We measure the temperature of the past

When scientists talk about climate archives, they don't have a real book or a library in mind. Instead, they mean glacier or ground ice, fossilised animal remains such as mussel shells or sediment samples from the lake or sea bed that they have brought to the surface with drills. All three "archives" have one thing in common. They contain either water (usually in the form of ice) or silicon dioxide (e.g. in the opal skeleton of diatoms). The special thing about this is that the geochemical fingerprint of both substances differs depending on the environmental conditions that prevailed when the diatoms grew or the snow trickled down Greenland's ice sheet. "Both water and silicon dioxide contain oxygen isotopes, the ratio of which we analyse in our stable isotope laboratory. Depending on the ambient temperature, the stable isotopes are incorporated in a certain ratio in an ice crystal lattice, in the shell of a mussel or in the framework of the diatom. We measure this ratio in our laboratory and can use our results to determine, for example, the air temperature that prevailed at the time of formation," says AWI climate researcher Dr Hanno Meyer. 

He heads the ISOLAB Stable Isotope Facility at the AWI Potsdam, where some of the world's most important temperature reconstructions of climate history have been carried out: for example, the analysis of the Dronning Maud Land ice core, which AWI researchers and their international partners recovered in the so-called EPICA project.

Dr. Hanno Meyer (Head of Laboratory)

The EPICA DML ice core had a length of 2.892 metres. In order to find out what temperature data is stored in the various ice layers of the core, the scientists took an ice sample every 50 millimetres - well over 10,000 in total. These samples were melted and then analysed in one of the laboratory's Delta-S mass spectrometers for their ratio between 16O and 18O isotopes. "The oxygen isotope ¹⁶O has fewer neutrons than the isotope ¹⁸O. It therefore has a lower mass and is deflected more strongly by the mass spectrometer's electromagnet than the heavy ¹⁸O isotope. In this way, we are able to separate the isotopes, measure them individually and determine their ratio precisely," explains Hanno Meyer. 

In 2006, after months of work on the EPICA ice core, the AWI researchers presented a temperature curve of the Antarctic going back some 200,000 years. A data set that has advanced our knowledge of the interactions between the climate in the southern hemisphere and that in the northern hemisphere like no other.

The significance of the analyses in the Potsdam ISOLAB Stable Isotope Facility extends far beyond pure temperature reconstruction: Hanno Meyer and his colleagues can use samples from Arctic ground ice to reconstruct exactly where the snow that fell tens of thousands of years ago in the Russian tundra came from. "Every precipitation has its own geochemical fingerprint, which is preserved when the snow melts, seeps into the ground and refreezes there, for example as a ice vein in a frost crack. When we take samples from the permafrost today and analyse the isotope ratio of the ground ice in our laboratory, we also make this fingerprint visible and can trace the precipitation fields back to their origin," says Hanno Meyer.

While the isotope analysis of ice and water samples does not require extensive preparatory work, the scientists spend most of their time analysing diatoms. "We need the purest sample material for the analysis. This means that we sieve the sediment sample, add various chemicals to it to dissolve organic material and carbonate and finally wash it in ultra-pure water so that only the super-pure diatom skeletons remain at the end," explains the laboratory manager. Hanno Meyer needs just 1.5 milligrams of this ultra-pure material. He shoots it with a laser and reacts the sample in a fluorine atmosphere. During this process, the oxygen isotopes stored in the silicon dioxide are released. The gas sample then only needs to be fed into the mass spectrometer, which is used to determine the exact isotope ratio. "Such oxygen isotope analyses of the diatoms allow us to make statements about the isotope composition and the temperature of the water in which these algae once lived," says Hanno Meyer proudly. The ISOLAB Stable Isotope Facility at the AWI Potsdam is one of only eight scientific centres of its kind in the world.