Rapid transport and high accumulation of amorphous silica in the Congo deep-sea fan: A preliminary budget

• Published in: Journal of marine systems 2015-01, Vol.141, p.71-79
• Main Authors: Raimonet, Mélanie, Ragueneau, Olivier, Jacques, Vincent, Corvaisier, Rudolph, Moriceau, Brivaëla, Khripounoff, Alexis, Pozzato, Lara, Rabouille, Christophe
• Format: Article
• Language: English
• Subjects: Biodiversity and Ecology; Canyons; Carbon; Congo canyon; Construction; Continental margin; Deep-sea fan; Earth Sciences; Environmental Sciences; Fluxes; Land–sea export; Marine; Ocean, Atmosphere; Rivers; Sciences of the Universe; Sediments; Silica cycle; Silicon; Silicon dioxide
• ISSN: 0924-7963; 1879-1573
• DOI: 10.1016/j.jmarsys.2014.07.010

Mechanisms controlling the transfer and retention of silicon (Si) along continental margins are poorly understood, but play a major role in the functioning of coastal ecosystems and the oceanic biological pump of carbon. Deep-sea fans are well recognized as carbon sink spots, but we lack knowledge about the importance of the fans in the global Si cycle. Here, we provide a first estimate of the role played by the Congo deep-sea fan, one of the biggest in the world, in the Si cycle. Sediment cores sampled in the deep-sea fan were analyzed to build a Si mass balance. An exceptionally high accumulation rate of amorphous silica aSiO2 (2.29±0.58molSim−2y−1) was found, due to a high sedimentation rate and the presence of aluminum in the sediments. Although favored by bioirrigation, recycling fluxes remained low (0.3molSim−2y−1) and reconstructed input fluxes could only be explained by lateral inputs coming from the canyon. Preliminary calculations show that the rapid transport of aSiO2 through the canyon and the excellent preservation efficiency in the sediments imply that 50% of aSiO2 river inputs from the Congo River accumulate annually in the deep-sea fan. Si:C ratios in deep-sea fan sediments were very low (0.2) and only three times as high as those measured in the river itself, which suggests that material from the river and the continental shelf was delivered directly through the canyon, with very little time for Si and C cycle decoupling to take place. •Amorphous silica intensively flows through the submarine canyon to the Congo deep-sea fan.•Turbidite currents lead to the highest silica burial rates ever found in oceanic areas.•Burial in the deep-sea fan accounts for >50% of the amorphous silica riverine flux.•Low Si:C ratios highlight fast particle transfer and weak decoupling between Si and C cycles.