Oceanographic data, oxygen and methane concentrations, and C-CH4 isotope ratios measured at Jaco Scar

Methane (CH4) concentrations and CH4 stable carbon isotopic composition (d13CCH4) were investigated in the water column within Jaco Scar. It is one of several scars formed by massive slides resulting from the subduction of seamounts offshore Costa Rica, a process that can open up structural and stratigraphical pathways for migrating CH4. The release of large amounts of CH4 into the adjacent water column was discovered at the outcropping lowermost sedimentary sequence of the hanging wall in the northwest corner of Jaco Scar, where concentrations reached up to 1,500 nmol L-1. There CH4-rich fluids seeping from the sedimentary sequence stimulate both growth and activity of a dense chemosynthetic community. Additional point sources supplying CH4 at lower concentrations were identified in density layers above and below the main plume from light carbon isotope ratios. The injected CH4 is most likely a mixture of microbial and thermogenic CH4 as suggested by d13CCH4 values between -50 and -62 per mil Vienna Pee Dee Belemnite. This CH4 spreads along isopycnal surfaces throughout the whole area of the scar, and the concentrations decrease due to mixing with ocean water and microbial oxidation. The supply of CH4 appears to be persistent as repeatedly high CH4 concentrations were found within the scar over 6 years. The maximum CH4 concentration and average excess CH4 concentration at Jaco Scar indicate that CH4 seepage from scars might be as significant as seepage from other tectonic structures in the marine realm. Hence, taking into account the global abundance of scars, such structures might constitute a substantial, hitherto unconsidered contribution to natural CH4 sources at the seafloor.本研究对雅科凹地（Jaco Scar）水柱中的甲烷（CH₄）浓度及甲烷稳定碳同位素组成（δ¹³C-CH₄）展开了系统调查。雅科凹地是哥斯达黎加近海海山俯冲作用引发大规模滑坡所形成的多处凹地之一，该俯冲过程可形成构造与地层通道，为甲烷运移提供有利路径。研究人员在雅科凹地西北角断层上盘出露的最下部沉积序列处，发现有大量甲烷释放至邻近水柱，此处甲烷浓度最高可达1500 nmol·L⁻¹。该区域从沉积序列中渗出的富甲烷流体，可促进密集化能合成群落的生长与代谢活性。通过轻碳同位素比值特征，在主羽流上下的密度层中还识别出了其他低浓度甲烷点源。根据δ¹³C-CH₄值介于-50‰至-62‰（维也纳佩德贝莱姆石，Vienna Pee Dee Belemnite，VPDB），可知注入的甲烷大概率为微生物成因与热成因甲烷的混合物。该甲烷沿等密度面在整个凹地区域扩散，并因与海水混合及微生物氧化作用导致浓度逐渐降低。6年多的监测中多次在凹地内检测到高浓度甲烷，表明甲烷补给过程具有持续性。雅科凹地的最大甲烷浓度及平均过剩甲烷浓度表明，凹地的甲烷渗漏量或许与海洋环境中其他构造结构的渗漏量相当。因此，考虑到全球范围内凹地的分布规模，这类构造或许构成了海底甲烷自然来源中一项此前未被重视的重要贡献。