Elsevier

Chemical Geology

Volume 543, 20 June 2020, 119588
Chemical Geology

Geochemical evidence of methane seepage in the sediments of the Qiongdongnan Basin, South China Sea

https://doi.org/10.1016/j.chemgeo.2020.119588Get rights and content

Highlights

  • High quality samples drilled from high-flux active cold seep area were investigated.

  • AOM caused Mo/U enrichments and authigenic carbonate precipitation.

  • Sedimentary Mg/Ca and Sr/Ca ratios revealed the dominant presence of high-Mg calcite.

  • Three methane release events were identified using combined geochemical proxies.

Abstract

In this paper, the QS-1 sediments (3.64 m long; at 1500 m water depth) collected from the Qiongdongnan Basin were used to analyze the relationship between sedimentary geochemical anomalies and methane seepage. Mo-U covariation, Ce anomaly, and Mg/Ca-Sr/Ca end-member model were analyzed to identify the variations of redox environment and authigenic carbonate precipitation, finally constructing the methane release events in this study area. These observations suggest that: 1) Significant Mo/U enrichments and Ba front indicate the presence of high-flux active methane seepage with SMI of 260 cmbsf in this study area. The depositional environment evolved as oxic-anoxic-reducing-sulfide environment from bottom to top, as evidenced by Mo-U covariation and Ce anomaly; 2) Based on Sr/Ca and Mg/Ca end-member model proposed by Bayon et al. (2007), the high-Mg calcite was the dominant authigenic carbonate currently which usually precipitated behind the methane seep events; and 3) The active cold seep can be divided into three methane release events (MREs): MRE I, the most intense methane seepage occurred at 22.1 ka BP; MRE II, three low-intensity methane seepage occurred at 19.3, 16.5, and 11.3 ka BP, respectively; and MRE III, the continuous methane seep began at 9.3 ka BP until now. The synthesis of geochemical indicators has significant implications for tracing methane seepage of varying intensity and duration.

Introduction

Cold seeps escape upwards from sediment-water interface, which refer to fluids consisting of water, hydrocarbon (e.g., methane), and H2S (Klaucke et al., 2010; Feng and Chen, 2015). The anaerobic oxidation of methane (AOM, CH4 + SO42 → HCO3 + HS + H2O) is a key reaction at the sulfate-methane transition zone (SMTZ), consuming the methane-rich fluids and sulfate and then generating HS and HCO3 (Boetius et al., 2000; Reeburgh, 2007; Boetius and Wenzhöfer, 2013; Feng et al., 2018). This process strengthens the alkalinity of pore water, which facilitates the precipitation of authigenic carbonate (Ca2+ + HCO3 → CaCO3 + H+), pyrite, and sulfate mineral (Berner, 1980; Pierre, 2017). Unlike normal marine carbonate, authigenic carbonate exhibits extremely negative δ13C (+4‰ to –66.7‰) (Campbell et al., 2008; Kazutaka et al., 2010; Li et al., 2018), positive δ18O (+0.4‰ to +7.87‰) (Conti and Fontana, 2011; Anka et al., 2012; Wang et al., 2019), and opposing sulfide- and sulfate-sulfur isotope [+2.3‰ to –46.0‰ (extremely negative) and +19.6‰ to +77.1‰ (extremely positive), respectively] (Aharon and Fu, 2000, Aharon and Fu, 2003; Peckmann and Thiel, 2004; Li et al., 2018), which is the direct indicator of methane seepage. The vertical concentration gradient of SO42− in pore water can be used to estimate the depth of the sulfate-methane interface (SMI), which is the bottom interface of the SMTZ (Borowski et al., 1999; Torres et al., 2004; Gay et al., 2006; Kastner et al., 2008; Luo et al., 2013; Hu et al., 2015; Ye et al., 2016). In addition, major/trace elements, and rare earth elements (REEs) are often employed as geochemistry indicators for gas hydrate. Redox-sensitive elements (e.g., Molybdenum and Uranium) (Algeo et al., 2012; Tribovillard et al., 2012; Hu et al., 2015), Ba content (Feng and Chen, 2007; Vanneste et al., 2013), Sr/Ca and Mg/Ca ratios (Bayon et al., 2007; Nöthen and Kasten, 2011; Yang et al., 2014), and REE anomalies (Budakoglu et al., 2015; Zhu et al., 2019) can be used to recognize variations of the redox conditions and mineral precipitation, reconstructing the history of methane seepage and deposition.

