Sources of methane inferred from pore-water δ¹³C of dissolved inorganic carbon in Pockmark G11, offshore Mid-Norway

详细信息    查看全文

• 作者：Yifeng Chen ; William Ussler III ; Haflidi Haflidason ; Aivo Lepl ; Leif Rise ; Martin Hovl ; Berit O. Hjelstuen
• 关键词：δ ; ¹³C_DIC of pore-water ; Microbial methane ; Recycling of fossil methane hydrate ; Episodic methane fluxes ; Pockmark G11 ; Vø ; ring Plateau
• 刊名：Chemical Geology
• 出版年：2010
• 出版时间：August 2010
• 年：2010
• 卷：275
• 期：3-4
• 页码：127-138
• 全文大小：1555 K

文摘

Pockmark G11 is the most spectacular one among the pockmarks located at the southern border of Vøring Plateau and 1–2 km away from the northern flank of the Storegga Slide, mid-Norwegian continental margin. For the first time, detailed pore-water geochemical studies were conducted to address methane hydrate occurrence, methane seepage and associated geochemical processes, and methane characteristics in the pockmark. Pore-waters collected from five sediments cores inside and one sediment core outside the pockmark, were analyzed for dissolved Cl⁻, sulfate (SO₄²⁻), total hydrogen sulfide (ΣH₂S), Ca²⁺, Mg²⁺, dissolved inorganic carbon (DIC), δ¹³C_DIC and δ¹⁸O. Methane hydrates were recovered in all sediments below 0.75 m in a core inside pockmark G11, which is in good accordance with heavy oxygen isotope (1.9 to 2.3‰SMOW) and low Cl⁻concentrations (84.9 to 16.1 mM) in pore-waters. Pore-water profiles indicate that upward-migrating methane fluids are spatially variable in the pockmark, with methane fluxes ranging from below detection in the center and outside, to 0.30–0.54 mol m^− 2 a^− 1 inside. In the cores with active methane fluxes, maximum DIC concentrations (19.4 to 21.5 mM) and corresponding minimum δ¹³C_DIC values (− 52.3 to − 54.6 ‰ PDB) occur within sulfate-methane-transition (SMT) zones from  0.40 to 0.50 m below seafloor (mbsf), close to the seafloor. Simple mass balance modeling and δ¹³C_DIC measurements within the SMT zones suggest that methane in shallow sediments within pockmark G11 is microbial in origin. Pore-water geochemistry and seabed observations suggest that methane fluxes inside pockmark G11 are episodic, and derived mostly from the recycling of methane hydrate at depth during sediment burial.