Use of natural diamonds to monitor 14C AMS instrument backgrounds

To examine one component of the instrument-based background in the University of California Keck Carbon Cycle AMS spectrometer, we have obtained measurements on a set of natural diamonds pressed into sample holders. Natural diamond samples (N = 14) from different sources within rock formations with geological ages greatly in excess of 100 Ma yielded a range of currents (∼110–250 μA 12C− where filamentous graphite typically yields ∼150 μA 12C−) and apparent 14C ages (64.9 ± 0.4 ka BP [0.00031 ± 0.00002 fm] to 80.0 ± 1.1 ka BP [0.00005 ± 0.00001 fm]). Six fragments cut from a single diamond exhibited essentially identical 14C values – 69.3 ± 0.5 ka–70.6 ± 0.5 ka BP. The oldest 14C age equivalents were measured on natural diamonds which exhibited the highest current yields.