Using sediment geochemistry to infer temporal variation of methane flux at a cold seep in the South China Sea

Accepted Manuscript Using sediment geochemistry to infer temporal variation of methane flux at a cold seep in the South China Sea Niu Li, Dong Feng, Linying Chen, Hongbin Wang, Duofu Chen PII:

S0264-8172(16)30252-5

DOI:

10.1016/j.marpetgeo.2016.07.026

Reference:

JMPG 2633

To appear in:

Marine and Petroleum Geology

Received Date: 26 January 2016 Revised Date:

11 July 2016

Accepted Date: 26 July 2016

Please cite this article as: Li, N., Feng, D., Chen, L., Wang, H., Chen, D., Using sediment geochemistry to infer temporal variation of methane flux at a cold seep in the South China Sea, Marine and Petroleum Geology (2016), doi: 10.1016/j.marpetgeo.2016.07.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Using sediment geochemistry to infer temporal variation of methane flux at a cold seep in the South China Sea

CAS Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry,

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a

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Niu Li a,b, Dong Feng c*, Linying Chen d, Hongbin Wang e, Duofu Chen d*

Chinese Academy of Sciences, Guangzhou 510640, China

Department of Earth Sciences and Geological Engineering, Sun Yat-Sen University,

Guangzhou 510275, China c

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b

Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology,

Chinese Academy of Sciences, Guangzhou 510301, China

Hadal Science and Technology Research Center, College of Marine Sciences,

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d

Shanghai Ocean University, Shanghai 201306, China Guangzhou Marine Geological Survey, Guangzhou 510740, China

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e

* Corresponding authors.

ACCEPTED MANUSCRIPT E-mail addresses: [email protected] (N. Li), [email protected] (D. Feng), [email protected] (D. Chen).

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Abstract: Release of methane from the seafloor throughout the world’s oceans and the biogeochemical processes involved may have significant effects on the marine

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sedimentary environment. Identification of such methane release events in marine

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sediment records can hence provide a window into the magnitude of ancient seeps. Here, we report on analysis of the geochemical composition of samples in a 12.3 m long sediment core (DH-5) collected from a seep site in the South China Sea (SCS). Our aim has been to investigate whether the evidence for the presence of methane

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release event within sediments is discernible from solid-phase sediment geochemistry. We show that sedimentary total sulfur (TS), δ34S values of chromium reducible sulfur (δ34SCRS) along with total organic carbon (TOC) and total inorganic carbon (TIC)

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content can be used to infer the presence of methane release events in cold seep

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settings. At least three methane release events were identified in the studied core (Unit I at 400-550 cm, Unit II at 740-820 cm, and Unit III at 1000-1150 cm). According to the characteristic of redox-sensitive elements (eg., Mo, U and Mn), we suggest that methane flux has been changed from relatively high (Unit I) to low (Unit II and III) rates. This inference is supported by the coupled occurrence of 34S-enriched sulfides in Unit II and III. AMS 14C dates from planktonic foraminifera in Unit I suggest that high methane flux event occurred at ~15.4 to 24.8 kyr BP, which probably resulted in

ACCEPTED MANUSCRIPT locally-focused aerobic methane oxidation. Overall, our results suggest that TS, TOC, TIC and δ34SCRS have potential for identifying present and fossil methane release

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events in marine sediments.

Keywords: Cold seep, Anaerobic oxidation of methane, Sulfur isotope, South China

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Sea

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1. Introduction

The seepage of hydrocarbon-rich fluids (mainly methane) in marine sediments is common in continental margins worldwide (e.g. Campbell et al., 2002; Campbell, 2006). A large quantity of methane could release into the water column and

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atmosphere owing to the glacial-interglacial sea level fluctuations and bottom water temperature changes driving massive gas hydrate destabilization (Watanabe et al., 2008; Westbrook et al., 2009; Ménot and Bard, 2010; Han et al., 2014; Feng et al.,

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2014). As a greenhouse gas, methane is 25 times more effective in trapping heat than

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carbon dioxide (Chang et al., 2012). Some high temperature events in geological time, such as the Paleocene-Eocene Thermal Maximum (PETM) are likely related to substantial methane release due to gas hydrate dissociation (Dickens, 1995; Zachos et al., 2005). The latest study shows that the activity of Bacteria and Archaea in seep areas may contribute significant organic matter to the deep ocean (Pohlman et al., 2011; Coffin et al., 2014; Coffin et al., 2015). Modern seep activity can be detected through seabed observations, pore water

ACCEPTED MANUSCRIPT analysis, methane anomaly detection, and many other methods (Borowski et al., 1996, 1999; Borowski, 2004; Newman et al., 2008; Bayon et al., 2009b, 2011; Mazumdar et al., 2012a, 2014; Brothers et al., 2013, 2014; Lemaitre et al., 2014; Skarke et al, 2014),

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but temporal variations in methane flux in the past are not well constrained because it is difficult to select the appropriate indicators in quantifying and age determining the

occurrence of seep activity. Consequently, little is known about the impact of methane

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seepage on the surrounding sediment environment in seep settings.

