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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/gmd-2019-344
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gmd-2019-344
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: development and technical paper 27 Feb 2020

Submitted as: development and technical paper | 27 Feb 2020

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A revised version of this preprint is currently under review for the journal GMD.

Calibration of key temperature-dependent ocean microbial processes in the cGENIE.muffin Earth system model

Katherine Anne Crichton1,a, Jamie Devereux Wilson2, Andy Ridgwell3, and Paul N. Pearson1 Katherine Anne Crichton et al.
  • 1School of Earth and Ocean Sciences, Cardiff University, UK
  • 2BRIDGE, School of Geographical Sciences, University of Bristol, Bristol, UK
  • 3Department of Earth Sciences, University of California, Riverside, CA 92521, USA
  • anow at: School of Geography, University of Exeter, EX4 4RJ, UK

Abstract. Temperature is a master parameter in the marine carbon cycle, exerting a critical control on the rate of biological transformation of a variety of solid and dissolved reactants and substrates. Although in the construction of numerical models of marine carbon cycling, temperature has been long-recognised as a key parameter in the production and export of organic matter at the ocean surface, it is much less commonly taken into account in the ocean interior. There, bacteria (primarily) transform sinking particulate organic matter into its dissolved constituents and thereby consume dissolved oxygen (and/or other electron acceptors such as sulphate) and release nutrients, which are then available for transport back to the surface. Here we present and calibrate a more complete temperature-dependent representation of marine carbon cycling in the cGENIE.muffin Earth system model, intended for both past and future climate applications. In this, we combine a temperature-dependent remineralisation scheme for sinking organic matter with a biological export production scheme that also includes a temperature-dependent limitation on nutrient uptake in surface waters (and hence phytoplankton growth). Via a parameter ensemble, we jointly calibrate the two parameterisations by statistically contrasting model projected fields of nutrients, oxygen, and the stable carbon isotopic signature (δ13C) of dissolved inorganic carbon in the ocean, with modern observations.

We find that for the present-day, the temperature-dependent version shows as-good-as or better fit to data than the existing tuned non-temperature dependent version of the cGENIE.muffin. The main impact of adding temperature-dependent remineralisation is in driving higher rates of remineralisation in warmer waters and hence a more rapid return of nutrients to the surface there – stimulating organic matter production. As a result, more organic matter is exported below 80 m in mid and low latitude warmer waters as compared to the standard model. Conversely, at higher latitudes, colder water temperature reduces the rate of nutrient supply to the surface as a result of slower in-situ rates of remineralisation.

We also assess the implications of including a more complete set of temperature-dependent parameterisations by analysing a series of historical transient experiments. We find that between the pre-industrial and the present day, in response to a simulated air temperature increase of 0.9 °C and ocean warming of 0.12 °C (0.6 °C in surface waters and 0.02 °C in deep waters), a reduction in POC export at 80 m of just 0.3 % occurs. In contrast, with no assumed temperature-dependent biological processes, global POC export at 80 m falls by 2.9 % between the pre-industrial and present day as a consequence of ocean stratification and reduced nutrient supply to the surface. This suggests that increased nutrient recycling in warmer conditions offsets some of the stratification-induced surface nutrient limitation in a warmer world, and that less carbon (and nutrients) then reaches the inner and deep ocean. This extension to the cGENIE.muffin Earth system model provides it with additional capabilities in addressing marine carbon cycling in warmer past and future worlds.

Katherine Anne Crichton et al.

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Katherine Anne Crichton et al.

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Short summary
Temperature is a key controller of metabolic processes, and also therefore a controller of the ocean's carbon cycle. We present and calibrate a temperature-dependent version of marine carbon cycling in the cGENIE Earth system model. We also present results for transient simulations of the historical industrial period, finding a reduction in biological carbon pump efficiency and an increase in carbon and nutrient recycling rates in warming waters.
Temperature is a key controller of metabolic processes, and also therefore a controller of the...
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