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Calcium Isotope Fractionation and Its Controlling Factors Over Authigenic Carbonates in the Cold Seeps of the Northern South China Sea

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Calcium Isotope Fractionation and Its Controlling Factors Over Authigenic Carbonates in the Cold Seeps of the Northern South China Sea Article Oceanology April 2012 Vol.57 No.11: 13251332 doi: 10.1007/s11434-012-4990-9 Calcium isotope fractionation and its controlling factors over authigenic carbonates in the cold seeps of the northern South China Sea WANG ShuHong1, YAN Wen1*, MAGALHÃES H Vitor2, CHEN Zhong1, PINHEIRO M Luis2 & GUSSONE Nikolaus3 1 CAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China; 2 Centre for Environmental and Marine Studies (CESAM) and Geosciences Department, University of Aveiro, Aveiro 3810-193, Portugal; 3 Institut für Mineralogie, Universität Münster, Münster 48149, Germany Received November 7, 2011; accepted December 23, 2011; published online February 21, 2012 In this study, we analyzed stable calcium isotope results of authigenic carbonates from two cold seep areas of the Dongsha area and the Baiyun Sag in the northern South China Sea. The stable isotopes of carbon and oxygen as well as the mineral composition 44/40 of authigenic carbonates were used to investigate control calcium isotope fractionation. The Ca ratios of the southwestern Dongsha area samples ranged from 1.21‰ to 1.52‰ and the ratio of the Baiyun Sag sample was 1.55‰ of the SRM915a isotope standard. X-ray diffraction analysis showed that the carbonate samples consisted of dolomite, calcite and aragonite, with small amounts of high-Mg calcite and siderite. The 13C values of the carbonates of the southwestern Dongsha area varied between 18 49.21‰ and 16.86‰ of the Vienna PeeDee Belemnite (VPDB) standard and the O values ranged from 2.25‰ to 3.72‰ 13 18 13 VPDB. The C value of the Baiyun Sag sample was 2.36‰ VPDB and the O value was 0.44‰ VPDB. The C values of the carbonates of the southwestern Dongsha area revealed there is methane seeping into this area, with a variable contribution of 13 methane-derived carbon. The sampled carbonates covered a range of C values suggesting a dominant methane carbon source 13 for the light samples and mixtures of C values for the heavier samples, with possibly an organic or seawater carbon source. The 18 18 O values indicated that there is enrichment in O, which is related to the larger oxygen isotope fractionation in dolomite com- pared to calcite. The results of the Baiyun Sag sample exhibited normal seawater carbon and oxygen isotopic values, indicating 44/40 that this sample is not related to methane seepage but instead to precipitation from seawater. The relatively high Ca values indicated either precipitation at comparatively high rates in pore-water regimes with high alkalinity, or precipitation from an evolved heavy fluid with high degrees of Ca consumption (Raleigh type fractionation). The dolomite samples from the Dongsha area revealed a clear correlation between the carbon and calcium isotope composition, indicating a link between the amount and/or rate of carbonate precipitation and methane contribution to the bicarbonate source. The results of the three stable isotope systems, mineralogy and petrography, show that mineral composition, the geochemical environment of authigenic carbonates and carbon source can control the calcium isotope fractionation. authigenic carbonate, cold seeps, calcium isotope, northern South China Sea, gas hydrates Citation: Wang S H, Yan W, Magalhães H V, et al. Calcium isotope fractionation and its controlling factors over authigenic carbonates in the cold seeps of the northern South China Sea. Chin Sci Bull, 2012, 57: 13251332, doi: 10.1007/s11434-012-4990-9 Cold seeps occur extensively along active and passive con- [1,2]. The ascending methane is mainly derived from mi- tinental margins worldwide, and are characterized by the crobial methanogenesis or from thermochemical decompo- expulsion of hydrocarbon-rich fluids (especially methane) sition of organic matter at depth. The emanation of gas from from the sedimentary column to the seafloor at seepage sites the deep subsurface into the water column has a profound effect on the marine environment, providing an environment *Corresponding author (email: [email protected]) for chemosynthetic biota along the seafloor local to the © The Author(s) 2012. This article is published with open access at Springerlink.com csb.scichina.com www.springer.com/scp 1326 Wang S H, et al. Chin Sci Bull April (2012) Vol.57 No.11 seepage sites, as well as on the chemistry of the sediments 1 Geological setting [3]. Carbonate precipitation at hydrocarbon seepage sites results from anaerobic oxidation of methane by archaea The South China Sea is one of the largest marginal seas in microbes and sulfate reduction of heavier hydrocarbons by the western Pacific, with an average water depth of about sulfate reducing bacteria [4,5]. Authigenic carbonates are 1200 m and a maximal depth of about 5380 m. Its bottom therefore common at cold vent sites [6], and can be used to topography is complex, showing spatial variability with unravel