A long record of atmospheric ¹⁴C concentration, from 45 to 11 thousand years ago (ka), was obtained from a stalagmite with thermal-ionization mass-spectrometric ²³⁰Th and accelerator mass-spectrometric ¹⁴C measurements. This record reveals highly elevated Δ¹⁴C between 45 and 33 ka, portions of which may correlate with peaks in cosmogenic ³⁶Cl and¹⁰Be isotopes observed in polar ice cores. Superimposed on this broad peak of Δ¹⁴C are several rapid excursions, the largest of which occurs between 44.3 and 43.3 ka. Between 26 and 11 ka, atmospheric Δ¹⁴C decreased from ∼700 to ∼100 per mil, modulated by numerous minor excursions. Carbon cycle models suggest that the major features of this record cannot be produced with solar or terrestrial magnetic field modulation alone but also require substantial fluctuations in the carbon cycle.