CONCLUSIONS

Chemical and isotopic measurements of interstitial water and headspace gas samples, together with XRD and isotope studies of siderite concretions from Site 1146 have shown the following:

  1. The interstitial water above 400 mbsf trapped in the sediments has a typical seawater composition. The increase of lithium below 400 mbsf and a slight decrease of chlorine below 560 mbsf indicate the contribution of other fluids migrated from elsewhere.
  2. Anomalously high methane concentrations (with a maximum of 85,000 ppmv at 563.2 mbsf) were detected in headspace gas below 400 mbsf, together with the presence of ethane and propane below 500 mbsf with peak values at 572 mbsf. The carbon isotopic composition of methane and molecular ratios (C1/C2+C3) indicate that the hydrocarbon is thermogenic gas or mixed gas.
  3. Authigenic siderite concretions with high 18O values, found at ~600 mbsf, are probably related to the decomposition of gas hydrate. Their carbon isotopic compositions indicate that they have no relation with bacterial fermentation.
  4. All these results suggest that a pore fluid with high hydrocarbon and low chlorine concentrations was probably produced by decomposition of gas hydrate near the area and migrated to Site 1146 along faults or bedded planes.

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