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Blackwell Publishing AsiaMelbourne, AustraliaIARIsland Arc1038-48712007 Blackwell Publishing Asia Pty Ltd? 200716193104Research Article Study on sediment of the Ulleung BasinJ.-H. Kim et al.

Island Arc (2007) 16, 93–104

Research Article Geochemical characterization of the organic matter, pore water constituents and shallow methane gas in the eastern part of the Ulleung Basin, East Sea (Japan Sea) JI-HOON KIM,1 MYONG-HO PARK,2 URUMU TSUNOGAI,3 TAE-JIN CHEONG,1 BYONG-JAE RYU,1 YOUNG-JOO LEE,1 HYUN-CHUL HAN,1 JAE-HO OH1 AND HO-WAN CHANG4* 1

Petroleum and Marine Resources Research Division, Korea Institute of Geoscience and Mineral Resources, 30 Gajeong-dong, Yuseong-gu, Daejeon, 2Petroleum Technology Institute, Korea National Oil Corporation, Anyang, Gyeonggi-do, Korea, 3Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan and 4School of Earth and Environmental Sciences, Seoul National University, San 56-1 Shilim-dong, Kwanak-gu, Seoul 151-742, Korea (email: [email protected])

Abstract Geochemical analyses of sediments, pore water and headspace gas of the piston cores taken from the eastern part of Ulleung Basin of the East Sea (Japan Sea) were carried out to assess the origin of the sedimentary organic matter and interstitial fluid. Several tephra layers within the core are identified as the Ulleung-Oki (10.1 ka), the AiraTanzawa (23 ka) and the Ulleung-Yamato (30.9 ka) tephras. With the exception of these volcanic layers, the cores consist predominantly of muddy sediments that contain >0.5% total organic carbon. Atomic C/N ratios and δ13Corg values suggest that the organic matter originated from marine algae rather than from land vascular plants, whereas Rock-Eval pyrolysis suggests that the organic matter is thermally immature and comes from a land vascular plant (Type III). These conflicting results seem to be caused by the heavy oxidization of the marine organic matter. Sulphate concentration profiles of pore waters show strongly linear depletion (r2 > 0.97) with sediment depth. The estimated sulphate–methane interface (SMI) depth using the sulphate concentration gradient was nearly 3.5 m below seafloor (mbsf) in the southern part of the study area, and deeper than 6 mbsf in the northern part of the area. The difference in SMI depths is likely associated with the amount of the methane flux. The methane concentration below the SMI in the two southern cores increases rapidly, implying the occurrence of methanogenesis and anaerobic methane oxidation (AMO). In contrast, the two northern cores have a low methane concentration below the SMI. d13CCH4 values measured from all cores were in the range of −83.5 to −69.5‰, which suggests that the methane derives from a methanogenic microbe. d13CCH4 values become decreased toward SMI, but increased below SMI; therefore, d13CCH4 has eventually the minimum value near the SMI. The d13CCH4 values are also decreased when the methane concentration is increased. These phenomena support the typical occurrence of AMO in the study area. Key words: anaerobic methane oxidation, atomic C/N ratio, δ13Corg, East Sea (Japan Sea), Rock-Eval pyrolysis, sulphate–methane interface, Ulleung Basin. INTRODUCTION Geochemical transformations in marine sediments are directly or indirectly produced by microbes. *Correspondence. Received 16 January 2006; accepted for publication 23 May 2006. © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd

The early diagenesis of marine sediments involves microbially mediated processes that sequentially bring about the oxidation of organic matter through the use of oxygen, nitrate, Mn (IV) oxides, Fe oxides, sulphate and carbon dioxide (Froelich et al. 1979) with sulphate reduction and methanogenesis as the main anoxic biogeochemical prodoi:10.1111/j.1440-1738.2007.00560.x

