Marine radiocarbon reservoir age simulations for the past 50000 years revisited

Prior to about 14 ka BP, the most recent radiocarbon (14C) calibration curve IntCal20 is based on a combination of terrestrial and marine 14C archives. To gain insight into the spatio-temporal evolution of the involved marine 14C records and their systematic 14C concentration differences from the atmosphere, IntCal20 has considered marine reservoir age (MRA) simulations of the LSG ocean general circulation model. The LSG model was not fully coupled to the atmosphere and did not include a prognostic sea ice component. Instead, it applied various stadial and interstadial climate boundary conditions to assess upper and lower bounds of past climate variations and the associated effects on past MRAs. Here, we present results of new long-term MRA simulations which overcome this limitation. We apply the Earth system model of intermediate complexity CLIMBER-X which we have equipped with 14C and noble gas tracers. CLIMBER-X is forced with insolation, greenhouse gas concentrations, and continental ice sheets. Radiocarbon is prescribed in the atmosphere according to IntCal20. While the new simulations confirm some of the LSG model results at the global scale, there are considerable regional differences. For example, we find weaker inhibition of marine 14CO2 uptake in the presence of sea ice and hence lower polar MRAs than the LSG model. Moreover, we find that continental ice sheet forcing affects MRAs at the ocean-basin scale. This is particularly the case during the last deglaciation for which some meltwater discharge reconstructions could be questioned according to our results.