Methane-Rich Plumes in the Suruga Trough (Japan) and Their Carbon Isotopic Characterization

ELSEVIER Earth and Planetary Science Letters 160 (1998) 97±105

Methane-rich plumes in the Suruga Trough (Japan) and their carbon isotopic characterization

U. Tsunogai a,Ł,J.Ishibashia, H. Wakita a,1,T.Gamob a Laboratory for Earthquake Chemistry, Faculty of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan b Ocean Research Institute, University of Tokyo, Minami-dai, Nakano-ku, Tokyo 164, Japan Received 18 August 1997; accepted 27 April 1998

Abstract

13 The carbon isotopic compositions (Ž CCH4) of the methane-rich buoyant plumes, observed in the oxygenated hemipelagic sea waters of the Suruga Trough, Japan, are discussed in relation to their sources. During a survey made in May 1996, two layers of anomalous methane-rich plumes, both of which centred at the same station about a few tens of kilometres off the coast, were found in the Suruga Trough. The deeper plume (ca. 2100 m depth, with a maximum methane concentration of 13 nmol=kg) had already been detected by a previous survey in 1986 at the same station, whereas the shallower plume (ca. 1000 m depth, with a maximum methane concentration of 10 nmol=kg) was newly discovered. 13 The estimated end-member Ž CCH4 value (59 š 3½ PDB) for the deeper plume suggests a microbial origin of the methane, probably derived from some shallow (surface) layer of sediment. The plume could be supplied from a continuous cold ¯uid seepage on the sea ¯oor of the Suruga Trough. On the other hand, the shallower plume is characterized by more 13 13 C-enriched end-member methane (Ž CCH4 D38 š 2½ PDB), presumably produced by the thermogenic degradation of organic matter. Since thermogenic methane should originate from a deeper part (more than 1000 m) of the sedimentary layer, it is unlikely that the thermogenic methane reaches the sea water by normal transport processes. The shallower plume may be a result of some sudden, catastrophic event on the sea ¯oor, such as earthquakes.  1998 Elsevier Science B.V. All rights reserved.

Keywords: methane; carbon; isotopes; Suruga Bay; earthquakes

1. Introduction front of the Philippine sea plate, subducting beneath the plates of the Japanese Islands (Fig. 1). During a 1.1. Suruga Trough and anomalous methane hydrographic survey in November 1986, anomalous enrichment methane enrichment (a methane plume) was detected in the bottom sea water at 34ë400N, 138ë360E (station The Suruga Trough, which is the northern exten- 18 in Fig. 1) in the Suruga Trough (Fig. 2). Methane sion of the Nankai Trough, is the northern convergent enrichment in sea water, together with heat, Fe, Mn,

Ł Corresponding author. Present address: Department of Environmental Physics and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan. Tel.: C81 (45) 924-5555; Fax: C81 (45) 924-5554; E-mail: [email protected] 1 Present address: Faculty of Intercultural Studies, Gakushuin Women's College, 3-20-1 Toyama, Shinjuku-ku, Tokyo 162-8650, Japan.

