Isotope constraints on the origin of pore waters and salinity in the permafrost and gas hydrate core intervals of the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well

Abstract

Pore waters have been extracted from sediments in the permafrost interval (110-176 m) and the gas hydrate interval (886-952 m) of the JAPEX/JNOC/GSC Mallik 2L-38 drill core and analyzed for d18O, d2H, and geochemistry. Pore waters from the permafrost interval have d18O values of -19.5 ± 0.5 o/oo (upper permafrost) and -23.1 ± 1.0 o/oo (lower permafrost) indicating the likely origin to be local, contemporary meteoric waters infiltrating these sediments during a period of subaerial exposure. Pore waters in the subpermafrost gas hydrate zone are isotopically depleted from seawater values, with d18O ranging between -14 o/oo and -8 o/oo. A weak correlation between d18O and Cl- exists in the gas-hydrate-bearing sands, consistent with the combined effect of isotopic depletion during gas hydrate formation, and enrichment associated with gas hydrate decomposition. The upper silt and deeper clayey silt sections also retain a minor correlation between isotopes and Cl-, and show strong variability in both d18O and Cl- with depth, suggesting a history of gas hydrate formation, decomposition, and fluid migration. The Cl--d18O relationships demonstrate that the original pore waters are a mixture of seawater with greater than 50% meteoric water.