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

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 CH 4 hydrate particles rising in a turbulent flow may explain the high heterogeneity of methane concentration and CH 4 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 H 2S and CO 2 are co-clathrated with CH 4 in cages. Analyses of hydrate water show depletions of Cl, SO 4, 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 CO 2 (0.83%) and heavier gases such as C 2H 6 (0.043%) and H 2S (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 δ 13C of −69.3‰ (PDB) and δ D of −199‰ (V-SMOW), and the hydrate CO 2 has δ 13C of −17.5‰ (PDB). These values indicate a primarily microbial origin for the CH 4, which is generated through bacterial CO 2 reduction, as previously observed on many continental margins where solid gas hydrates were sampled.