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Detection of low-chloride fluids beneath a cold seep field on the Nankai accretionary wedge off Kumano, south of Japan
Earth and Planetary Science Letters 228 (2004) 37 – 47 www.elsevier.com/locate/epsl
Detection of low-chloride fluids beneath a cold seep field on the Nankai accretionary wedge off Kumano, south of Japan T. Tokia,b,*, U. Tsunogaia, T. Gamoa,b, S. Kuramotoc, J. Ashid a
Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan b Marine Inorganic Chemistry Group, Department of Chemical Oceanography, Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164-8639, Japan c Center for Deep Earth Exploration, Japan Marine Science and Technology Center, 2-15, Natsushima-cho, Yokosuka 237-0061, Japan d Department of Ocean Floor GeoScience, Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164-8639, Japan Received 4 November 2003; received in revised form 28 June 2004; accepted 3 September 2004 Available online 26 October 2004 Editor: E. Boyle
Abstract Chemical and isotopic characteristics were determined for interstitial waters extracted from surface sediments in and around dense biological communities on the seafloor of the Nankai accretionary prism off Kumano, south of Japan. We found the following unique features when compared with usual interstitial water samples of normal seafloor in those of samples from bacterial mats on the Oomine Ridge, one of the outer ridge in the Nankai accretionary prism: (1) significant depletion of chloride concentration (maximum 10% depletion from bottom seawater), (2) high concentrations of CH4 and ACO2 (more than 660 Amol/kg and 60 mmol/kg, respectively), (3) sulfate depletion (more than 90% depletion compared to bottom seawater), and (4) dDH2O and d 18OH2O depletion [more than 4x and 0.7x depletion, respectively, compared to standard mean ocean water (SMOW)]. The highest CH4 value among these samples was comparable to the highest value so far reported at one of the most active seep areas in the Nankai Trough, suggesting that these sites should also be regarded as one of the most active seep sites in the Nankai Trough. The chemical compositions of the samples taken from the Oomine Ridge strongly suggest that the fluid originates not from normal sediment–seawater interaction at the sediment surface of hemipelagic environments, but from active seepage of fluids that are rich in CH4 and ACO2, depleted in Cl and SO42, and low in dDH2O and d 18OH2O compared to normal seawater. Values for the carbon isotopic composition (d 13CCH4) of the dissolved methane in the interstitial fluid [less than 70x PeeDee Belemnite (PDB)] and for the C2H6/CH4 ratio (less than 103) suggest that the methane originates from microbial production in a relatively shallow layer of sediment, not from the deep sedimentary layer of higher temperature than 60 8C at the depth of more than 300 m below the seafloor. The Cl=0 mmol/kg extrapolated end-member dDH2O and d 18OH2O values of low-chloride fluids were 46F7x and 6.3F0.7x SMOW, respectively, suggesting that land-derived groundwater could be one of the possible sources for the low-Cl fluids. Depth profiles of chloride concentrations of
* Corresponding author. Marine Inorganic Chemistry Group, Department of Chemical Oceanography, Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164-8639, Japan. Tel.: +81 3 5351 6453; fax: +81 3 5351 6452. E-mail address: [email protected] (T. Toki). 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.09.007
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interstitial fluids show the heterogeneity of end members and upward fluid flow velocities suggest that active fluid seepage on the Oomine Ridge seems to be a localized phenomenon. Assuming steady-state emission of fluid from the cold seep vent, upward fluid flow velocities from the seeping vent are estimated to be 40–200 cm year1, comparable to the previously reported values within the bacterial mats in the Nankai Trough accretionary wedge. Development of bacterial mat might favor slower advection, which might allow longer time for diagenetic reactions in the vent conduits, and consequently, carry more reductive compounds in the fluids. D 2004 Elsevier B.V. All rights reserved. Keywords: Nankai accretionary prism; microbial methane; groundwater; localized seepage; fluid flow velocity
