolf, or a reindeer, or bears (especially in Canada, where they also do a mean line in moose). All the time the mosquitoes attack, and the bugs grab flesh and gorge themselves. If you don’t crawl into camp with a face and arms bitten to look like red bubble wrap, you haven’t been working. And there’s no sleep in a 24-hour effort, just half-dozes between sampling alarms, while eternally slapping off the biters.
Then, after checking we haven’t sucked in any mozzies, we ship the samples back to the lab to analyse. If our samples have a range of methane contents, we can then plot a “Keeling plot”. This is a graph of the C-12 to C-13 ratio in the methane against 1/methane concentration, the reciprocal of the concentration. If you plot up a lot of points with variable C-12:C-13 and concentration, you typically get a nice straight line. On the axis, where 1/concentration is zero (i.e. source concentration is infinite), the intercept of the line gives the C-12:C-13 value of the typical source.
In the 2013 summer campaigns, we’re measuring swamps in a variety of places. Dave Lowry is driving across northern Sweden and Finland, mapping out the methane concentration and C-12 to C-13 ratios across the source areas. Rebecca Fisher is working in swamps, bogs and fens around the Abisko National Park in north-west Sweden. Euan Nisbet and Mary Fowler have just come back from a major field trip across the muskeg of Canada, in northern Ontario and northern Saskatchewan, in collaboration with Environment Canada, to characterise the C-12 to C-13 signature there, and will now cross the wetlands of Sweden and Finland just north of the Gulf of Bothnia. All of them will be generously feeding the bugs with excellent new bites.
Up in the air, James France, Rebecca and Dave will be in the plane, collecting air samples. Mathias Lanoisellé, recently back from a Atlantic boat profile, will be helping. By comparing the C-12 to C-13 ratios in methane from the airplane samples with the C signatures measured on the ground, and by back-projecting the wind movements of the air masses that have been sampled, we can figure out what the sources were.
For example, both in previous work and in 2012 flights to Spitsbergen, we determined that the summer methane source in the easterly air was almost entirely from wetlands. Compared to background, the high summer input of methane that came from the east (northern Russia) was almost all biological, from swamps. This was at a time when the gasfields were at a low ebb, and luckily there were few fires. The result meant that other more geological sources were quiescent.
One inference from this is that there was very little methane input from decomposing methane hydrates. Since hydrate decomposition is not seasonal, then if large hydrate inputs were absent in summer they were likely absent in winter too. Of course that may change, and our winds came from central northern Siberia, not the far east, but we found no strong hydrate signal. Hydrate methane emissions have been much in the press recently, but they’re shy in the Arctic wind.
Now has anyone got any anti-histamine?
Previously in this series:
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