Exclusion of 2-methylbutane (isopentane) during crystallization of structure II gas hydrate in sea-floor sediment, Gulf of Mexico

Structure II gas hydrate is abundant across the central Gulf of Mexico continental slope. Sediment that directly overlies nodular gas hydrate was collected with a piston core at 1920 m water depth in the Atwater Valley (AT) 425 area of the lower continental slope. The gas hydrate has C1–C5 molecular and isotopic properties consistent with structure II gas hydrate that crystallized from relatively unaltered thermogenic vent gas. The gas hydrate contains mainly methane, ethane, propane and butanes, with 2-methylbutane (isopentane ) as a minor component (< 0.2%). Sediment that closely overlies the gas hydrate (within < 1 m) is characterized by an anomalous abundance of 2-methylbutane (as much as 9.6%). Because the molecular diameter of 2-methylbutane is too large for structure II gas hydrate, the 2-methylbutane appears to accumulate preferentially in adjacent sediment as a direct consequence of massive gas hydrate crystallization. The 2-methylbutane is interpreted to be a molecular marker of recent or ongoing net accumulation of structure II gas hydrate. Abundant 2-methylbutane in sediment also could be a precursor to the natural occurrence of structure H gas hydrate, and other new gas hydrate structures not yet discovered in the geologic environment.