THE various reservoirs of the global carbon cycle, with their very different residence times, are linked by a complex and evolving system of exchanges for which natural radiocarbon is the most robust tracer(1). Any change in the sizes of these reservoirs, or the exchange rates between them, could perturb the C-14/C-12 ratio of each other reservoir, and the smallest of them--the atmosphere--would be the most sensitive. In particular, high-resolution reconstructions of past atmospheric C-14/C-12 ratios may provide important clues to the mechanisms of abrupt climate change. Annually laminated lake sediments potentially provide an optimal record in this respect, as they preserve information about both past atmospheric C-14 levels and climate changes, providing absolutely dated material beyond the range of tree-ring chronologies and, unlike corals, directly monitor C-14 concentrations in atmospheric CO2. Here we report the relationship between C-14 concentration and climate changes during the Younger Dryas and early Holocene periods, derived from analyses of the annually laminated sediments of Lake Gosciaz, in central Poland. We find that atmospheric C-14 concentrations during the Younger Dryas were abnormally high, which we interpret as a reduced ventilation rate of the deep ocean, most probably as a result of a decrease in intensity of the North Atlantic Deep Water formation.