Strong particle dynamics counteract the nutrient-pumping effect leading to weak carbon flux in a cyclonic eddy

Sinking particles play a crucial role in transferring carbon from the atmosphere to the deep ocean. However, due to intensive particle transformations such as aggregation, disaggregation, and remineralization, only a small portion of the organic carbon produced in the euphotic zone ends up being sequestered in the deep ocean or sediment. Mesoscale eddies can significantly impact the surface ocean nutrient budget, primary production, and carbon export. Despite this, there is still a lack of research on how particle dynamics in eddy-impacted regions affect the efficiency of carbon export. In this study, we used observations of thorium isotopes (²³⁴Th and ²²⁸Th) and particulate organic carbon (POC) at two stations in the South China Sea (TS1: a decaying-eddy-impacted station and SEATS: an oligotrophic station) and an inverse model to investigate the impact of particle dynamics on particle export efficiency. Our findings indicate that particle remineralization/fragmentation was enhanced inside the eddy, which counteracted the nutrient pumping effect that promotes surface ocean productivity and eventually led to even lower carbon flux compared to the oligotrophic station.