SHALLOW GAS FLUX RATES FROM THE RÍAS BAIXAS SEABEDS (NW SPAIN)

The presence of shallow gas in marine sediments has a significant effect in their geoacoustic behaviour. Gas seepages affect seawater chemistry and have a poorly assessed effect on atmospheric carbon and methane levels.

The Rías Baixas of Galicia (Ría de Vigo, Pontevedra, Arousa and Muros) are located on the passive Atlantic margin of southwestern Galicia (NW Spain).

Objective layout grids of shallow seismic (Sparker, Uniboom, 3.5 kHz profiler), Echo sounder and Side scan sonar have been acquired from 1991 until 2001 in the Rías Baixas (Ría de Vigo, Pontevedra, Arousa and Muros). Detailed interpretations of the whole records have been allowed mapping of shallow gas accumulations and gas escape features in the four rías. X-ray photographs of gravity cores and vibrocores have been used as semi-direct evidence of the presence of gassy sediments.

Mapping of fluid escape areas shows that these are located either above the shallower gas accumulations or in the outermost zones of the gas accumulations fields, as well as at intersections of the main fault systems.

The spatial distribution of the gas escapes; gas accumulations and their vertical variations are interpreted as evidence of a sedimentary facies control, its permeability and the quantity of gas within them.

GC-MS analysis of direct gas samples shows the presence of methane in the shallow gas of the rías.

Quantitative data on gas flux rates from seabed seepages has been performed in the Rías. Calculations in the rías indicate mean densities of: 1.7 acoustic plumes km–2 from the Ría de Vigo; 7.9 acoustic plumes km–2 from the Ría de Pontevedra ;10.9 acoustic plumes km–2 from the Ría de Arousa and 7.4 acoustic plumes km–2 from the Ría de Muros.

Applying a similar calculation to that used by Judd et al. (1997) to estimate the flux of gas to the water column, we obtain a result of 206.25 Tm(C)/yr to 5907 Tm(C)/yr towards the water column in the Ría de Vigo; 45 Tm(C)/yr to 1289 Tm(C)/yr in the Ría de Pontevedra; 670 Tm(C)/yr to 19188 Tm(C)/yr in the Ría de Arousa and 74 TmC/yr to 2112 Tm (C)/yr in the Ría de Muros.

Assuming that a 30 % of gas is dissolved in the water column a total of 144 Tm(C)/yr to 4135 Tm(C)/yr get to the atmosphere in Ría de Vigo; 31.5 Tm(C)/yr to 902 Tm(C)/yr in the Ría de Pontevedra ;469 Tm(C)/yr to 13431 Tm(C)/yr in the Ría de Arousa and 51.625 Tm (C)/yr to 1478.54 Tm (C)/yr escape to the atmosphere in the Ría de Muros.