Abstract
Sinking biogenic particles are central to transporting carbon to depth. To date, studies have focused on quantifying the downward export flux from the epipelagic (0-100 m), often neglecting particle fate in the mesopelagic (100–1000 m) due to sampling issues. Particle fate is set by sinking speed and flux attenuation which determine penetration depth. Characterising particle penetration depth is essential to quantify atmospheric return times for biologically-fixed carbon, hence the influence of the biological pump on climate. Here, a profiling float-based imaging sensor, measuring particle abundance over 14 size-classes (0.1–2.58 mm), revealed conspicuous particle export pulses, from two annual phytoplankton blooms, with size-dependent sinking speeds from 3 to 136 m d− 1. Penetration depth of small slow-sinking particles (< 0.6 mm) was < 200 m, accounting for 66% of POC attenuation across all size-classes over the mesopelagic. Larger particles, penetrating to > 900 m, resulted in only a small increase in POC attenuation to 77% at 200 m. This attenuation exceeded that from respiration (42%), derived from float-based oxygen measurements, suggesting that POC attenuation was jointly controlled by remineralisation and fragmentation. This float-based approach can assess the downward and return pathway of the biological pump.