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

• Main Author: Badger, Marcus P. S.
• Format: Text
• Language: English
• Published: 2020
• Subjects: ice core
• Online Access: https://doi.org/10.5194/bg-2020-356; https://bg.copernicus.org/preprints/bg-2020-356/

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 CO 2 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 CO 2 reconstructions from multiple sites over the Pleistocene, which allows comparison to be made with ice core CO 2 records. Interrogating these records reveal that the relationship between proxy- and ice core-CO 2 breaks down when local aqueous CO 2 concentration falls below 7 μmol L −1 . The recognition of this threshold explains why many alkenone based CO 2 records fail to accurately replicate ice core CO 2 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.