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Geochemical proxies such as stable carbon isotope records of authigenic carbonates have been applied to evaluate the temporal variation of methane seep, owing to their very negative δ13C (Peckmann and Thiel, 2004; Han et al., 2004, 2008, 2013; Mazumdar et al., 2009, 2011; Bayon et al., 2009a, 2013, 2015; Crémière et al.,

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2013; Tong et al., 2013; Tribovillard et al., 2013; Feng et al., 2014, 2015a, 2015b). However, such carbonates are most intense at intermediate flow rates, and usually do not form in low and high flux conditions (Luff and Wallmann, 2003; Karaca et al.,

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2010). Moreover, low dissolved methane content of fluids, high bioturbation rates,

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and/or high sedimentation rates can also result in the absence of authigenic carbonates in marine sediments (Luff et al., 2004; Bayon et al., 2007). In fact, numerous recent studies reveal that seep-impacted sediment cores generally have relatively complete sequence and may serve as another important archive to reconstruct the evolution of past seepage activities (Bayon et al., 2007; Peketi et al., 2015). Methane-involved biogeochemical processes at seep sites mainly include anaerobic oxidation of methane (AOM) and aerobic oxidation of methane (Reeburgh,

ACCEPTED MANUSCRIPT 2007): CH4+SO42-
HCO3-+HS- +H2O

(1)

In low-flux seep environments the methane may be completely consumed by AOM.

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In high-flux seep environments the sulfate methane transition zone (SMTZ) is typically very shallow, and methane may escape directly into the bottom waters (Paull et al., 2005; Castellini et al., 2006). In this scenario, most of the methane that escapes

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is then consumed by methanotrophic aerobic microbes in the bottom waters, resulting

2015): CH4+2O2
CO2+2H2O

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locally in anoxic conditions (Niemann et al., 2006; Reeburgh, 2007; Consolaro et al.,

(2)

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The AOM process in cold seep systems are increasingly understood (e.g. Boetius and Wenzhöfer, 2013), but little is known about the process of aerobic oxidation of methane and its impact on sedimentary environment.

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In this study, we describe the behavior of the coupled C–S–Fe geochemistry and

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redox-sensitive elements, as well as chromium reducible sulfur (CRS) and δ34SCRS features in sediments collected from the cold seep area of the Dongsha on the northern continental slope of the SCS. The purpose of this study is to: 1) evaluate the dynamics of methane release over the course of sediment formation in seep environments, and 2) provide insight into impacts on the local sediment environment.

2. Background

ACCEPTED MANUSCRIPT 2.1 Pyrite and pyrite sulfur isotope as a proxy of anaerobic oxidation of methane In normal marine environment, pyrite is typically formed through sulfate reduction producing hydrogen sulfide, which reacts with reactive iron (Berner, 1970,

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1984). Therefore, the sedimentary organic carbon supply is commonly the primary control on pyrite formation (Canfield, 1991; Jørgensen, 1982; Lim et al., 2011;

Tribovillard et al., 2015). However, in methane-rich environment, AOM at the SMTZ

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could produce additional hydrogen sulfide that is ultimately fixed as sulfide minerals,

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such as sulfur, So; iron monosulfides, FeS; and pyrite, FeS2 in sediments. However, owing to the ephemeral of elemental sulfur and FeS minerals, so that it is pyrite usually survives as the main phase of the sulfide minerals (Borowski et al., 2013). Therefore, the content of sulfide minerals near the SMTZ could indicate the historical

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AOM and methane migration (Lim et al., 2011). Moreover, enrichments in sulfate phases (mainly barite) based on bulk sediment Ba profiles has also been used to assess the temporal variation of methane seeping (Dickens, 2001). Owing to a small

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fractionation of only about 1‰ in the transfer of sulfur from dissolved sulfide into

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sulfide minerals, the sulfur isotope of sulfide minerals sulfur isotope values represent good estimates of the isotopic composition of pore water HS- (Price and Shieh, 1979). Consequently, the sulfide minerals sulfur isotope values could record the AOM-induced sulfate reduction process. The high values of sulfide minerals sulfur isotope (up to +21‰) in sediment, is attributed to the isotopically-enriched HS- pool through AOM, which was fuelled by an upward vertical methane flux (Borowski et al., 2013). Enrichment of 34S in residual pore water sulfate and the HS- is the result of

ACCEPTED MANUSCRIPT higher rate of sulfate reduction than the rate of sulfate diffusion into the sediment pore water from overlying seawater in a closed system (Peketi et al., 2012). In normal marine environment, early diagenetic alteration of organic matter by sulfate reduction

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actually controlling the δ34S composition of pore waters. Thus, 34S-enriched sulfide minerals, similar to that of seawater sulfate (21‰) preserved in modern and ancient marine sediments, may identify former AOM-related processes and present-day and

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past locations of the SMTZ in cold seep affected sediments (Borowski et al., 2013).

2.2 Redox-sensitive elements as a proxy of aerobic oxidation of methane Under different redox conditions, modern and ancient marine sediments are typically characterized by relative enrichment or depletion of various trace metals, such as

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redox-sensitive elements Mn, Mo, U , V, Cd and Re, which thus can be used as indicators of paleo-redox conditions (Warning and Brumsack, 2000; Böning et al., 2004; Tribovillard et al., 2008; Mazumdar et al., 2012b). V/Sc, U/Th, Mn/Al, Mo/ Al

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and U/Al ratios are frequently used as indicators of oxygenation conditions of the

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water column (Algeo and Maynard, 2004; Tribovillard et al., 2006; Chun et al., 2010; Pälike et al., 2014). Therefore, it has been known that we can use these redox-sensitive elements as proxies for identifying past events of aerobic oxidation of methane (Chun et al., 2010; Pälike et al., 2014). Under anoxic conditions U is preferentially taken up into the sediment, forming less soluble UO2, U3O7, or U3O8 (Klinkhammer and Palmer, 1991; Morford et al., 2001) because U(VI) is reduced at the Fe(II)-Fe(III) redox boundary (Jones and

