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

• Main Authors: Dentith, Jennifer E., Ivanovic, Ruza F., Gregoire, Lauren J., Tindall, Julia C., Robinson, Laura F., Valdes, Paul J.
• Format: Text
• Language: English
• Published: 2019
• Subjects: North Atlantic Deep Water; North Atlantic
• Online Access: https://doi.org/10.5194/bg-2019-365; https://www.biogeosciences-discuss.net/bg-2019-365/

Constraining ocean circulation and its temporal variability is crucial for understanding changes in surface climate and the carbon cycle. Radiocarbon ( 14 C) is often used as a geochemical tracer of ocean circulation, but interpreting ∆ 14 C in geological archives is complex. Isotope-enabled models enable us to directly compare simulated ∆ 14 C values to Δ 14 C measurements and investigate plausible mechanisms for the observed signals. We have added three new tracers (water age, abiotic 14 C, and biotic 14 C) 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 ∆ 14 C 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 ∆ 14 C in geological archives from shallow-to-intermediate water depths across the North Atlantic. The model captures the large-scale structure and range of ∆ 14 C values (both spatially and temporally) suggesting that, on the whole, the uptake and transport of 14 C 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 14 C tracer as a sensitive, independent tuning diagnostic. We also examine the importance of the biological pump for deep ocean 14 C concentrations and assess the extent to which 14 C can be interpreted as a ventilation tracer. Comparing the simulated biotic and abiotic δ 14 C, we infer that biology has a spatially heterogeneous influence on 14 C 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 14 C ages in FAMOUS demonstrates that interpreting proxy ∆ 14 C measurements in terms of ventilation alone could lead to erroneous conclusions about palaeocean circulation. Specifically, our results suggest that ∆ 14 C is only a faithful proxy for water age in regions with strong convection; elsewhere, the temperature dependence of the solubility of CO 2 in seawater complicates the signal.