M.A. Plummer, F.M. Phillips, J. Fabryka-Martin, H.J. Turin, P.E. Wigand, P. P. Sharma

Science 277 (1997) 538-541 www.sciencemag.org

Knowledge of the production history of cosmogenic nuclides, which is needed for geological and archaeological dating, has been uncertain. Measurements of chlorine-36/chlorine(36Cl/36Cl) ratios in fossil packrat middents from Nevada that are radiocarbon dated between about 38 thousand years ago (ka) and the present showed that 36Cl/Cl ratios were higher by a factor of about 2 before ~11 ka. This raises the possibility that cosmogenic production rates just before the close of the Pleistocene were up to 50% higher than is suggested by carbon-14 calibration data. The discrepancy could be explained by addition of low-carbon-14 carbon dioxide to the atmosphere during that period, which would have depressed atmospheric radiocarbon activity. Alternatively, climatic effects on 36Cl deposition may have enhanced the 36Cl/Cl ratios.