94 J.-H. Kim et al.

cesses (Claypool & Kaplan 1974; Berner 1980). Another important and well-documented biogeochemical process is anaerobic methane oxidation (AMO), which involves the oxidation of CH4 to CO2 using SO42– as a terminal electron acceptor. Anaerobic methane oxidation occurs at the sulphate–methane interface (SMI) (Reeburgh 1976; Borowski et al. 1996, 1997; Fossing et al. 2000; Jørgensen et al. 2001). The East Sea (Japan Sea), composed of three main basins (Japan, Yamato and Ulleung), is one of the largest isolated marginal seas in the northwestern Pacific. During the Last Glacial Maximum, the East Sea (Japan Sea) became nearly isolated from the world ocean. Consequently, the paleoenvironments of the East Sea (Japan Sea) have been much affected by sea level oscillations, changing climate conditions and sediment depositions. Many seismic, stratigraphic and sedimentological studies of Quaternary sediments have been carried out in an effort to understand the paleoenvironmental changes in the Ulleung Basin of the East Sea (Japan Sea) (Lee et al. 1996; Chough et al. 2000; Kim et al. 2000; Park et al. 2003; Lee et al. 2004). However, only a few geochemical studies have been conducted in this basin (Park et al. 2005; Kim et al. 2006) that characterize and identify shallow gas and organic matters. Therefore,

the purpose of this study is to: (i) characterize organic matter, pore water and headspace gas chemistry using core sediments taken from the eastern Ulleung Basin (Fig. 1); (ii) to identify their origin; and (iii) to elucidate their interrelationship based on geochemical and stable isotopic analyses of pore water, gas and core sediments. MATERIALS AND METHODS MATERIALS

Four piston core samples were taken from the eastern part of the Ulleung Basin in water depths of 1197 m to 2179 m using R/V Tamhae II of the Korea Institute of Geoscience and Mineral Resources (KIGAM) in 2003 (Fig. 1). The cores were cut in the laboratory at KIGAM. One half of each core was preserved as archived core and the other half was further processed. ROCK-EVAL AND ELEMENTAL ANALYSIS

Sediment samples for elemental and Rock-Eval analyses were taken at 10 cm intervals and where changes in sediment lithology occurred. After freeze drying for 24 h, the sediment was powdered in an agate mortar and analyzed for total organic carbon (TOC) and total nitrogen (TN). Rock-Eval

Fig. 1 Map showing location of four cores (03GHP-01, 03GHP-02, 03GHP03 and 03GHP-04) from the eastern Ulleung Basin, East Sea (Japan Sea) (HB, Hupo Bank; KP, Korea Plateau; UB, Ulleung Basin; UIG, Ulleung Interplain Gap; OB, Oki Bank; U-Oki, Ulleung-Oki; UYm, Ulleung-Yamato). Bathymetry in meters. © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd

Study on sediment of the Ulleung Basin 95

6 pyrolysis was used as a rapid screening tool for the determination of the hydrocarbon source-rock potential of the sediment sample (Tissot & Welte 1984; Peters 1986). The free and adsorbed hydrocarbons that were released by the programmed heating of a sample were recorded as the first peak in a pyrogram (S1) under a low temperature (300°C). The second peak (S2) in the pyrogram represents a hydrocarbon released by the kerogen cracking when the sample was heated to 550°C. The temperature at the maximum S2 peak is defined as Tmax. CO2 shown as the third peak (S3) in the pyrogram was also generated by the kerogen degradation. When these components are normalized to the organic carbon content, the S2 peak becomes the hydrogen index (HI; S2 × 100/TOC), and the S3 becomes the oxygen index (OI; S3 × 100/ TOC). Total organic carbon is measured by adding the pyrolyzed carbon (PC) and residual carbon (RC) (Arthur et al. 1998; Lafargue et al. 1998). Total nitrogen content was also measured by the combustion method using a LECO CHN-900 apparatus with a detection limit of 0.01%. Powdered samples were pretreated with 3 N HCl to remove carbonate (CaCO3) to analyze the organic carbon isotope ratios (δ13Corg) in the sediments. δ13Corg were measured with a VG prism stable isotope ratio mass spectrometer at the Korea Basic Science Institute. The reported analytical reproducibility for the usual δ notation relative to the VPDB was ±0.2‰ for δ13C. PORE WATER ANALYSIS

