Aliphatic hydrocarbons and triterpenes of the Congo deep sea fan

Type Article
Date 2017-08
Language English
Author(s) Mejanelle Laurence1, Riviere Beatrice1, Pinturier Laurence2, Khripounoff AlexisORCID3, Baudin Francois4, Dachs Jordi5
Affiliation(s) 1 : Sorbonne Univ, UPMC Univ Paris 06, CNRS, Lab ECObiogeochim Benth,UMR 8222, 18 Ave Fontaule, F-66650 Banyuls Sur Mer, France.
2 : Norge AS, Total E&P, Finnestadveien 44, N-4029 Stavanger, Norway.
3 : IFREMER, Ctr Bretagne, Inst Carnot, Lab Environm Profond,EDROME,REM EEP LEP, F-29280 Plouzane, France.
4 : Sorbonne Univ, UPMC Univ Paris 06, Inst Sci Terre Paris, CNRS, 4 Pl Jussieu, F-75005 Paris, France.
5 : Inst Environm Assessment & Water Res IDAEA CSIC, Dept Environm Chem, Barcelona, Catalonia, Spain.
Source Deep-sea Research Part Ii-topical Studies In Oceanography (0967-0645) (Pergamon-elsevier Science Ltd), 2017-08 , Vol. 142 , P. 109-124
DOI 10.1016/j.dsr2.2017.06.003
WOS© Times Cited 9
Keyword(s) Congo fan, Hydrocarbons, NAH, Hopanes, Hopenes, Sediments, Turbidite system
Abstract

Hydrocarbons were analyzed in sediments from the Congo River deep-sea fan, from the Congo River, and in sinking particles collected by sediment traps 40 m above the sediment. Studied sites encompassed three lobes of decreasing age of formation along the canyon: sites A, F and C and a another lobe system, disconnected from the active channel since 4 ka, Site E. Terrestrial long-chain odd n-alkanes were dominant in all sediments of the lobe system. Unsaturated terpenoids sourced by higher plants, such as gammacerene, lupene, ursene and oleanene, were also detected. At site C, characterized by high accumulation rates (10–20 cm yr−1), the organic matter spends less time in the oxic layer than at other sites and high phytadiene concentrations (10 to 17 μg gOC−1) evidenced recent terrestrial and phytoplanktonic remains reworked in anaerobic conditions. In these sediments, organic carbon-normalized concentrations of terrestrial alkanes and terpenoids were several fold higher than in the lobe sediments with lower accumulation rates (sites A and F), arguing for a more rapid degradation of terrestrial hydrocarbons than bulk organic carbon in the first steps of pre-diagenesis. Ample variations in the contributions of biomarkers from higher plants, ferns, bacteria and angiosperms, indicate an heterogeneous contribution of the soil and vegetation detritus delivered to the Congo lobe sediments. Lower concentrations in terrestrial hydrocarbons at site E, 45 km away from the active canyon, indicated that river particles are still admixed to the dominant marine organic matter. Diploptene and hop-7(21)ene have a dual origin, from terrestrial and marine microorganisms. Scatter in their relationship to gammacerene argues for a contribution of marine microorganisms, in addition to soils-sourced microorganisms. The close distribution patterns of diploptene, hop-21-ene, hop-7(21)ene and neohop-13(18)-ene is in line with the hypothesis of sequential clay-catalyzed isomerisation of bacterial hopenes. Terrestrial biomarker accumulation fluxes at site C are one order of magnitude higher than vertical pelagic flux, demonstrating the magnitude of the inputs delivered through turbiditic transport in the submarine canyon. Crude oil contamination was evidenced at the disconnected site E (UCM, C21 to C26 tricyclic diterpanes, CPI) and, in smaller amounts, in some sediments from sites A and C. It may be related to marine crude oil extraction and transport. A short-chain mode of alkanes with an even predominance is evidenced in sediments of the lobe complex and likely sources, crude oil, microorganisms and ferns, are discussed.

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