A number of years ago, Groningen physicists from the Centre for Isotope Research sprayed a snow layer in Greenland that has gradually come to lie deeper in subsequent years. Research on the diffusion of oxygen and hydrogen isotopes to neighbouring layers appears to open up new opportunities for climate research.
Ice cores from deep snow layers have been bored in polar regions for decades. They can tell us a lot about the climate in the past, because by measuring the oxygen and hydrogen isotopes we can discover which temperatures there were on earth thousands or even hundreds of thousands of years ago. Valuable knowledge about the turbulent process that is climate history can thus be gathered.
The first ice core samples date from the 1960s, when more became known in the world of physics about how water molecules with heavy isotopes transform from water into water vapour in the air. This occurs during the process where water evaporates in the tropics to eventually fall as rain or snow at higher latitudes.
The Groningen physicist Prof. Harro Meijer has been involved in climate research for a long time now, as head of the University’s Centre for Isotope Research (CIO). ‘The water vapour slowly loses heavy isotopes during that journey because precipitation tends to include that type of isotope’, he explains when talking about the principle behind the research. ‘When it arrives at the North Pole, the remaining water vapour contains fewer heavy isotopes. How much has been lost exactly depends on the temperature on the way. So once you know how much has been lost, you can in principle calculate the temperature.’
Although this phenomenon is what makes it possible to relate isotope concentrations at different depths in snow or ice layers to temperatures from long ago, correctly interpreting the measurements is by no means simple. One problem is that the various isotopes of oxygen and hydrogen in a snow or ice layer can diffuse to neighbouring layers. This is rather inconvenient because this ‘smearing’ of the original signal can even mean that seasonal variations vanish.
In Groningen this week, Gerko van der Wel will be awarded a PhD for research on this process. It’s a tricky issue because the diffusion is influenced by a series of factors, for example the density and porosity of the snow. A few years ago, however, remarkable news was published. A Danish colleague researcher proposed that it was possible to use diffusion in a positive way as well. When the diffusion rates of two different isotopes are compared, it turns out that the temperature in the snow layer is the only factor that influences this difference.
This opens up new possibilities, because the diffusion process itself can now serve as a climate archive. The concentration of the isotopes can say something about the temperatures on the way; knowledge about the diffusion process improves the calibration of that, and adds information about the temperature in the polar regions at the time the snow fell.
In his thesis, Van der Wel describes laboratory research on this phenomenon using snow layers he stored in the freezer. Parallel to this, he and colleagues from the Centre for Isotope Research have collected data over the years during fieldwork at Camp Summit, located at a height of 3200 metres in the middle of Greenland.
In 2007 the Groningen researchers sprayed a first snow layer there, which they created with a high pressure sprayer using water with an artificially changed isotope composition. In subsequent years samples were regularly taken from this layer, which was gradually covered by new snowfall. In the meantime, the diffusion process was also at work. Meijer: ‘Smearing works fastest in the first few years, because the snow is then still very porous. That gradually declines. Once the snow lies at a depth of 80 metres it has become ice and the diffusion process stops.’
In the first place, the research confirms measurements gathered during the 1980s at Camp Summit and elsewhere using borings, Meijer observed. ‘These measurements turn out to be corrected rather well for diffusion.’ What he finds more important is that there is now a solid foundation on which to build further research. ‘We now know all the ins and outs of the diffusion process. The technology to use diffusion ourselves as a climate indicator is finally open to us.’
Gerko van der Wel will be awarded his PhD on 2 March 2012. The Willem Barentsz Polar Institute of the University of Groningen is organizing a mini symposium around the PhD ceremony on ‘Isotope in ice cores and their relation to climate history’, with speakers from the Netherlands, Sweden and Switzerland. For more information: symposium website
For more information: Prof. Harro Meijer
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