Physical and chemical characterization of gas hydrates and associated methane plumes in the Congo–Angola Basin

Charlou, J.L., Donval, J.P., Fouquet, Y., Ondreas, H., Knoery, J., Cochonat, P., Levaché, D., Poirier, Y., Jean-Baptiste, P., Fourré, E., Chazallon, B. and Gay, A. (2004) Physical and chemical characterization of gas hydrates and associated methane plumes in the Congo–Angola Basin. Chemical Geology, 205 (3-4), 405-425. (doi:10.1016/j.chemgeo.2003.12.033).

Abstract

As a part of the ZAIANGO IFREMER/TOTAL-FINA-ELF program, gas hydrates were collected from a gravity sediment core on the Congo–Angola margin during the ZAI-ROV cruise (December 2000). Gas hydrates, associated with a deep giant (800 m in diameter) active depression called “pockmark,” occurred from the sediment surface down to 12 m depth, at 3160 m water depth. Elevated concentrations of particles, manganese, iron, and methane detected in the water column close to the sediment surface reveal intense fluid circulation transporting methane-rich turbid fluids into the overlying bottom waters. Free gas is liberated via diffusion and/or advection and from destabilizing gas hydrates. The gas hydrates occur as small fragments and massive crystal aggregates, mostly disseminated irregularly in the highly disturbed sediment and escaping in the overlying deep seawater, creating methane-rich plumes. The dissociation of solid CH4 hydrate particles rising in a turbulent flow may explain the high heterogeneity of methane concentration and CH4 peaks measured in vertical profiles. Raman spectroscopy revealed that the gas hydrates recovered from the Congo–Angola are mainly 100% methane gas hydrate of structure I cubic with a lattice constant of a=12 A°, but H2S and CO2 are co-clathrated with CH4 in cages. Analyses of hydrate water show depletions of Cl, SO4, Na, Mg, Ca, and Sr, and enrichments of Si, Cs, and Ba, compared to the ambient deep seawater. Gas analysis shows that methane is the major component (99.1%), but CO2 (0.83%) and heavier gases such as C2H6 (0.043%) and H2S (0.02%) are also present as traces. Helium concentrations were in the range of 0.04–1.3 ppm, with slightly radiogenic 3He/4He ratios between 6.6×10?7 and 7.6×10?7. The hydrate methane has ?13C of ?69.3‰ (PDB) and ?D of ?199‰ (V-SMOW), and the hydrate CO2 has ?13C of ?17.5‰ (PDB). These values indicate a primarily microbial origin for the CH4, which is generated through bacterial CO2 reduction, as previously observed on many continental margins where solid gas hydrates were sampled.

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