the evolution of past cold seep activity. In particular, several plateaus, troughs, valleys and island reefs. The stable isotopes of carbon and oxygen have been proven to northern margin is composed of fault controlled terraces and be useful proxies in such carbonate minerals [7,8]. In addi- basins (Figure 1), where the deposition of sediments with tion, during the last decade, the use of the isotope composi- abundant organic matter favors gas (mainly methane) gen- tion of Ca has shown considerable potential as a geological eration with subsequent hydrate formation and conservation tracer. In the marine realm, Ca isotopes have been used for within the gas hydrate stability zone. In early 1984, evi- the reconstruction of the marine Ca-budget [9–11], paleo- dence for bottom simulating reflectors (BSR) in the Nansha temperature reconstructions [12–14], characterization of Trough of the southern South China Sea was discovered relationships in ancient food webs [15,16], the study of bi- based on seismic data. These are mainly located at 300–600 m omineralization and related Ca isotope fractionation mecha- below the seabed along the southeastern slopes of the nisms [17–21], and the study of processes during early dia- Nansha Trough, at water depths between 1500–2800 m [34]. genesis [22–24]. The northern South China Sea is presently a key area for The study of the Ca isotope ratios of authigenic car- the exploration and research of gas hydrate resources within bonate minerals can contribute to our understanding of ge- China. In 1998, evidence of BSRs was reported in the ochemical processes. This is because the ratio of Ca iso- northern South China Sea [37]. Several areas of gas hy- topes is influenced by several variables including tempera- drates occurrence, including two of high concentration, the ture [25,26], precipitation rate [26,27], the crystal structure northeastern Dongsha area and Xisha Trough, were discovered of the minerals [28,29], the composition of the Ca in the based on the presence of BSRs and geochemical anomalies fluid and the fraction of Ca removed during precipitation [36,38–42]. In recent years, many gas hydrate cold seeps [22]. have been found in the northern South China Sea. Occur- The calcium isotope research of cold seeps and gas hy- rences of gas hydrates by the observation of BSRs and cold drate is a frontier field [22,24,30,31]. At cold seep sites, seeps have also been reported in southwestern Taiwan [43–45]. methane-derived authigenic carbonates are a significant Chen et al. [46] reported for the first time the occurrence of calcium sink in the sediments. The strong depletion of dis- carbonates originating from cold seeps in the South China solved Ca in the pore-waters of shallow sedimentary sec- Sea and showed that those carbonates probably formed at tions (centimeters-tens of meters) because of carbonate presently active gas seepage sites. The presence of active formation has been widely observed [31]. Where CaCO3 gas seepage sites has direct implications for gas exploration precipitation is faster than diffusion, the carbonate precipi- and hydrate accumulation in that area. The Guangzhou Ma- tation causes 44Ca enrichment in the surrounding pore-waters 40 rine Geological Survey has discovered large seafloor areas because of the preferred incorporation of Ca into the pre- covered by authigenic carbonates, bacterial mats, tube worms cipitate [25,26,32,33]. It was shown by Teichert et al. [22] and bivalves in the northeastern Dongsha area, which were that the combined application of the stable isotopes of C, O interpreted as evidence of gas hydrates occurrence [47–49]. and Ca on cold seep carbonates provides insight into multi- The discovery of large carbonates of massive and irregular ple factors influencing the isotopic composition of the au- forms, carbonate crusts and pavements, carbonate chimneys thigenic carbonate. In addition, the application of these iso- and tube worms in the southwestern Dongsha area, took topes provides a tool to record the evolution of the geo- place during cruises organized by the South China Sea In- chemical environment of gas hydrate systems. stitute of Oceanology of the Chinese Academy of Sciences The aim of this study was to understand the mechanisms in 2005 [36]. Authigenic carbonates were also found in the controlling Ca-isotope fractionation during the biogeo- gas hydrate prospect area of Qiongdongnan. These discov- chemical processes at cold seeps. This was achieved eries constitute important evidence of the widespread pres- through characterization of authigenic carbonates by use of ence of methane seepage in this area [36]. petrography, mineralogy and stable carbon and oxygen iso- tope results. We focused on environmental parameters and processes that govern Ca isotope fractionation in the authi- 2 Materials and methods genic carbonates and investigated if they can be used to 2.1 Samples indicate the formation and destabilization of gas hydrate. The 44/40Ca values of the authigenic carbonates represent The authigenic carbonates analyzed in this study were col- the first Ca isotope data of cold seep carbonates derived lected with a grab sampler.
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