Variations of Younger Dryas atmospheric radiocarbon explicable without ocean circulation changes

The concentration of radiocarbon, C-14, in the atmosphere depends on its production rate by cosmic rays, and on the intensity of carbon exchange between the atmosphere and other reservoirs, for example the deep oceans. For the Holocene (the past similar to 11,500 years), it has been shown that fluctuations in atmospheric radiocarbon concentrations have been caused mostly by variations in the solar magnetic field(1-3). Recent progress in extending the radiocarbon record backwards in time(4-10) has indicated especially high atmospheric radiocarbon concentrations in the Younger Dryas cold period, between 12,700 and 11,500 years before the present. These high concentrations have been interpreted as a result of a reduced exchange with the deep-ocean reservoir, caused by a drastic weakening of the deep-ocean ventilation(7-9,11,12). Here we present a high-resolution reconstruction of atmospheric radiocarbon concentrations, derived from annually laminated sediments of two Polish lakes, Lake Gosciaz and Lake Perespilno. These records indicate that the maximum in atmospheric radiocarbon concentrations in the early Younger Dryas was smaller than previously believed, and might have been caused by variations in solar activity. If so, there is no indication that the deep-ocean ventilation in the Younger Dryas was significantly different from today's.