Geochemical evidence of methane seepage in the sediments of the Qiongdongnan Basin, South China Sea
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.
References (90)
- et al.
Sulfur and oxygen isotopes of coeval sulfate–sulfide in pore fluids of cold seep sediments with sharp redox gradients
Chem. Geol.
(2003) - et al.
Paleoceanographic applications of trace-metal concentration data
Chem. Geol.
(2012) - et al.
Environmental analysis of paleoceanographic systems based on molybdenum-uranium covariation
Chem. Geol.
(2009) - et al.
Hydrocarbon leakage through focused fluid flow systems in continental margins
Mar. Geol.
(2012) - et al.
Sr/Ca and Mg/Ca ratios in Niger Delta sediments: implications for authigenic carbonate genesis in cold seep environments
Mar. Geol.
(2007) - et al.
Anaerobic methane oxidation on the Amazon Shelf
Geochim. Cosmochim. Acta
(1995) - et al.
Global and local variations of interstitial sulfate gradients in deep-water, continental margin sediments: sensitivity to underlying methane and gas hydrates
Mar. Geol.
(1999) - et al.
The rare earth element geochemistry on surface sediments, shallow cores and lithological units of Lake Acıgöl basin, Denizli, Turkey
J.Asian Earth Sci.
(2015) Controls on marine carbonate cement mineralogy: review and reassessment
Chem. Geol.
(1993)- et al.
Hydrocarbon seep-carbonates of a Miocene forearc (east coast basin), North Island, New Zealand
Sediment. Geol.
(2008)
Evidence of intense methane seepages from molybdenum enrichments in gas hydrate-bearing sediments of the northern South China Sea
Chem. Geol.
Sulfate profiles and barium fronts in sediment on the Blake Ridge: present and past methane fluxes through a large gas hydrate reservoir
Geochem. Cosmochim. Acta
Authigenic carbonates from an active cold seep of the northern South China Sea: new insights into fluid sources and past seepage activity
Deep Sea Res. II: Topical Studies in Oceanography
Seafloor facies related to upward methane flux within a Giant Pockmark of the Lower Congo Basin
Mar. Geol.
A study of the chemistry of pore fluids and authigenic carbonates in methane seep environments: Kodiak Trench, Hydrate Ridge, Monterey Bay, and Eel River Basin
Chem. Geol.
Molybdenum geochemistry in a seasonally dysoxic Mo-limited lacustrine ecosystem
Geochim. Cosmochim. Acta
Impact of anaerobic oxidation of methane on the geochemical cycle of redox-sensitive elements at cold-seep sites of the northern South China Sea
Deep Sea Res. II: Topical Studies in Oceanography
Genitic types and migration-accumulation dynamics of natural gases in the Ying- Qiong Basin, the South China Sea
Biogeochemical signatures and microbial activity of different cold-seep habitats along the Gulf of Mexico deep slope
Deep-Sea Res. II: Topical Studies in Oceanography
Geochemical constraints on the origin of the pore fluids and gas hydrate distribution at Atwater Valley and Keathley Canyon, northern Gulf of Mexico
Mar. Petrol. Geol.
Temporal variability of gas seeps offshore New Zealand: multi-frequency geoacoustic imaging of the Wairarapa area, Hikurangi Margin
Mar. Geol.
Comparison of marine gas hydrate in sediments of an active and passive continental margin
Mar. Petrol. Geol.
Paleo-cold seep activity in the southern South China Sea: evidence from the geochemical and geophysical records of sediments
J. Asian Earth Sci.
Variations of methane induced pyrite formation in the accretionary wedge sediments offshore southwestern Taiwan
Mar. Petrol. Geol.
Simulation of long-term feedbacks from authigenic carbonate crust formation at cold vent sites
Chem. Geol.
Pockmark activity inferred from pore water geochemistry in shallow sediments of the pockmark field in southwestern Xisha Uplift, northwestern South China Sea
Mar. Petrol. Geol.
Reconstructing changes in seep activity by means of pore water and solid phase Sr/Ca and Mg/Ca ratios in pockmark sediments of the Northern Congo Fan
Mar. Geol.
Carbon cycling at ancient methane-seeps
Chem. Geol.
