Incorporating the stable carbon isotope 13C in the ocean biogeochemical component of the Max Planck Institute Earth System Model

• Published in: Biogeosciences
• Main Authors: Liu, Bo, Six, Katharina D., Ilyina, Tatiana
• Format: Text
• Language: English
• Published: 2021
• Subjects: Eide; Indian; Pacific; North Atlantic
• Online Access: https://doi.org/10.5194/bg-18-4389-2021; https://bg.copernicus.org/articles/18/4389/2021/

The stable carbon isotopic composition ( δ 13 C) is an important variable to study the ocean carbon cycle across different timescales. We include a new representation of the stable carbon isotope 13 C into the HAMburg Ocean Carbon Cycle model (HAMOCC), the ocean biogeochemical component of the Max Planck Institute Earth System Model (MPI-ESM). 13 C is explicitly resolved for all oceanic carbon pools considered. We account for fractionation during air–sea gas exchange and for biological fractionation ϵ p associated with photosynthetic carbon fixation during phytoplankton growth. We examine two ϵ p parameterisations of different complexity: <math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="italic">ϵ</mi><mi mathvariant="normal">p</mi><mi mathvariant="normal">Popp</mi></msubsup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="27pt" height="19pt" class="svg-formula" dspmath="mathimg" md5hash="21ffb3d4283b593a5c32b93ad4c83b61"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-4389-2021-ie00001.svg" width="27pt" height="19pt" src="bg-18-4389-2021-ie00001.png"/></svg:svg> varies with surface dissolved CO 2 concentration ( Popp et al. , 1989 ) , while <math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="italic">ϵ</mi><mi mathvariant="normal">p</mi><mi mathvariant="normal">Laws</mi></msubsup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="27pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="ef0e7fefce486a332941c8042f82bff5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-4389-2021-ie00002.svg" width="27pt" height="17pt" src="bg-18-4389-2021-ie00002.png"/></svg:svg> additionally depends on local phytoplankton growth rates ( Laws et al. , 1995 ) . When compared to observations of δ 13 C of dissolved inorganic carbon (DIC), both parameterisations yield similar performance. However, with regard to δ 13 C in particulate organic carbon (POC) <math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="italic">ϵ</mi><mi mathvariant="normal">p</mi><mi mathvariant="normal">Popp</mi></msubsup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="27pt" height="19pt" class="svg-formula" dspmath="mathimg" md5hash="a642317d1f7c8c34fa247910c51e1ec5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-4389-2021-ie00003.svg" width="27pt" height="19pt" src="bg-18-4389-2021-ie00003.png"/></svg:svg> shows a considerably improved performance compared to <math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="italic">ϵ</mi><mi mathvariant="normal">p</mi><mi mathvariant="normal">Laws</mi></msubsup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="27pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="ef117615e532cfdac081eb046738c445"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-4389-2021-ie00004.svg" width="27pt" height="17pt" src="bg-18-4389-2021-ie00004.png"/></svg:svg> . This is because <math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="italic">ϵ</mi><mi mathvariant="normal">p</mi><mi mathvariant="normal">Laws</mi></msubsup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="27pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="f89f65639b16a81d9805bf36c55af4b8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-4389-2021-ie00005.svg" width="27pt" height="17pt" src="bg-18-4389-2021-ie00005.png"/></svg:svg> produces too strong a preference for 12 C, resulting in δ 13 C POC that is too low in our model. The model also well reproduces the global oceanic anthropogenic CO 2 sink and the oceanic 13 C Suess effect, i.e. the intrusion and distribution of the isotopically light anthropogenic CO 2 in the ocean. The satisfactory model performance of the present-day oceanic δ 13 C distribution using <math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="italic">ϵ</mi><mi mathvariant="normal">p</mi><mi mathvariant="normal">Popp</mi></msubsup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="27pt" height="19pt" class="svg-formula" dspmath="mathimg" md5hash="600f5ab346414439396331297d4e8fc3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-4389-2021-ie00006.svg" width="27pt" height="19pt" src="bg-18-4389-2021-ie00006.png"/></svg:svg> and of the anthropogenic CO 2 uptake allows us to further investigate the potential sources of uncertainty of the Eide et al. ( 2017 a ) approach for estimating the oceanic 13 C Suess effect. Eide et al. ( 2017 a ) derived the first global oceanic 13 C Suess effect estimate based on observations. They have noted a potential underestimation, but their approach does not provide any insight about the cause. By applying the Eide et al. ( 2017 a ) approach to the model data we are able to investigate in detail potential sources of underestimation of the 13 C Suess effect. Based on our model we find underestimations of the 13 C Suess effect at 200 m by 0.24 ‰ in the Indian Ocean, 0.21 ‰ in the North Pacific, 0.26 ‰ in the South Pacific, 0.1 ‰ in the North Atlantic and 0.14 ‰ in the South Atlantic. We attribute the major sources of underestimation to two assumptions in the Eide et al. ( 2017 a ) approach: the spatially uniform preformed component of δ 13 C DIC in year 1940 and the neglect of processes that are not directly linked to the oceanic uptake and transport of chlorofluorocarbon-12 (CFC-12) such as the decrease in δ 13 C POC over the industrial period. The new 13 C module in the ocean biogeochemical component of MPI-ESM shows satisfying performance. It is a useful tool to study the ocean carbon sink under the anthropogenic influences, and it will be applied to investigating variations of ocean carbon cycle in the past.