The role of subduction and gravitational sinking in particle export, carbon sequestration, and the remineralization length scale in the California Current Ecosystem

Particles and aggregates created in the surface layers of the ocean are transported not only by gravity, but also by the horizontal and vertical advection of the surrounding water. Subduction, in particular, can transport organic matter from the surface ocean to the mesopelagic in a manner that is not likely to be detected by typical in situ carbon export measurements (e.g., sediment traps and U-238-Th-234 disequilibrium). To assess the importance of subduction to the biological pump, we combined in situ sediment trap, thorium, primary productivity, and particulate organic carbon (POC) measurements with a data-assimilative physical circulation model and a Lagrangian particle tracking model. We develop a simple parameterization of two alternative particle sinking processes (Phytoplankton-Fecal Pellet [PFP] and Aggregation) using results from 13 extensively sampled water parcels in the California Current Ecosystem. Both parameterizations suggested that subduction is an important, at times dominant, mechanism of POC vertical export in the region (median 44% and 23% contribution to total POC export for PFP and Aggregate parameterizations at the 100-m depth horizon). The percentage contribution of subduction was highly variable across water parcels (ranging from 7% to 90%), with subduction typically more important in offshore, oligotrophic regions. On average the fate of particles that are passively transported out of the surface layer by advection is different from that of particles that sink across the 100-m depth horizon. Subducted particles were predominantly remineralized shallower than 150 m, while approximately 50% of gravitationally exported POC was remineralized at depths > 500 m.