Sinking rates and ballast composition of particles in the Atlantic Ocean: implications for the organic carbon fluxes to the deep ocean

Abstract

Abstract. The flux of materials to the deep sea is dominated by larger, organic-rich particles with sinking rates varying between a few meters and several hundred meters per day. Mineral ballast may regulate the transfer of organic matter and other components by determining the sinking rates, e.g. via particle density. We calculated particle sinking rates from mass flux patterns and alkenone measurements applying the results of sediment trap experiments from the Atlantic Ocean. We have indication for higher particle sinking rates in carbonate-dominated production systems when considering both regional and seasonal data. During a summer coccolithophorid bloom in the Cape Blanc coastal upwelling off Mauritania, particle sinking rates reached almost 570 m per day, most probably due the fast sedimentation of densely packed zooplankton fecal pellets, which transport high amounts of organic carbon associated with coccoliths to the deep ocean despite rather low production. During the recurring winter-spring blooms off NW Africa and in opalrich production systems of the Southern Ocean, sinking rates of larger particles, most probably diatom aggregates, showed a tendency to lower values. However, there is no straightforward relationship between carbonate content and particle sinking rates. This could be due to the unknown composition of carbonate and/or the influence of particle size and shape on sinking rates. It also remains noticeable that the highest sinking rates occurred in dust-rich ocean regions off NW Africa, but this issue deserves further detailed field and laboratory investigations. We obtained increasing sinking rates with depth. By using a seven-compartment biogeochemical model, it was shown that the deep ocean organic carbon flux at a mesotrophic sediment trap site off Cape Blanc can be captured fairly well using seasonal variable particle sinking rates. Our model provides a total organic carbon flux of Correspondence to: G. Fischer (gerhard.fischer@uni-bremen.de) 0.29 Tg per year down to 3000 m off the NW African upwelling region between 5 and 35 • N. Simple parameterisations of remineralisation and sinking rates in such models, however, limit their capability in reproducing the flux variation in the water column.

Aggregation as a function of and mineral particles

“…The relatively lower TEP content (Engel and Schartau 1999;Azetsu-Scott and Passow 2004) and the higher illite content (Ploug et al 2008a,b;Fischer and Karakas 2009;Iversen and Ploug 2010) of the aggregates formed at high initial suspended concentrations of illite should have increased excess density and hence sinking velocity of aggregates in the high-clay treatments. But incorporation of minerals also reduces the size of aggregates (Passow and De La Rocha 2006;De La Rocha et al 2008), which diminishes sinking velocity (McCave 1975;Hamm 2002;De La Rocha and Passow 2007).…”

Bathypelagic particle flux signatures from a suboxic eddy in the oligotrophic tropical North Atlantic: production, sedimentation and preservation

“…Considering the synchronicity of peaks in BSi and other bulk components, a fast vertical transport of the surface particle flux signature into the meso-and bathy-pelagic is expected within the eddy. Given the 29-day sampling interval of the traps, the particle settling rate for the bathypelagic water column should at least reach 150 m d −1 , applying the methods described in Karakas (2009) andBerelson (2002). Fischer and Karakas (2009) provided a compilation of particle settling rates with a mean of 235 ± 71 m d −1 , using several particle flux patterns from the mesotrophic Cape Blanc moorings, located in the EBUEs and, hence, closer to the coast.…”

“…Given the 29-day sampling interval of the traps, the particle settling rate for the bathypelagic water column should at least reach 150 m d −1 , applying the methods described in Karakas (2009) andBerelson (2002). Fischer and Karakas (2009) provided a compilation of particle settling rates with a mean of 235 ± 71 m d −1 , using several particle flux patterns from the mesotrophic Cape Blanc moorings, located in the EBUEs and, hence, closer to the coast. For the sediment trap mooring sites south of Cabo Verde (CV 1-2; Ratmeyer et al, 1999), an even higher mean sinking speed of 416 m d −1 was estimated (Fischer and Karakas, 2009).…”

“…Fischer and Karakas (2009) provided a compilation of particle settling rates with a mean of 235 ± 71 m d −1 , using several particle flux patterns from the mesotrophic Cape Blanc moorings, located in the EBUEs and, hence, closer to the coast. For the sediment trap mooring sites south of Cabo Verde (CV 1-2; Ratmeyer et al, 1999), an even higher mean sinking speed of 416 m d −1 was estimated (Fischer and Karakas, 2009). The latter authors argued that high organic carbon fluxes in the Canary Current compared to other EBUEs are at least partly due to high particle settling rates, which result in low carbon respiration rates , most probably favoured by a high ballast content.…”

“…Ansmann et al 2011;Moulin et al, 1997) and the addition of nitrate, phosphate and iron to the surface ocean (e.g. Jickells et al, 1998), but also for the ballasting of organic-rich particles (Ittekkot, 1993;Armstrong et al, 2002;Ploug et al, 2008;Fischer and Karakas, 2009;Bressac et al, 2014) formed in the surface ocean. Lithogenic material attributed to mineral dust has been shown to contribute between 1/3 and 1/2 to the total deep ocean mass flux off Cape Blanc and south of Cabo Verde (CV-1-2 trap, ca.…”