Unlocking polar climate archives

Thanks to a new high-tech tool, the Alfred Wegener Institute can finally unlock many polar climate archives for the first time. At its C-14 dating laboratory, AWI researchers are using radioactive carbon to determine samples’ age. Aiding them in their efforts is one of the most cutting-edge accelerator mass spectrometers currently available on the market.

Determining the “birthday” of e.g. microfossils, wood, leaves, bivalves or other components found in samples gathered from the seafloor requires high-precision analytical work – a challenge that the Alfred Wegener Institute’s C-14 lab has now overcome. To do so, the scientists use a method called radiocarbon dating, also known as the C-14 method.

The C-14 method involves measuring the proportion of the radioactive carbon ¹⁴C isotope (C-14) to the stable ¹²C isotope. Radioactive carbon is formed in the atmosphere and subsequently absorbed by all life on Earth. In living organisms, the ¹⁴C level largely remains stable, since they are in constant contact with the atmosphere. When they die, however, this exchange with the atmosphere comes to an end. The ¹⁴C content is then measured using the radioactive decay, which progresses on the basis of a specific half-life, while the stable carbon isotopes remain unchanged. In this way, measuring the isotope proportion ¹⁴C/¹²C can tell us how old a given sample is.

Normally an accelerator mass spectrometer is used for the ¹⁴C method – traditionally a massive machine that takes up an entire room and has to be constantly supervised by several personnel. Accordingly, in the past, samples were always sent to major labs equipped with such systems, though doing so was quite expensive. However, thanks to the evolution of the conventional accelerator mass spectrometer, for some time now much smaller units, specially designed for the ¹⁴C method, have also been available. “The high costs of external analyses are one of the reasons why the AWI now has its own ¹⁴C lab,” says Gesine Mollenhauer, a marine geochemist and head of the laboratory. To date, the AWI is the only institute in northwest Germany with a next-generation ¹⁴C accelerator mass spectrometer, which can perform analyses with even the smallest amounts of carbon; the main reason the AWI chose to invest in the new technology.

The ¹⁴C method can reliably date sample material that is between 300 and 50,000 years old – regardless of its physical state. Whereas in the past, ¹⁴C analyses could only be made using a sample made of graphite (pure carbon), we can now also use material in gaseous form as CO₂. “This allows us to perform dating on samples that consist of just a few micrograms,” explains Gesine Mollenhauer. That’s especially valuable for sediment cores from the polar regions, since the deposits often contain only very small amounts of datable material.

The only disadvantage: the results for CO₂ aren’t as precise as those for graphite. “Nevertheless, this is a success; it’s better than having no way at all to date these samples,” the head of the laboratory maintains. “In the past, the samples simply weren’t large enough to test. But since the new unit can analyse even the smallest samples, we can now work with a number or archives that weren’t datable before.”

Text: Helena Kreiensiek