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Marine and Petroleum Geology 28 (2011) 1829e1837

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Marine and Petroleum Geology

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Variations of methane induced pyrite formation in the accretionary wedgesediments offshore southwestern Taiwan

Yee Cheng Lim a, Saulwood Lin a,*, Tsanyao Frank Yang b, Yue-Gau Chen b, Char-Shine Liu a

a Institute of Oceanography, National Taiwan University, TaiwanbDepartment of Geosciences, National Taiwan University, Taiwan

a r t i c l e i n f o

Article history:Received 13 March 2010Received in revised form3 April 2011Accepted 24 April 2011Available online 30 April 2011

Keywords:Gas hydrateMethaneSulfate reductionPyriteAnaerobic methane oxidation

* Corresponding author. Tel./fax: þ886 2 2363 642E-mail address: swlin@ntu.edu.tw (S. Lin).

0264-8172/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.marpetgeo.2011.04.004

a b s t r a c t

The accretionary wedge of offshore southwestern Taiwan contains abundant deposits of gas hydratebeneath the sea floor. High concentrations of methane in pore waters are observed at several locationswith little data concerning historical methane venting available. To understand temporal variation ofmethane venting in sediments over geologic time, a 23-m-long Calypso piston core (MD05-2911) wascollected on the flank of the Yung-An Ridge. Pore water sulfate, dissolved sulfide, dissolved iron,methane, sedimentary pyrite, acid volatile sulfide, reactive iron, organic carbon and nitrogen as well ascarbonate d13C were analyzed.

Three zones with markedly different pyrite concentration were found at the study site. Unit I sedi-ments (>20 mbsf) were characterized with a high amount of pyrite (251e380 mmol/g) and a d13C-depleted carbonate, Unit II sediments (15e20 mbsf) with a low pyrite (15e43 mmol/g) and a high contentof iron oxide mineral and Unit III sediments (<10 mbsf) by a present-day sulfateemethane interface(SMI) at 5 mwith a high amount of pyrite (84e221 mmol/g) and a high concentration of dissolved sulfide.

The oscillation records of pyrite concentrations are controlled by temporal variations of methane flux.With an abundant supply of methane to Unit I and III, anaerobic methane oxidation and associatedsulfate reduction favor diagenetic conditions conducive for significant pyrite formation. No AOM signalwas found in Unit II, characterized by typical organically-limited normal marine sediments with littlepyrite formation. The AOM induced pyrite formation near the SMI generates a marked pyrite signature,rendering such formation of pyrite as a useful proxy in identifying methane flux oscillation in a methaneflux fluctuate environment.

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1. Introduction

Gas hydrate destabilization and methane gas expulsion may bea key factor of the climatic change (Kvenvolden, 1988, 1993;MacDonald, 1990). A great amount of methane could release intothe water column and atmosphere as a result of glacial-interglacialsea level variations and bottom water temperature fluctuationsdriving massive destabilization of the sea floor gas hydrate(Kennett et al., 2003; Paull et al., 1991,1996). Geological recordssuch as negative excursions of carbon isotope in benthic forami-nifera, collapse features of seafloor, and chemoherm carbonatebuildups registered past gas hydrate dissociation and methaneventing during the glacial-interglacial oscillations (Dickens et al.,1995; Dillon et al., 2001; Hesselbo et al., 2000; Kennett et al.,2000; Teichert et al., 2005). As a greenhouse gas, methane is

4.

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more effective than carbon dioxide. The huge amount of methanerelease due to gas hydrate dissociation could play an important rolein global warming over geologic time (Kvenvolden, 1988, 1993;MacDonald, 1990).

Anaerobic methane oxidation (AOM) is the predominantprocess in methane-rich marine environment. Upward methaneflux could control pore water sulfate depletion and the subsequentdepth of sulfateemethane interface (SMI) (Borowski et al.,1996,1999). Under correct conditions, active methane fluxes canbe quantified by sulfate gradients driven by AOM (Borowski et al.,1996, 1999). A close relationship between methane flux andsulfate reduction has been observed in many regions, in particular,at gas hydrate-bearing cold seeps (Boetius and Suess, 2004;Borowski et al., 1999; Suess et al., 1999; Tsunogai et al., 2002),mud volcanoes (Niemann et al., 2006), coastal upwelling areas(Niewöhner et al., 1998; Treude et al., 2005), and shallow riverdeltas (Blair and Aller, 1995). However, methane venting may dis-continue because of closure and opening of gas migration conduits(Aharon et al., 1997; Teichert et al., 2003), and climate induced

Y.C. Lim et al. / Marine and Petroleum Geology 28 (2011) 1829e18371830

dissociation and formation of deep-seated gas hydrate (Judd et al.,2002; Kvenvolden, 1993). Historical temporal variations inmethane flux are not well documented because difficulties mayarise with proper proxies in quantifying and age determining theoccurrence of methane migration. Geochemical indicators such asauthigenic barite formation and calcium carbonate precipitationhave been proposed to assess the temporal variation of methaneventing (Dickens, 2001; Teichert et al., 2003; Torres et al., 1996). Inthis study, we propose diagenetic pyrite as an additionalgeochemical proxy for the occurrence of methane venting and gashydrate dissociation in marine sediments.

