14 C Variations During the Upper Pleistocene

Abstract

Parallel determinations of 14 C and ionium dates on a stalagmite from the Cango Caves provide evidence of variations in the 14 C content of the atmosphere beyond the range of the California tree-ring sequence. During the Holocene growth period the 230 Th dates are compatible with the tree-ring calibrated 14 C dates. At 18,000 BP and between 30,000 and 40,000 BP the 14 C ages are markedly younger than the 230 Th ages, suggesting that the 14 C level of the atmosphere was considerably higher at these times. Between the 230 Th ages of 35,000 and 29,000 BP the 14 C ages remain nearly constant at 29,500 BP, indicating that 14 C production must have been drastically reduced during this period. The 14 C fluctuation is greater than that predicted by Barbetti (1980) but it may be explained by postulating a substantial increase in the geomagnetic dipole field, for which there is mounting evidence.