Soil organic carbon is one of the largest surface pools of carbon that humans can manage in order to partially mitigate annual anthropogenic CO 2 emissions. A significant element to assess soil sequestration potential is the carbon age, which is evaluated by modelling or experimentally using carbon isotopes. Results, however, are not consistent. The 14 C derived approach seems to overestimate by a factor of 6-10 the average carbon age in soils estimated by modeling and 13 C approaches and thus the sequestration potential. A fully independent method is needed. The cosmogenic chlorine nuclide, 36 Cl, is a potential alternative. 36 Cl is a naturally occurring cosmogenic radionuclide with a production that increased by three orders of magnitude during nuclear bomb tests. Part of this production is retained by soil organic matter in organochloride form and hence acts as a tracer of the fate of soil organic carbon. We here quantify the fraction and the duration of 36 Cl retained in the soil and we show that retention time increases with depth from 20 to 322 years, in agreement with both modelling and 13 C-derived estimates. This work demonstrates that 36 Cl retention duration can be a proxy for the age of soil organic carbon. Soil organic carbon (SOC) is one of the largest surface pools of carbon 1 , with estimates ranging from 1500 to 2500 gigatons for the first meter of the soil 2. This pool is one of the very few that humans can manage to partly mitigate annual anthropogenic CO 2 emission ("4 per 1000" 3 initiative launched by France during the COP21 based on a calculation made by Balesdent and Arrouays 4. SOC can also foster food production and ecosystem stability 5 can also be fostered. To achieve these multiple objectives, understanding SOC dynamics is crucial to unravel carbon soil sequestration potential 6. Soil organic matter is a complex mixture of molecules that evolve more or less rapidly towards mineralisation (CO 2 emission) or towards more complex or stabilized molecules. The carbon sequestration potential of soil depends on the balance between SOC mineralization and stabilization over a longer or shorter period. Depending on the length of time that soil organic matter remains undecomposed, soil carbon is considered as falling into different pools. This is, however, only a conceptual framework of how SOC works. The estimation of the carbon content of these reservoirs is not achievable by experimentation and their modelling remains challenging. The representation of SOC in different pools is the basis used in Earth system models (ESMs) 7. In order to validate this modelling approach, the SOC ages yielded by the models are compared to actual ages of SOC experimentally estimated from 14 C 6,8 and stable carbon isotope 9 approaches. Meta-analyses using natural and nuclear bomb 14 C peaks on soil profiles show that the mean derived-SOC ages for the first meter of the soil range from 3100 to 4800 ± 1800 years 6,8. The 14 C measured SOC age is more than six times higher than the SOC age yielded by the models (430 ± 50 years), resulting in a difference of 40 ± 27% in the soil's potential to sequester atmospheric carbon 6. On the other hand, Balesdent et al. 9 estimated the SOC age distribution over the soil profile using the stable carbon isotopic signature. Their results provided a mean SOC age for the first meter of the soil of 489 ± 173 years, close to that yielded by ESM models 7 but very different from those obtained by 14 C dating 6,8. The method used by Balesdent et al. 9 is based on the proportion of new carbon atoms that was determined after a natural change in the stable carbon isotope signature of the vegetation at a known age. This approach is therefore not applicable everywhere. There is thus still a need to better assess the SOC age in order to discriminate between the two sets of SOC ages provided by 14 C dating 6,8 and C stable isotopes 9. Chlorine (Cl) is a highly mobile element that is slightly retained in soils in organochloride form 10 , with the notable exception of saline soils where Cl is also encountered as salt. The chlorination process occurs during the very first steps of soil organic matter degradation, mainly mediated by fungal activity 11,12. The Cl forms covalent