The ballasting effect of Saharan dust deposition on aggregate dynamics and carbon export: Aggregation, settling, and scavenging potential of marine snow

Abstract

Lithogenic material such as Saharan dust can be incorporated into organic aggregates and act as ballast, potentially enhancing the marine carbon export via increased sinking velocities of aggregates. We studied the ballasting effects of Saharan dust on the aggregate dynamics in the upwelling region off Cape Blanc (Mauritania). Aggregate formation from a natural plankton community exposed to Saharan dust deposition resulted in higher abundance of aggregates with higher sinking velocities compared to aggregate formation with low dust. This higher aggregate abundance and sinking velocities potentially increased the carbon export 10‐fold when the aggregates were ballasted by Saharan dust. After aggregate formation in the surface waters, subsequent sinking through suspended Saharan dust minerals had no influence on aggregate sizes, abundance, and sinking velocities. We found that aggregates formed in the surface ocean off Mauritania were already heavily ballasted with lithogenic material and could therefore not scavenge any additional minerals during their descent. This suggests that carbon export to the deep ocean in regions with high dust deposition is strongly controlled by dust input to the surface ocean while suspended dust particles in deeper water layers do not significantly interact with sinking aggregates.

Changes in the Dust‐Influenced Biological Carbon Pump in the Canary Current System: Implications From a Coastal and an Offshore Sediment Trap Record Off Cape Blanc, Mauritania

“…Organic carbon was well correlated to BSi at the coastal site CBeu; individual BSi maxima revealed a peak‐to‐peak correlation to the dust fluxes in winter–spring (Figure ). We propose that the ballasted organic‐rich aggregates in the surface waters react immediately to any additional dust supply with aggregation followed by rapid sedimentation (Van der Jagt et al, . ), Organic carbon fluxes and its relative contribution to total mass fluxes decreased from 2003 to 2016 at the coastal upwelling site CBeu during all seasons (Figures and ).…”

“…Distinctive short‐term peaks in BSi and lithogenic materials (dust) in the winter–spring season show a perfect temporal match (Figure ). However, the relationship between the amplitudes of BSi and lithogenic components remains weak, suggesting complex interactions between the biological carbon pump and different types of ballast minerals (Le Moigne et al, ; Van der Jagt et al, ). BSi and organic carbon were highly correlated to the dust fluxes, except in fall when dust supply is rather low.…”

“…As emphasized by Bory et al (), a high‐frequency temporal coupling between atmospheric and oceanic fluxes is assumed to be primarily production‐dependent. Due to an almost continuously high biomass and productivity in the permanent upwelling zone off Cape Blanc (Cropper et al ), any supply of dust particles to the surface ocean may increase aggregate formation until a certain threshold or carrying capacity is reached (Van der Jagt et al, ) and particles settle to depth. Then, a coupled sedimentation of both biogenic and non‐biogenic particles may be recorded in the traps (Fischer et al, ).…”

“…Coastal upwelling under the influence of the NE trade winds system is well developed in the Canary Current (CC) EBUEs off NW Africa, in particular at certain upwelling filaments of which the Cape Blanc filament is the largest (Pelegrí et al, 2005). The "giant Cape Blanc filament" (Gabric et al, 1993;Van Camp et al, 1991) is a tongue of cold upwelled waters, spreading several hundreds of kilometers offshore. Upwelling filaments transport nutrient-rich coastal waters to the oligotrophic ocean (Àlvarez-Salgado et al, 2001) and are ©2019.…”

“…The interactions of organic‐rich marine snow particles with mineral ballast, in particular the high dust availability in the NW African upwelling system, may result in a relatively high carbon sequestration flux compared to the other EBUEs (Fischer, Karakas, et al, ; Iversen et al, ; Van der Jagt et al, ). During winter and spring, dust is most frequently transported within the low‐level NE trade winds to coastal Mauritania and deposited offshore above the sediment‐trap sites (Bory & Newton, ; Friese et al, ; Ratmeyer, Fischer, et al, , Ratmeyer, Balzer, et al, ; Stuut et al, ; Korte et al, ).…”