Fig. 1. Map showing the locations of sampling stations (circle D November, 1986 and square D 29 May, 1996), bottom topography of the Suruga Trough, and the epicentre of a magnitude 4.7 earthquake on 27 May, 1996 (star). Sectional contour map of methane concentration in Fig. 3 is constructed along the central axis of the trough (line A±B). and 3He enrichment, are well known indicators for methane (maximum concentration: ca. 200 nmol=kg) sea-¯oor hydrothermal activity, because hydrother- on the southern slope of Funka Bay, Japan, at a depth mal ¯uids are highly enriched in these compounds of about 50 m. relative to background sea water (e.g. [1]). However, In order to study the geochemical cycle of in spite of an observed high methane concentration methane in the ocean, we must clarify the geo- of more than 15 nmol=kg, this plume did not show chemical, geological, geophysical, and hydrologi- any of the other hydrothermal signatures. The plume, cal characteristics of sources of such methane-rich therefore, may be caused by some cold venting ¯uids plumes. Thus on 29 May, 1996, we revisited the Su- other than hydrothermal ¯uids, a possibility indi- ruga Trough during the KT96-8 Expedition of R=V cated also by later discoveries of numerous methane- Tansei Maru (University of Tokyo) in order to bet- rich cold ¯uid seepage sites at the adjacent Nankai ter understand the origins of methane in the plume Trough (e.g. [2]). observed in 1986. Observations of such anomalous, probably non- hydrothermal plumes in sea water have also been 1.2. Stable carbon isotopic composition of methane reported elsewhere. At a depth of several hundred metres in the Northwest Caribbean Sea, Brooks [3] The stable carbon isotopic composition of found methane-rich plumes (at a maximum concen- methane offers useful information for discerning the tration of more than 100 nmol=kg), possibly supplied origin of methane. The methane emitted by sea- from the Jamaica Ridge. Horibe et al. [4] have found ¯oor hydrothermal systems in sediment-poor envi- a methane-rich plume (at a maximum concentra- ronments is extremely enriched in 13C with a carbon 13 tion of ca. 3 nmol=kg) at a depth of approximately isotopic ratio (Ž CCH4) ranging from 25 to 8 3000 m in an open-ocean water column in the Mari- ½PDB, indicating an inorganic origin [7,8]. On the ana Trough, western Paci®c, of unknown origin [5]. other hand, organic-rich marine sediments produce Watanabe et al. [6] have found a large source of extremely 13C-depleted methane with a Ž13Cof50 U. Tsunogai et al. / Earth and Planetary Science Letters 160 (1998) 97±105 99

residual methane [17]) is suf®ciently slow relative to the diffusion of methane in sea water, we can study the origin of methane in methane-rich plumes by 13 Ž CCH4 measurements. Except for some recent works [18±21], however, previous studies of the marine geochemistry of methane have lacked carbon isotopic data because they have dealt with oxygenated sea water which has 13 insuf®cient CH4 for traditional Ž CCH4 determination. New isotope-ratio-monitoring gas chromatog- raphy=mass spectrometry (irm±GC=MS) systems re- quire much smaller samples for isotopic analyses (e.g. [22]). We have developed an analytical system which is capable of on-line simultaneous analyses of concentration and Ž13C values of low-level methane in sea water using irm±GC=MS, similar to the method developed by Popp et al. [19].