Pore water was extracted from core samples by centrifuging for 30 min at 10 000 rpm. Water was collected with a syringe and filtered using 0.45-µm membrane filters. The chloride (Cl–) concentration was determined by the Mohr titration with silver nitrate using potassium chromate/potassium dichromate as an indicator (Gieskes et al. 1991). Sulphate (SO42–) was analyzed using ion chromatography (Dionex DX-500 IC) with an AS-40 auto sampler at Seoul National University. HEADSPACE GAS AND CARBON ISOTOPE ANALYSIS

A 5-cm3 sample was taken from each core as soon as it was retrieved from the sea, and this sample was placed in a 20-cm3 glass vial to analyze headspace gas as described by Pimmel and Claypool (2001). The gas extracted through the septum was injected with a glass syringe into an high performance 5890II gas chromatograph (GC) at KIGAM.

The stable carbon isotope ratios ( d13CCH4) of the headspace gas were analyzed using an isotope ratio-monitoring gas chromatograph/mass spectrometer (GC/MS) at Hokkaido University, Japan (Tsunogai et al. 2000). The accuracy and precision of this isotope analysis were examined by comparison with a standard gas. The detection limit was 200 pmol for the isotope ratio with a standard deviation of 0.3‰ Vienna Pee Dee Belemnite (V-PDB).

RESULTS AND DISCUSSION TEPHRA LAYERS AND SEDIMENTARY FACIES IN THE CORES

Retrieved core sediments from the Ulleung Basin generally showed a lithologic change from late Pleistocene turbidites to Holocene hemipelagic muds, in which several volcanic layers are interbedded (Furuta et al. 1986; Lee et al. 1996; Machida & Arai 2003; Kim et al. 2006). Several tephra layers provide a very useful tool for stratigraphic correlation in the Ulleung Basin (Chun et al. 1997; Park et al. 2005). The tephra layers found in the study cores are the Ulleung-Oki (UOki), Aira-Tanzawa (AT) and Ulleung-Yamato (UYm) layers, erupted at approximately 10.1 ka, 23 ka and 30.9 ka, respectively (Fig. 2; Park et al. 2006). These volcanic layers range in thickness from a few millimeters to several centimeters, but may be as thick as 40 cm (Fig. 2). The cores consist predominantly of muddy sediments, and the color of muddy sediments varies from olive gray to dark olive gray (5GY 5/1–5GY 4/1). The muddy sediments can be classified into four lithologic units. The most common lithology is the bioturbated mud facies, which is characterized by burrows that are circular, oval or tubular in shapes. The laminated mud facies, homogenous mud facies, and slightly laminated mud facies are also found in the cores. The laminae of the latter are generally less sharp and more irregular than those in the laminated mud. Poorly sorted beds occur within the homogenous mud facies that usually lack visible primary structures. In addition, dark laminated mud (DLM) layers are present in the three cores (03GHP-02, 03GHP03 and 03GHP-04) (Fig. 2). These DLM layers (15.4 ka) can be also used for the stratigraphic correlation. According to previous studies (e.g. Park et al. 2005), these layers commonly have high Si and Al contents, suggesting that the DLM layers generally contain significant amounts of finegrained silicates and/or aluminosilicates. However, © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd

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02, 42–459 mgHC/gTOC for core 03GHP-03 and 37–409 mgHC/gTOC for core 03GHP-04. The OI varies from 50 to 464 mgCO2/gTOC for core 03GHP-01, 104–234 mgCO2/gTOC for core 03GHP02, 43–387 mgCO2/gTOC for core 03GHP-03 and 78–458 mgCO2/gTOC for core 03GHP-04. In general, HI values below approximately 300 mgHC/ gTOC are typical of terrigenous organic matter (Type III), whereas HI values from 600 to 900 mgHC/gTOC are typical of marine organic matter (Tissot & Welte 1984). Thus, most sedimentary organic matter lies along the Type III evolution path (Fig. 3). The results from the Rock-Eval pyrolysis indicate that most organic matter belongs to Type III (land-derived organic matter). This is in agreement with previous work (Lee et al. 1999). Tmax

Fig. 2 Lithology of the core sediment and inferred stratigraphic correlation based on tephra and dark laminated mud (DLM) layers. mbsf, meters below sea floor.

the DLM layer is not found in the core 03GHP-01, most likely because the sediment interval including the DLM layer has been partially disturbed or reworked (Fig. 2).