ACCEPTED MANUSCRIPT Manning, 1994; Zheng et al., 2002; Tribovillard et al., 2006). In oxic settings V exists as V(V), forming soluble vanadate ionic species such as HVO42- and H2VO4(Tribovillard et al., 2006). In anoxic environments, V(V) is reduced first to V(IV),

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forming vanadyl ion VO2+, related hydroxyl species VO(OH)3-, or insoluble hydroxides VO(OH)2, and under more strongly reducing environments, the presence of free H2S causes V reduced to V(III) form insoluble oxides V2O3 or hydroxides

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V(OH)3 (Algeo and Maynard, 2004; Tribovillard et al., 2006). Many researches

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indicated that the Mo in the water column could convert to thiomolybdates under the condition of high HS- activity, which then would be captured by iron sulfides and organic matter (Helz et al., 1996; Erickson and Helz, 2000; Tribovillard et al., 2004; Vorlicek et al., 2004). Generally, the concentration of Mo up to 20 µg/g in bulk

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sediments, accompanied with low Mn content indicates anoxic conditions and free hydrogen sulfide in the pore water. Moreover, Mo concentrations up to 60 µg/g suggest a euxinic condition (Scott and Lyons, 2012). Mn is primarily present as

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insoluble Mn(IV) oxides in oxic conditions (Calvert and Pedersen, 1993). Below the

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oxic-anoxic interface, Mn(IV) oxides are reduced to soluble Mn(II) species, and the upward diffusion of Mn2+ can lead to either Mn2+ escape to the bottom water or are reoxidized and precipitated of Mn oxides when pore water oxygen is encountered (Morford and Emerson, 1999; Xiong et al., 2012; Tribovillard et al., 2012). However, in carbonate-rich sedimentary environments, upward migrating Mn can lead to MnCO3 precipitation at the oxic-suboxic interface (Thomson et al., 1993; Morford et al., 2001).

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3. Materials and methods 3.1. Study area

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The Dongsha area on the northern continental slope of the South China Sea (SCS) lies in the southwestern Taiwan Basin. The seafloor morphology of study area is

characterized by deeply incised, and NW-SE trending canyons (Ding et al., 2004;

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Suess et al., 2005). Here, normal faults, thrust faults, mud volcanoes, and mud diapirs

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are well-developed (Suess et al., 2005), show evidence of methane-charged fluids and their migration. There are extensive bottom-simulating reflectors (BSRs) covered most of the study area (Fig. 1), indicating gas hydrate in the strata (Suess et al., 2005; Ge et al., 2010). In addition to authigenic carbonates, chemosynthetic communities

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were visually observed, and shallow SMTZS were observed in this area (Lim et al., 2011; Feng et al., 2015b). Gas hydrate was recovered from drilling sites near the study area at 600 to 1100 m depth during the scientific expedition conducted by the China

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Marine Geological Survey in 2013 (Zhang et al., 2014).

3.2. Sampling and analyses A 12.3-m-long sediment core, DH-5, was recovered using the Haiyang-6 (Oct. 7th, 2012) from a water depth of 2000 m from the Dongsha cold seep area (21.9465°N, 118.9782°E) (Fig. 1). All core sediments were cut into 2 cm long sections and freeze-dried, and some were powdered manually in an agate mortar for subsequent geochemical analysis. Three authigenic carbonate nodules with diameter of less than

ACCEPTED MANUSCRIPT 0.5 cm were recovered at the location of core 400 cm, 680 cm and 1130 cm, respectively. Total carbon (TC), total organic carbon (TOC), total nitrogen (TN), and total

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sulfur (TS) were measured using a Heraeus CHN-O Rapid elemental analyzer at Guangzhou Institute of Geochemistry (GIG), Chinese Academic of Sciences (CAS). After TC measurement, the sediment was decalcified with 10% HCl, washed twice

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with de-ionized water and dried at 60 °C for TOC determination. The total inorganic

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carbon (TIC) content of the sediment was calculated by substracting TOC from TC. Precision and accuracy of the TOC, TC, TS and TN were better than 3%. Samples for major and trace element analysis of sediments were placed in Teflon beakers, dissolved by a mixture of concentrated, ultra-clean HNO3, HCl, and HF (0.8,

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0.1 and 0.2 ml, respectively). The sealed beakers were then heated to 185 °C for about 36 h, then the solutions were evaporated on a hotplate overnight and then treated with 2 ml of HNO3 and 3 ml of ultrapure distilled water. The beakers were sealed again

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and heated to 135 °C for about 5 h to dissolve the residue. Finally, the samples were

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dissolved in 3% HNO3 spiked with an internal Rhodium standard (10 ppb) for trace elements analysis. Major and trace elements were determined using a Varian Vista Pro ICP-AES and a Perkin-Elmer Sciex ELAN 6000 ICP-MS following the method described in Qi et al. (2005). All samples were analyzed at the Institute of Geochemistry, CAS. Certified reference materials (GSR-1, OU-6, 1633-a, GXR-2, GXR-5) were used for quality control. The precision and accuracy were both better than 5% for Al, Ti, Mn, Cr, Ba, Cu, Ni, V, Th, Sc, U, and 10% for Mo.

