d13CO2 measured in Antarctic ice cores provides constraints on oceanic and terrestrial carbon cycle processes linked with millennial-scale changes in atmospheric CO2. However, the interpretation of d13CO2 is not straightforward. Using carbon isotope-enabled versions of the LOVECLIM and Bern3D models, we perform a set of sensitivity experiments in which the formation rates of North Atlantic Deep Water (NADW), North Pacific Deep Water (NPDW), Antarctic Bottom Water (AABW), and Antarctic Intermediate Water (AAIW) are varied. We study the impact of these circulation changes on atmospheric d13CO2 as well as on the oceanic d13C distribution. In general, we find that the formation rates of AABW, NADW, NPDW, and AAIW are negatively correlated with changes in d13CO2: namely, strong oceanic ventilation decreases atmospheric d13CO2. However, since large-scale oceanic circulation reorganizations also impact nutrient utilization and the Earth's climate, the relationship between d13CO2 levels and ocean ventilation rate is not unequivocal. In both models atmospheric d13CO2 is very sensitive to changes in AABW formation rates: increased AABW formation enhances the transport of low d13C waters to the surface and decreases atmospheric d13CO2. By contrast, the impact of NADW changes on atmospheric d13CO2 is less robust and might be model dependent. This results from complex interplay between global climate, carbon cycle, and the formation rate of NADW, a water body characterized by relatively high d13C.
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