Radiocarbon activity of dissolved inorganic carbon has been measured in the northwestern Black Sea. Both continental shelf and open-sea profiles show that surface waters are in equilibrium with the atmosphere. The observed distribution of 14 C activity shows a weak contribution of the deep 14 C-depleted CO 2 to the photic zone. Such a distribution of 14 C within the water column is unable to explain the aging of sedimentary organic matter and reservoir ages greater than 500 yr. A contribution of production by chemoautotrophic bacteria feeding on 14 C-depleted methane at the boundary of the oxic and anoxic zones is a realistic hypothesis. Also, a contribution to sedimentary organic carbon estimated at <15% of the photosyn-thetic primary production could explain 14 C reservoir ages greater than 1300 yr.