Simulating oceanic radiocarbon with the FAMOUS GCM: implications for its use as a proxy for ventilation and carbon uptake

Abstract. Constraining ocean circulation and its temporal variability is crucial for understanding changes in surface climate and the carbon cycle. Radiocarbon (¹⁴C) is often used as a geochemical tracer of ocean circulation, but interpreting ∆¹⁴C in geological archives is complex. Isotope-enabled models enable us to directly compare simulated ∆¹⁴C values to Δ¹⁴C measurements and investigate plausible mechanisms for the observed signals. We have added three new tracers (water age, abiotic ¹⁴C, and biotic ¹⁴C) to the ocean component of the FAMOUS General Circulation Model to study large-scale ocean circulation and the marine carbon cycle. Following a 10 000 year spin-up, we prescribed the Suess effect (the isotopic imprint of anthropogenic fossil fuel burning) and the bomb pulse (the isotopic imprint of thermonuclear weapons testing) in a transient simulation spanning 1765 to 2000 CE. To validate the new isotope scheme, we compare the model output to direct ∆¹⁴C observations in the surface ocean (pre-bomb and post-bomb) and at depth (post-bomb only). We also compare the timing, shape and amplitude of the simulated marine bomb spike to ∆¹⁴C in geological archives from shallow-to-intermediate water depths across the North Atlantic. The model captures the large-scale structure and range of ∆¹⁴C values (both spatially and temporally) suggesting that, on the whole, the uptake and transport of ¹⁴C are well represented in FAMOUS. Differences between the simulated and observed values arise due to physical model biases (such as weak surface winds and over-deep North Atlantic Deep Water), demonstrating the potential of the ¹⁴C tracer as a sensitive, independent tuning diagnostic. We also examine the importance of the biological pump for deep ocean ¹⁴C concentrations and assess the extent to which ¹⁴C can be interpreted as a ventilation tracer. Comparing the simulated biotic and abiotic δ¹⁴C, we infer that biology has a spatially heterogeneous influence on ¹⁴C distributions in the surface ocean (between 18 and 30 ‰), but a near constant influence at depth (≈ 20 ‰). Nevertheless, the decoupling between the simulated water ages and the simulated ¹⁴C ages in FAMOUS demonstrates that interpreting proxy ∆¹⁴C measurements in terms of ventilation alone could lead to erroneous conclusions about palaeocean circulation. Specifically, our results suggest that ∆¹⁴C is only a faithful proxy for water age in regions with strong convection; elsewhere, the temperature dependence of the solubility of CO₂ in seawater complicates the signal.