Red Wood-Ant Nests and Fault-Related Methane Micro-Seepage 2016

We measured methane (CH4) and stable carbon isotope of methane (ẟ13C-CH4) concentrations in ambient air and within a red wood-ant (RWA; Formica polyctena) nest in the Neuwied Basin (Germany) using high-resolution in-situ sampling to detect microbial, thermogenic, and abiotic fault-related micro-seepage of CH4. Methane degassing from RWA nests was not synchronized with earth tides, nor was it influenced by micro-earthquake degassing or concomitantly measured RWA activity. Two ẟ13C-CH4 signatures were identified in nest gas: −69‰ and −37‰. The lower peak was attributed to microbial decomposition of organic matter within the RWA nest, in line with previous observations that RWA nests are hot-spots of microbial CH4. The higher peak has not been reported in previous studies. We attribute this peak to fault-related CH4 emissions moving via fault networks into the RWA nest, which could originate either from thermogenic or abiotic CH4 formation. Sources of these micro-seepages could be Devonian schists, iron-bearing “Klerf Schichten,” or overlapping micro-seepage of magmatic CH4 from the Eifel plume. Given the abundance of RWA nests on the landscape, their role as sources of microbial CH4 and biological indicators for abiotically-derived CH4 should be included in estimation of methane emissions that are contributing to climatic change.

本研究采用高分辨率原位采样（high-resolution in-situ sampling）技术，在德国诺伊韦德盆地（Neuwied Basin）的环境空气及红褐林蚁（red wood-ant, RWA; Formica polyctena）巢穴内，测定了甲烷（methane, CH4）浓度与甲烷稳定碳同位素（stable carbon isotope of methane, δ¹³C-CH4）组成，以探测甲烷的微生物成因、热成因及非生物断层相关微渗漏。红褐林蚁巢穴的甲烷脱气现象既未与地球潮汐同步，也未受微地震脱气或同期监测的蚂蚁活动影响。在巢穴气体中共检测到两种甲烷稳定碳同位素特征值：-69‰与-37‰。较低的同位素峰值被归因于红褐林蚁巢穴内有机质的微生物分解，这与此前观测到的红褐林蚁巢穴是微生物成因甲烷富集热点的结论一致。较高的峰值则为既往研究未曾记载的新发现，本研究将其归因于通过断层网络渗透至红褐林蚁巢穴的断层相关甲烷排放，该排放的成因可分为热成因或非生物成因甲烷两类。此类微渗漏的源区可能为泥盆纪片岩、含铁的“克莱夫层（Klerf Schichten）”，或是来自埃菲尔地幔柱的岩浆成因甲烷叠加微渗漏。鉴于地表红褐林蚁巢穴分布广泛，其作为微生物成因甲烷源区以及非生物成因甲烷生物指示物的作用，应当被纳入影响气候变化的甲烷排放估算体系。