Observations and modeling of slow-sinking particles in the twilight zone

The biological carbon pump (BCP) transfers carbon from the surface ocean into the oceans' interior, mainly in the form of sinking particles with an organic component, and thereby keeps atmospheric CO2 at significantly lower levels than if the oceans were abiotic. The depth at which these sinking particles are remineralized is a key control over atmospheric CO2. Particle sinking speed is likely to be a critical parameter over remineralization depth. Carbon export is usually controlled by large, rapidly sinking particles (>150 m·d−1); however, under some circumstances sinking velocity distributions are strongly bimodal with a significant fraction of total flux being carried by slowly (<10 m·d−1) sinking particles. Therefore, there is an interest in determining sinking particle velocities and their variations with depth, as well as in understanding the interplay between sinking velocity distributions and carbon export. Here, we use profiles of total and particulate concentrations of the naturally occurring radionuclide pair 210Po-210Pb from the Porcupine Abyssal Plain (PAP) site (48°N, 16.5°W) to estimate depth variation in particle sinking speed using a one-box model and inverse techniques. Average sinking speeds increase from 60 ± 30 m·d−1 at 50 m, to 75 ± 25 m·d−1 and 90 ± 20 m·d−1 at 150 and 500 m. Furthermore, a sensitivity analysis suggests that at the PAP site the measured 210Po profiles are inconsistent with the usually assumed sinking velocities of 200 m·d−1. We hypothesize that a trend of increasing velocity with depth might be caused by a gradual loss of slow-sinking material with depth, a factor with significant implications for regional carbon budgets.