Evidence of intense methane seepages from molybdenum enrichments in gas hydrate-bearing sediments of the northern South China Sea
Graphical abstract
Introduction
The seepage of methane-rich fluids (namely cold seeps) at the seafloor is a widespread phenomenon along continental margins worldwide (e.g., Campbell, 2006, Judd and Hovland, 2007, Suess, 2014). Cold seeps usually breed chemosynthesis-based communities, develop authigenic carbonates, and are associated with the possible occurrence of gas hydrates (Judd and Hovland, 2007, Suess, 2014). The key biogeochemical process at seeps is the anaerobic oxidation of methane (AOM) coupled with sulfate reduction (Boetius et al., 2000, Boetius and Wenzhöfer, 2013). This process produces dissolved bicarbonate and hydrogen sulfide (H2S) that increase pore water alkalinity, thus favoring the precipitation of authigenic carbonates (Berner, 1980). The dynamics of fluid flows at seeps are characterized by changes in flow intensity and episodic seepages as well, which might be controlled by factors such as the exhaustion of hydrocarbon sources, sea level variations and bottom water temperature fluctuations that drive the dissociation of gas hydrates (e.g., Kvenvolden, 1993, Aharon et al., 1997, Judd et al., 2002, Kennett et al., 2003, Teichert et al., 2003, Feng et al., 2010, Bayon et al., 2015). The reconstruction of past variations in methane fluxes is largely dependent on proxies that help define the intensities of methane seepages. Numerous studies have attempted to use geochemical proxies such as the content and isotopic anomalies of authigenic carbonates, barites and pyrites to constrain methane seepage intensities and their variations (e.g., Torres et al., 1996, Aharon et al., 1997, Dickens, 2001, Teichert et al., 2003, Bayon et al., 2007, Nöthen and Kasten, 2011, Feng et al., 2010, Lim et al., 2011, Peketi et al., 2012, Borowski et al., 2013).
Authigenic carbonates formed at seeps can be used to identify periods of methane seepage and/or gas hydrate dissociation and variations in methane fluxes by carbon and oxygen isotopic and elemental analyses (e.g., Aharon et al., 1997, Bohrmann et al., 1998, Naehr et al., 2000, Greinert et al., 2001, Teichert et al., 2003, Peckmann and Thiel, 2004, Feng et al., 2010, Han et al., 2014, Feng and Chen, 2015). However, the most important proxy, strong 13C-depletion in carbonates, might be altered and even shifted to 13C-enrichment by the incorporation of upraised 13C-rich fluids from the methanogenic zone (e.g., Peckmann and Thiel, 2004, Roberts et al., 2010). In addition, barium (Ba) fronts typically form slightly above sulfate methane transition zones (SMTZs; e.g., Torres et al., 1996, Dickens, 2001, Riedinger et al., 2006, Snyder et al., 2007, Vanneste et al., 2013). It has been shown that Ba fronts can record present and past fluid seepage events and the evolution of SMTZs in sedimentary columns (e.g., Dickens, 2001, Riedinger et al., 2006, Vanneste et al., 2013). In systems where the upward Ba2 + flux exceeds the downward barite flux, a prominent Ba front develops just above the depth of SO42 − depletion. A new Ba front will develop below a “paleo” front if the SO42 − gradient deepens (Dickens, 2001). That formation condition is uncommon and occasionally leads to the absence of Ba fronts at SMTZs (Peketi et al., 2012), thus limiting the application of Ba fronts to the reconstruction of past seepage events. The potential of sulfur isotopes in pyrites as a proxy for identifying AOM-related processes and seepage events has been proposed (Jørgensen et al., 2004, Peketi et al., 2012, Borowski et al., 2013, Formolo and Lyons, 2013). The application of sulfur isotopes in pyrites, however, will be limited by a complex interplay of factors that include iron availability and non-steady-state sedimentation (Borowski et al., 2013, Formolo and Lyons, 2013). Accordingly, more geochemical proxies are needed to better constrain methane seepages and their variabilities.