1. Introduction Since benthic communities based on chemosynthesis were first discovered at the Galapagos Spreading Center in 1977, chemosynthetic communities have been reported from hydrothermal vent fields along mid-ocean ridges around the world (e.g., Ref. [1]). They are dependent on the primary production by chemoautosynthetic bacteria that oxidize chemically reduced components, such as H2S and CH4. Similar chemosynthetic communities, free of the direct influences of heat and materials from hot magmas, have been found along passive margins (e.g., the Gulf of Mexico [2]) and along active continental margins (e.g., Oregon subduction zone [3]). These observations have demonstrated discharge of low-temperature fluids (cold seeps), containing H2S and/or CH4, and upward migration of fluids from deeper zones of the margins [4]. Chemical and isotopic compositions of the expelled fluids at the seafloor are controlled by two sets of processes: deep reservoir conditions and secondary processes during ascent. Chemically reactive constituents should record physicochemical changes prior to seepage from the seafloor, whereas chemically inert species could provide information about their sources. Circulation of interstitial fluids within accretionary wedges has been identified as an important process controlling the earthquake stress cycle [5]. It is expected that the chemical and isotopic compositions of dissolved constituents in the seeping fluids should contain imprints of their subsurface history and thus provide useful information on the structure of accretionary wedges, especially the flow paths of the fluids. We are going to provide answers to the following questions: Where do the fluids come
from? How are the fields of seepage distributed on the seafloor? In Nankai Trough, numerous chemosynthetic communities have been found on the accretionary prism, indicating the existence of cold seeps and possible fluid migration from deep zones of the accretionary prism [6]. We collected interstitial fluids from 27 push cores (up to 30 cm long) both inside and outside of dense biological communities on the seafloor of the Nankai accretionary prism off Kumano and analyzed them for CH 4 , ACO 2 , d 13 C CH4 , d 13CCO2, chloride, sulfate, dDH2O, and d 18OH2O. The purpose of the present study is to elucidate the origin of cold seep fluids, as well as quantify fluid velocities from seepage vents on the seafloor of the Nankai accretionary wedge. On this basis, we discuss the distribution of seepage at the seafloor.
2. Geological setting The Nankai Trough is an active plate margin, where the Philippine Sea plate subducts beneath the Eurasian plate (Fig. 1). The Nankai accretionary prism forms on the landward slope of the Nankai Trough margin, and is associated with great earthquakes along the subduction zone. In order to reveal the mechanism of the earthquakes, dive surveys and dense topographic surveys have been conducted from geological, geophysical, geochemical, and biological points of view along the Nankai Trough margin [7–9]. Multichannel seismic reflection surveys have clearly imaged splay faults branching upward from the plate–boundary interface in the area. The faults break through the accretionary prism to reach the seafloor just seaward of the outer ridge, such as the
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Fig. 1. (a) A topographic map of the Nankai Trough off Kumano area, together with the locations of Site 808 (star), and the Ryuyo Canyon (triangle). Contour interval is 100 m. The box represents the outline of Fig. 1b. (b) A bathymetry map with the specific positions of the Oomine Ridge site and the Oodai Ridge site (circle) and the reference sites (dots). Contour interval is 50 m. The box represents the outline of Fig. 1c. (c) A topographic map of the Oomine Ridge together with sampling sites of cores, which show two groupings: the dots symbols denote samples characterized by CH4 enrichment and Cl depletion and the triangle ones denote samples characterized by slight CH4 enrichment and little Cl depletion, in comparison with normal seawater. Contour interval is 50 m. The shaded area represents the area where we observed many bacterial mats scattered at intervals of several hundred meters at water depths ranging from 2500 to 2550 m.
Oomine Ridge and the Oodai Ridge as shown in Fig. 1 [7]. On the Oomine Ridge in particular, recent submersible exploration has revealed the distribution of chemosynthetic communities (shell fragments, living tube worms, and bacterial mats) around the seafloor scarp of the splay faults [8,10]. The thermal gradient inside of a chemosynthetic community observed in the Oomine Ridge showed anomalously
high heat flow, suggesting the existence of active fluid discharge at this site [9].
