CL-36 IN FOSSIL RAT URINE - AN ARCHIVE OF COSMOGENIC NUCLIDE DEPOSITION DURING THE PAST 40,000 YEARS

Knowledge of the production history of cosmogenic nuclides, which is n eeded for geological and archaeological dating, has been uncertain. Me asurements of chlorine-36/chlorine (Cl-36/Cl) ratios in fossil packrat middens from Nevada that are radiocarbon-dated between about 38 thous and years ago (ka) and the present showed that Cl-36/Cl ratios were hi gher by a factor of about 2 before similar to 11 ka. This raises the p ossibility that cosmogenic production rates just before the close of t he Pleistocene were up to 50% higher than is suggested by carbon-14 ca libration data. The discrepancy could be explained by addition of low- carbon-14 carbon dioxide to the atmosphere during that period, which w ould have depressed atmospheric radiocarbon activity. Alternatively, c limatic effects on Cl-36 deposition may have enhanced the Cl-36/Cl rat ios.