2. Experiment

Water was sampled on 29 May 1996 at stations 10, 11, 18, 19, and 20 (Fig. 1), which are located al- Fig. 2. Vertical CH4 pro®les at station 18 in the Suruga Trough most on the axis (deepest line) of the Suruga Trough. measured in 1986 (circle) and 1996 (square). The samples were collected with a 12-port Rosette multisampler (10-l Niskin bottles) attached to a Neil to 110½ PDB, as a result of microbial production Brown Mark III CTD system with a sonar pinger. Se- in the anoxic sedimentary layer (e.g. [9]). Methane- rial hydrographic measurements were made with the 13 rich cold seep ¯uids show similar Ž CCH4 values CTD system. The sampling was focused on depths as marine sediments, which are the main source of below 1000 m, because the methane plume was pre- methane in the ¯uids [10±13]. Sediment-rich hy- viously observed at depths below 1500 m (Fig. 2). drothermal systems supply methane with intermedi- Discrete sample analyses of salinity and dissolved ate Ž13C values, ranging from 30 to 50½ PDB oxygen were made using a salinometer (Autolab [7,14], due to thermogenic degradation of organic Industries) and the standard Winkler method. matter in the sediment. Methane in deeply buried ma- For CH4 content and isotopic analysis, a water rine sediments sometimes show similar Ž13Cvalues sample was slowly transferred into a ca. 530-ml as sediment-rich hydrothermal ¯uids for two possi- glass vial, which contained a Te¯on coated magnetic ble reasons: (1) thermal degradation of organic mat- stirrer of 5 cm length. After an approximately 2-fold ter under the in¯uence of elevated temperature (e.g. volume over¯ow to prevent air contamination, 3 [11,15]); (2) microbial production using extremely ml of saturated HgCl2 solution (6%wt) was slowly 13 C-enriched CO2 [16]. In the Nankai Trough, the added as a preservative. The vial was then sealed southern extension of the Suruga Trough, one may with a grey butyl rubber stopper and stored in the ®nd such 13C-enriched thermogenic methane in the dark at room temperature until analysis. In order to sedimentary layer about 1000 m below the sea ¯oor prevent seal breakage due to increases in volume [15]. If we can assume that some secondary alter- caused by temperature increases after sealing, about 13 nation of Ž CCH4 in the seawater column (i.e. bio- 1 ml of sample in each bottle was discarded without logical consumption in oxygenated seawater which contact with air, using a needle syringe through the 13 may result in enhancement of Ž CCH4 values of rubber stopper within an hour after sealing. 100 U. Tsunogai et al. / Earth and Planetary Science Letters 160 (1998) 97±105

