VARIATIONS IN C-14 RESERVOIR AGES OF BLACK SEA WATERS AND SEDIMENTARY ORGANIC CARBON DURING ANOXIC PERIODS: INFLUENCE OF PHOTOSYNTHETIC VERSUS CHEMOAUTOTROPHIC PRODUCTION

Radiocarbon activity of dissolved inorganic carbon has been measured in the northwestern Black Sea. Both continental she If and open-sea profiles show that surface waters are in equilibrium with the atmosphere. The observed distribution of C-14 activity shows a weak contribution of the deep C-14-depleted CO2 to the photic zone. Such a distribution of C-14 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 C-14-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 photosynthetic primary production could explain C-14 reservoir ages greater than 1300 yr.