The role of subduction and gravitational sinking in particle export, carbon sequestration, and the remineralization length scale in the California Current Ecosystem

Abstract

Particles and aggregates created in the surface layers of the ocean are transported not only by gravity, but also by the horizontal and vertical advection of the surrounding water. Subduction, in particular, can transport organic matter from the surface ocean to the mesopelagic in a manner that is not likely to be detected by typical in situ carbon export measurements (e.g., sediment traps and 238 U-234 Th disequilibrium). To assess the importance of subduction to the biological pump, we combined in situ sediment trap, thorium, primary productivity, and particulate organic carbon (POC) measurements with a data-assimilative physical circulation model and a Lagrangian particle tracking model. We develop a simple parameterization of two alternative particle sinking processes (Phytoplankton-Fecal Pellet [PFP] and Aggregation) using results from 13 extensively sampled water parcels in the California Current Ecosystem. Both parameterizations suggested that subduction is an important, at times dominant, mechanism of POC vertical export in the region (median 44% and 23% contribution to total POC export for PFP and Aggregate parameterizations at the 100-m depth horizon). The percentage contribution of subduction was highly variable across water parcels (ranging from 7% to 90%), with subduction typically more important in offshore, oligotrophic regions. On average the fate of particles that are passively transported out of the surface layer by advection is different from that of particles that sink across the 100-m depth horizon. Subducted particles were predominantly remineralized shallower than 150 m, while approximately 50% of gravitationally exported POC was remineralized at depths > 500 m.

Effects of Eddy‐Driven Subduction on Ocean Biological Carbon Pump

“…However, because these events are relatively rare and limited in space and time, their contribution to the annual and regional carbon export is still about 10 times smaller than the gravitational pump (Figure ). This result is consistent with the study of Stukel, Song, et al (), which showed that, in the California Current, sinking particles were more efficiently exported at depth than subducted particles.…”

“…The compensation between upward and downward transport is, however, not complete. In line with previous observations and modeling studies (Llort et al, ; Omand et al, ; Stukel, Aluwihare, et al; Stukel, Song, et al), we find that the downward transport of POC across the euphotic layer (8 mg C·m −2 ·day −1 ) exceeds the upward transport of POC (7 mg C·m −2 ·day −1 ) because of the asymmetric effect of upward and downward velocities on the vertical gradient in particles (sharp decline with depth, Figure ). The integrated effect of eddy‐driven POC subduction is, however, small and accounts for less than 2% of the annual POC export at the regional scale.…”

“…All terms are computed as a function of depth. Note that in the literature, subduction sometimes includes the mixed-layer pump and the eddy pump (Stukel, Song, et al 2017), the mixed-layer pump and the Ekman pump (Lévy et al, 2013), or only the eddy pump (Omand et al, 2015).…”

“…Fine‐scale measurements have, however, revealed that POC export can be intensified at the center of ocean mesoscale eddies or in frontal structures at their periphery (Benitez‐Nelson et al, ; Buesseler et al, ; Estapa et al, ; Guidi et al, ; McGillicuddy et al, ; Newton et al, ; Omand et al, ; Waite et al, ; Stukel, Aluwihare, et al ; Stukel, Song, et al ). Observations have also highlighted the presence of localized maxima in POC concentrations below the base of the euphotic layer, which are interpreted as evidence of small‐scale physical subduction of POC (Llort et al, ; Omand et al, ; Stukel, Aluwihare, et al).…”

“…Here we will use the generic term ”eddy‐driven” vertical velocity, to refer to the range of (sub)‐mesoscale dynamics, that is, in the scale range 1–100 km. This eddy pump of POC could account for 20% to 70% of the local organic carbon export at the base of the euphotic layer (Llort et al, ; Omand et al, ; Stukel, Aluwihare, et al, Stukel, Song, et al ; Stukel & Ducklow, ) and has been identified as a potential pathway to transfer carbon to depths below the reach of the seasonal mixed layer (Erickson & Thompson, ). Nevertheless, previous observational and fine‐scale model estimates of the eddy pump were limited in space and time (typically a few eddy structures over a few weeks/months and the upper 100 m) and excluded the eddy effects on the subduction of DOC.…”