The role of biologically-mediated fragmentation in carbon flux attenuation in a 1-D mechanistic particle model

Abstract. The efficiency of particulate organic carbon (POC) transport through the water column is the foundation of the biological carbon pump’s capability to sequester atmospheric CO₂ into the ocean interior over prolonged timescales. However, constraining the magnitude and composition of POC flux reaching ~ 1 km depth remains challenging, due to diverse biotic and abiotic processes that alter particle morphology, composition and sinking speed. Recent studies have highlighted biologically-mediated fragmentation, which breaks up particles into smaller, slower-sinking POC, as a potential primary driver for POC flux attenuation in the mesopelagic zone. Here, we use a vertical mechanistic particle model, based on the SISSOMA mixed-layer model, to assess the role of biologically-mediated fragmentation on diverse aggregates and POC flux attenuation. Our model resolves a spectrum of particles across size and density, incorporating a representation for particle associated copepodsas key agents of fragmentation below the mixed layer. Sensitivity experiments show fragmentation is critical to reproducing the observed initial decrease in particle size with depth. Fragmentation is shown to have a greater contribution to POC flux loss when export is increased and is composed of relatively larger particles, suggesting it is likely to be a more significant process for flux attenuation in more productive regions. We also demonstrate that the initial slope of particle size distribution at the export depth significantly influences the extent to which transformation processes affect particle size profiles and flux attenuation. While our results highlight the necessity of including fragmentation in biological pump models, fragmentation alone cannot fully account for observed profiles in particle size structure and transfer efficiency. We propose that additional sources of POC (e.g., fecal pellets) and transformation processes (e.g., differential remineralisation, ballasting) likely contribute to the discrepancies between our model and observations. Our results highlight the importance of explicitly representing fragmentation and particle diversity in biological pump models to improve mechanistic understanding and predictions of ocean carbon sequestration.