level*

The cosmogenic isotope ⁴¹Ca with a half-life of 99,000 years can in principle serve as tracer for environmental processes at an age scale beyond the reach of ¹⁴C. With accelerator mass spectrometry, the ratio of ⁴¹Ca/Ca has been measured down to the 10^-15 level in natural samples. A wide range of potential applications, such as burial dating of bones and exposure dating of rocks, require measuring even smaller ⁴¹Ca/Ca ratios in the range of 10^-16 to 10^-15. Here we achieved this by employing the atom trap trace analysis method in which individual ⁴¹Ca atoms are selectively captured in a magneto-optical trap and counted by detecting their fluorescence. We realized a precision of 12% on the ⁴¹Ca/Ca ratio at the level of 10^-16 and achieved a detection limit at the level of 10^-17, which is below the distribution of natural abundances. We verified the accuracy of the ⁴¹Ca/Ca results through a series of measurements of reference samples, and performed demonstration analyses on bone, rock and seawater samples. Our table-top method has the potential to explore the suitability of ⁴¹Ca as tracer.