In normal marine environment, sulfate reduction is a majorpathway for organic matter degradation (Berner, 1982; Canfieldet al., 1993; Jørgensen, 1982). Organic carbon is the energy sourceand consumed by sulfate reducing bacteria in the followingreaction:

2ðCH2OÞ þ SO2�4 /2HCO�

3 þ H2S (1)

Sulfate reduction produces hydrogen sulfide, which subsequentlyreacts with reactive iron to produce pyrite (FeS2) (Berner,1970,1984):

2FeOOHþ 3H2S/2FeSþ S0 þ 4H2O (2)

FeSþ S0/FeS2 (3)

Consequently, organic carbon supply is typically the primarycontrol on sulfate reduction and pyrite formation in the normalmarine environments (Canfield, 1991; Jørgensen, 1982; Lin et al.,2002b; Lin and Morse, 1991). Additionally, anaerobic methaneoxidation utilizes a major fraction of sulfate in anoxic sediments(Hinrichs and Boetius, 2002; Nauhaus et al., 2002; Treude et al.,2003). Methane is oxidized by a consortium of anaerobic meth-anotrophic archaea and sulfate reducing bacteria with concurrentsulfate reduction according to the following net reaction (Boetiuset al., 2000; Hinrichs et al., 1999):

CH4 þ SO2�4 /HCO�

3 þ HS� þH2O (4)

The addition of methane from the upward diffusion and/oradvection from gas hydrate dissociation may provide extra carbonto the sulfate reducing bacteria with maximum rates exceedingthose in the anoxic marine environment (Aharon and Fu, 2000;Jorgensen et al., 2001; Knab et al., 2009; Orcutt et al., 2004).Anaerobic methane oxidation at the SMI could produce additionalhydrogen sulfide and thus enhance pyrite formation in sediments.Consequently, the accumulation of pyrite near the SMI could bea useful indicator in deciphering the historical anaerobic methaneoxidation and methane migration in marine environment.

The purpose of this study is to report temporal geochemicalvariations of methane migration as recorded in sediments, inparticular, the pyrite signature. Pore water methane, sulfate, dis-solved sulfide, dissolved iron, sedimentary reduced sulfideminerals(acid volatile sulfide, pyrite), reactive iron, d13C of carbonate insediments were analyzed to examine temporal variations betweenanaerobic methane oxidation and pyrite formation. Specialemphasis is placed on utilizing variations in the amounts of diage-netic pyrite as a proxy in assessing historical methane migration.

2. Materials and methods

2.1. Study area

The active accretionary wedge offshore southwestern Taiwanlies between the Luzon Arc and the China passive continental

margin. The area is near the border of the Luzon subduction systemtransforming into an incipient arc-continent collision with thePhilippine Sea Plate moving at a rate of w8 cm/yr toward theEurasia Plate (Huang et al., 2000). A series of semi-parallel north-south trend anticlinal ridges is separated by normal and thrustfaults (Liu et al., 1997). Fluid seepages and gassy sediments as wellas mud diapirs were identified (Chiu et al., 2006). Normal faults,thrust faults, mud volcanoes, and mud diapirs could provide goodconduits for methane-charged fluids migrating from deeper sedi-ments (Liu et al., 2006). Liu et al. (2006) showed that bottom-simulating reflectors (BSRs) covered most of the study area,indicating abundant gas hydrate at depth in the region. Chuanget al. (2006) showed that the area is characterized by highconcentrations of methane in sediments. In addition to highconcentrations of methane in the pore water, steep sulfate gradi-ents, shallow sulfateemethane interfaces, high content of ironsulfides (i.e., pyrite) and authigenic carbonates were observed inthis area (Lin et al., 2006). It also receives large quantities ofterrigenous sediment from the adjacent Kaoping, Erjen andTsengwen Rivers, delivering a combined total river particle load of120 Mtons/yr (Dadson et al., 2003).

