Seasonal variation in particulate organic carbon sequestration in subarctic and subtropical gyres of the western North Pacific

The ocean's biological carbon pump regulates climate by transferring a portion of surface-fixed CO₂ to the deep ocean through sinking particulate organic carbon (POC). Although this flux is strongly attenuated in the twilight zone, the seasonal controls on attenuation remain poorly understood. We examined seasonal variations in POC flux, the nitrogen isotope ratio of sinking particulate nitrogen (δ¹⁵N_sink), and mineral composition using sediment traps observations at subarctic (K2) and subtropical (S1) stations in the western North Pacific. POC sequestration efficiency at 500 m [Seq₍₅₀₀₎ = POC flux/NPP] was quantified, with net primary productivity (NPP) reconstructed from δ¹⁵N_sink using an empirical framework. Seq₍₅₀₀₎ remained nearly constant at K2 (7.4-8.1%) but varied substantially at S1 (3.4-6.5%). At K2, CaCO₃ and opal contents exhibited complementary seasonal patterns, whereas S1 showed pronounced variability primarily in CaCO₃. We propose that mineral composition modulates aggregate settling velocity and adhesive strength, thereby regulating POC attenuation through fragmentation processes. These findings indicate that seasonal shifts in surface ecosystem structure influence the physical properties of sinking aggregates and ultimately control the fate of NPP in the twilight zone.