Note Exclusion of 2-methylbutane (isopentane) during crystallization of structure II gas hydrate in sea-¯oor sediment, Gulf of Mexico Roger Sassen *, Stephen T. Sweet, Debra A. DeFreitas, Alexei V. Milkov Geochemical and Environmental Research Group (GERG), Texas A&M University, College Station, Texas 77845, USA Received 28 July 2000; accepted 11 September 2000 (returned to author for revision 30 August 2000) Abstract 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 C 1 ±C 5 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. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Gas hydrate; Structure II gas hydrate; Hydrocarbons; 2-Methylbutane; Isopentane; Seeps; Recent sediment; Gulf of Mexico 1. Introduction Gas hydrate is an ice-like crystalline mineral in which hydrocarbon and non-hydrocarbon gases are held within rigid cages of water molecules. Structure I gas hydrate has a body-centered cubic lattice, structure II gas hydrate has a diamond lattice, and structure H gas hydrate has a hexagonal lattice (Sloan, 1998). Structure I gas hydrate, which occurs in the Gulf of Mexico and many other basins, is frequently dominated by bacterial methane (Kvenvolden, 1995). Thermogenic gas hydrates of the Gulf slope contain oil-related hydrocarbon gases that migrate to shallow sediments from deeply buried Mesozoic petroleum source rocks (Wenger et al., 1994). Naturally occurring structure II hydrate mainly includes C 1 ±C 4 hydrocarbons (methane through butanes) and structure H hydrate includes C 1 ±C 5 hydrocarbons (methane through 2-methylbutane) as major compo- nents (Sassen and MacDonald, 1994). In contrast to simple bacterial methane, thermogenic hydrocarbons preserve complex information on the ori- gin of gas hydrates because multiple hydrocarbon molecules of varying properties are held within the crystal lattice (Sassen et al., 1999a). The Gulf of Mexico is one of the few areas outside of the Caspian Sea (Ginsburg and Soloviev, 1998) where oil-related struc- ture II gas hydrate is abundant at shallow depth in sea ¯oor sediment. In earlier studies, attention focused on the molecular and isotopic properties of thermogenic vent gas and the structure II gas hydrate commonly 0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(00)00144-3 Organic Geochemistry 31 (2000) 1257±1262 www.elsevier.nl/locate/orggeochem * Corresponding author. Fax +1-979-862-2361. E-mail address: sassen@gerg.tamu.edu (R. Sassen).