Radiocarbon in modern carbon cycle research

Atmospheric nuclear weapon testing in the 1950s and 1960s has been worrying, however, in many aspects it was extremely beneficial for environmental sciences. The artificial production of more than 6 x 10²⁸ atoms or about 0.6 tons of radiocarbon (¹⁴C), leading to a doubling of the ¹⁴C/C ratio in tropospheric CO₂ of the Northern Hemisphere, has generated a prominent spike in 1963. This “bomb-spike” has been used as transient tracer in all compartments of the carbon cycle, but also to study atmospheric dynamics, such as inter-hemispheric and stratosphere-troposphere air mass exchange. Moreover, our attempt to accurately determine total bomb produced ¹⁴C led to improved estimates of the atmosphere-ocean gas exchange rate and to a new constraint of the residence time of carbon in the terrestrial biosphere. Today, the transient bomb-radiocarbon signal has levelled off, and the anthropogenic input of radiocarbon-free fossil fuel CO₂ into the atmosphere has become the dominant driver of the ¹⁴C/C ratio in global atmospheric CO₂. The observed decreasing ¹⁴C/C trend in atmospheric CO₂ may thus help scrutinising the total global release of fossil fuel CO₂ into the atmosphere. On the local and regional scale, atmospheric ¹⁴C/C measurements are already routinely conducted to separate fossil fuel from biogenic CO₂ signals and to estimate trends of regional fossil fuel CO₂ emissions. Some prominent examples where the bomb ¹⁴CO₂ disturbance has been successfully used to study dynamic processes in the carbon cycle are discussed as well as our current activities applying this unique isotope tracer for continental scale carbon cycle budgeting.