A long core, approximately 23.5 m in length, was taken at siteMD05-2911 (Fig. 1) using the Calypso giant piston corer on boardthe R/V Marion Dufresne. The core site lies on the southwesternflank of the Yung-An Ridge at a water depth of 1076 m. Yung-AnRidge is a thrust ridge (Lin et al., 2008) located near the deforma-tion front on the continental slope offshore southwestern Taiwan. Aprominent BSR beneath as observed in seismic profiles indicatespresence of gas hydrate and free gas under the ridge sediments (Liuet al., 2006). Gas plumes, authigenic carbonate buildups, andchemosynthetic communities were visually observed using a deep-towed camera system (TowCam, WHOI) on top of the ridgeshowing clear evidence of strong methane migration in sedimentsat present (Lin et al., 2009).

2.2. Sampling and analyses

After core recovery on deck, sediments were sectioned into2-cm intervals and stored in polyethylene (PE) tubes. Part of thesamples was centrifuged and filtered on board to obtain porewater.A fraction of pore waters was used for dissolved sulfide analysis onboard. Part of the pore water samples was stored in pre-cleaned PEvials for sulfate, dissolved iron (nitric acid addition) and other porewater determinations in the shore-based laboratory. Sedimentsamples in PE tubes were stored at �20 �C for further laboratoryanalysis.

Part of the wet sediment samples was used for acid volatilesulfide and pyrite sulfide analyses. A fraction of sediments wasfreeze-dried using a Labconco freeze-dryer (model Lyph-Lock 6) fora week. After freeze-drying, sediments were ground to fine powderusing an agate mortar and a Fritsch planetary micro mill (modelPulverisette 7) and stored in small PE vials for later analyses ofoxalate extractable and total iron, total organic carbon andnitrogen, and stable carbon isotope (d13C) of carbonate.

Pore water sulfate concentrations were measured by a Dionexion chromatograph (model 4500i) equipped with a suppressedconductivity detector (Dionex, model CDM-2) and an IonPac AS4Aanion exchange column (Dionex). A mixed solution of 1.7 mMNaHCO3 (Merck) and 1.8 mM Na2CO3 (Merck) was used as theeluent and 0.025 N H2SO4 (Merck) as the regeneration fluid. Porewater samples were usually diluted 80-fold with de-ionized water(Milli-Q, 18 MU cm) prior to analysis.

Dissolved sulfide concentrations were measured by the absor-bance of methylene blue complex at 670 nm (Cline, 1969) usinga Hitachi spectrophotometer (model U-2001).

Figure 1. Study area and location of giant piston core MD05-2911 (black star) on the Yung-An Ridge offshore southwestern Taiwan.

Y.C. Lim et al. / Marine and Petroleum Geology 28 (2011) 1829e1837 1831

Dissolved iron concentrations were measured by a graphitefurnace atomic absorption spectrometer (model Z-5700). Porewater samples were diluted, usually 10 folds, with 0.1 N HNO3(Merck) prior to analysis. A calibration curve was prepared froma Standard Metal Solution (Merck).

Methane concentrations in core sediment were collected onboard by the headspace method and measured by an SRI gaschromatograph (model 8610C) (Chuang et al., 2006).

Total organic carbon and nitrogen contents were analyzed usinga CHN elemental analyzer (Thermo Electron, model Flash EA 1112)equippedwith a high temperature resistance furnace and a thermalconductivity detector (TCD). Combustion and reduction werecarried out in a single reactor filled with chromium trioxide(Elemental Microanalysis), silvered cobaltous cobaltic oxide(Elemental Microanalysis) and metallic copper (Elemental Micro-analysis). Total organic carbon and nitrogen contents were deter-mined using approximately 25 mg of pre-acidified samples.Sediment acidification was carried out by adding 50 ml of 2 N HCl(Merck) for 4 h and thoroughly rinsed with de-ionized water (Milli-Q, 18 MU cm). An acetanilide Standard (Thermo Electron) was usedto calibrate the carbon and nitrogen content of samples.

