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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-365
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-365
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 07 Oct 2019

Submitted as: research article | 07 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

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

Jennifer E. Dentith1, Ruza F. Ivanovic1, Lauren J. Gregoire1, Julia C. Tindall1, Laura F. Robinson2, and Paul J. Valdes3 Jennifer E. Dentith et al.
  • 1School of Earth and Environment, University of Leeds, Leeds, UK, LS2 9JT
  • 2School of Earth Sciences, University of Bristol, Bristol, UK, BS8 1RJ
  • 3School of Geographical Sciences, University of Bristol, Bristol, UK, BS8 1SS

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.

Jennifer E. Dentith et al.
Interactive discussion
Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Jennifer E. Dentith et al.
Data sets

Supplementary material for the thesis “Modelling carbon isotopes to examine ocean circulation and the marine carbon cycle” J. E. Dentith https://doi.org/10.5518/621

Model code and software

Supplementary material for the thesis “Modelling carbon isotopes to examine ocean circulation and the marine carbon cycle” J. E. Dentith https://doi.org/10.5518/621

Jennifer E. Dentith et al.
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Short summary
We have added three new tracers (a dye tracer and two representations of radiocarbon, 14C) into the ocean of the FAMOUS climate model to study large-scale circulation and the marine carbon cycle. The model performs well compared to modern 14C observations, both spatially and temporally. Proxy 14C records are interpreted in terms of water age, but comparing our dye tracer to our 14C tracer, we find that this is only valid in certain areas; elsewhere, the carbon cycle complicates the signal.
We have added three new tracers (a dye tracer and two representations of radiocarbon, 14C) into...
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