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

Abstract

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

Simulating stable carbon isotopes in the ocean component of the FAMOUS general circulation model with MOSES1 (XOAVI)

“…Specifically, FAMOUS does not capture the observed structure in the Atlantic basin because, in this generation of the model, the AMOC is characterised by an overdeep North Atlantic Deep Water (NADW) cell and insufficient Antarctic Bottom Water formation (Dentith et al, 2019b). FAMOUS also simulates weak (less than 3 Sv) ventilation to depths of 2000 m in the North Pacific Ocean (Dentith et al, 2019b), which prevents the accumulation of old, 12 C-enriched (low δ 13 C DIC ) waters at inter- Figure 8. Zonally averaged mean annual mixed layer δ 13 C POC during the 1990s: observations (Goericke and Fry, 1994; red), the std simulation (black), the L95 simulation (grey), and the L97 simulation (blue).…”

“…Crucially, this study only ran each perturbed parameter simulation for 200 years and neglected to evaluate the strength and structure of the AMOC. The optimal parameter set therefore had small but important imbalances in the surface climate, which caused the AMOC to collapse over longer (multi-millennial) timescales (Dentith et al, 2019b).…”

“…modifying the remineralisation rate and/or the export ratio). Longer term, we propose that the addition of carbon isotope ratios as tuning targets (both the δ 13 C presented here and the new 14 C tracer for FAMOUS discussed by Dentith et al, 2019a) would improve the work of Williams et al (2013) because they provide an objective and straightforward way of assessing whether the balance between all of the ecological processes in the model is correct.…”

“…There are three naturally occurring carbon isotopes: the stable isotopes 12 C (98.9 %) and 13 C (1.1 %), and the radioactive isotope 14 C (1.2 × 10 −10 %), which is also known as radiocarbon (Key, 2001). In this study, we focus on the stable isotopes, with 14 C being discussed in detail elsewhere (Dentith et al, 2019a). The relative proportions of 12 C and 13 C in a given oceanic pool (e.g.…”

“…We added 13 C as an optional passive tracer into the ocean component of FAMOUS, using the Met Office Surface Exchange Scheme (MOSES) version 1 (Cox et al, 1999) generation of the model to evaluate our scheme. Although a newer version of the land surface model exists, which includes the terrestrial carbon cycle and interactive vegetation (MOSES2.2; Essery et al, 2001Essery et al, , 2003Williams et al, 2013;Valdes et al, 2017), problems have been identified with its representation of meridional overturning circulation (MOC) in multi-millennial simulations with constant pre-industrial boundary conditions (Dentith et al, 2019b). Specifically, FAMOUS-MOSES2.2 simulates a collapsed Atlantic MOC (AMOC) and a strong, deep Pacific MOC when the run length exceeds 6000 years, resulting in spurious ocean tracer distributions.…”