The analytical system consisted sequentially of Table 1 a He-sparging bottle of water (80 ml=min carrier Observed concentration (CH4 in nmol=kg) and carbon isotopic Ž13 gas ¯ow), a CO2-trapping port with alkaline so- composition ( C in ½ PDB) of methane together with depth (D), water temperature (T), salinity (S) and oxygen concentration lution, a gas dryer (Na®on tubing), a liquid N2 (O ) temperature trap (150-mm-long column packed with 2 13 Porapak-Q), and a liquid N2 temperature cryofocus- D (m) T (ëC) S (PSU) O2 (ml=l) CH4 Ž C ing (5-mm-long column packed Porapak-Q) [13,19]. Station 10 (34ë00.200N, 138ë29.320E) The concentrated methane portion was processed by 0 ± ± ± 2.30 52 š 2 GC separation using a PoraPLOT-Q analytical capil- 50 15.49 34.58 5.03 3.15 45 š 2 lary column at a temperature of 20ëC. The methane 99 12.49 34.52 4.62 3.85 49 š 2 497 6.83 34.23 2.51 2.73 34 š 2 was then quantitatively converted to CO2 by passing = 992 3.44 34.42 1.52 1.12 36 š 3 it through a 850ëC combustion (CuO Pt catalyzer). 1486 2.59 34.52 1.93 1.20 48 š 3 The carbon content and carbon isotope ratios were 1980 2.04 34.60 2.51 4.73 48 š 2 measured by a Finnigan MAT Delta S isotope-ratio- 2275 1.74 34.64 2.94 1.23 43 š 3 monitoring mass spectrometer. Each sample analysis 2472 1.64 34.65 3.14 1.67 46 š 3 took about 60 min. 2669 1.57 34.66 3.28 1.51 58 š 3 2815 1.56 34.66 3.29 2.20 47 š 2 Methane content in the sample was calculated by 2913 1.53 34.66 3.37 1.30 53 š 3 44 comparing the CO2 output with that of a working 2978 1.53 34.66 3.37 1.90 66 š 3 standard gas, containing 100 ppm methane in nitrogen Station 11 (34ë19.970N 138ë33.130E) that had been calibrated by Takachiho Trading Co. 10 18.75 34.50 5.70 3.77 65 š 2 Ltd. The precision of the concentration determination 992 3.36 34.43 1.54 1.46 49 š 3 (1 sigma value for ten determinations) was estimated 1486 2.58 34.53 1.95 0.98 48 š 4 to be 6.5% at a concentration level of 2 nmol=kg. 1782 2.25 34.58 2.30 1.86 45 š 3 1980 2.07 34.60 2.51 4.14 48 š 2 The accuracy and precision of this isotope analy- 2176 1.86 ± 2.82 11.24 59 š 1 sis were examined by using a gas, which had been 2374 1.69 34.65 3.07 4.38 53 š 2 standardized by conventional method [8,23], and by 2473 1.59 34.66 3.19 2.17 52 š 2 analyzing it in the mass spectrometer in a similar 2570 1.57 34.67 3.24 1.65 44 š 3 way as with the solution sample. The detection lim- 2629 1.57 34.66 3.30 1.94 50 š 3 2678 1.56 34.67 3.30 1.51 52 š 3 its were 5 nmol and 0.5 nmol for isotope ratio standard deviations of 1½ and 4½, which corresponds Station 18 (34ë39.970N 138ë35.720E) to about 10 and 1 nmol=kg of sea water sample, 10 18.88 34.52 5.69 3.89 57 š 2 992 3.30 34.44 1.65 10.02 38 š 1 respectively, for a 530-ml sample. Systematic bias 1239 2.84 34.50 1.88 2.21 37 š 2 in the measured isotope ratio depending on sample 1486 2.51 34.54 2.13 1.35 35 š 3 size was negligible for the sample sizes between 0.5 1685 2.30 34.57 2.23 1.44 44 š 3 and 100 nmol. Analytical blanks associated with the 1782 2.18 34.59 2.35 1.98 48 š 2 method are also negligible, being less than 10 pmol. 1881 2.10 34.59 2.49 4.94 55 š 1 1979 1.97 34.60 2.65 8.34 55 š 1 2078 1.86 34.61 2.78 12.85 64 š 1 2169 1.79 34.63 2.91 12.67 54 š 1 3. Results and discussion Station 19 (34ë49.820N 138ë37.910E) 10 18.22 34.50 5.40 3.25 47 š 2 3.1. Distribution of methane plumes in the Suruga 992 3.46 34.43 1.68 4.88 38 š 1 Trough: temporal variability 1091 3.22 34.46 1.75 3.92 36 š 2 1190 3.01 34.48 1.82 2.10 36 š 2 Ž13 1289 2.83 34.50 1.93 2.02 35 š 3 The vertical CH4 pro®les of contents and C 1348 2.72 34.51 1.99 2.00 35 š 3 were determined at all ®ve stations (Table 1). Both 1408 2.67 34.52 2.00 1.95 37 š 3 13 the content and Ž CCH4 showed wide variation, 1457 2.60 34.53 2.01 1.83 38 š 3 ranging from 1 to 13 nmol=kg and from 16 š 3to 1502 2.51 34.54 2.12 1.76 38 š 3 66 š 3½ PDB, respectively. 1541 2.46 34.54 2.13 2.12 41 š 2 U. Tsunogai et al. / Earth and Planetary Science Letters 160 (1998) 97±105 101

