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To achieve more accurate Earth system model projections of diverse climate scenarios, researchers need observation-based data on the movement of carbon between reservoirs, and especially across tropical regions. The Tropical Low-Pressure Belt (TLPB) is a key driver of atmospheric circulation across lower latitudes. While the TLPB shifts across the east–west extent of northern Africa, the extent to which 14C concentrations apply to Afrotropical forests remains untested, restricting our understanding about other carbon feedbacks. Here, we present a high-precision atmospheric 14C record (1940–2012) from a lowland tropical tree species (Entandrophragma utile) in Cameroon. We included 107 measurements from whole rings and 15 intra-annual slices. The intra-annual 14C data from 1962, 1963, and 1964 confirm a 7-month long growing season (April–November) with a photosynthetic profile typical of Northern Hemisphere (NH) sites, and showing no nonstructural-carbohydrate interference. The full 14C record reveals that air masses reaching the site were derived primarily from Southern Hemisphere (SH) readings followed by recycled bomb-14C signals from soil and litter. Radiocarbon results were substantiated by HYSPLIT model trajectories coupled with NCEP/NCAR reanalysis data. The paradox of finding that tropical NH trees grow using 14CO2 of SH air masses and land-surface respiration challenges existing zonal 14C classifications. Our findings highlight an essential role for robust observational 14C data in refining atmospheric models and improving carbon-cycle assessments across distinct climate zones. 

To achieve more accurate Earth system model projections of diverse climate scenarios, researchers need observation-based data on the movement of carbon between reservoirs, and especially across tropical regions. The Tropical Low-Pressure Belt (TLPB) is a key driver of atmospheric circulation across lower latitudes. While the TLPB shifts across the east–west extent of northern Africa, the extent to which 14C concentrations apply to Afrotropical forests remains untested, restricting our understanding about other carbon feedbacks. Here, we present a high-precision atmospheric 14C record (1940–2012) from a lowland tropical tree species (Entandrophragma utile) in Cameroon. We included 107 measurements from whole rings and 15 intra-annual slices. The intra-annual 14C data from 1962, 1963, and 1964 confirm a 7-month long growing season (April–November) with a photosynthetic profile typical of Northern Hemisphere (NH) sites, and showing no nonstructural-carbohydrate interference. The full 14C record reveals that air masses reaching the site were derived primarily from Southern Hemisphere (SH) readings followed by recycled bomb-14C signals from soil and litter. Radiocarbon results were substantiated by HYSPLIT model trajectories coupled with NCEP/NCAR reanalysis data. The paradox of finding that tropical NH trees grow using 14CO2 of SH air masses and land-surface respiration challenges existing zonal 14C classifications. Our findings highlight an essential role for robust observational 14C data in refining atmospheric models and improving carbon-cycle assessments across distinct climate zones.