ACCEPTED MANUSCRIPT Sulfide minerals in marine sediments (mainly sulfur, So; iron monosulfides, FeS; and pyrite, FeS2) (Berner, 1967; Morse and Cornwell, 1987; Rickard and Luther, 1997) were extracted for content and isotope analysis. Each sample was digested in 6 mol/l

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HCl at 50 °C for 12 h to dissolve acid volatile sulfur and the residue was analyzed for bulk sedimentary CRS (So, FeS and FeS2) at the GIG, CAS using a modified version

of the Cr-reduction method (Canfield et al., 1986). CRS were extracted using 6N HCl

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and 1M CrCl2 in the presence of a 100% N2 atmosphere, and H2S produced by

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reduction of sulfide was trapped as Ag2S in AgNO3 solution for gravimetric and isotopic measurements. Gravimetric sulfide capture and measurement reproducibility was better than 2% based on analysis of pyrite standards. The recovered Ag2S was analyzed at Indiana University where Ag2S was converted to SO2 using an Elemental

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Analyzer (EA) at 980 °C and subsequent a Thermo-Electron Delta V Plus Advantage mass spectrometer in a continuous-flow mode. The standard deviation associated with δ34S analysis was ±0.3 ‰, and are reported relative to the VCDT (Vienna Canyon

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Diablo Troilite) standard.

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Authigenic carbonate nodules for stable carbon isotopic analysis were conducted using a Thermo Finnigan Delta V Advantage at the Oxy-Anion Stable Isotope Consortium (OASIC) at Louisiana State University. The CO2 was liberated by reacting with 100% phosphoric acid at 970 °C. Values are reported in permil (‰) using standard δ notation relative to the Vienna Pee Dee Belemnite (V-PDB). Precision was better than 0.06‰ (2σ) for δ13C values. Wet sediment samples were washed through a 63 µm, 100 µm and 1 mm mesh

ACCEPTED MANUSCRIPT size sieve. The residues were dried at 40°C. Planktonic foraminifera were handpicked from the > 100 µm size fraction, counted and identified for statistics analysis and 14C dating. At least 12 mg of planktonic foraminifera (Globigerinoides ruber and

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Globigerinoides sacculifer) were handpicked under a binocular microscope in the critical layers 408-410 cm and 554-556 cm, to provide 0.2 to 1.2 mg of carbonate

material. Because of the influence of cold seep, we didn’t pick the foraminifera of

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other sulfide-zone for 14C dating. The AMS 14C dating was completed commercially

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at Beta Analytic Inc. (Miami USA). A detailed description of the operating procedure can be found in www.radiocarbon.com/analytic.htm. Beta Analytic used the INTCAL09 database in September 2011 to calibrate radiocarbon age to calendar

4. Results

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years.

4.1. Bulk CNS contents and carbon isotopic compositions

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Profiles of TOC content, TIC content, and TS content for 121 samples of

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sediment from core DH-5 are plotted in Fig. 2. TOC content ranges from 0.40 to 0.99 wt. % (Fig. 2; Table S1). TOC content decreased from 0.74 wt. % to 0.49 wt. % from 0 to 400 cm, then highest TOC (0.76-0.99 wt. %) occurs from 400 to 550 cm. Below 550 cm, TOC showed slight variation with a mean content of 0.52 wt. %. The vertical TIC distribution within the sediment provides almost a mirror image to that of the TOC, except for those samples from 400 cm to 550 cm. TIC content ranged from 0.4 to 1.2 wt. %. From 400 to 550 cm, with increasing sediment depth,

ACCEPTED MANUSCRIPT TIC content increased from 0.7 to 1.09 wt. % (Fig. 2). δ13C values of authigenic carbonates varied from -11.2‰ to -7.9‰. Three distinct TS enrichment zones were identified. Unit I (400-550 cm)

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sediments were characterized by high TS content, ranging from 0.37 wt. % to 0.78 wt. % and averaging 0.49 wt. %. In Unit II (740-820 cm), the TS content range from 0.31 wt. % to 0.52 wt. %, and average is 0.43 wt. %. In Unit III (1000-1150 cm), the

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TS content vary between 0.11 wt. % and1.89 wt. %, and average is 0.49 wt. %.

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Moreover, C/S ratios were typically less than 2 in these three TS enrichment zones. In the remaining sections of the core, sediments had very low TS contents (as low as <0.29 wt. %), ranging from 0.09 wt. % to 0.29 wt. % and averaging 0.15 wt. %, and relatively high C/S ratios (>4) except from 550 to 740 cm with C/S ratios similar to

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modern marine oxic environment sediments (Berner et al., 1982). C/N molar ratios varied between 6.0 and 10.0, with an average of 7.4 (Fig. 2), which indicated same land derived organic matter in the sediments (Meyers, 1994).

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Relativly higher C/N molar ratios were observed in sediments from 600 to 860 cm

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and vary between 6.8 and 9.3.

4.2. Major and trace metals Major element concentrations in the investigated core are 2.4–3.5% for Ca,

7.1–9.8% for Al, and 0.3–0.4% for Ti, respectively (Fig. 3). Mn/Al ratios below 600 cm are greater than that of upper crust value (McLennan, 2001), and showed slight variation with a mean value of 75 ×10−4. An average Mn/Al ratio of 52×10−4 was

ACCEPTED MANUSCRIPT observed in sediments within the top 600 cm. The lowest Mn content occurs in Unit I. Mo content shows enrichment in Unit I and Unit III, and the highest values are 2.7 µg/g and 5.4 µg/g, respectively (Fig.3; Table S2). The remaining part is characterized

4.3. Chromium reducible sulfur and δ34SCRS

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than that of Mo; high values also occur outside of Unit I.