Acid volatile sulfide (AVS) concentrations in sediments weredetermined by the cold 6 NHCl method (Cornwell andMorse,1987;Lin et al., 2002b). AVS was extracted as H2S evolved by reacting wetsamples (w5 g) for 1 h in 6 N HCl (Merck) in a stirred reaction flaskpurged with high purity N2 gas. The evolved H2S gas was trapped in25 ml of sulfide anti-oxidant buffer and analyzed by potentiometrictitration with 30 mM Pb(ClO4)2 (Aldrich) using a sulfur titratorequipped with Metrohm Titroprocessor (model 686) and MetrohmDosimat (model 665). Chromium-reducible sulfur (CRS) wasdetermined by chromium(II) reduction technique (Canfield et al.,1986; Lin et al., 2002b) with sediment in boiling chromium(II)chloride and 6 N HCl (Merck) for 1 h. The evolved H2S was analyzedby the potentiometric titration same as the AVS determination.Sedimentary pyrite, elemental sulfur and a fraction of the totalgreigite were extracted using the Cr(II) digestion procedure. In thisstudy, CRS was defined as pyrite sulfide assuming elemental sulfur

and greigite were small in proportion of the total sulfur followingCornwell and Morse (1987).

Oxalate extractable iron and total iron concentrations weremeasured by flame atomic absorption spectrometry. For oxalateextractable iron, approximately 2.5 g of well-mixed ground sedi-ments were extracted with 20 ml of a buffer solution of 0.2 Mammonium oxalate (Merck) and 0.1 M oxalic acid (Merck) for 4 h inthe dark (Canfield, 1989; Chou and Zhou, 1983; Lin et al., 2002b).For total iron, sediment was digested using a CEM microwave(model MDS-2000) with a two-step digestion method (Lin et al.,2002a). As the first step, approximately 0.25 g of ground sedi-ment sample was digested with 2.5 ml of a mixed reagent ofconcentrated nitric acid (Merck) and hydrofluoric acid (Merck)(HNO3/HF: 5/2), followed by an addition of 10 ml of 4% boric acid(Merck) in the second step of digestion. Iron concentrations inextractant and digest solutions were determined using a PerkinElmer flame atomic absorption spectrometer (model PE-3300). Acalibration curve was prepared from a Standard Metal Solution(Merck). Reagent blank and Standard Reference Material NIST-1646a Standard Sediment (National Institute of Standards andTechnology) were used in each batch digestion for the analyticalquality control.

3. Result

3.1. Sulfate, methane and dissolved sulfide

Results of pore water sulfate, methane, and dissolved sulfide inthe coreMD05-2911 are shown in Figure 2a and b. Sulfate depletiontogetherwith high concentrations of methane and dissolved sulfidewere observed at the study site. Pore water sulfate concentrationshowed a concave-up profile, decreasing from a seawater value ofabout 28 mM within the upper 1 mbsf (meter below seafloor), andfollowing by a rapid linear decrease from 3 mbsf to near depletionat 5.5 mbsf. Sulfate concentrations remained at a level of less than1mMbelow 5.5mbsf. Porewatermethane concentration increasedrapidly with increasing sediment depth, reaching a high value of

Figure 2. Profiles of pore water and sedimentary geochemical parameters of the giant piston core MD05-2911: (a) pore water sulfate and dissolved sulfide; (b) pore water methane;(c) sedimentary organic carbon and organic nitrogen; (d) sedimentary pyrite and acid volatile sulfide; (e) pore water dissolved iron, sedimentary oxalate extractable and total iron;(f) carbonate d13C of bulk sediment; (g) C/N molar ratio; (h) C/S ratio; (i) degree of iron pyritization (DOP) variations with depth. Dashed lines are the depth of the presentsulfateemethane interface (SMI). Dot-dashed lines indicate boundaries of the Unit I, II, and III, labeled at right side.

Y.C. Lim et al. / Marine and Petroleum Geology 28 (2011) 1829e18371832

Y.C. Lim et al. / Marine and Petroleum Geology 28 (2011) 1829e1837 1833

720 mM at the SMI where pore water sulfate was almost completelydepleted (<1 mM). Methane values varied considerably below5.5 mbsf, probably a result of gas loss during core recovery, similarto those documented in Dickens et al. (1997) and Paull et al. (2000).Concentration of dissolved sulfide increased rapidly from anundetectable value of above 2.5 mbsf to a peak value of 2410 mM at5.5 mbsf, the same depth as the SMI. Concentration of dissolvedsulfide decreased steeply to zero at 10 mbsf where AVS started toincrease, indicating its subsequent removal by iron sulfideprecipitation.

3.2. Total organic carbon and nitrogen

Organic carbon and nitrogen in bulk sediment are shown inFigure 2c. Total organic carbon concentrations were in the range of0.43e0.87%. The highest concentration of total organic carbon(w0.87%) was observed at the surface. With increasing sedimentdepth, organic carbon decreased to 0.45% at 13 mbsf. Below13 mbsf, total organic carbon showed slight variation with a meanconcentration of 0.47 � 0.03%. Total organic nitrogen concentra-tions were in the range of 0.06e0.12%, exhibiting a trend similar tothe total organic carbon concentration. An average of 0.11 � 0.01%of total organic nitrogen concentration was observed in sedimentsabove the top 10 mbsf. However, relatively lower concentrations oftotal organic nitrogen were observed in sediments between 13 and18 mbsf with an average value of 0.06 � 0.01%.