3. Sampling Fig. 1 shows the sediment sampling locations. During the cruise YK02-02 (5/30–6/17 2002) and
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YK03-03 (4/7–5/1 2003) of R/V Yokosuka (JAMSTEC), push core samples were collected by the manned submersible bShinkai 6500Q (JAMSTEC). Supplemental Table 1 summarizes brief descriptions of the core samples collected in this study. In a laboratory on board the ship, core samples were treated for the extraction of interstitial fluids, which were distributed to sample bottles within 5 h after their recovery with an effort to avoid evaporation, degassing, and contamination. Details about sampling strategy and handling are given in Tsunogai et al. [11]. In brief, the water over the sediment–water interface of a corer was first taken by siphon, and then the interstitial water was extracted from the subsampled sediment using the procedures and equipment described by Manheim [12]. About 2 cm3 of the retrieved interstitial fluid was immediately transferred to a 3-cm3 serum glass vial, poisoned with HgCl2, and capped with a gray butyl rubber septum as a preservative for subsequent shore-based analysis of the concentration and stable isotopic composition of dissolved CH4, C2H6, and ACO2. The remaining fluids were preserved in polypropylene bottles for the measurement of major anion concentrations and dD and d 18O values of water. All the samples were refrigerated in the dark and kept at 4 8C until analysis.
4. Analysis 4.1. Concentration and carbon isotopic composition of hydrocarbons and RCO2 The concentrations and the carbon isotopic analyses of hydrocarbons (CH4 and C2H6) in interstitial fluid samples were performed using isotope-ratio-monitoring-gas chromatography/mass spectrometry system [11]. A portion of the headspace gas of each vial was transferred into a gas loop using a gas-tight needle syringe and injected into a gas chromatograph (Hewlett-Packard 6890 equipped with a 25-m-long 32 Am ID PoraPLOT-Q capillary column). Separated hydrocarbon compounds were quantitatively oxidized to CO2 using an oxidizing furnace equipped with a copper oxide wire and platinum wire as a catalyst (T=960 8C). Each CO2 gas was transferred by carrier gas via a splitting system into the ion source of a high-precision mass
spectrometer (FINNIGAN MAT 252). A working standard gas containing ca. 875 ppm CH4 and ca. 44 ppm C2H6 in nitrogen, made from the NIST RM 8560 (IAEA NGS2) standard, was used for calculation of the hydrocarbon contents. Analytical precision of the concentration determination (1 r value for 10 determinations) was estimated to be 6.5% for CH4 and C2H6. The detection limit of the isotope analysis was 200 pmol with isotope ratio standard deviation of 0.3x. The analytical blank being less than 1 pmol was negligibly small. Isotope ratios (13C/12C) are reported in the common dnotation relative to the PeeDee Belemnite (PDB) standard: d 13CCH4={((13CCH4/12CCH4)sample/(13CCH4/12CCH4)PDB) 1}1000. The concentrations of ACO2 in interstitial fluid samples were measured by the above-mentioned isotope-ratio-monitoring GC/MS system in a similar way as that described by Miyajima et al. [13]. The precision of the concentration determination was estimated to be 7%. 4.2. Major components, dD and d 18O of water The Cl and SO42 concentrations in the interstitial fluid samples were measured with an ion chromatograph [14]. Analytical precision was estimated to be 0.4% for these components by five repeated measurements of the same sample, and accuracy was estimated to be 3% from the measurements of IAPSO standard ocean water. For measurement of hydrogen isotopic composition, the interstitial water was reduced to hydrogen using zinc [15]. The obtained hydrogen was introduced into dual inlet mass spectrometer (Finnigan MAT 252). Analytical precision was estimated to be within 1x. Results are reported in the usual delta notation relative to standard mean ocean water (SMOW): dD={(D/H)sample/(D/H)SMOW1}1000x. The oxygen isotopic compositions of water were analyzed repeatedly three times with a MAT252 continuous flow isotope ratio mass spectrometer
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following the technique described by Ijiri et al. [16]. The interstitial water was introduced into a glass vial together with solid NaHCO3 for H2O–CO2 equilibration. The oxygen isotopic composition of CO2 in the gas phase in the vial was analyzed by using the abovementioned isotope-ratio-monitoring GC/MS system. Analytical precision was estimated to be within 0.1x. Results are given relative to SMOW: d 18O={(18O/16O)sample/(18O/16O)SMOW1}1000x.