Origin of the authigenic gypsum and pyrite from active methane seeps of the southwest African Margin
Chem. Geol.
Geochemistry of deep sea sediments at cold seep sites in the Nankai Trough: insights into the effect of anaerobic oxidation of methane
Mar. Geol.
Authigenic carbonates from seeps on the northern continental slope of the South China Sea: new insights into fluid sources and geochronology
Mar. Petrol. Geol.
Barite fronts in continental margin sediments: a new look at barium remobilization in the zone of sulfate reduction and formation of heavy barites in diagenetic fronts
Chem. Geol.
Fluid seepage along the San Clemente Fault scarp: basin-wide impact on barium cycling
Earth Planet. Sci. Lett.
Gas hydrate growth, methane transport, and chloride enrichment at the southern summit of Hydrate Ridge, Cascadia margin off Oregon
Earth Planet. Sci. Lett.
Analysis of marine environmental conditions based on molybdenum–uranium covariation—applications to Mesozoic paleoceanography
Chem. Geol.
Authigenic barite records of methane seepage at the Carlos Ribeiro mud volcano (Gulf of Cadiz)
Chem. Geol.
Formation of methane-derived carbonates during the last glacial period on the northern slope of the South China Sea
J.Asian Earth Sci.
Pore water geochemistry in shallow sediments from the northeastern continental slope of the South China Sea
Mar. Petrol. Geol.
Stable isotopes and rare earth element compositions of ancient cold seep carbonates from Enza River, northern Apennines (Italy): implications for fluids sources and carbonate chimney growth
Mar. Petrol. Geol.
Microbial sulfate reduction rates and sulfur and oxygen isotope fractionations at oil and gas seeps in Deepwater Gulf of Mexico
Geochim. Cosmochim. Acta
Reprint of: new applications of trace metals as proxies in marine paleoenvironments
Chem. Geol.
The occurrence and formation of dolomite in organic-rich continental margin sediments
AAPG Bull.
Early Diagenesis-A Theoretical Approach
Barite solubilities and thermodynamic quantities up to 30 °C and 1400 bars
Am. Mineral.
Seafloor oxygen consumption fuelled by methane from cold seeps
Nat. Geosci.
Cited by (15)
Discerning the sulfur geochemical features of turbidites and methane-rich sediments from the South China sea
2024, Marine and Petroleum GeologyGeochemical record of methane seepage in carbon cycling and possible correlation with climate events in the Qiongdongnan basin, South China Sea
2023, Marine and Petroleum GeologyCitation Excerpt :This illustrated that the primary productivity has little effect on the carbon isotope compositions of benthic foraminifera in core QS-1. Furthermore in core QS-1, accompanied by the enrichments of Mo and U (Liu et al., 2020), the high values of the CaCO3 contens (Fig. 2) and the increasing trend of TS contents (Fig. 2) in the same intervals, the negative bias of δ13CU.spp indicate that the light methane isotope is the significant cause of the negative carbon isotope compositions in foraminifera, further confirm that methane released from gas hydrate decomposition was recorded by foraminifera shells. The intensities of SD-AOM varied with the scope of methane seepage (Feng and Chen, 2015).
Quantitative assessment of dissolved inorganic carbon cycling in marine sediments from gas hydrate-bearing areas in the South China Sea
2022, Marine and Petroleum GeologyTracing the paleo-methane seepage activity over the past 20,000 years in the sediments of Qiongdongnan Basin, northwestern South China Sea
2021, Chemical GeologyCitation Excerpt :In the South China Sea, the research on the paleo-methane seepage geochemistry of sediments is relatively late and little, and it is mainly concentrated in the Site F sedimentary area (also known as Formosa Ridge) (Chen et al. 2016; Li et al. 2016; Li et al. 2018). However, there are a few reports on the ancient methane seep activity in another modern active cold seep sedimentary area, ‘Haima seep’, which was discovered in 2016 in the Qiongdongnan Basin (Wang et al. 2018; Liu et al. 2020). Therefore, in this paper, the main/trace elements, TOC, TS, TIC, TN, magnetic susceptibility, grain size, and AMS-14C were measured and analyzed for the core sediments obtained in the ‘Haima seep’ cold seepage deposition area of the Qiongdongnan Basin.