ORIGIN AND MATURATION OF ORGANIC MATTER

Temperature for the maximum S2 peak (Tmax) is found to vary with the thermal evolution formerly undergone by the rock sample under analysis (Tissot & Espitalié 1975; Espitalié et al. 1977). Because mature organic matter is more condensed, it is more difficult to pyrolyze as a result of higher activation energies, thus, Tmax is inherently linked to the kinetics of organic matter cracking. In cases of Types II and III, organic matter reaches catagenesis at a Tmax of 435°C (Nali et al. 2000). The present results show that Tmax in ranges from 403 to 418°C for core 03GHP-01, from 314 to 415°C for core 03GHP-02, from 314 to 426°C for core 03GHP-03 and from 329 to 419°C for core 03GHP-04 (Fig. 4). As shown in Figure 4, Tmax for all cores is less than 435°C. These results indicate that the organic matter is thermally immature; i.e. it is in the early stages of diagenesis. TOC

The TOC contents are higher than 0.5% with the exception of the tephra layer; the overall range of the TOC content varies between 0.10 and 4.20% for core 03GHP-01, 0.03 and 4.96% for core 03GHP-02, 0.01 and 5.09% for core 03GHP-03, and 0.02 and 3.91% for core 03GHP-04 (Fig. 4). Accordingly, TOC values are used as geochemical proxies to locate the tephra layer in the retrieved core samples.

ROCK-EVAL PYROLYSIS HI and OI

The HI has a range of 89–226 mgHC/gTOC for core 03GHP-01, 79–238 mgHC/gTOC for core 03GHP© 2007 The Authors Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd

ATOMIC C/N RATIO Atomic C/N ratios have been widely used as a proxy to identify changes in the proportions of


Fig. 3 van Krevelen diagrams of sedimentary organic matter for cores (a) 03GHP-01, (b) 03GHP-02, (c) 03GHP-03, (d) 03GHP-04. HI, hydrogen index; OI, oxygen index.

sedimentary organic matter derived from marine algae versus land vascular plants (Prahl et al. 1980; Premuzic et al. 1982; Ishiwatari & Uzaki 1987; Jasper & Gagosian 1990; Meyers et al. 1996). The organic matter from marine algae typically has an atomic C/N ratio of 4–10, whereas the organic matter from land vascular plants has a ratio of 20 or higher (Emerson & Hedges 1988; Meyers 1994). The average atomic C/N ratios are 8 ± 0.8 for core 03GHP-01, 9.2 ± 1.2 for core 03GHP-02, 8.3 ± 1.4 for core 03GHP-03 and 8.4 ± 2.9 for core 03GHP-04 (Fig. 5). These results suggest that the organic matter from the sampled sediments has predominantly originated from marine algae with some admixture from land-derived vascular plants.

d13C VALUE FOR ORGANIC MATTER The carbon isotopic values for the organic matter (δ13Corg) in the core sediments also support a marine algal origin. Organic matter produced from atmospheric CO2 (δ13Corg = −8‰ PDB; Keeling et al. 1995) by land plants through the C3 pathway, including almost all trees and most shrubs, usually has an average δ13Corg value of −27‰ PDB (range = −32 to −21‰ PDB; Deines 1980). Many tropical savannah grasses and sedges which carbon cycles through the C4 pathway have an average δ13Corg value of approximately −14‰ PDB (range = −17 to −9‰ PDB; Deines 1980). Typical values of δ13Corg in marine organic matter range from −23 to −16‰ PDB, as marine algae use dissolved bicarbonate, which has a δ13Corg value of © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd

98 J.-H. Kim et al.

Fig. 4 Rock-Eval pyrolysis results of (a) Tmax and (b) total organic carbon (TOC) for the cores with a depth (dash line: 435°C Tmax). mbsf, meters below sea floor.