Table 1 (continued) rocal of CH4 concentration) for the samples within the plumes. The shallower plume shows systemati- 13 D (m) T (ëC) S (PSU) O2 (ml=l) CH4 Ž C 13 cally higher Ž CCH4 values than the deeper plume, Station 20 (34ë54.990N 138ë39.100E) suggesting different methane origins. The observa- 11 18.93 34.45 5.79 3.22 48 š 2 tion that all the data points are located approximately š 50 16.25 34.58 5.25 4.42 46 2 along the respective straight lines in Fig. 4 indicates 199 11.47 34.45 3.95 3.13 ± 497 5.91 34.26 2.19 2.35 38 š 2 that a simple mixing process between sea water and 992 3.47 34.43 1.71 3.12 34 š 2 other end-members has occurred. 13 1091 3.23 34.46 1.81 2.27 26 š 2 The study of Ž CCH4 distribution within a CH4 1190 3.01 34.49 1.90 1.62 16 š 3 plume, in which the methane has been continuously š 1290 2.86 34.50 1.94 1.71 21 3 emitted from the same source into oxygenated sea 1388 2.75 34.51 2.01 2.24 30 š 2 1467 2.59 34.53 2.06 2.10 38 š 2 water, is useful in evaluating the biological consump- 1516 2.54 34.54 2.13 1.98 35 š 3 tion effect within the oxygenated sea water column, 12 1548 2.53 34.54 2.08 1.98 39 š 3 because CH4 is preferentially utilized by methane- ± D not measured. oxidizing bacteria [17]. If signi®cant biological consumption occurs in a plume, then one may expect 12 13 various levels of CH4 depletion (Ž CCH4 enhance- A sectional contour map of the methane concen- ment), in proportion to the age within sea water, 13 tration along the central axis of the Suruga Trough resulting in heterogeneous Ž CCH4 value distribu- (line A±B on Fig. 1) is presented in Fig. 3. Two tions of emitted CH4 within the plume. 13 maxima can be seen at depths of ca. 1000 m and In our case, 1=CH4 and Ž CCH4 are linearly cor- ca. 2100 m, both centred at station 18. Outside of related, irrespective of sampling stations and depths. these areas, the methane values range from 1 to 4 This result certi®es our ®rst assumption that the nmol=kg, comparable to levels ordinarily observed biological consumption rate in the oxygenated sea in hemipelagic oxygenated sea waters [24]. water here is suf®ciently slow relative to the diffu- Comparing the methane pro®le of May 1996 at sion velocity of the methane-rich plume. If biological 13 station 18 with that of November 1986 (Fig. 2), the consumption had occurred, then the Ž CCH4 value deeper plume is commonly recognized with a similar would have increased with decreasing ln(CH4) [17]. maximum methane concentration of 15š2nmol=kg. The ratio is determined by a kinetic isotopic fraction- This suggests that the deeper plume in the Suruga ation factor. If we assume that the kinetic isotopic Trough has been maintained steadily for at least 10 fractionation factor of the biological methane con- years. In contrast, there is a remarkable difference sumption in the area is more than 1.005 (e.g. [17]), between the 1986 and 1996 methane data with regard it is unlikely that there is signi®cant methane con- to the shallower plume: there is no indication of sumption within the plumes. The linear correlation this shallower plume in the 1986 pro®le. Since it is a result of simple mixing, and the original Ž13C is dif®cult to assume some in-situ methanogenic composition of methane, which is derived from the activities within a sea water column of 1000 m end-member plume ¯uid, is preserved from the sec- depth (e.g. [25]), this episodic and large (ca. 50 km) ondary alternation during diffusion. Thus, we may 13 shallower plume may be a kind of `megaplume', estimate the end-member Ž CCH4 values from the as has been observed in hydrothermal plumes [26] y-axis intersects (1=CH4 D 0 values) of the linear derived from some sudden, catastrophic event on the regression ®ts; they are 59š3and38š2½ PDB sea ¯oor. This possibility is discussed in Section 3.3. for the deeper and shallower plumes, respectively.

