An improved model of particle attenuation reduces estimates of Southern Ocean carbon transfer efficiency

The downward transport of organic particles produced by marine organisms is a key control on the ocean's carbon storage. Measurements of particle attenuation through the water column have historically been used to infer the sequestration potential of the biological carbon pump. While often modelled using a single power-law fit, this simplification may obscure important depth-dependent processes. Using data from biogeochemical-Argo floats in the Southern Ocean, here we show that splitting the water column into two distinct regimes captures depth-dependent variability in particle attenuation more effectively. Compared to the single power-law fit the model reveals greater attenuation just below the productive layer, reducing particulate organic carbon flux into the mesopelagic (200 - 400 m) by 40 - 60 %. Our findings suggest a more mechanistic representation of particle attenuation is needed to improve estimates of carbon transport and the durability of biologically-based marine carbon dioxide removal technologies, and to reduce uncertainties in ocean-climate feedbacks and future climate projections.