Simulated radiocarbon cycle revisited by considering the bipolar seesaw and benthic 14C data

Carbon cycle models used to calculate the marine reservoir age of the non-polar surface ocean (called Marine20) out of IntCal20, the compilation of atmospheric ${\Delta }^{14}$C, have so far neglected a key aspect of the millennial-scale variability connected with the thermal bipolar seesaw: changes in the strength of the Atlantic meridional overturning circulation (AMOC) related to Dansgaard/Oeschger and Heinrich events. Here we implement such AMOC changes in the carbon cycle box model BICYCLE-SE to investigate how model performance over the last 55 kyr is affected, in particular with respect to available ¹⁴C and CO₂ data. Constraints from deep ocean ¹⁴C data suggest that the AMOC in the model during Heinrich stadial 1 needs to be highly reduced or even completely shutdown. Ocean circulation and sea ice coverage combined are the processes that almost completely explain the simulated changes in deep ocean ¹⁴C age, and these are also responsible for a glacial drawdown of ∼60 ppm of atmospheric CO₂. We find that the implementation of abrupt reductions in AMOC during Greenland stadials in the model setup that was previously used for the calculation of Marine20 leads to differences of less than ±100 ¹⁴C yrs. The representation of AMOC changes therefore appears to be of minor importance for deriving non-polar mean ocean radiocarbon calibration products such as Marine20, where atmospheric carbon cycle variables are forced by reconstructions. However, simulated atmospheric CO₂ exhibits minima during AMOC reductions in Heinrich stadials, in disagreement with ice core data. This mismatch supports previous suggestions that millennial-scale changes in CO₂ were probably not driven directly by the AMOC, but rather by biological and physical processes in the Southern Ocean and by contributions from variable land carbon storage.