Observations and modeling of slow-sinkingparticles in the twilight zone

Abstract

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 CO2atsignificantly lower levels than if the oceans were abiotic. The depth at which these sinking particles areremineralized is a key control over atmospheric CO2. Particle sinking speed is likely to be a critical parameter overremineralization depth. Carbon export is usuallycontrolled by large, rapidly sinking particles (>150 m·d 1);however, under some circumstances sinking velocity distributions are strongly bimodal with a significant fractionof totalflux being carried by slowly (<10 m·d 1) sinking particles. Therefore, there is an interest in determiningsinking particle velocities and their variations with depth, as well as in understanding the interplay betweensinking velocity distributions and carbon export. Here, we use profiles of total and particulate concentrations ofthe naturally occurring radionuclide pair210Po-210Pb from the Porcupine Abyssal Plain (PAP) site (48°N, 16.5°W) toestimate depth variation in particle sinking speed using a one-box model and inverse techniques. Averagesinking speeds increase from 60 ± 30 m·d 1at 50 m, to 75 ± 25 m·d 1and 90 ± 20 m·d 1at 150 and 500 m.Furthermore, a sensitivity analysis suggests that at the PAP site the measured210Po profiles are inconsistent withthe usually assumed sinking velocities of 200 m·d 1. We hypothesize that a trend of increasing velocitywith depth might be caused by a gradual loss of slow-sinking material with depth, a factor with significantimplications for regional carbon budgets.