© 2007 The Authors Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd


Fig. 5 Cross-plots of atomic C/N ratio versus TOC for cores: (a) 03GHP-01 (b) 03GHP-02 (c) 03GHP-03, (d) 03GHP-04. Atomic C/N ratio in the range of 4–10 generally identify marine sources (Emerson & Hedges 1988; Meyers 1994).

approximately 0‰ PDB (Jasper & Gagosian 1990; Meyers 1994). δ13Corg values were measured from a subset of 14 samples, seven samples per core with a 1 m interval from cores 03GHP-02 and 03GHP03. These values range from −22.9 to −21.5‰ PDB in core 03GHP-02 and −22.7 to −21.2‰ PDB in core 03GHP-03 (Fig. 6a). In addition, they have a relatively constant value and do not vary with the sediment depth (Fig. 6a). These measured δ13Corg values do not clearly identify between marine algal organic matter and a mixed origin between the marine algal and land plant sources. However, cross-plots of both δ13Corg values and the atomic C/N ratio clearly show that the organic matter is derived from a marine source (Fig. 6b) MARINE ALGAL SOURCE The results of Rock-Eval pyrolysis show that most organic matter seems to be land-derived (Type

III), whereas the atomic C/N ratio and the δ13Corg data suggest a marine algal origin. Additionally, the similar results have been reported in the other areas, such as the western North Atlantic or the Iberian margin (Meyers et al. 1996; Çagatay et al. 2001). According to these studies, such discrepancies could be a result of the heavy oxidation of the organic matter, whereas the marine algae were sinking in the water column or exposed on the seafloor. If hydrocarbon-rich organic matter (Type I or II) is oxidized, its hydrogen content decreases, while its oxygen content increases, and it takes on the HI-OI characteristics of Type III organic matter in van Krevelen diagrams (Espitalié et al. 1977; Peters 1986). Therefore, the evidence supports a marine algal source for organic matter. SULPHATE DEPLETION AND METHANE PRODUCTION

Sulphate concentration near the sediment–water interface varies by a factor of two (Table 1, Fig. 7). © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd

100 J.-H. Kim et al. Table 1 Sulphate concentration in pore water and methane concentration and its δ13C value of headspace gas Core 03GHP-01

03GHP-02

03GHP-03

03GHP-04

Depth (mbsf )

SO42– (mM)

CH4 (ppm)

d 13CCH4 (‰)

0.0 0.7 1.6 2.6 3.6 4.5 5.5 0.0 0.6 1.5 2.4 3.4 4.3 5.2 0.0 0.7 1.6 2.6 3.6 4.5 5.4 0.0 1.2 3.3 4.5 5.0 6.2 7.3

13.3 10.9 7.4 4.9 0.2 0.4 1.0 15.0 11.1 7.4 2.3 0.4 1.1 2.2 15.7 12.9 11.0 9.1 7.5 5.9 5.0 28.5 25.2 14.4 9.8 4.9 0.5 1.8

n. d. 83.3 165.4 593.0 9 277.3 23 479.2 23 634.3 n. d. 90.3 149.1 2 909.7 20 363.4 26 049.2 35 138.6 n. d. n. d. 69.1 135.6 378.0 78.8 564.8 n. d. n. d. 102.1 161.8 704.4 60.8 785.7

– – −71.7 −73.6 −81.5 −83.2 −83.5 – – −71.2 −82.2 −81.7 −80.9 – – – −72.4 −72.9 −80.0 −69.5 −82.6 – – – – – – –

n.d. no detection, –, no measurement.

Fig. 6 (a) The δ13Corg profiles for cores 03GHP-02 and 03GHP-03. (b) Relationship between δ13Corg and atomic C/N ratio for cores 03GHP-02 and 03GHP-03. Dashed boxes show the fields for marine algae, and C4 and C3 land plants. mbsf, meters below sea floor; PDB, Pee Dee Belemnite; mbsf, meters below sea floor.

