Sinking dynamics of particulate matter in the subarctic and subtropical regions of the western North Pacific

Abstract Sinking particles collected by drifting sediment trapsat 100 and 200 m depths at two observation sites, K2 and S1, located in the subarctic and subtropical gyres in the western North Pacific, respectively, were fractionated in 5 ranges of sinking velocities between 5 and 1000 m d −1 using an elutriationsystem. The velocity distributions were divided into two distinct types over both sites: type-S containing more particles at the relatively slow end of the velocity range and type-F including a peak in the middle range (50–150 m d −1 ). These distributions showed little change between 100 and 200 m, although fluxes of particulate organic carbon (POC) decreased vertically. The averaged sinking velocities ( w poc ) calculated from the velocity distributions of POC were 31 ± 16 and 63 ± 26 m d −1 at K2 and S1, respectively. For S1 particles, a positive correlation was found between w poc and content of CaCO 3 . This result indicates that particles containing large amounts of denser CaCO 3 sink faster than those containing large amounts of organic matter with low densities. Particles at K2, which were mainly composed of opal and organic matter, did not exhibit a clear relationship between w poc and the content of denser opal. Instead of opal, w poc had a positive correlation with δ 15 N of the sinking particles and was small (large) when the surface layer was stratified (well-mixed). The attenuation of POC flux with depth resulted from physical fragmentation of particles by turbulence at K2 and from biological decomposition and fragmentation at S1.