Variations of methane induced pyrite formation in the accretionary wedge sediments offshore southwestern Taiwan Yee Cheng Lim a , Saulwood Lin a, * , Tsanyao Frank Yang b , Yue-Gau Chen b , Char-Shine Liu a a Institute of Oceanography, National Taiwan University, Taiwan b Department of Geosciences, National Taiwan University, Taiwan article info Article history: Received 13 March 2010 Received in revised form 3 April 2011 Accepted 24 April 2011 Available online 30 April 2011 Keywords: Gas hydrate Methane Sulfate reduction Pyrite Anaerobic methane oxidation abstract The accretionary wedge of offshore southwestern Taiwan contains abundant deposits of gas hydrate beneath the sea floor. High concentrations of methane in pore waters are observed at several locations with little data concerning historical methane venting available. To understand temporal variation of methane venting in sediments over geologic time, a 23-m-long Calypso piston core (MD05-2911) was collected on the flank of the Yung-An Ridge. Pore water sulfate, dissolved sulfide, dissolved iron, methane, sedimentary pyrite, acid volatile sulfide, reactive iron, organic carbon and nitrogen as well as carbonate d 13 C were analyzed. Three zones with markedly different pyrite concentration were found at the study site. Unit I sedi- ments (>20 mbsf) were characterized with a high amount of pyrite (251e380 mmol/g) and a d 13 C- depleted carbonate, Unit II sediments (15e20 mbsf) with a low pyrite (15e43 mmol/g) and a high content of iron oxide mineral and Unit III sediments (<10 mbsf) by a present-day sulfateemethane interface (SMI) at 5 m with a high amount of pyrite (84e221 mmol/g) and a high concentration of dissolved sulfide. The oscillation records of pyrite concentrations are controlled by temporal variations of methane flux. With an abundant supply of methane to Unit I and III, anaerobic methane oxidation and associated sulfate reduction favor diagenetic conditions conducive for significant pyrite formation. No AOM signal was found in Unit II, characterized by typical organically-limited normal marine sediments with little pyrite formation. The AOM induced pyrite formation near the SMI generates a marked pyrite signature, rendering such formation of pyrite as a useful proxy in identifying methane flux oscillation in a methane flux fluctuate environment. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Gas hydrate destabilization and methane gas expulsion may be a key factor of the climatic change (Kvenvolden, 1988, 1993; MacDonald, 1990). A great amount of methane could release into the water column and atmosphere as a result of glacial-interglacial sea level variations and bottom water temperature fluctuations driving massive destabilization of the sea floor gas hydrate (Kennett et al., 2003; Paull et al., 1991,1996). Geological records such as negative excursions of carbon isotope in benthic forami- nifera, collapse features of seafloor, and chemoherm carbonate buildups registered past gas hydrate dissociation and methane venting during the glacial-interglacial oscillations (Dickens et al., 1995; Dillon et al., 2001; Hesselbo et al., 2000; Kennett et al., 2000; Teichert et al., 2005). As a greenhouse gas, methane is more effective than carbon dioxide. The huge amount of methane release due to gas hydrate dissociation could play an important role in global warming over geologic time (Kvenvolden, 1988, 1993; MacDonald, 1990). Anaerobic methane oxidation (AOM) is the predominant process in methane-rich marine environment. Upward methane flux could control pore water sulfate depletion and the subsequent depth of sulfateemethane interface (SMI) (Borowski et al., 1996,1999). Under correct conditions, active methane fluxes can be quantified by sulfate gradients driven by AOM (Borowski et al., 1996, 1999). A close relationship between methane flux and sulfate reduction has been observed in many regions, in particular, at gas hydrate-bearing cold seeps (Boetius and Suess, 2004; Borowski et al., 1999; Suess et al., 1999; Tsunogai et al., 2002), mud volcanoes (Niemann et al., 2006), coastal upwelling areas (Niewöhner et al., 1998; Treude et al., 2005), and shallow river deltas (Blair and Aller, 1995). However, methane venting may dis- continue because of closure and opening of gas migration conduits (Aharon et al., 1997; Teichert et al., 2003), and climate induced * Corresponding author. Tel./fax: þ886 2 2363 6424. E-mail address: swlin@ntu.edu.tw (S. Lin). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2011.04.004 Marine and Petroleum Geology 28 (2011) 1829e1837