13 3.2. End-member Ž CCH4 of the plumes 3.3. Sources of the methane-rich plumes

13 The two plumes were not only at different depths The Ž CCH4 value of 59 š 3½ PDB in the 13 13 but also distinguishable in their Ž CCH4 values. deeper plume indicates that the C-depleted end- 13 Fig. 4 shows Ž CCH4 plotted against 1=CH4 (recip- member may be produced by microbial activity in 102 U. Tsunogai et al. / Earth and Planetary Science Letters 160 (1998) 97±105

Fig. 3. Sectional contour map of methane concentration along the central axis of the Suruga Trough (line A±B in Fig. 1). The horizontal axis shows the relative distance from the northern coast. The dashed contour lines for depths less than 1000 m are partly speculative for lack of suf®cient data. a shallow (surface) sedimentary layer. As mentioned low altitudes. The low vertical velocity of the ¯uid in Section 1.2, microbial methane is a common ori- venting from the sea ¯oor, together with little density gin of methane in cold seep ¯uids. Many active cold anomaly of the seeping ¯uid, would result in the low, seep sites associated with methane-rich ¯uids have almost zero altitude of the plume. been found in the Nankai Trough (southern extension The 13C-enriched end-member methane with 13 of the Suruga Trough) and Sagami Trough (eastern Ž CCH4 D38š2½ PDB in the shallower plume is extension of the Suruga Trough) [2,12,27]. Judg- within the range of thermogenic CH4 values. There ing from the microbial signature of the dissolved are two possible processes to produce a thermogenic methane and the continuous emission of the plume, methane plume in sea waters: (1) the plume is pro- some unknown methane-rich cold seep vent located duced by elevated temperatures in a deep (more than on the sea ¯oor close to station 18 seems to be the 1000 m) sedimentary layer below the sea ¯oor and source of the deeper plume. methane is subsequently brought up to the sea ¯oor The low altitude of the deeper plume from the sea by an upward pore water migration caused either ¯oor also suggests calm and gentle cold seep activity by density (salinity, temperature) difference, or some at station 18. In contrast with hydrothermal plumes catastrophic event (earthquake, landslide, etc.); (2) in the Paci®c Ocean which are usually distributed the plume is produced in a shallow sedimentary layer several hundred metres higher than their vents [28], due to elevated temperatures caused by magmatic ac- the plumes over the cold seep zones of the Sagami tivity, and the methane is subsequently moved up to Trough [29] and Nankai Trough [27] commonly have the sea ¯oor by an upward hydrothermal ¯uid mi- U. Tsunogai et al. / Earth and Planetary Science Letters 160 (1998) 97±105 103

in groundwater discharge (especially in the number of self-spouting springs), accompanied by ¯uctuations in groundwater chemistry, at the epicentral region of the Kobe earthquake in January, 1995, in the Kansai district, Japan [32,34]. The new self- spouting springs emitted water vigorously for several days after the earthquake [34]. The observed increases in groundwater discharge and ¯uctuations in groundwater chemistries have been attributed to an increased ¯ow of groundwater from deeper regions [34]. Similar changes (i.e. new vents on the sea ¯oor and emission of ¯uids from deeper regions) could be expected after the earthquake of magnitude 4.7 . In the case of the Borah Peak earthquake in Oc- tober, 1983 (western United States; Ms D 7:0/,a total of 3 ð 108 m3 excess water discharge was observed at the epicentral region [33]. Compared with 13 3 3= Fig. 4. Ž CCH4 vs. 1=(CH4 concentration) for water samples the ¯uid ¯ux of less than 10 m day [12] from collected from the shallower plume (open marks) and the deeper a cold seep vent of the Sagami Trough, which re- plume (solid marks). Sampling depths for the shallower plume sulted in the large CH4 plume in a sea water column are from 900 to 1500 m at station 18 (open square), from 900 to (about 400 m thickness) [29], an earthquake-induced 1100 m at station 19 (open circle), and from 900 to 1000 m at station 20 (open triangle). Those for the deeper plume are from crustal ¯uid discharge may be suf®cient to account 1500 m to bottom (2169 m) at station 18 (solid square), from for the CH4-rich ¯uids in the shallower plume (a few 1700 to 2600 m at station 11 (solid circle), and from 1900 to hundred metres thick). 2100 m at station 10 (solid triangle). Although more studies (such as investigations using a submersible) are needed to ascertain the source of the CH4-rich plumes, this study shows 13 gration. In any case, the episodic shallower plume that Ž CCH4 data may offer useful and sensitive cannot occur under normal conditions of the ocean information. Continuous and extensive surveys of 13 and sea ¯oor; it must, therefore, be a result of some CH4 distribution, including Ž CCH4 determination, sudden events on the sea ¯oor. The high altitude of in coastal and hemipelagic sea water may reveal the the shallower plume also suggests the occurrence of sources of CH4-rich plumes in the ocean and the an unusually large event for plume generation. behaviour of ¯uids migrating below the sea ¯oor. 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