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Abstract Constraining radiocarbon ( 14 C) reservoir age offsets is critical to deriving accurate calendar-age chronologies from 14 C dating of materials which did not draw carbon directly from the atmosphere. The application of 14 C dating to such materials is severely limited in hydrologically sensitive environments like the Black Sea because of the difficulty to quantify reservoir age offsets, which can vary quickly and significantly through time, due to the dynamics of the biogeochemical cycling of carbon. Here we reconstruct 14 C reservoir age offsets (R shell-atm ) of Holocene bivalve shells from the coastal Black Sea relatively to their contemporaneous atmosphere. We show that the 14 C reservoir age offset and the stable carbon isotope composition of bivalve shells are linearly correlated in this region. From a biogeochemical standpoint, this suggests that inorganic stable carbon isotope and 14 C compositions of Black Sea coastal waters are controlled by the balance between autochthonous primary productivity and heterotrophic respiration of allochthonous pre-aged terrestrial organic matter supplied by rivers. This provided an important implication for Black Sea geochronology as the reservoir age offset of 14 C-dated bivalve shell can be inferred from its stable carbon isotope composition. Our results provide a fundamental and inexpensive geochemical tool which will considerably improve the accuracy of Holocene calendar age chronologies in the Black Sea.