Three of the cores (cores 03GHP-01, 03GHP-02 and 03GHP-03) display values of ∼15 mM, whereas core 03GHP-04 has a value of 28 mM, close to the seawater value of ∼29 mM (Wilson 1975). Obviously, sulphate depletion is rapid in most of the cores whose values are similar to those in the Black Sea (Jørgensen et al. 2001) and the Gulf of Mexico (Arvidson et al. 2004). It is possible that the former (cores 03GHP-01, 03GHP-02 and 03GHP-03) has a more anoxic condition at the water–sediment interface than the latter (core 03GHP-04), although the exact reason for this is not known. A plot of sulphate concentration versus depth shows a linear relationship (r2 > 0.97). Using linear least-squares regression, sulphate gradients and intercept depths were calculated for all cores © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd

(Table 2). The intercept depth defines the base of sulphate reduction zone, or SMI, where pore water sulphate concentrations are at or near zero. Nonzero sulphate values below the SMI are likely because of seawater contamination occurring during piston coring (Borowski et al. 1996). Linear sulphate profiles imply focused depletion of sulphate at the SMI, caused by AMO (Borowski et al. 1996, 1999). Anaerobic methane oxidation links the pore water sulphate and methane pools by the coconsumption of methane and sulphate by the following net reaction (Reeburgh 1976): CH 4 + SO24− → HCO3− + HS − + H 2O

(1)

Thus, it is inferred that AMO is a significant sulphate sink in these sediments, in addition to sulphate reduction occurring through the microbial oxidation of organic matter. Interstitial methane concentration generally increases in the methanogenic zone below the SMI (Table 1, Fig. 7), in which microbes produce methane as a metabolic byproduct (e.g. Martens & Berner 1974). Core 03GHP-03 does not penetrate the SMI, whereas the other cores sample the


Fig. 7 Sulfate () and methane () concentration profiles of pore waters for cores: (a) 03GHP-01, (b) 03GHP-02, (c) 03GHP-03, (d) 03GHP-04. mbsf, meters below sea floor. Table 2 Calculation of sulfate gradients and sulphate– methane interface (SMI) depth by least-squares regression, and core length Core

03GHP-01 03GHP-02 03GHP-03 03GHP-04

Sulfate gradient (mM/m)

SMI depth (m)

Core length (m)

3.60 4.39 1.89 4.69

3.73 3.17 7.55 6.28

5.52 5.22 5.44 7.34

methanogenic zone. Methane concentration is typically at or near zero ppm in most of the sulphate reduction zones with some sporadic increases closer to the SMI. Cores 03GHP-01 and 03GHP-02 have the highest methane concentration values, as they penetrate far below the SMI (Table 1). ORIGIN OF METHANE

The carbon isotopic value of methane ( d13CCH4) gives clues about the origin of interstitial methane (e.g. Whiticar et al. 1986; Borowski et al. 1997). The d13CCH4 values range from −83.5 to −69.5‰ PDB in samples taken within the methanogenic zone (Table 1, Fig. 8). The specific ranges are: −83.5 to −71.7‰, core 03GHP-01; −82.2 to −71.2‰, core 03GHP-02; −82.6 to −69.5‰, core 03GHP-03. These ( d13CCH4) values suggest a microbial origin for the methane as empirically noted by Whiticar et al. (1986) (Fig. 8). Cross-plots of log [CH4] versus d13CCH4 show a linear relationship in all cores sampling the methanogenic zone; core 03GHP-01, r 2 = 0.99; core 03GHP-02, r 2 = 0.76; core 03GHP-03, r 2 = 0.93. This result supports that d13CCH4 values

Fig. 8 (a) The d 13CCH4 values for cores 03GHP-01, 03GHP-02 and 03GHP-03. The range of d 13CCH4 for microbial, mixed and thermogenic sources is based on data from Whiticar et al. (1986). (b) Relationship between d 13CCH4 and methane concentration for cores 03GHP-01, 03GHP-02 and 03GHP-03. mbsf, meters below sea floor. © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd

102 J.-H. Kim et al.

are affected by the AMO as proposed by Tsunogai et al. (2002). Additionally, according to Whiticar (1999), the microbial uptake of CH4 is also associated with a kinetic isotope effect that enriches residual CH4 in 13C. Therefore, the methane could originate from a microbial source through AMO rather than through a thermogenic origin. ROLE OF AMO

