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Thermogenic and bacterial hydrocarbon gases (free and sorbed) in Middle Valley, Juan De Fuca Ridge, Leg 139

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  • The sorbed gases at all four Ocean Drilling Program Leg 139 sites (855, 856, 857, and 858) showed the unmistakable presence of thermogenic hydrocarbons. No indication of abiogenic gas was found. The evidence for the thermogenic hydrocarbons includes elevated contents of higher hydrocarbons (i.e., C₁/[C₂ + C₃] ca. 2-20), δ¹³CCH₄ between -30‰ and -45‰ (PDB), δ¹³CC₂H₆ between -18.7‰ and -26. l‰, and δ¹³CC₃H₈ from -20.5‰ to -25. l‰. The carbon isotope ratios of C₂ and C₃ indicate that the organic matter that generated these hydrocarbons is mature to overmature (1.8% to 4% Rο equivalent) reflecting the range in higher heat flow in the region. Considering the geologic setting, it is highly probable that the thermogenic gas was formed by hydrothermal processes. The presence of considerable amounts of ethene and propene (up to 7.1 hydrocarbon percent [h.c.%]) along with the accelerated maturation of organic matter support this conclusion. Some of the sorbed gases, especially at Sites 857 and 858, have a possible bacterial gas component admixed with the thermogenic, as is also seen in the free gases. The free gases are generally distinct from the sorbed gases and have C₁/(C₂+ C₃) ratios up to 200 and δ¹³CCH₄ as light as -56. l‰. The bacterial hydrocarbons in the free gas are most prominent at depths where bacterial sulfate reduction has removed most or all of the dissolved sulfate. It is uncertain from the geochemical information whether or not the hydrothermal gases are autothonous or have migrated into the sediments vertically or laterally. However, the general increase in hydrocarbon concentration with depth could indicate a limited upward diffusion or advection which is consistent with the heat and fluid flow information for these holes.
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  • Whiticar, M.J., Faber, E., Whelan, J. K., Simoneit, B.R.T. (1994). Thermogenic and bacterial hydrocarbon gases (free and sorbed) in Middle Valley, Juan De Fuca Ridge, leg 139. Proceedings of the Ocean Drilling Program, Scientific Results, 139, 467-477.
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  • 139
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