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by low Mo values with a narrow range of 0.4 to 2.1µg/g. The variation of U is higher

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The depth profiles for CRS concentrations and its sulfur isotopic composition in core DH-5 are shown in Figure 4 and Table 2. CRS content varied between 0.01 wt. % and 2.12 wt. % with an average of 0.30 wt. %. High CRS content was observed in the TS enrichment unit. The δ34SCRS showed wide variation ranging from −32.6‰ to

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34.2‰. The lowest δ34SCRS value occurs at a depth of 380 cm, whereas the highest δ34SCRS is observed at the depth of 810 cm. The δ34SCRS values are typically high in Units II and III, and range from -5‰ to 34.2‰ and -5.8‰ to 24.9‰, respectively.

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However, the δ34SCRS in Unit I show low values and range from −24.1‰ to −11.7‰.

4.4. Radiocarbon dating The foraminifers collected from 408-410 cm and 554-556 cm of the core yielded

radio carbon ages of 15,390±200 cal. yr BP 24,770±300 cal. yr BP, respectively (Table 1).

ACCEPTED MANUSCRIPT 5. Discussion 5.1. Anaerobic oxidation of methane (AOM) signals in the sediments The supply of organic carbon is the primary control on sulfate reduction and

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reduced sulfur contents in marine sediments (Canfield, 1991; Jørgensen, 1982; Lim et al., 2011). In oxic and suboxic marine sediments, the reduced sulfur and organic

carbon contents typically show a positive correlation with an average S/C ratio of 0.36

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(Berner, 1982). The S/C ratios are high in euxinic condition, generally greater than

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2.05, just like the Black Sea (Leventhal, 1983), when compared to the freshwater condition where sulfate is limiting (Lim et al., 2011). Moreover, in organically-limited and methane-rich environment, where AOM is the predominant process, AOM induced the pyrite formation may significantly increase the S/C ratio of sediments

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(Lim et al., 2011; Sato et al., 2012).

High CRS contents were observed in the TS enrichment Units. This observation indicates sulfide mineral mostly of So, FeS and FeS2, only a very small portion of

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organic sulfur. This may be due to the low organic carbon content in the study core

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(<1%). TS and TOC show a positive correlation from 0 to 400 cm and 850 to 990 cm in our study sediments, and have an average S/C ratio of 0.14 (Fig. 5) which is lower than 0.36. Since the organic carbon content is low (<1%), and the C/N ratio indicate same land derived organic matter in the sediments (Meyers, 1994), the low reactivity of land-derived organic matter may lower S/C ratio (Berner, 1984). The low S/C ratio and low CRS content from 0 to 400 cm and 850 to 990 cm may be attributed to the sediments formed in a normal oxic marine environment or not affected by the cold

ACCEPTED MANUSCRIPT seep activity. The TS enriched sediments in Unit I, II and III show the lack of a positive correlation between TS and TOC and very high S/C ratios, ranging from 0.5 to 2.1, are similar to those sediments found in the euxinic Black Sea and Cariaco

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basin (Raiswell and Berner, 1986; Lim et al., 2011). The high CRS contents in sediments significant increased S/C ratios in Unit I, II and III (Fig. 5). However, the

very low TOC content in our study core (<1%) exclude the mechanism of additional

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CRS formation in a euxinic environment. Another possible reason for increased S/C

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ratio in DH-5 may be related to the AOM. The sediments from Argentine Basin and Nankai Trough show S/C ratios of 0.5 to 2.5 where significant AOM take place (Hensen et al., 2003; Sato et al., 2012). In AOM process, the generated hydrogen sulfide fixed as reduced sulfur, such as pyrite in sediments, resulting in significant

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increase of S/C ratio in the very low TOC contents environment. Consequently, AOM may take place in sediments from Unit I, II and III. The highly depleted sulfur isotope ratios (-18 to -32.6‰) of CRS observed from 0

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to 400 cm (Fig. 4), like most values for continental margin sediments, are the result of

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early sulfur cycling close to the sediment water interface where the diffusion rate of sulfate from bottom water into the underlying pore water exceeds the rate of sulfate consumption (Peketi et al., 2015). Large S isotopic fractionation of 39–53.6 ‰ from 0 to 400 cm relative to the average sea water sulfate δ34S of ~21 ‰ (Rees et al., 1978), is likely related to the microbial sulfur disproportionation of sulfur intermediates–So, SO32-, S2O32-, that are produced during the oxidative part of sulfur cycling (Canfield and Thamdrup, 1994; Peketi et al., 2012). Sulfur disproportionation is a particular

ACCEPTED MANUSCRIPT type of redox reaction in which sulfur is simultaneously reduced and oxidized to form two different products (Peketi et al., 2015). However, up to 70‰ 34S depletion without disproportionation of sulfur intermediates may also be caused by

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dissimilatory sulfate reduction in marine sediments (Wortmann et al., 2001; Brunner and Bernasconi, 2005; Sim et al., 2011; Peketi et al., 2012, 2015). The exact

mechanism producing large sulfur fractionations and very negative δ34S values in

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marine sediments is not critical, here. It is only but the deviation from this baseline of

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large depletions in 34S that is significant. Therefore, the high values of δ34SCRS, generally greater than average seawater sulfate δ34S of ~21‰ in Unit II and Unit III are attributed to the presence of isotopically enriched HS- pool at anaerobic methane oxidation coupled with sulfate reduction process (Borowski et al., 1996, 2013;

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Jørgensen and Parkes, 2010; Peketi et al., 2012, 2015). Similar pyrite δ34S enrichments have been observed in cold seep areas of Blake Ridge, Black Sea, and Bay of Bengal (Jørgensenet al., 2004; Peketi et al., 2012, 2015; Borowskia et al.,

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2013). Therefore, the high values of δ34SCRS combined with the high CRS contents in

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Unit II and Unit III, as well as the presence of authigenic carbonates suggest the occurrence of AOM and past locations of the SMTZ at about 740-820 cm and 1000-1150 cm, respectively. However, depending on dissolved Fe availability, the zone of sulfide mineralized associated with anaerobic oxidation of methane may be considerably below the actual SMTZ (Jørgensen et al., 2004).

