Gas seep induced interstitial water circulation: observations and environmental implications

O'Hara, S.C.M., Dando, P.R., Schuster, U., Bennis, A., Boyle, J.D., Chui, F.T.W., Hatherell, T.V.J., Niven, S.J. and Taylor, L.J. 1995 Gas seep induced interstitial water circulation: observations and environmental implications. Continental Shelf Research, 15 (8). 931-948.

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An interstitial water circulation, generated by gas flow through a permeable sediment, was observed at an intertidal site on the Kattegat coast of Denmark. Concentrations of methane dissolved in the interstitial water of the near-surface sediment decreased sharply only centimetres away from gas seeps venting almost pure methane (circs 99% methane). Water was driven out of the sediment by the rising bubbles of gas at the seep and was replaced by an equivalent draw-down of overlying, oxygenated water into the surrounding sediment. This process steepened the chemical gradients close to the gas flow channel, with the effects progressively diminishing with increasing distance from the seep. The position of the redox potential discontinuity (RPD) moved by as much as 7 cm deeper into the sediment close to the seep: this effect was less marked, but still detectable, 50 cm away. The degree of displacement from the “normal” sediment profiles depended on the magnitude of the interstitial flow rate. The distribution of pore water pH and sulphate:sodium ratios were also dependent on the flow rate of the circulating water. The concentrations of sulphide, thiosulphate and sulphite in the interstitial water from the top 10 cm of sediment, were high at a seep, decreased to a minimum at 20-30 cm distance, then increased again at 40-50 cm distance. Laboratory experiments confirmed that gas bubbling through a fluid filled permeable matrix generated a flow, out of the sediment at the gas exit and into the sediment over the peripheral surfaces surrounding the outlet. Experimentally determined rates of dispersion, for gas flow rates of 3-20 ml/min, for a 40 g/l sodium chloride solution, were 62.5 x 10-9 to 540 x 10-9 m2 s-l, 40-400 times the molecular diffusion coefficient. Linear interstitial fluid velocities of 3-12 mm/min, were recorded at 14-3 cm from the seep axis respectively, with a gas flow rate of 5 ml/min. Two-dimensional modelling of the experimental system confirmed the flow patterns determined visually with dye. Implications of this process with regard to the recycling rates of elements generally, and of nutrient and waste materials,in particular, are discussed.

Item Type: Publication - Article
Subjects: Chemistry
Earth Sciences
Ecology and Environment
Marine Sciences
Divisions: Marine Biological Association of the UK > Other (MBA)
Depositing User: Professor Paul Dando
Date made live: 21 Dec 2021 10:31
Last Modified: 21 Dec 2021 10:31

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