Gases in Taiwan mud volcanoes: Chemical composition, methane carbon isotopes, and gas fluxes Hung-Chun Chao a,b , Chen-Feng You a,b, * , Chih-Hsien Sun c a Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan, ROC b Earth Dynamic System Research Center, National Cheng Kung University, Tainan, Taiwan, ROC c Exploration and Production Research Institute, Chinese Petroleum Corporation, Taiwan, ROC article info Article history: Received 18 August 2009 Accepted 24 December 2009 Available online 7 January 2010 Editorial handling by Dr. R. Fuge abstract Mud volcanoes are important pathways for CH 4 emission from deep buried sediments; however, the importance of gas fluxes have hitherto been neglected in atmospheric source budget considerations. In this study, gas fluxes have been monitored to examine the stability of their chemical compositions and fluxes spatially, and stable C isotopic ratios of CH 4 were determined, for several mud volcanoes on land in Taiwan. The major gas components are CH 4 (>90%), ‘‘air” (i.e. N 2 +O 2 + Ar, 1–5%) and CO 2 (1–5%) and these associated gas fluxes varied slightly at different mud volcanoes in southwestern Taiwan. The Hsiao- kun-shui (HKS) mud volcano emits the highest CH 4 concentration (CH 4 > 97%). On the other hand, the Chung-lun mud volcano (CL) shows CO 2 up to 85%, and much lower CH 4 content (<37%). High CH 4 content (>90%) with low CO 2 (<0.2%) are detected in the mud volcano gases collected in eastern Taiwan. It is sug- gestive that these gases are mostly of thermogenic origin based on C 1 (methane)/C 2 (ethane) + C 3 (pro- pane) and d 13 C CH4 results, with the exception of mud volcanoes situated along the Gu-ting-keng (GTK) anticline axis showing unique biogenic characteristics. Only small CH 4 concentration variations, <2%, were detected in four on-site short term field-monitoring experiments, at Yue-shi-jie A, B, Kun-shui-ping and Lo-shan A. Preliminary estimation of CH 4 emission fluxes for mud volcanoes on land in Taiwan fall in a range between 980 and 2010 tons annually. If soil diffusion were taken into account, the total amount of mud volcano CH 4 could contribute up to 10% of total natural CH 4 emissions in Taiwan. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Mud volcanoes are a common geological phenomenon in tec- tonically compressed areas on land and offshore, widely spread along the Tethys suture zone (the Alps – the Black Sea – the Cas- pian Sea – the Himalayas) and around the Pacific Ocean (Milkov, 2000; Kopf, 2002), in countries such as Azerbaijan, Georgia, Indo- nesia, Italy, Pakistan, Romania, Taiwan, Trinidad and Ukraine. Mud volcanoes are diapiric structures resulting from the vertical migration of gases and fluids originating from a deep accretionary wedge and have mainly been formed in an environment where thick, soft, and fine-grained mud deposited rapidly with tectonic stress, and with hydrocarbon gases generated continuously, result- ing in the enhancement of fluid buoyancy and the weakening of the formation (Milkov, 2000; Kopf, 2002; Dimitrov, 2002). Faults and fractures in anticlines also provide a ‘‘freeway” for the gas and fluid to migrate upward. Molecular composition and C isotopic ratio of CH 4 (in terms of C 1 /(C 2 +C 3 ) vs. d 13 C CH4 ) are widely used for delineating the origin of hydrocarbon gases (Bernard et al., 1976, 1978; Etiope et al., 2007, 2009). However, secondary post-diagenetic processes, such as oxidation or molecular fractionation during migration, may alter these signals (Schoell, 1983; Whiticar, 1999; Etiope et al., 2007, 2009). Thus, the molecular speciation should be used with caution for discussing gas origin and migration processes. Recently, attention has been shifted to the environmental impact of mud volcanoes, such as greenhouse gas emission and geo-hazards (Etiope and Klusman, 2002; Dimitrov, 2002; Mazzini et al., 2007). It has been estimated that the global natural CH 4 emission from biogenic sources (wetlands, oceans, termites, natu- ral biomass burning) range from 150 and 237 Tg/a (Fung et al., 1991; Hein et al., 1997; Lelieveld et al., 1998; Kvenvolden and Rog- ers, 2005). However, the potential importance of mud volcano gases was not taken into account in these calculations. Etiope and Milkov (2004) estimated that global CH 4 flux to the atmo- sphere from mud volcanoes on land is 6–9 Tg/a, which is 3–6% of the natural CH 4 source and is the same emission level attributed to ocean and gas hydrates. Before 1960, research on Taiwan mud volcanoes was mainly related to petroleum exploration. Shih (1967) reported informa- tion regarding fluid pH, temperature, mud flow, grain size and minerals expelled from mud volcanoes on land. The mineralogical 0883-2927/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2009.12.009 * Corresponding author. Address: No. 1, University Road, Tainan City 701, Taiwan, ROC. Tel.: +886 6 2757575x65438; fax: +886 6 2758682. E-mail address: cfy20@mail.ncku.edu.tw (C.-F. You). Applied Geochemistry 25 (2010) 428–436 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem