Anaerobic Oxidation of Methane in Coastal Sediment from Guishan Island (Pearl River Estuary), South China Sea

Anaerobic oxidation of methane in coastal sediment from Guishan Island (Pearl River Estuary), South China Sea Zijun Wu1,2,∗, Huaiyang Zhou1, Xiaotong Peng1, Nan Jia2, Yuhong Wang3 and Linxi Yuan2 1State Key Laboratory of Marine Geology, Tongji University, Shanghai 200 092, China. 2Institute of Polar Environment, University of Science and Technology of China, Hefei, Anhui 230 026, China. 3National Institute of Health, Bethesda, Maryland 20892, USA. ∗e-mail: [email protected] 2− The concentrations of CH4,SO4 , ΣCO2 and the carbon isotope compositions of ΣCO2 and CH4 in the pore-water of the GS sedimentary core collected from Guishan Island (Pearl River Estuary), South China Sea, were determined. The methane concentration in the pore-water shows dramatic changes and sulfate concentration gradients are linear at the base of the sulfate reduction zone for the station. The carbon isotope of methane becomes heavier at the sulfate-methane transition 12 13 (SMT) likely because of the Raleigh distillation effect; CH4 was oxidized faster than CH4,and 13 13 this caused the enrichment of residual methane δ Candδ C–ΣCO2 minimum. The geochemical profiles of the pore-water support the existence of anaerobic oxidation of methane (AOM), which is mainly controlled by the quality and quantity of the sedimentary organic matter. As inferred from the index of δ13C–TOC value and TOC/TN ratio, the organic matter is a mix of mainly refractory terrestrial component plus some labile alga marine-derived in the study area. A large amount of labile organic matter (mainly labile alga marine-derived) is consumed via the process of sedimentary organic matter diagenesis, and this reduces the amount of labile organic matter incorporated into the base of the sulfate reduction zone. Due to the scarcity of labile organic matter, the sulfate will in turn be consumed by its reaction with methane and therefore AOM takes place. Based on a diffussion model, the portion of pore-water sulfate reduction via AOM is 58.6%, and the percentage of ΣCO2 in the pore-water derived from AOM is 41.4%. Thus, AOM plays an important role in the carbon and sulfur cycling in the marine sediments of Pearl River Estuary. 1. Introduction available electron donors such as acetate, H2 and low-molecular weight organic matter (Froelich et al Methane (CH4) is considered as the most potent 1979; Burns 1998). Once sulfate is removed from greenhouse gas, and it contributes about 22% of the pore fluids, methanogenesis begins and results in climate forcing by all long-lived greenhouse gases high concentrations of dissolved CH4 at relatively in the past 150 years (Lelieveld et al 1998). The shallow depths for organic-rich coastal sediments CH4 concentration in the atmosphere has more (e.g., Martens and Berner 1977; Martens et al than doubled in the post-industrial era, and an 1998, 1999). After diffusing into overlying sedi- increasing amount of attention has been paid to ment, most of CH4 is reoxidized to CO2 using 2− the production and fate of CH4 in different envi- SO4 as the terminal electron acceptor, resulting ronments. In marine sediments, sulfate-reducing in a zone of coupled sulfate reduction-methane bacteria outcompete mathanogenic bacteria for oxidation near the base of the sulfate reduction Keywords. Anaerobic oxidation of methane (AOM); sulfate reduction; pore-water; South China Sea. J. Earth Syst. Sci. 117, No. 6, December 2008, pp. 935–943 © Printed in India. 935 936 Zijun Wu et al zone (Martens and Berner 1977; Reeburgh 1980; Moore et al 2004; Treude et al 2005). This process mentioned above is called anaerobic oxidation of methane (AOM), it occurs through the 2− − − net reaction CH4 +SO4 → HCO3 +HS +H2O (Reeburgh 1976), and it is mediated by a con- sortium of sulfate-reducing bacteria and archea (Boetius et al 2000; Orphan et al 2001). AOM can effectively remove methane from marine sediments before it reaches the sediment– water interface, and it may consume 25–94 Tg of methane in marine sediments each year, and the amount is between 5% and 20% of the global flux of methane to the atmosphere (Reeburgh and Alperin 1988). Hence, AOM plays a significant role in the global budgets of methane and the emission of methane into the atmosphere, where CH4 acts as a strong greenhouse gas. Furthermore, methane is oxidized with concurrent sulfate reduction during AOM, producing inorganic carbon and volatile sulfur, and in this way it affects the biogeochemi- cal cycle of carbon and sulfur in marine sediments (Hensen et al 2003; Jørgensen et al 2004; Moore et al 2004). Figure 1. Map of the Pearl River Estuary, South China Sea AOM could leave a distinctive imprint on the