Origin and distribution of the organic matter in the distal lobe of the Congo deep-sea fan – A Rock-Eval survey

Abstract

International audienceThe Congo River, the second largest river in the world, is a major source of organic matter for the deep Atlantic Ocean because of the connection of its estuary to the deep offshore area by a submarine canyon which feeds a vast deep-sea fan. The lobe zone of this deep-sea fan is the final receptacle of the sedimentary inputs presently channelled by the canyon and covers an area of ~2500 km². The quantity and the source of organic matter preserved in recent turbiditic sediments from the distal lobe of the Congo deep-sea fan were assessed using Rock-Eval pyrolysis analyses. Six sites, located at approximately 5000 m water-depth, were investigated. The mud-rich sediments of the distal lobe contain high amounts of organic matter (~3.5 to 4% Corg), the origin of which is a mixture of terrestrial higher-plant debris, soil organic matter and deeply oxidized phytoplanktonic material. Although the respective contribution of terrestrial and marine sources of organic matter cannot be precisely quantified using Rock-Eval analyses, the terrestrial fraction is dominant according to similar hydrogen and oxygen indices of both suspended and bedload sediments from the Congo River and that deposited in the lobe complex. The Rock-Eval signature supports the 70% to 80% of the terrestrial fraction previously estimated using C/N and δ13Corg data. In the background sediment, the organic matter distribution is homogeneous at different scales, from a single turbiditic event to the entire lobe, and changes in accumulation rates only have a limited effect on the quantity and quality of the preserved organic matter. Peculiar areas with chemosynthetic bivalves and/or bacterial mats, explored using ROV Victor 6000, show a Rock-Eval signature more or less similar to background sediment. This high organic carbon content associated to high sedimentation rates (> 2 to 20 mm.yr−1) in the Congo distal lobe complex implies a high burial rate for organic carbon. Consequently, the Congo deep-sea fan represents an enormous sink of terrestrial organic matter when compared to other turbiditic systems over the world

Análisis de facies en depósitos de grano fino asociados a flujos de fango. Formación Vaca Muerta (Tithoniano-Valanginiano), Cuenca Neuquina central, Argentina

“…Por lo general, estos ambientes suelen presentar una productividad orgánica relativamente baja y, asimismo, el aporte de materia orgánica proveniente de plumas boyantes tiene un potencial de preservación muy bajo, debido al largo camino que debe recorrer a través de la extensa columna de agua (Arthur et al, 1984;Stow et al, 2001). Durante los últimos años, el desarrollo de expediciones oceanográficas en sistemas depositacionales actuales permitió obtener una mejor comprensión acerca de los mecanismos más eficaces para la transferencia de fango rico en materia orgánica hacia el interior de las cuencas oceánicas (Baudin et al, 2010(Baudin et al, , 2017aBiscara et al, 2011;Stetten et al, 2015;Mignard et al, 2017). Estas contribuciones FIG.…”

Relative Abundance and Diversity of Bacterial Methanotrophs at the Oxic–Anoxic Interface of the Congo Deep-Sea Fan

“…Elemental and isotopic analysis have shown that the nature of OM in the recent lobe complex is dominated (>80%) by a terrestrial source containing a mixture of higher plant debris and soil derived OM and that signatures of sediments from the recent lobe complex and from the Congo River were similar (Stetten et al, 2015; Baudin et al, 2017b). Furthermore, uncommon elevated values of a specific bacteriohopanepolyols (BHPs) produced by type I MOB were reported down to 115 mbsf in marine sediments located on the lower Congo fan (ODP Site 1075) (Talbot et al, 2014).…”

Routing of terrestrial organic matter from the Congo River to the ultimate sink in the abyss: a mass balance approach (André Dumont medallist lecture 2017)

“…Additionally, numerous sedimentological and organic geochemical data were obtained during the shore-based studies on the 145 sediment cores collected in different parts of the Congo turbidite system, from the canyon head to the terminal lobe complex, by ca. 5000 m of water depth (Droz et al, 1996(Droz et al, , 2003Gervais et al, 2001;Babonneau, 2002;Babonneau et al, 2002Babonneau et al, , 2004Babonneau et al, , 2010Ferry et al, 2004;Migeon et al, 2004;Bonnel, 2005;Treignier, 2005;Marsset et al, 2009;Rabouille et al, 2009Rabouille et al, , 2017aRabouille et al, , 2017bBaudin et al, 2010Baudin et al, , 2017aBaudin et al, , 2017bPicot, 2015;Stetten et al, 2015;Picot et al, 2016Picot et al, , 2019Croguennec et al, 2017;Méjanelle et al, 2017;Pastor et al, 2017;Pozzato et al, 2017;Schnyder et al, 2017;Taillefert et al, 2017). The resulting data allows a good description and understanding of the entire Congo turbidite system.…”

“…5). Detailed organic geochemical investigations, including molecular signatures and palynofacies observations, were conducted in the upper and lower channellevee areas (Baudin et al, 2010) and in the terminal lobe complex (Stetten et al, 2015;Baudin et al, 2017b;Méjanelle et al, 2017;Pruski et al, 2017;Schnyder et al, 2017). All geochemical and palynological proxies suggest that at least 80-90% of the organic matter preserved in marine sediment along the turbidite channel has a terrestrial origin.…”

“…Despite these subtle variations, the organic matter content distribution appears relatively homogeneous at different scales, from a single cm-to dm-thick turbiditic event to several m-long core all along the turbidite system. Changes in accumulation rates have a limited effect on the quantity and quality of preserved organic matter, whether in channel-levees system (Baudin et al, 2010) or in the terminal lobe zone (Stetten et al, 2015;Baudin et al, 2017b;Schnyder et al, 2017). Consequently, it was possible to consider POC as a conservative parameter of the system and to use it to compute a budget of organic carbon in a S2S approach.…”