Cold seeps occur on continental margins within the depth from a few hundred to approximately 3500 m (Chen et al., 2002; Xi et al., 2017). The cold seeps of China are common in the Qiongdongnan and Taixinan Basins, and the Shenhu region of the South China Sea (SCS) (Xi et al., 2017; Feng et al., 2018). Based on seismic data collected in the Qiongdongnan Basin, Wang et al. (2010) discovered the presences of bottom-simulating reflector (BSR), gas chimneys, mud diapirism, and polygonal faults which facilitate the upward migration of hydrocarbon, indicating the presence of gas hydrate. Recent studies have focused on the geochemical characteristics of authigenic carbonate, pyrite, and ion concentration in pore water related to methane seepage in the Qiongdongnan Basin. And the studies on geochemical indicators show the presences of 13C-depleted and 18O-enriched authigenic carbonate and framboidal pyrite (Wu et al., 2009a; Li et al., 2018). Wu et al. (2007) discovered that Ca2+, Mg2+, Mn2+, Sr2+, and SO42− concentrations in pore water decrease with increasing depth. Deng et al. (2017) observed that surface sediments in the Qiongdongnan Basin exhibit Mo and U enrichments, which indicates a strong reducing environment and the upward migration of methane-enriched fluids in this region, also reported in the Dongsha region (Hu et al., 2015; Chen et al., 2016).

The relationship between sedimentary geochemical anomalies (e.g., redox-sensitive elements, element ratios, and REE anomalies) and methane seepage has the potential to be preserved in the geological records, serving as a valuable tool for identifying methane seepage. The main purpose of this study is the reconstruction of methane release events (MREs), using Mo-U covariation, Ba front, REE anomalies, and Mg/Ca versus Sr/Ca as the geochemical indicators from the QS-1 sediments. This paper focuses primarily on the recognitions of redox environment and authigenic carbonate, and we show that a synthesis of geochemical indicators can be used as proxies for methane seepage.

Section snippets

Geological background

The SCS is the largest marginal sea of the Western Pacific Ocean, as an important part of the Western Pacific gas hydrate metallogenic belt. The Qiongdongnan Basin is a superimposed basin primarily formed by continental margin rifts during the Cenozoic Era (He et al., 2006). Extreme thick strata and high depositional rate in the Qiongdongnan Basin are conducive for the preservation of organic matter, leading to the formation of microbial and thermal methane (Kvenvolden, 1985; Yan et al., 2006;

Grain sizes

The grain sizes exhibit fluctuating variations in the 93–210 cmbsf interval (average median grain size: 16.58 μm) with anomalously large grain sizes in some layers (Fig. 2). The particles in 210–240 cmbsf interval were generally quite coarse (average median grain size: 45.66 μm; maximum: 79.31 μm) with the highest sand content of 47%. The depth profile of C/M ratios show relatively low values in the 93–240 cmbsf interval in comparison with the other intervals. The values of C/M ratios vary from

C/M ratios

The median grain sizes vary significantly in the 93–240 cmbsf interval with sudden increases in many layers, and are extremely high in the 210–240 cmbsf interval as well as the sand content (Fig. 2). Due to the natural stratigraphic deposition of the QS-1 sediments, the C-M patterns (Fig. 5) are illustrated to study the cause of grain-size variations in the 93–240 cmbsf interval. The overall trends of the 93–210 and 210–240 cmbsf intervals are parallel to the C = M baseline (Fig. 5A),

Conclusion

Sedimentary geochemical anomalies recorded by the QS-1 sediments resulted from high-flux active cold seep in this study area. At the SMI with a shallow depth of 260 cmbsf, intense AOM reactions consumed the high-flux methane fluids, causing the coincident presences of Mo/U enrichments and the increase of alkalinity in pore water which favored authigenic carbonate precipitation. Based on the end-member model developed by Bayon et al. (2007), high-Mg calcite was the dominant authigenic carbonate

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The QS-1 sediments were collected by the Guangzhou Marine Geological Survey (GMGS). We thank the scientists and crew for their hard work in collecting the core samples. We appreciate Prof. N. Li (South China Sea Institute of Oceanology, CAS) for his thoughtful suggestions on a revised draft of the manuscript, We thank Karen Johannesson (Editor-in-Chief of Chemical Geology) and two anonymous reviewers for their constructive comments, which considerably improved the quality of the manuscript.

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