5.2 Aerobic oxidation of methane in an AOM-dominated environment

ACCEPTED MANUSCRIPT In the study core, the high values of TOC, V/Sc, U/Th, Mo/Al and U/Al ratios and low values of Mn/Al ratios in Unit I (Fig. 3), suggest a suboxic environment. The median particle size values of the sediments in Unit I range from 6.5 to 8.2 µm (Li et

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al., unpublished data), which exclude the influence of turbidity. However, no significant enrichment of Mo was found in Unit I, with most values less than 2 µg/g, which is similar to the value of average continental crust (1 µg/g) (Taylor and

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McLennan, 1985). This suggests the H2S appeared in pore water (Fig. 3). The suboxic

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condition in the bottom water may be due to the high productivity or changes in deepwater ventilation. However, the paleoproductivity proxies of Ni/Al and Cu/Al ratios in core DH-5 have no significant change (Table S2), indicating the paleoproductivity was not the main reason for the development of suboxic bottom

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water conditions in Unit I. The deep waters of the study area originated from deep water current from the Pacific into the SCS through the Luzon Strait (Liu et al., 2014). During the Last Glacial Maximum, rates of deepwater formation were elevated for

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deep water current from the northern SCS (Wan et al., 2014). In addition, according to

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the redox-sensitive elements and TOC content, the sediments in the nearby station HD-196A are formed in oxic conditions (Deng et al., 2006) (Fig. 6). Therefore, changes in paleoproductivity and deepwater ventilation in the study station are unlikely the reason for the development of suboxic bottom water environments during Unit I formation. Aerobic oxidation of methane can influence bottom water redox conditions in the ocean (Consolaro et al., 2015). Evidence of abnormal concentrations of Mn and U

ACCEPTED MANUSCRIPT during PETM suggested that an elevated upward methane flux increased oxygen consumption and thus contributed to development of suboxic bottom water conditions (Chun et al., 2010). The intensity of upward methane flux can influence the shape of

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sulfate profiles and depth of SMTZ in a diffusion of methane (Consolaro et al., 2015). The high content of TOC, TS, and the enrichment of redox-sensitive elements in Unit I is ascribed to the aerobic oxidation of methane induced suboxic conditions. In

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addition, the absence of benthic foraminifera in Unit I is also suggested aerobic

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oxidation of methane induced suboxic conditions (Li et al, unpublished data). The abundance of benthic foraminifera in modern intensive cold seep activity environment is typically low (Panieri et al., 2014). The absence of authigenic carbonate in study core may be due to the low efficient AOM, which induce the low alkalinity, or acidic

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environment caused by aerobic oxidation of methane, just like the cold seeps in convergent margin off Costa Rica and calcium carbonate dissolved in PETM (Zachos et al., 2005; Karaca et al., 2010; Consolaro et al., 2015). In this case, the upward

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methane flux is high and the SMTZ is very shallow and close to the seafloor. The

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occurrence of AOM and SMTZ located at only a few centimeters below the seafloor at the central part of Regab, which characterized by high methane fluxes (Ristova et al., 2012; Consolaro et al., 2015). The development of suboxic bottom water conditions and enrichment of TOC may be owing to the lower rates of AOM and higher methane flux into the water column (Fig. 7A) (Consolaro et al., 2015). The negative δ34SCRS values (from -11.5 to -24.1‰) in Unit I may be related to the shallow SMTZ. In this case, the sulfate diffusion into the sediment pore water from overlying

ACCEPTED MANUSCRIPT seawater is easier than in a closed system. However, low sedimentation rates, and the resulting low detrital iron fluxes may also form such low pyrite δ34S values (Formolo and Lyons, 2013).

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The high TS contents and high values of δ34SCRS (from -5.8 to 34.2‰) in Unit II and Unit III sediments (Fig. 7B) can be attributed to the low methane flux and deep SMTZ. In this case, the oxidation of methane was efficient, and H2S generated by

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AOM was released into the pore water and caused the enrichment of Mo (Fig. 3).

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AMS 14C dates indicating that Unit I in the study core formed at ~15.4 to 24.8 kyr BP, which cover the Last Glacial Maximum of the late Pleistocene Wisconsinan glacial stage. It was proposed that cold seeps were particularly active at low sea-level stands due to reduction in hydrostatic in the SCS (Tong et al., 2013; Han et al., 2014). Thus,

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we hypothesize that the period of Unit I sediment formation may be related to the reduced hydrastastic pressures during sea-level lowstands. Moreover, this study has examined only limited samples from a specific site. Additional work would hence be

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needed to assess the cause of such dynamics.

6. Conclusion

Elemental and stable sulfur isotope data have been reported for sediments from

cold seep environments in the South China Sea. Total sulfur (TS), total organic carbon (TOC), total inorganic carbon (TIC) content and δ34SCRS values exhibit large variations in the studied sediments, which may reflect temporal variation of methane flux. Three methane release events has been suggested; relatively high methane flux at

ACCEPTED MANUSCRIPT depth of 400-550 cm (Unit I) and low methane flux at depth of 740-820 cm (Unit II) at 1000-1150 cm (Unit III), respectively. High methane flux occurred at ~15.4 to 24.8 kyr BP in Unit I have led to aerobic methane oxidation. Presumably, this would result

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in the high content of TOC and TS, and the enrichment of redox-sensitive elements. Relatively low methane fluxes in the lower sediment layers (Unit II and II) were

probably cause the high TS contents and high δ34SCRS values. Overall, our results

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suggest that sediment geochemistry in cold seep settings can be modified markedly by

and stable isotope analyses.

