Bulk Particle Sinking Speeds Across the Atlantic Ocean From Year-Round Autonomous Observations

Sinking particulate organic carbon (POC) is thought to be the primary vehicle of the biological carbon pump, accounting for at least 50% of carbon export. There is high confidence that POC export to the deep ocean will shift under a changing climate, yet our confidence in the scale and sign of this shift is low. An inability to accurately model future magnitudes of POC export restricts our capacity to prepare and respond to climate change. Sinking POC export is a function of particle carbon concentration and particle sinking speed; refining our measurements of either parameter increases the accuracy of calculated carbon export. Stokes Law theoretically links particle size and density to particle sinking speed; however, empirical relationships that capture this theory in situ are elusive, in large part due to the difficulties of observing particle behaviour in situ. To address this observational gap, we have identified over 17 plumes of sinking material captured by 7 profiling floats in the Atlantic Ocean and employ a plume tracking method to derive the bulk sinking speed of particles across 18 size classes, ranging from 80 - 4000 µm. We observe strong inter- and intra-annual variability in particle sinking speed across particle size classes and test three hypothesised drivers behind this variability: particle size, and proxies for both particle density and prevailing plankton community composition. For particles greater than 600 µm in diameter, we find that particle size is a strong driver of particle sinking speeds. Below 600 µm in diameter, we find that fragmentation plays an increasing role in shaping plume evolution, obfuscating the link between particle size and particle sinking speed in the mesopelagic. The empirical links found here can reduce uncertainties in our estimates of POC export and improve model representations of sinking particles.