Preprints
https://doi.org/10.5194/egusphere-2025-2566
https://doi.org/10.5194/egusphere-2025-2566
08 Jul 2025
 | 08 Jul 2025
Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Earth system models overestimate the sensitivity of apparent oxygen utilisation to age change in the deep ocean

Damien Couespel, Xabier Davila, Nadine Goris, Emil Jeansson, Siv K. Lauvset, and Jerry Tjiputra

Abstract. The biological carbon pump (BCP), involving photosynthesis at the surface and remineralisation at depth, maintains a significant vertical gradient in dissolved inorganic carbon (DIC), promoting the ocean's ability to absorb atmospheric CO2. Remineralised DIC is a good indicator of the strength of the BCP. It can be estimated from apparent oxygen utilisation (AOU) that measures the deficit of oxygen compared to saturation. AOU is projected to increase under climate change due to changes in remineralisation rates and circulation. However, the amplitude of the change is still uncertain. Here, we identify linear relationships between AOU trends and age trends in the deep ocean in simulations of the contemporary (1972–2013) and future (2015–2099) periods from five Earth system models (ESMs). Linear relationships identified within observational data for the contemporary period indicate that ESMs overestimate the sensitivity of AOU to age changes in the deep ocean. The study highlights the stability over time of the AOU sensitivity to age changes, suggesting an overestimation of the BCP strengthening inferred from AOU. Furthermore, our analysis underscores the substantial role of circulation slowdown in increasing remineralised DIC. These insights emphasise the challenges and opportunities to constrain future BCP projections due to circulation uncertainties.

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Damien Couespel, Xabier Davila, Nadine Goris, Emil Jeansson, Siv K. Lauvset, and Jerry Tjiputra

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Damien Couespel, Xabier Davila, Nadine Goris, Emil Jeansson, Siv K. Lauvset, and Jerry Tjiputra
Damien Couespel, Xabier Davila, Nadine Goris, Emil Jeansson, Siv K. Lauvset, and Jerry Tjiputra

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Short summary
Remineralised carbon storage is projected to increase along the 21st century, but the magnitude of increase varies depending on the Earth system models. To constrain the projections, we explore the relation between remineralised carbon and circulation in the deep ocean. Comparing model simulations and observations, we show that models overestimate the sensitivity of remineralised carbon storage to circulation slowdown, suggesting an overestimation of the future remineralised carbon increase.
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