C/Nmolar ratios were in the range between 6.6 and 9.7 (Fig. 2g),showing varied admixtures of marine and terrigenous organicmatters (Meyers, 1994). Relative higher C/N ratios were observed insediments above 4 mbsf (8.6 � 0.4) and between 15 and 18 mbsf(9.5� 0.2). C/N ratio increased from a low (7.1�0.5) at 4e8mbsf toa high at 15e18 mbsf, followed by a decrease to another low of7.1 � 0.4 at depth greater than 18 mbsf.

3.3. Pyrite and acid volatile sulfide

Sulfide minerals compositions (pyrite and acid volatile sulfide)in the core MD05-2911 are shown in Figure 2d. Pyrite concentra-tions varied between 15 and 380 mmol/g. High pyrite concentra-tions (84e220 mmol/g) were observed in the top 13 mbsf witha peak high at 4 mbsf near the SMI, followed by a gradual decreaseto approximately 100 mmol/g at 13 mbsf. As depth increases, pyriteconcentration decreased rapidly to a level of less than 25 mmol/gbetween 15 and 20 mbsf. Another high concentration of pyrite wasfound at depth between 20 and 23 mbsf, from a low of 25 mmol/g at19 mbsf to a high of 380 mmol/g at 23 mbsf within 4 m.

Acid volatile sulfide concentrations (Fig. 2d), in the range of0e82 mmol/g, weremuch lower than pyrite concentrations with theexception of sediments between 10 and 15 mbsf. Higher concen-trations of AVS (up to 82 mmol/g) were observed in sedimentsbetween 10 and 15 mbsf, whereas most AVS concentrations insediments above 10mbsf and below 15mbsf weremuch lower, lessthan 6 mmol/g.

3.4. Dissolved iron, oxalate extractable and total iron

Oxalate extractable and total iron as well as dissolved ironprofiles are shown in Figure 2e. Oxalate extractable irons, includingamorphous iron hydroxide, ferrihydrite, and lepidocrocite repre-sent iron oxide minerals with high reactivity in dissimilatory ironreduction and iron sulfidization (Canfield, 1989). Oxalate extract-able iron concentrations also showed a wide variation between 12and 190 mmol/g. The variation, however, was in an opposite patternas that of pyrite. Reactive iron also showed three different zones ofdistribution in the study site: at above 10 mbsf, a relatively low

concentration of 69 � 9 mmol/g was observed; at between 10 and20 mbsf, an increase to a higher concentration of 162 � 17 mmol/galong with a low concentration of pyrite; at below 20 mbsf, a rapiddepletion to a concentration of onlyw12 mmol/g where high pyritecontents were also found.

Total iron concentrations were in the range of 678e814 mmol/g.Approximately 680 mmol/g of total iron was found in Unit III sedi-ments. The maximum total iron (814 mmol/g) was found at17.5 mbsf in Unit II, about 10% higher than that in Unit I and III. Totaliron concentration decreased in sediments deeper than 18 mbsf to678 mmol/g at 22 mbsf in Unit I.

Dissolved iron concentrations displayed a near surface highervalue of about 4 mM between the sediment/water interface and3 mbsf as well as a peak concentration with a maximum about13 mM at 16e19 mbsf. This peak concentration began to increasefrom a depth of 14 mbsf and decreased to a depth of 21 mbsf. Thedepth where peak concentrations of dissolved iron appeared isright beneath the depth of the AVS peak.

3.5. Bulk Sediment carbonate d13C

Carbonate d13C in bulk sediments of core MD05-2911 are shownin Figure 2f. Carbonate d13C in bulk sediments above 21 mbsf wereapproximately �1.5 � 0.9&, reflecting isotopic values of typicalbiogenic carbonates in equilibriumwith seawater. Lower carbonated13C values (as low as �29&) were found in sediments between21 mbsf and 23.5 mbsf in Unit I, which indicate anaerobic methaneoxidation in the sediments. No similar trend was found in Unit IIIwhere the present-day AOM appeared.