showing the location of the sampling station GS. The inset sedimentary geochemical record as revealed by the shows a large-scale map of China with a small box marking slope changes in methane and sulfate concentra- the region expanded in the main map. tion profiles (e.g., Reeburgh 1976; Niewöhner et al 13 1998), the δ C–ΣCO2 minima due to the special isotope composition of methane (Blair and Aller from Asian regions) at the present time. Thus, 1995; Martens et al 1999; Tsunogai et al 2002), neither is much known about the magnitude and and the peaks of in situ AOM and sulfate reduc- the control mechanisms that drive AOM in the dif- tion rates measured at the same depth (Devol 1983; ferent marine sediments nor how important it is Iversen and Jørgensen 1985; Treude et al 2005). concerning the global budgets of C and S. The portion of total sulfate reduction fuelled by In this study, one marine sediment core from Pearl River Estuary, South China Sea was sampled. methane oxidation varies among locations. Numer- 2− ous studies have demonstrated the importance of We analyzed the pore-water profiles of CH4,SO4 , AOM in the consumption of sulfate in marine sedi- ΣCO2, pH and the carbon isotope of the CH4 and ments (Devol 1983; Iversen and Jørgensen 1985; ΣCO2, identified the process of AOM, and exami- Niewöhner et al 1998; Treude et al 2005). For ned AOM’s role in the consumption of sulfate and Saanich Inlet sediments, 23–40% of the down- the production of ΣCO2 in this area. ward sulfate fluxes were attributed to AOM according to a coupled reaction diffusion model (Devol and Anderson 1984). From interstitial flux cal- 2. Material and methods culations, approximately 50% of the downward sulfate fluxes at Cariaco Trench station could be 2.1 Sampling sites accounted for AOM (Reeburgh 1976). Iversen and Jørgensen (1985) reported that 61–89% of the Guishan Island is situated in Wanshan district, downward sulfate fluxes consumed by AOM at the Zhuhai city, Guangdong province. The island is 17 base of sulfate reduction zone for the Kattegat nautical miles from Macao and Xiangzhou to the and Skagerrak sediments using direct rate mea- west and three nautical miles from Lantau Island surements with radiotracer labeled method. Thus, in Hong Kong to the north, forming an arc as a AOM needs to be investigated throughout the pro- natural sea gate to the mouth of the Pearl River ductive marine regions worldwide, from littoral to Estuary, South China Sea (figure 1). bathyal depths, to enhance the understanding of During an investigation cruise on the Haijian 83 this complex controlled process and to evaluate its in the South China Sea, on 15 March 2006, a sedi- quantitative importance. However, investigations ment gravity core (labeled as GS) was recovered on global AOM are still incomplete (especially from the shallow water station (at a water depth Anaerobic oxidation of methane in coastal sediment 937 of 27 m) near the Guishan Island. At the sam- reported here. Relative precision was estimated at pling site, the temperature of the surface and bot- ±3.0% for TOC and ±5.0% for TN. tom water is 25.4◦Cand15.0◦C, respectively; the Samples for δ13C analysis were placed in the salinity of the bottom water is 25.9.Thissedi- CE EA1112 C/N/S Analyzer and burned at 960◦C ment core mainly consists of silt clay, and the color in a combustion tube under an O2 atmosphere. changes from gray for the upper 12 cm sediments Combustion gases were swept through a reduction to black at the bottom. An odorous smell of H2S column by a stream of inert He gas and passed was detected from 50 cm and below. Furthermore, into a gas chromatography where CO2, still in the the bottom water is anoxic as dissolved oxygen can He stream, was separated from other gases. The be rapidly depleted by both chemical and biologic gas stream then entered a DELTAplusXL mass processes (Peng and Chen 1994; Jia and Peng 2003; spectrometer, and the CO2 gas was analyzed and Dai et al 2006; Hu et al 2006). compared to NBS-22 reference gas. The internal gas standard carbon isotope ratios in sediment 2.2 Sampling methods organic matter are expressed in conventional delta (δ) notation, the per mil () deviation from the Sediment samples were collected from the gra- standard Pee Dee Belemnite (PDB). vity core GS (ID 7.4 cm, length 4 m). The core barrel contained two rows of predrilled 1 cm ID 2.3.2 Methane, sulfate and dissolved holes so that subsamples could be taken at every inorganic matter (ΣCO2) 4 cm interval. The holes were covered with plastic tape on both, inside and outside of the core bar- The level of methane in pore-water was determined rel, and upon retrieval of the core, the tape was according to Jørgensen et al (2001).

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