Changes in C-14 activity over time during vacuum distillation of carbon from rock pore water

The radiocarbon activity of carbon collected by vacuum distillation from a single partially saturated tuff began to decline after approximately 60% of the water and carbon had been extracted. Disproportionate changes in C-14 activity and delta(13)C during distillation rule out simple isotopic fracti onation as a causative explanation. Additional phenomena such as matrix dif fusion and ion exclusion in micropores may play a role in altering the isot opic value of extracted carbon, but neither can fully account for the obser ved changes. The most plausible explanation is that distillation recovers c arbon from an adsorbed phase that is depleted in C-14 relative to DIC in th e bulk pore water.