Acknowledgments

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methane release events and such impact can be well constrained by paired elemental

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We are grateful to the crew of the RV Haiyang-6 for their assistance in collecting the core. We would also like to thank L. Qi (Institute of Geochemistry, CAS) for major and trace metals analysis, J.Z. He and Y. Hu (GIG, CAS) for carbon and sulfur

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analysis, Y.B. Peng (Louisiana State University) for sulfur isotope measurement.

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Journal editor H. Tian and reviewers W.S. Borowski, G. Bayon, and N. Tribovillard provided valuable input. The research was partially supported by the NSF of China (Grants: 91228206, 41422602, and 41373085) and the “Hundred Talents Program” of CAS.

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Thomson, J., Higgs, N.C., Croudace, I.W., Colley, S., Hydes, D.J., 1993. Redox zonation of elements at an oxic/post-oxic boundary in deep-sea sediments. Geochim. Cosmochim. Acta 57, 579–595.

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Figure captions

Fig. 1. Map showing the locations of the sediment core DH-5 and HD-196A (yellow

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circle) in this study. Shadows are BSR-occurring areas mapped by Ge et al. (2010).

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Fig. 2. Depth profiles with in core DH-5 for the concentration of total organic carbon (TOC), total inorganic carbon (TIC), and total sulfur (TS) expressed in weight percent (wt. %), as well as carbon isotope of authigenic carbonates. Also shown are carbon/nitrogen (C/N) molar ratios and carbon/sulfur (C/S) ratios. Shaded horizontal

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bars indicate three TS enrichment zones, Units I, II and III.

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Fig. 3. Depth profiles within core DH-5 for the concentration of Ca, Al, Ti, Mo, U, Mn and V/Sc, U/Th, Mn/Al, U/Al, Mo/Al, TOC/Al and Sr/Ca ratio. The dashed

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vertical lines show the Mn/Al ratios for upper crust, respectively (McLennan, 2001). Red lines represent the content of Mn, U and Mo, and black lines represent the Mn/Al, U/Al and Mo/Al ratios, respectively. Shaded horizontal bars indicate three TS enrichment zones, Units I, II and III.

Fig. 4. Downcore profiles for the concentration of chromium reducible sulfur (CRS) and its sulfur isotopic composition (δ34S) expressed in permil (‰) units using the

ACCEPTED MANUSCRIPT standard, Vienna Canyon Diablo Troilite (VCDT). Shaded horizontal bars indicate three TS enrichment zones coincident with those in Figures 2 and 3.

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Fig. 5. Scatter plot of total organic carbon (TOC) vs. total sulfur (TS). Line showing the TOC/TS ratio of oxic–suboxic marine sediments is from Berner (1982).

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and Mn/Al (from Deng et al., 2006).

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Fig. 6. Depth profiles within core HD-196A for the concentration of TOC, TIC, Al, Si,

Fig. 7. Conceptual illustration showing the temporal variation of methane flux and its record in seep sediments of the study area (after Borowski et al., 1996 and Consolaro

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et al., 2015). Depth profiles with in core DH-5 for the concentration of total organic carbon (TOC), total inorganic carbon (TIC), and total sulfur (TS) expressed in weight percent (wt. %). Also shown are carbon/nitrogen (C/N) molar ratios and carbon/sulfur

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(C/S) ratios. Shaded horizontal bars indicate three TS enrichment zones, Units I, II

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and III. (A) Under relatively high methane flux (Unit I, core DH-5), the SMTZ is close to the seafloor. The sediment in this unit is enriched in total sulfur and redox sensitive elements (e.g. U), and have negative δ34SCRS values. Meantime, a certain amount of methane may release into the water column that could result in the suboxic bottom water conditions and enrichment of TOC. (B) Under relatively low methane flux (Unit II and Unit III in core DH-5), the SMTZ moved down in the sediment. The sediments show enrichment of total sulfur and positive δ34SCRS values. The H2S

ACCEPTED MANUSCRIPT generated during AOM could cause the enrichment of Mo. (Arrow sizes are proportional to downward sulfate and upward methane fluxes. SWI: sediment-water

Table captions Table 1. AMS 14C age-dates of planktonic foraminifera.

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interface; SMTZ: sulfate methane transition zone.)

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Table 2. Content of chromium reducible sulfur (CRS) and δ34SCRS values in core

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DH-5.

Table S1. TOC, TIC, TN and TS content of the sediment samples of core DH-5.

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Table S2. Major and trace elements of sediment samples in core DH-5.

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Sample DH5-1 DH5-2

Sample depth (cm bsf) 408-410 554-556

Planktonic foraminifera G.ruber+G.sacculifer G.ruber+G.sacculifer

AMS14Cage (yr B.P.)

Calibratedage* (yr B.P.±2σ)

12880±50 20570±80

15390±200 24770±300

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*Calibrated calendar age using the corrected 14C age, the CALIB 6.1.0 program (Stuiver and Reimer, 1993), and the Marine 13 calibration data (Reimer et al., 2013). 0 Cal. ka B.P.=1950 AD.