4. Discussion

4.1. Anaerobic methane oxidation

Upward migration of methane provides a high concentration ofmethane for anaerobic methane oxidation resulting in a depletionof sulfate with a shallow SMI in the study site. In order to deter-minate the AOM rate, diffusive sulfate fluxwas calculated from databetween 3 and 5 mbsf that are linear (r2 ¼ 0.99) according to Fick’sdiffusion equation:

J ¼ FDSðdC=dzÞ; (5)

where J is flux of sulfate across the SMI, f measured porosity, Ds thediffusion coefficient of sulfate in sediment and dC/dz, concentrationgradient of sulfate. Sediment diffusion coefficient (Ds ¼ 3.56 � 10�10

m2 s�1) of sulfate was calculated according to Boudreau (1997) fromthe measured porosity (f ¼ 0.75) and the diffusion coefficient inseawater (D0) corrected for in situ temperature (D0¼ 5.61�10�10 m2

s�1 at 4.5 �C) (Schulz, 2006).

DS ¼ D0=�1� 1n

�f2

��(6)

Calculated sulfate flux is approximately 81 mmol m�2 y�1,slightly lower than those measured directly using the radioactive35S method in the same region (113 vs 81 mmol m�2 y�1, Lin,unpublished results). As a comparison, this flux is at least 10 timeshigher than those sites without AOM in adjacent area (Lin et al.,2006). This flux is also higher than those reported in otherknown gas hydrate environments; for example, the Carolina Riseand Blake Ridge (2.6e19 mmol m�2 y�1) (Borowski et al., 1999;Dickens, 2001), the Namibian slope (22e62 mmol m�2 y�1)(Niewöhner et al., 1998) but comparable to the Chilean upwellingmargin (46.0e100 mmol m�2 y�1) (Treude et al., 2005) and BlackSea (40e104 mmol m�2 y�1) (Jørgensen et al., 2001).

Y.C. Lim et al. / Marine and Petroleum Geology 28 (2011) 1829e18371834

The higher sulfate downward flux in the study site, as comparedto other environments, demonstrated that large quantity ofmethane is migrating upward from below at the present time.Furthermore, the concave-up sulfate profile at the top 3 mbsf of thestudy site is similar to the sulfate profiles of the Argentine Basin byHensen et al. (2003), who proposed the event driven profile as anindicative of increasing methane flux at the present time. It is likelythat methane flux may increase with time, inducing AOM whichgradually drive the SMI upward at this study site resulting ina pyritization front moving upward.

4.2. Pyrite formation

Three distinct zones were identified at the study site based onsediment geochemical properties of pyrite, reactive iron, C/S ratio,C/N ratio. Unit I (>20 mbsf) sediments is characterized by highpyrite concentrations (as high as 380 mmol/g), a very low reactiveiron and a low C/S and C/N ratio. Unit II sediments (15e20mbsf), bycontrast, consisted of very low pyrite concentrations (as low as15 mmol/g) together with higher reactive iron contents and higherC/S and C/N ratio. Unit III sediments (<10 mbsf), high pyriteconcentrations (up to 220 mmol/g) and relatively low reactive ironcontents were found. Since an organic carbon limited condition dis-favored an existence of high pyrite concentration, AOM playa dominant role for the observed large differences in pyriteconcentrations in this study site.

4.2.1. C/S ratioSedimentary organic carbon is, in general, the primary control

on sulfate reduction and pyrite abundance (Canfield, 1991;Jørgensen, 1982; Lin and Morse, 1991; Lin et al., 2002b). Sedimen-tary organic carbon and reduced sulfur contents in the normalmarine sediment show a positive correlation with a relativelyconstant C/S ratio of 2.8 � 0.8 (Berner, 1982; Lin and Morse, 1991;Morse and Berner, 1995; Raiswell and Berner, 1986). In euxinicenvironment, e.g. Black Sea, C/S ratios are low as compared to thehigh C/S ratio of the freshwater environment where sulfate islimiting. In organically-lean and methane-rich sediment, AOMmaybecome predominant and contribute a significant fraction of pyriteleading to a lowering of C/S ratio in sediments (Kaneko et al., 2010;Lin et al., 2006).

Sedimentary organic carbon content is low in the study site,varied within a slight range of 0.43%e0.87% (Fig. 2c), representinga typical organically-limited normal marine sediments. However,the C/S ratio varied significantly in the study sediments (Fig. 2h),which indicates that other factor, i.e. AOM, may play important rolein pyrite formation. In Unit III sediments (<10 mbsf), the AOMwitha shallow SMI together with sulfate reduction driven by organicmatter degradation are the reasons for the observed low C/S ratio.The C/S ratio of 1.8 � 0.4 in Unit III sediments is lower than those ofthe normal marine sediment. Anaerobic methane oxidation at thepresent-day SMI in Unit III sediments provides additional hydrogensulfide in favor of pyrite formation. High concentrations of dis-solved sulfide diffused upward resulted in a higher pyrite concen-tration and consequently lower the C/S ratio in sediments above10 mbsf.