Sulphate depletion and SMI depth depend on several factors such as the quantity and quality of organic matter, as well as the amount of downward diffusing sulphate and upward diffusing methane (Borowski et al. 1996). Mean sedimentation rates are a proxy for the delivery of organic matter and could be estimated using DLM (Park et al. 2006) and three tephra layers (U-Oki, AT and U-Ym) (Machida & Arai 2003; Park et al. 2005). The average sedimentation rate is 16.3 cm/ky for core 03GHP-02, 20.7 cm/ky for core 03GHP-03 and 12.4 cm/ky for core 03GHP-04. If the SMI depth is controlled solely by the sedimentation rate, it should be deeper in the order of cores 03GHP-03, 03GHP-02 and 03GHP-04. However, the SMI depth was found to be shallower in the order of cores 03GHP-03, 03GHP-04 and 03GHP-02. This implies that there is no direct relationship between the sedimentation rate and the SMI depth, and further suggests that the sedimentation rates do not solely control the SMI depth. Moreover, the quality of organic matter does not affect SMI depth, as the δ13Corg and C/N ratios are especially similar for all sediments at all core sites. Among four cores, core 03GHP-02 has the highest methane concentration and upward methane flux, and core 03GHP-01 has the second highest, with the other cores showing the lower values, although the methane could be lost when the piston core is recovered. Comparing the upward methane flux and the SMI depth suggests that they are strongly related to each other; i.e. the SMI depth is shallower when the methane flux is higher, which is in good agreement with the results from other areas (Borowski et al. 1996, 1999; Niewöhner et al. 1998). As large amounts of methane diffuse upward from depth, the SMI depth should be shifted towards the sediment surface to increase the sulphate flux to the SMI, as the diffusive fluxes of methane and sulphate should be equal at steady-state conditions (Borowski et al. 1996). Therefore, the data from the present study collectively suggest that AMO is a major sulphate depletion mechanism, as influenced by upward © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd

methane flux, in addition to the oxidation of organic matter through sulphate reduction.

CONCLUSIONS 1. Several tephra layers were found at the four coring sites. Among these, previously wellknown tephra layers such as U-Oki, AT and U-Ym were identified and were used to stratigraphically correlate the cores. Except for the lapilli or ash layers, the cores consist predominantly of silty mud, which can be further divided into four sedimentary facies: i.e. bioturbated, laminated, slightly laminated and homogenous mud facies. These muddy sediments generally contain more than 0.5% TOC. 2. Atomic C/N ratios of sedimentary organic matter range from 4 to 10, and δ13Corg values vary from −22.9 to −21.2‰ PDB. These results indicate that organic matter in core sediments originated from marine algae rather than from vascular land plants. However, Rock-Eval pyrolysis results suggest thermally immature organic matter with a land-plant (Type III) origin. These apparently conflicting results are resolved if the marine algal source is heavily oxidized because of its exposure within the water column or on the seafloor. 3. Linear sulphate concentration profiles of pore waters imply that sulphate depletion is related to the AMO. Moreover, the SMI depth was approximately 3.5 mbsf for cores 03GHP-01 and 03GHP-02, and was deeper than 6 mbsf for cores 03GHP-03 and 03GHP-04. Methane concentration below the SMI depth in cores 03GHP-01 and 03GHP-02 is rapidly enriched although the other cores show a relatively low and constant methane concentration, as they do not reach the SMI and methanogenic zones. 4. d13CCH4 values from core sediments range from −86.5 to −69.5‰ PDB, implying that interstitial methane has a microbial origin, likely through CO2 reduction, rather than thermogenic source.

ACKNOWLEDGEMENTS The authors thank the Korea Institute of Geoscience and Mineral Resources for providing the piston cores. We would like to thank Professor W. S. Borowski, Professor A. Waseda, Professor S. R. Wallis and Professor T. Sakai for their helpful and constructive comments. The current study was


financially supported by the Gas Hydrate Development Project and BK21 Program, School of Earth and Environmental Sciences, Seoul National University.

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