Assessing the ability of the 14C projection-age method to constrain the circulation of the past in a 3-D ocean model

Radiocarbon differences between benthic and planktonic foraminifera (B-P ages) and radiocarbon projection ages are both used to determine changes of the past ocean circulation rate. A global 3-D ocean circulation model with a constant modern ocean circulation is used to study which method is less influenced by atmospheric Delta(14)C variations. Three factors cause uncertainties: first, the long equilibration time of the ocean after atmospheric Delta(14)C changes; second, different mixing processes in the ocean, which cause an ocean response of smaller amplitude than the atmospheric forcing; and third, the unknown source region and corresponding initial surface (14)C reservoir age of subsurface waters. The model suggests that BP ages and projection ages have lower uncertainties the closer they are to deepwater formation zones. In the North Atlantic the B-P age method is less influenced by atmospheric Delta(14)C variations than the projection-age method. Projections ages vary less in the Pacific as long as atmospheric Delta(14)C decreases linearly. A more irregular atmospheric Delta(14)C evolution leads to age variations of similar magnitude with both methods. On the basis of the model experiment, we suggest a potential improvement of the projection-age method.