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Geological Society, London, Special Publications; 1998; v. 137; p. 161-165;
DOI: 10.1144/GSL.SP.1998.137.01.13
© 1998 Geological Society of London

Worldwide Gas Hydrate Occurrences and Regional Case Studies

Geochemistry of gas hydrates and associated fluids in the sediments of a passive continental margin. Preliminary results of the ODP Leg 164 on the Blake Outer Ridge

R. Thiéry1, R. Bakker2 & C. Monnin3

1 CREGU, BP 23, 54 501 Vandoeuvre-les-Nancy, France
2 Geologisches-Palaeontologisches Institut, Heidelberg, Germany
3 Laboratoire de Gochimie, Université Paul Sabatier, rue des Trente-Six Ponts, Toulouse, France

Marine sediments containing gas hydrates have been drilled up to a depth of 750 m below the sea floor (mbsf) at the Blake Outer Ridge during the ODP Leg 164. Gas hydrates are present between 190 and 450 mbsf. Shipboard analyses of gases and interstitial waters are interpreted with the help of thermodynamic models for gas hydrates and aqueous solutions. Two aspects of the influence of gas hydrates on the chemistry of associated fluids are investigated here. The first one is related to the methane/ethane ratio, which exhibits a sudden change of trend at the base of the gas hydrate zone. This phenomenon could be due to gas hydrates, which act as a concentration barrier for ethane. The second aspect concerns the presence of interstitial waters less saline than sea water above and inside the gas hydrate zone. This could result from the upward expulsion of saline waters during the compaction of sediments in the gas hydrate zone.