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https://doi.org/10.5194/bg-2020-356
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-356
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  02 Oct 2020

02 Oct 2020

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This preprint is currently under review for the journal BG.

Alkenone isotopes show evidence of active carbon concentrating mechanisms in coccolithophores as aqueous carbon dioxide concentrations fall below 7 μmol L−1

Marcus P. S. Badger Marcus P. S. Badger
  • School of Environment, Earth & Ecosystem Sciences, The Open University, Milton Keynes, MK7 6AA, UK

Abstract. Coccolithophores and other haptophyte algae acquire the carbon required for metabolic processes from the water in which they live. Whether carbon is actively moved across the cell membrane via a carbon concentrating mechanism, or passively through diffusion, is important for haptophyte biochemistry. The possible utilisation of carbon concentrating mechanisms also has the potential to over-print one proxy method by which ancient atmospheric CO2 is reconstructed using alkenone isotopes. Here I show that carbon concentrating mechanisms are likely used when aqueous carbon dioxide concentrations are below 7 μmol L−1. I use published alkenone based CO2 reconstructions from multiple sites over the Pleistocene, which allows comparison to be made with ice core CO2 records. Interrogating these records reveal that the relationship between proxy- and ice core-CO2 breaks down when local aqueous CO2 concentration falls below 7 μmol L−1. The recognition of this threshold explains why many alkenone based CO2 records fail to accurately replicate ice core CO2 records, and suggests the alkenone proxy is likely robust for much of the Cenozoic when this threshold was unlikely to be reached in much of the global ocean.

Marcus P. S. Badger

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Marcus P. S. Badger

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Latest update: 26 Oct 2020
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Short summary
Reconstructing ancient atmospheric CO2 is an important aim of palaeoclimate science in order to understand the Earth's climate system. One method, the alkenone proxy based on molecular fossils of coccolithophores, has been recently shown to be ineffective at low to moderate CO2 levels. In this paper I show that this is likely due to changes in the biogeochemistry of the coccolithophores when there is low carbon availability, but for much of the Cenozoic the alkenone proxy should have utility.
Reconstructing ancient atmospheric CO2 is an important aim of palaeoclimate science in order to...
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