Parameterization of sinking velocity rates in the Atlantic Ocean

Particles sinking on the ocean constitute the vehicles of the Biological Carbon Pump (BCP). As these particles descend, they give form to a complex mixture of biogeochemical materials, each characterised by distinct size, density, porosity, and morphology. Consequently, the velocity of particle sinking (SV) and the flux of particulate organic carbon (POC) exhibit significant variability, influenced by factors such as depth, season, and the characteristics of the ecosystem. The flux of POC and the SV are interconnected parameters; besides, the profile of POC flux attenuation, i.e. the rate at which sinking particles are remineralised and degraded by bacteria and zooplankton, is also strongly dependent on the rate at which the particles sink. Intuitively, faster sinking particles would reach the Twilight Zone in a greater proportion than slow sinking particles; however, this simple correlation is not globally observed in the ocean. Overall, SV is a key variable directly impacting on the strength of the BCP, in spite of that, the methods to estimate particle SV are not standardized and this variable remains poorly measured in the ocean. Therefore, its influence is not properly quantified, nor is how to incorporate this parameter to ocean biogeochemical models.

The utilisation of the disequilibrium between radioactive pairs, ²³⁴Th-²³⁸U and ²¹⁰Po-²¹⁰Pb, allows obtaining both average SV and downward POC flux. In this study, disequilibrium profiles from 15 cruises in the Atlantic and Southern Oceans were examined (including data from COMICS, CUSTARD, APERO and EXPORTS programs), encompassing biogeochemically contrasting sites and various stages of the bloom. This analysis led to a novel compilation of POC flux and SV, coupled with satellite-driven net primary production (NPP) and including export efficiency and transfer efficiency, when available. The objective of this synthesis is to understand the mechanisms associated with the spatial and temporal variation of the SV and to look for patterns in the Biological Carbon Pump efficiency and, ultimately, ocean carbon storage.