Recent studies have reported molybdenum (Mo) enrichments in seep-impacted sediments (Peketi et al., 2012, Sato et al., 2012), especially Mo enrichments at SMTZs (Peketi et al., 2012), which has been suggested to be capable of identifying H2S seepages associated with methane (Peketi et al., 2012). Molybdenum behaves conservatively under oxic conditions. Under a sulfidic environment with the generation of free H2S, dissolved Mo is scavenged from solution via organic materials or via Mo capture by iron sulfide phases (Helz et al., 1996, Helz et al., 2011, Zheng et al., 2000, Tribovillard et al., 2006). Although Mo enrichment is a promising proxy for tracing methane seepages, some SMTZs have shown an absence of Mo enrichments (Peketi et al., 2012), which complicates the relationship between Mo enrichments and methane (or H2S) seepages, and further work is therefore needed to reveal the condition for the enrichment of Mo at seeps and its potential as a proxy for tracing methane seepages.
Here we investigate the solid geochemistry of sediments and associated authigenic carbonates obtained from gas hydrate-bearing sediment cores as long as ~ 95 m at gas hydrate drilling sites from the northern South China Sea (SCS). The carbon and oxygen isotopes and contents of authigenic carbonates and major and trace elements of the sediments were determined. We show below that Mo enrichments in sediments can provide unique information on the intensity of methane seepages and its possible relationship to the local dissociation of gas hydrates.
Section snippets
Regional setting
The northern SCS is a passive continental margin controlled by complex interactions between the Eurasian, Pacific, and Indian-Australian plates (Taylor and Hayes, 1983). The study site is situated in the Dongsha Area of the northern SCS (Fig. 1). The widely observed faults and mud diapirs in the study area can serve as effective pathways for hydrocarbon migration (Suess, 2005, Wu et al., 2005, Yan et al., 2006). In the vicinity of the study site, a number of bottom simulating reflectors,
Sampling and analytical methods
The sediment samples were obtained from a drilling program during the GMGS-2 gas hydrate expedition (Fig. 1). The GMGS08 site is one of 13 drilled sites and has a water depth of 801 m. Five sediment cores (08B, 08C, 08E, 08F, and 08G) were drilled and collected, and the drilling depth reached ~ 95 m below the seafloor (mbsf). Considering the very close proximities of the collected cores and the purpose of this study (Fig. 1), hereafter the cores are taken together as a single vertical profile. The
TOC contents
The TOC contents in the sediments is relatively low and ranges from 0.52% to 1.49% (mean = 0.94%, sd = 0.23%, and n = 78; Supplementary Table 1). The TOC contents in upper cores 08B (mean = 1.11%, sd = 0.15%, and n = 8) and 08C (mean = 1.11%, sd = 0.20%, and n = 30) are slightly higher than those in cores 08G (mean = 0.72%, sd = 0.12%, and n = 15), 08E (mean = 0.86%, sd = 0.08%, and n = 10), and 08F (mean = 0.81%, sd = 0.18%, and n = 15).
Carbonate contents
The total carbonate contents (∑ Carb.) vary from 12.1% to 92.9% (mean = 21.0%, sd = 13.9%, and n =
Evidence of past intense seepages and gas hydrate dissociation
Authigenic carbonates that formed at cold seeps usually have highly depleted 13C, which serves as an important indicator of past methane seepage and the source of the seep fluids (e.g., Peckmann and Thiel, 2004, Chen et al., 2005, Roberts et al., 2010). The highly 13C-depleted authigenic carbonates in the five intervals of the sediment profile indicate that the authigenic carbonates are the products of AOM and reveal the presence of the past methane seepages in these intervals. The extremely
Conclusions
Elemental and stable isotopic data were reported for gas hydrate-bearing sediments and associated authigenic carbonates at the GMGS08 gas hydrate drilling site in the northern South China Sea. High contents and strongly negative δ13C values of authigenic carbonates occurred in the five intervals of the sediment profile, indicating the presence of methane seepages. The methane seepages are suggested to be intense, and methane may have been discharged into the water column based on the observed
Acknowledgements
We thank the crew and scientists of the vessel during the GMGS-2 gas hydrate drilling expedition. We are grateful to the technicians from Institute of Experimental Test (GMGS) and Prof. Y.B. Peng (Louisiana State University) for helping with the geochemical analysis. We thank the journal editor M.E. Böttcher, N. Tribovillard and one anonymous reviewer for constructive comments, which considerably improved the quality of the manuscript. This research was partially supported by the NSF of China
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