Shallow Microbial Recycling of Deep-Sourced Carbon in Gulf of Cadiz Mud Volcanoes

• Published in: Geomicrobiology journal 2008-01, Vol.25 (6), p.283-295
• Main Authors: Nuzzo, Marianne, Hornibrook, Edward R. C., Hensen, Christian, Parkes, R. John, Cragg, Barry A., Rinna, Joachim, von Deimling, Jens Schneider, Sommer, Stefan, Magalhã, es, Vitor H., Reitz, Anja, Brückmann, Warner, Pinheiro, Luis M.
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0149-0451; 1521-0529
• DOI: 10.1080/01490450802258196

Based upon the molecular and isotopic composition of hydrocarbons it has been proposed that the source of CH 4 in Gulf of Cadiz mud volcanoes (MV) is a mixture of deep sourced thermogenic CH 4 and shallow biogenic CH 4 . We directly investigated this possibility by comparing porewater CH 4 concentrations and their δ 13 C values with the potential for Archaeal methanogenesis in Gulf of Cadiz mud volcano (MV) sediments (Captain Arutyunov, Bonjardim, Ginsburg and Porto) using 14 C-rate measurements. The CH 4 has a deep sourced thermogenic origin ( δ 13 C ∼ −49) but becomes 13 C-depleted in and beneath the zone of anaerobic oxidation of methane (AOM) where the rates of hydrogenotrophic methanogenesis increase. Thus we infer that a portion of AOM-produced CO 2 is being recycled to CH 4 by methanogens yielding further 13 C-depleted CH 4 , which might be misinterpreted as indicative of a fully shallow biogenic origin for this gas. Production of H 2 is related to compositional changes in sedimentary organic matter, or to upward flux of substrate-enriched fluids. In contrast to other MVs in the Gulf of Cadiz, Ginsburg MV fluids are enriched in SO 4 2− and contain very high concentrations of acetate (2478 μ M below 150 cmbsf); however, the high levels of acetate did not stimulate methanogenesis but instead were oxidized to CO 2 coupled to sulphate reduction. Both anaerobic oxidation of thermogenic CH 4 linked to shallow methanogenesis and fluid geochemistry control the recycling of deep-sourced carbon at Gulf of Cadiz MVs, impacting near-surface δ 13 C-CH 4 values.