In addition, an evenmuch lower C/S ratio of only 0.49� 0.10wasfound in Unit I sediments (>20 mbsf), a pattern similar to thosefound in the euxinic Black Sea and Cariaco Trench environments(Leventhal, 1983; Raiswell and Berner, 1985). Again, this lower C/Sratio in Unit I is a result of higher pyrite concentrations in sedi-ments. Pyrite concentration is, in fact, even higher than some ofthose in the upwelling area off central Chile (192 mmol/cm3) (Zopfiet al., 2008) where organic carbon could reach up to a high of 8%(Thamdrup and Canfield, 1996). Unlike in the upwelling region, the

average organic carbon concentration in Unit I sediments is within0.46 � 0.03%, too low a concentration to support the observedadditional pyrite formation with a euxinic-like C/S ratio. Appar-ently, AOMwas a major mechanism for the pyrite formation in thisstudy sitewith plenty of methane. Another evidence of AOM in UnitI included the lower values of carbonate d13C. Stable carbon isotopicsignature in Unit I bulk sediments are depleted, down to �29&.Kulm and Suess (1990), Paull et al. (1992), and Treude et al. (2003)showed that carbon originated from anaerobic methane oxidationis usually characterized by a strongly depleted d13C. This study alsofound d13C-depleted carbonate carbon in Unit I sediments. Theeuxinic-like C/S ratio coupled with a d13C-depleted carbonatesuggested that the observed high pyrite was a result of an enhancedanaerobic methane oxidation at the paleo-SMI for some period oftime in the past. Anaerobic methane oxidation, therefore, was animportant process in registering the observed strong pyrite signa-ture in Unit I sediments.

Comparing to Unit I and III, C/S ratio of Unit II sediments(15e20 mbsf) is much higher, in an average of 8.36 � 1.31, whereaspyrite concentration appear much lower, at 15 mmol/g, 15e25 timeslower than those in Unit I and III sediments. The high C/S ratio andlow pyrite concentration indicated low sulfate reduction and pyriteformation in Unit II sediments. Similar conditions were reported inenvironments lacking methane migrating in adjacent area (Linet al., 2006) and comparable to those of the continental sloperegion in the Gulf of Mexico (Lin and Morse, 1991) and southernEast China Sea sediments (Lin et al., 2002b). The lack of pyriteformation in Unit II sediments indicates that sediments weredeposited in a normal marine diagenetic condition withoutsignificant anaerobic methane oxidation.

4.2.2. Reactive ironA lack of pyrite formation inmarine sedimentsmay be a result of

either iron limitation or an insufficient supply of organic carbon.Because there is little variation in organic carbon content in thestudy site, iron limitation may play a major role. Degree of ironpyritization (DOP) is commonly used in identifying the role of ironin pyrite formation (Berner, 1970; Canfield et al., 1992; Raiswellet al., 1988).

DOP ¼ Fepyrite=�Fepyrite þ Feextractable

�(7)

where Fepyrite ¼ Spyrite/2; and Feextractable represent reactive ironcontent.

Canfield et al. (1992) define reactive iron as the iron oxidephases that most easily react with dissolved sulfide to form pyrite.

As showed in Figure 2i, DOP values in core MD05-2911 is0.45 � 0.07 in Unit III sediments. Pyrite concentrations and DOPvalues in Unit I sediments are similar to those observed at themethane-dominated cold seep site in the continental slope of thenorthern Gulf of Mexico (Morse et al., 2002). Arvidson et al. (2004)suggested that abundant pyrite observed in the Gulf of Mexico coldseep sediments could attributed to anaerobic methane oxidationdriven by methane migrating upward from the base of gas hydratestability zone to the sulfate reduction zone. Similar to those foundin the euxinic sediments (Raiswell and Berner, 1985), the highvalues of DOP (0.89 � 0.05) in this study indicated that the avail-ability of reactive iron was nearing total depletion in Unit I sedi-ments. Furthermore, high concentrations of pyrite are comparableto those in the euxinic Black Sea (406 � 94 mmol/g) (Lyons, 1997).Novosel et al. (2005) pointed out that enhanced pyrite formationdriven by anaerobic methane oxidation was responsible forenhanced diagenetic transformation of iron minerals from ironoxides to pyrite in high methane flux region. Similarly, those highpyrite in Unit I sediments were a result of enhanced anaerobic

Y.C. Lim et al. / Marine and Petroleum Geology 28 (2011) 1829e1837 1835

methane oxidation, which produced additional hydrogen sulfideand later react with reactive iron in precipitating pyrite. As a result,strong depletion of reactive iron and accumulation of pyrite sulfidewere registered in Unit I sediments.

