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Paired measurements of C-14/C-12 and Th-230 ages from two Hulu Cave stalagmites complete a precise record of atmospheric C-14 covering the full range of the C-14 dating method (similar to 54,000 years). Over the last glacial period, atmospheric C-14/C-12 ranges from values similar to modern values to values 1.70 times higher (42,000 to 39,000 years ago). The latter correspond to C-14 ages 5200 years less than calibrated ages and correlate with the Laschamp geomagnetic excursion followed by Heinrich Stadial 4. Millennial-scale variations are largely attributable to Earth's magnetic field changes and in part to climate-related changes in the oceanic carbon cycle. A progressive shift to lower C-14/C-12 values between 25,000 and 11,000 years ago is likely related, in part, to progressively increasing ocean ventilation rates.