ACCEPTED MANUSCRIPT Table 2 Weight %

Depth

(cm bsf)

(‰VCDT)

CRS

(cm) 710 730 750 770 790 810 830 850 900 950 990 1000 1030 1050 1070 1090 1110 1130 1170 1210

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(‰VCDT) 1.6 3.4 2.7 3.9 -5 34.2 2.8 -2.1 -0.6 -7.6 -8.9 24.9 19.5 14.5 21.1 -1.4 17.6 -5.8 -7 -8.3

Weight % CRS

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0.01 0.03 0.06 0.02 0.07 0.08 0.2 0.37 0.34 0.35 0.47 0.45 0.52 0.44 0.24 0.18 0.2 0.23 0.15 0.22 0.17

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-18 -24.2 -32.5 -30.2 -32.2 -32.6 -14.5 -22.8 -21.6 -22 -24.1 -15.7 -11.7 -19.2 -10.6 -8.3 -7.6 -7 -3.3 -3.7 -2.7

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90 160 210 280 340 380 410 430 450 470 490 510 530 550 570 590 610 630 650 670 690

δ34SCRS

0.11 0.18 0.35 0.51 0.55 0.4 0.1 0.11 0.1 0.05 0.04 1.3 0.34 0.21 0.26 0.05 0.25 0.61 0.03 0.03

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δ34SCRS

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Depth

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South China

Taiwan 23ºN

22ºN

HD - 196A DH - 5 NE Dongsha

Water Depth ( m )

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Gas hydrate drilling

Shenhu

-800

BSR area

-1400

-2600

113ºE

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112ºE

114ºE

115ºE

0

116ºE

117ºE

19ºN

Sample site

South China Sea

-2000

111ºE

20ºN

SW Dongsha

-200

21ºN

118ºE

100

119ºE

200km

120ºE

18ºN

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δ13C (‰ VPDB ) 0

13

δ C TIC

400

І

І

600

П

П

800

1000

Ш 1200 0.4

0.6

0.8

1.0

Ш

TIC ( wt . %)

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TS ( wt . %)

0

200

400

І 600

П

П

Ш

Ш

800

1000

0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.4 0.8 1.2 1.6 2.0

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TOC ( wt . %)

І

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Depth ( cm )

200

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-12 -10 -8 -6 -4 -2 0

1200 0 1 2 3 4 5 6 7

C / N molar ratio

5

6

7

8

9

C / S ratio

10 11

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0

400

600

800

0.05

0.07

0 1

2.1 2.3 2.5 2.7 2.9

2

3

4

5 Mo

U U / Al

Mn / Al

Mo / Al 200

400

І

І 600

П

П

П

П

Ш

Ш

Ш

Ш

800

1000

1000

EP

1200

6.6

7.4

8.2

0.16

1200 0.20

0.24 0.004 0.006

U/Th

V / Sc

0.008

0.22

0.26

0.30 0.34 0.0

0.4

0.6 -4

-4

Mn/Al

0.2

U / Al ( 10 )

Mo/ Al ( 10 )

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0

0

Depth ( cm )

200

200

400

400

І

І

І

І

600

600

800

1000

П

П

П

П

Ш

Ш

Ш

Ш

800

1000

1200

1200 0.00

6 0

Mn

І

І

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Depth ( cm )

200

M o ( μg/g)

U (μg/g)

Mn (%)

0.03

0.04

0.08

TOC/Al

0.12 2.2

2.6

3.0

Ca (%)

3.4

6.5

7.5

8.5

Al (%)

9.5

0.30

0.40

Ti (%)

0.50 40

50

60

70 -4

Sr / Ca ( 10 )

80

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0

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0

200

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200

400

400

Depth ( cm )

І

600

П

800

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1000

0.0

0.5

1.0

1.5

EP

CRS ( wt . %)

2.0

800

1000

Ш

1200

AC C

600

1200 -40

-20

0

20

34 δ S CRS (‰ VCDT )

40

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2.0

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0 - 400cm and850 - 990cm

1.8

І: 400 - 550cm П : 740 - 829cm Ш : 1000 - 1150cm

1.6

1.2 1.0 0.8 0.6 0.4

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TS ( wt . %)

1.4

S / C = 0 . 36

0.2 0.0 0.4

0.5

0.6

0.7

0.8

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TOC ( wt . %)

0.9

1.0

1.1

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0

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0

400

600

0.5

1.0

1 . 5 2 . 0 0.2

0.6

0 . 8 1 . 0 7.0

TIC ( wt . %)

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TOC ( wt . %)

0.4

8.0

9.0

Al ( %)

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800 0.0

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Depth ( cm )

200

1 0 . 024

26

28

Si ( %)

200

400

600

800

30 0 . 003

0 . 006

Mn / Al

0 . 009

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B : Zone Ⅱ and Ⅲ

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A : Zone Ⅰ

→

CO 2+2H 2O

CH 4

SO 4

SWI

2-

U

SMTZ

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CH 4+2O 2

SWI

U

Mo

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SMTZ

High Methane Flux

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Iron sulfides with negative sulfur isotope values Iron sulfides with positive sulfur isotope values

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SO 42-

Mo Mo

Low Methane Flux Mo

U

Molybdenum , Uranium Methane bubble

Mo Mo

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Seep sediment TS, TOC, TIC and δ34S can be used to identify changes in methane flux. Anaerobic oxidation of methane caused Mo enrichment and positive δ34SCRS values.

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Aerobic oxidation of methane caused U enrichment and Mn depletion.

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Three methane release events were identified in gravity core from seep area.