A sharp contrast existed between Unit I and II. DOP in Unit II wasmuch lower, with an average of only 0.08 � 0.04. High reactive ironconcentrations found in Unit II sediments with little pyrite indicatethat reactive iron is abundant in Unit II and thus unlikely to limitpyrite formation. Moreover, relatively higher total iron concentra-tion and C/N ratio in Unit II sediments indicated a shift to a burial ofterrigenous materials during the Unit II deposition. With sufficientsupply of reactive iron, little pyrite formation in Unit II could bea result of lack of anaerobic methane oxidation. Pyrites in Unit IIremain low because of a lack of sulfate with the SMI located ata shallow depth keeping the overlying seawater sulfate above theSMI. The microbially-mediated anaerobic methane oxidation actedas a barrier in preventing sulfate migrates through the present-daySMI depth. This limited further pyrite formation in Unit II at thepresence of high concentrations of methane.

A peak concentration of AVS (up to 82 mmol/g) was observed inthe upper part of Unit II at between 10 and 15 mbsf (Fig. 2d)together with an slowly increase of pyrite concentration. Thisaccumulation of iron sulfides in the reactive iron-rich zone isa result of downward diffusion of dissolved sulfide and upwarddiffusion of dissolved iron from deeper part of the Unit II wherehigher concentration of reactive iron was found. The reactionbetween the downward diffusive dissolved sulfide and upwarddiffusion of dissolved iron leads to the precipitation of amorphousiron sulfide at the boundary of the reactive iron-rich zone (Böttcherand Lepland, 2000; Jorgensen et al., 2004; Kasten et al., 1998;Passier et al., 1996). This downward progressing sulfidizationfront has been observed in the transition of limnic and marineenvironment in Black Sea and Baltic Sea, where an oxic depositionlayer deeply buried beneath anoxic sediments (Jørgensen et al.,2004; Böttcher and Lepland, 2000). In our study site, the reactiveiron-rich Unit II was well preserved at depths between 10 and20 mbsf. High amount of iron oxide mineral in Unit II sedimentprovided sufficient source of dissolved iron, which react withdownward diffusive hydrogen sulfide from the present-day SMI inaccumulation of AVS in sediments at between 10 and 15 mbsf.Under this downward progressing sulfidization front, DOP (Fig. 2i)increased with decreasing sediment depth, which is similar tothose documented in Neretin et al. (2004) and Böttcher andLepland (2000). The appearance of an AVS peak at a depth deeperthan dissolved sulfide indicated an upward shift of the SMI. Theincrease in methane flux moved the SMI upward to the presentlocation (5.5 mbsf) leaving the AVS as a record of the past sulfid-ization front and the reactive iron-rich layer at depth.

5. Conclusion

Temporal variation of methane flux is the most likely factor incontrolling dynamic variations in enhancing or weakening of theanaerobic methane oxidation and pyrite formation in sediment.Three different zones of diagenetic oscillationwere identified in thestudy site with a strong pyrite formation at the top 10 m (Unit III)and at below 20 m (Unit I) in contrast to a limited pyrite formationat between 15 and 20 m (Unit II) based on pyrite abundance, C/Sratio, and reactive iron availability.

Temporal variations of methane migration in the study envi-ronment shifted position of the SMI where anaerobic methaneoxidation taking place and thus changes the degree of pyriteformation. Upward migration of methane regulated the observedvariations of anaerobic methane oxidation and pyrite formation inthe studied sediments. Under conditions of sufficient methane

migration, anaerobic methane oxidation and associated sulfatereduction generate diagenetic conditions conducive for significantpyrite formation near the SMI. Without anaerobic methaneoxidation, sediments were characterized by normal organically-lean marine sediments with little pyrite formation. Enhancedpyrite formation near the SMI with sufficient methane supply mayserve as a useful proxy for methane flux oscillation.

Acknowledgment

Special thanks go to Yvon Balut of r/v Marion DuFresne whohelped tremendously during coring operation and for successfullong cores obtained near Taiwan. The authors thank Prof. Ju-ChinChen for his kindness and wisdom in guiding the gas hydrateprogram. The authors thank reviewers and Dr. John Pohlman fortheir constructive comments and suggestions in improving thismanuscript. Financial supports of this research were from theTaiwan Central Geological Survey (CGS5226902000-05-94-02).

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