Studies of temporal changes of ocean circulation and deepwater ventilation often rely on δ13C records of epibenthic foraminifer Cibicidoides wuellerstorfi. However, primary productivity-related overprints may distort the signal and simulate a chemical age of ambient water mass that is too old and simulates poorly ventilated ambient bottom waters. To further constrain the use of C. wuellerstorfi δ13C records from high-productivity areas, we analyzed a 14CAMS-dated gravity core from the upwelling regime off northwest Africa at 12°N. We compare this new record with 37 radiocarbon dated δ13C records from the eastern Atlantic Ocean between 45°N and 25°S that are bathed by the same water mass. Only during Heinrich events 1 and 2, when the investigated core site off northwest Africa experienced year-round, sustained deposition of organic matter, the δ13C values at this site faithfully record deepwater ventilation states. During times of predominantly seasonal deposition of fresh phytodetritus, however, δ13C values were significantly lower than at the reference sites. This underscores that reconstruction of paleocirculation and deep ocean ventilation using C. wuellerstorfi δ13C from regions that experienced seasonal phytodetritus deposition needs to be validated by additional proxies that are not affected by local productivity.