GERBAM — Deuxième Colloque International de Bactériologie marine — CNRS, Brest, 1-5 octobre 1984
IFREMER, Actes de Colloques, 3, 1986, pp. 35-42
3
CHEMICAL AND MICROBIOLOGICAL ASPECTS OF ACOUSTICALLY
TURBID SEDIMENTS
G.D. FLOODGATE, G.W. JONES and M.R. YARRINGTON
Department fo marine sciences, UCNW, MENAI BRIDGE, ANGLESEY (UK)
ABSTRACT - The methane concentrations within both Holyhead harbour and Western Irish sea sediments were
consistent with the hypothesis that methane was the major component responsible for the acoustic turbidity, which
is in agreement with Anderson and Hampton (1980 a) and Vilks and Rashid (1977). Microcosm experiments
indicate that, for Holyhead harbour methanogenesis may be the major contributor to the methane pool held
within the sediment. Furthermore sulphate-reducing bacteria, although natural competitors
for the most probable subtrates of methanogenesis, may be limited in their activity by sulphate concentrations at
the depths where methane is being produced.
Key words: gassy sediment, methanogenesis, acoustic turbidity.
RÉSUMÉ - Les concentrations de méthane dans les sédiments du port de Holyhead et de la partie occidentale de
la mer d’Irlande sont en compatibles avec l’hypothèse selon laquelle le méthane est le principal composé
responsable de la turbidité acoustique, ce qui est en accord avec les résultats de Anderson et Hampton (1980a)
et Wilks et Rashid ( 1977). Des expériences en microcosme indiquent que dans le cas du port de Holyhead, sinon
pour la mer d’Irlande, la méthanogénèse peut être la source principale du méthane des sédiments. De plus, les
bactéries sulfato-réductrices, bien que compétiteurs naturels pour la plupart des substrats de la méthanogénèse,
peuvent être limitées par les concentrations des sulfates existant aux profondeurs où le méthane est produit.
Mots clés : sédiment, méthanogénèse, turbidité acoustique.
INTRODUCTION
Acoustically turbid sediments contain gas bubbles, minimum diameter between 1 µm and
10 µm, held within unconsolidated fine sediment, which cause scattering of sound waves
generated by subbottom profiling equipment (Anderson and Hampton, 1980 a) These gas
bubbles reduce the shear strength gradients in fine sediments. In North sea sediments gas
plumes have been linked to the production of pockmarks (cone shaped depressions
possibly caused by gas/fluid migration) which represent a threat to the stability of
structures placed on the seabed (Hovland and Gudmestad, 1984).
Our interest centres round the origin and nature of the gases within unconsolidated fine
sediments. Hovland and Gudmestad (1984) consider that two major sources of gas and
fluid exist capable of causing pockmarks, either biogenic (shallow) gas formed in shallow
surface layers, or thermogenic (deep seated) gas and fluid from hydrocarbon reserves. Gas
formed in organically rich sediments is usually a consequence of the anaerobic breakdown of organic matter, as oxygen is present in only the top few centimetres, and may
include carbon dioxide, methane, nitrogen, ammonia or hydrogen sulphide. The considerations governing bubble formation and gaseous composition were discussed by Anderson and Hampton (1980 a, b). It was demonstrated by Anderson and Hampton (1980 a),
35
IFREMER, Actes de Colloques, 3, 1986, pp. 35-42
3
CHEMICAL AND MICROBIOLOGICAL ASPECTS OF ACOUSTICALLY
TURBID SEDIMENTS
G.D. FLOODGATE, G.W. JONES and M.R. YARRINGTON
Department fo marine sciences, UCNW, MENAI BRIDGE, ANGLESEY (UK)
ABSTRACT - The methane concentrations within both Holyhead harbour and Western Irish sea sediments were
consistent with the hypothesis that methane was the major component responsible for the acoustic turbidity, which
is in agreement with Anderson and Hampton (1980 a) and Vilks and Rashid (1977). Microcosm experiments
indicate that, for Holyhead harbour methanogenesis may be the major contributor to the methane pool held
within the sediment. Furthermore sulphate-reducing bacteria, although natural competitors
for the most probable subtrates of methanogenesis, may be limited in their activity by sulphate concentrations at
the depths where methane is being produced.
Key words: gassy sediment, methanogenesis, acoustic turbidity.
RÉSUMÉ - Les concentrations de méthane dans les sédiments du port de Holyhead et de la partie occidentale de
la mer d’Irlande sont en compatibles avec l’hypothèse selon laquelle le méthane est le principal composé
responsable de la turbidité acoustique, ce qui est en accord avec les résultats de Anderson et Hampton (1980a)
et Wilks et Rashid ( 1977). Des expériences en microcosme indiquent que dans le cas du port de Holyhead, sinon
pour la mer d’Irlande, la méthanogénèse peut être la source principale du méthane des sédiments. De plus, les
bactéries sulfato-réductrices, bien que compétiteurs naturels pour la plupart des substrats de la méthanogénèse,
peuvent être limitées par les concentrations des sulfates existant aux profondeurs où le méthane est produit.
Mots clés : sédiment, méthanogénèse, turbidité acoustique.
INTRODUCTION
Acoustically turbid sediments contain gas bubbles, minimum diameter between 1 µm and
10 µm, held within unconsolidated fine sediment, which cause scattering of sound waves
generated by subbottom profiling equipment (Anderson and Hampton, 1980 a) These gas
bubbles reduce the shear strength gradients in fine sediments. In North sea sediments gas
plumes have been linked to the production of pockmarks (cone shaped depressions
possibly caused by gas/fluid migration) which represent a threat to the stability of
structures placed on the seabed (Hovland and Gudmestad, 1984).
Our interest centres round the origin and nature of the gases within unconsolidated fine
sediments. Hovland and Gudmestad (1984) consider that two major sources of gas and
fluid exist capable of causing pockmarks, either biogenic (shallow) gas formed in shallow
surface layers, or thermogenic (deep seated) gas and fluid from hydrocarbon reserves. Gas
formed in organically rich sediments is usually a consequence of the anaerobic breakdown of organic matter, as oxygen is present in only the top few centimetres, and may
include carbon dioxide, methane, nitrogen, ammonia or hydrogen sulphide. The considerations governing bubble formation and gaseous composition were discussed by Anderson and Hampton (1980 a, b). It was demonstrated by Anderson and Hampton (1980 a),
35