Observations and modeling of slow‐sinking particles 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 CO 2 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 CO 2 . 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 À1 ); however, under some circumstances sinking velocity distributions are strongly bimodal with a significant fraction of total flux being carried by slowly (<10 m·d À1 ) 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 210 Pb 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 À1 at 50 m, to 75 ± 25 m·d À1 and 90 ± 20 m·d À1 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 À1. 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.

Geographical, seasonal, and depth variation in sinking particle speeds in the North Atlantic

“…[] and Villa‐Alfageme et al . []. Cruise D354 was undertaken in July–August 2010 in the Irminger Basin (IrB) and Iceland Basin (IB) in the high‐latitude NA [ Le Moigne et al ., ].…”

“…We combined particulate and dissolved 210 Po activities into a one‐box model and calculated ASVs following the methods by Villa‐Alfageme et al . []. This model assumes steady state; i.e., both activities and 210 Po fluxes are assumed to be constant for time scales shorter than 138 days (the half‐life of 210 Po).…”

“…Villa‐Alfageme et al . [] analyzed the contribution of slow‐sinking particles to downward particle flux at different depths in the temperate NA during the postbloom season and reported AVS of 60 and 90 m d −1 at 50 and 500 m, respectively.…”

“…Yet very few studies have measured sinking speed of particles in situ using, e.g., the Marine Snow Catcher [ Riley et al ., ], imaging of particles in viscous polyacrylamide gels [ McDonnell and Buesseler , ], or Indented Rotating Sphere sediment traps [ Alonso‐Gonzalez et al ., ]. A novel method uses the 210 Po‐ 210 Pb disequilibrium in the water column, coupled with inverse modeling techniques and a one‐box model, as proxy to provide estimates of particle ASVs throughout the upper mesopelagic (euphotic zone depth to ~500 m depth) [ Villa‐Alfageme et al ., ].…”

“…We expand the limited data set by Villa‐Alfageme et al . [] and, using the 210 Po‐ 210 Pb method, present ASV estimates from both oligotrophic and eutrophic regions in the NA during different stages of the bloom. We discuss the contribution of fast‐ and slow‐sinking particles to carbon export and the variability in ASV distributions with depth, region, and season.…”