Convection injects labile particulate organic carbon to the deep ocean

Vertical fluxes of particulate organic carbon (POC) are central to the oceanic carbon cycle, yet the role and interannual variability of convective POC transport remain poorly constrained. Autonomous profilers drifting in the Labrador and Irminger Seas detected recurrent winter-spring chlorophyll a fluorescence peaks at 1000 meters during intense convection in 2014-2017. The bio-optical properties of the particles indicate deep mixing of surface POC rich in labile phytoplanktonic material, distinct from both background suspended particles and sinking aggregates. Model simulations show that the mixing-driven POC supply to the deep ocean (500 to 2000 meters) scales with convection volume (R² ≈ 0.88) and is typically dominated by planktonic biomass. According to the model, vertical mixing regionally contributes ∼33 to 44% of the combined POC supply from mixing and sinking to the 500 to 2000-meter layer during strong convection years. These convective injection episodes likely reshape deep-ocean carbon budgets by enhancing metabolism and driving lateral exports, linking upper-ocean dynamics to deep-ocean water masses and ecosystems.