Global patterns in small organic particle loss in the mesopelagic ocean

Abstract The majority of particulate organic carbon in the ocean’s mesopelagic zone is contained in particles smaller than 100 µm. However, the challenges of measuring their abundance and carbon content using shipboard techniques has limited our understanding of their role in the ocean carbon cycle. Here we use a 4D Biogeochemical-Argo based data product to investigate the spatial distribution and seasonality in small particles, as well as their loss in the mesopelagic zone. We find that small particle loss in the mesopelagic is greater at high latitudes than at low latitudes. We examine several hypotheses that might explain this observed pattern, although limitations in the extant data prevent us drawing definitive conclusions. Contrary to expectations, we find that temperature-driven remineralisation is unlikely to be the primary driver of the observed patterns, and instead find that particle lability may be a significant factor in explaining this unexpected result.