High-resolution physical-biogeochemical modelling in the Northeast Atlantic Ocean: mechanisms driving carbon flux attenuation in the mesopelagic realm

The biological carbon pump includes all the biological processes involving the production of organic carbon in the euphotic layer and its export towards the deep ocean, where it will be stored and isolated from the atmosphere for centuries. The main export pathway is via the sinking of particles by gravity, known as the gravitational pump. Another important one is the migrant pump, sustained by a specific behaviour called zooplankton Diel Vertical Migration (DVM). Migrant organisms feed in the euphotic layer at night and hide from predators at depth during daytime, thus actively transferring carbon from the surface to the migration depth. The export flux of carbon is attenuated along its way to the dark ocean, mainly by heterotrophic processes linked to prokaryotes and zooplankton. The strongest decline occurs in what is called the mesopelagic zone (approximately 100-1000 m). The study of Kwon et al. (2009) demonstrated the crucial control of this attenuation on atmospheric CO2 concentrations and thus on Earth’s climate. Yet, the processes at work in the mesopelagic realm remain poorly understood, as it was underlined by Wilson et al. (2022).

In the present study, we address this issue through a modelling approach. We perform realistic 3D coupled physical-biogeochemical simulations at an unusually high horizontal resolution (2 km). We use CROCO (Coastal and Regional Ocean Community, Mason et al., 2010) and PISCES (Pelagic Interactions Scheme for Carbon and Ecosystem Studies, Aumont et al., 2015) models, respectively for the physical and biogeochemical compartments. The simulations take place in the Northeast Atlantic Ocean during the years 2020-2024. This work is part of the APERO ANR (Assessing marine biogenic matter Production, Export and Remineralization: from the surface to the dark Ocean), which is based on a cruise that took place during the summer of 2023 near the PAP station (Porcupine Abyssal Plain, located southwest of Ireland).

We aim to shed light on the main mechanisms driving carbon flux attenuation in the mesopelagic realm, and to understand the role of small scales on carbon export and storage in the deep ocean. More specifically, using results from the APERO cruise, we will focus on how to improve the parametrisation of zooplankton DVM and particle sinking velocities in the PISCES model.