5. Results and discussion 5.1. Chemical and isotopic characteristics of the seeping fluids Analytical results of chemical and isotopic compositions are shown in supplemental Table 1. Fig. 2 shows vertical profiles of chemical and isotopic compositions of interstitial waters in the 27 sediment cores as well as of the overlying bottom waters. Among the 27 cores sampled in this study, a remarkable feature in the vertical chemical distribution was observed for the following samples: D674C4, C5, D744C1, C3, D752C2, D754C1, and D756C1, all retrieved from the Oomine Ridge. They were characterized by CH4 and ACO2 enrichment and Cl and SO42 depletion compared with the other samples. The CH4 value of 667 Amol/kg observed for the D674C5 samples (supplemental Table 1) is the highest so far reported in the Nankai Trough, suggesting that these sites on the Oomine Ridge should be regarded as one of the most active seep sites in the Nankai Trough. It is noteworthy that the minimum chloride concentration was as low as 490 mmol/kg (ca. 10% depletion compared to bottom seawater). Such anomalies in the chemical composition of the samples from the Oomine Ridge strongly suggest that the fluid originates not from normal sediment– seawater interaction at the sediment surface of hemipelagic environments, but from active seepage of fluids that are enriched in CH4 and ACO2 and depleted in Cl and SO42 relative to normal seawater. It is known that isotopic compositions of methane and C2H6/CH4 (C2/C1) ratios are useful in distinguishing between microbially produced methane and thermogenically produced methane [17,18]. The Oomine Ridge fluids are characterized by 12C-
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enriched CH4 (d 13CCH4b70x PDB) and a depleted C2/C1 ratio (less than 103) as shown in supplemental Table 1. It can, therefore, be deduced that most of the hydrocarbon gases at these sites on the Oomine Ridge are generated via microbial methanogenesis, not via thermal degradation of organic matter (e.g., Ref. [17]). The thermal decomposition of organic material occurs at temperatures more than 60 8C [19] and the thermal gradient in the Oomine Ridge has been estimated to be about 60–70 8C/km as background and up to about 200 8C/km within the bacterial mat [9]. The methane seemed to be added from the shallow sedimentary layer during the ascent of cold seep fluids, not from the deep sedimentary layer of higher temperature than 60 8C at the depth of more than 300 m below the seafloor (mbsf). 5.2. Origin of low-Cl fluids Previous studies of the Nankai Trough have widely noted the Cl depleted interstitial fluids present there [11,20–23]. At Site 808 during Leg 131 of the Ocean Drilling Program (ODP) in the Nankai accretionary wedge off Muroto (Fig. 1a), low-Cl fluids were collected at the depths from 620 to 1150 mbsf and were regarded to be the result of clay mineral dehydration [22,23]. On the other hand, Tsunogai et al. [11] explained the low-Cl fluids seeping onto the seafloor of the biological community area at the Ryuyo Canyon in the Nankai Trough, 30 km off the coast of Japan Islands (Fig. 1a), as the result of incorporation of land-derived groundwater. In view of recent findings on methane-rich seeping fluids on the seafloor in several convergent environments, gas hydrate dissociation and upward fluid migration are proposed as important sources for methane-rich cold seep fluids (e.g., Refs. [4,24–28]. In the Nankai Trough, gas hydrates are widespread as shown by the findings of bottom-simulating reflectors (BSRs) delineating the base of the gas hydrate stability zone [29]. The destabilization of gas hydrate is a highly probable source for the interstitial fluids at the Nankai Trough [30]. Dissociation of gas hydrate is known to produce a low-Cl fluid whose dDH2O and d 18OH2O should be higher than those of seawater because gas hydrates not only exclude Cl from their lattice structure but also preferentially incorporate the heavier isotopes of O and H in their cage water. The
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Fig. 2. Vertical profiles of (a) chloride, (b) sulfate, (c) methane concentration, (d) d 13CCH4, (e) dDH2O, and (f) d 18OH2O of interstitial fluid samples. Each symbol denotes the classification of sampling location: the Oomine Ridge site (solid diamond); open diamond; solid circle; solid triangle; open square; cross; open triangle; solid square; open circle), the Oodai Ridge site (plus: D676C1, C2, C3, C6, D746C1, C2, and D747C1), and the reference sites (dot: D675C1, C3, D680C3, D683C3, C4, D684C1, C2, C3, D685C1, C4, and D753C1). The exponential curves, a dashed line for D674C4 and a dotted line for D674C5, respectively, represent the best exponential fit to the dissolved chloride data, together with (a).
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values for dissociated hydrate samples corrected for any impurities from the surrounding interstitial water have been reported to be +22x for dDH2O and +2.4x for d 18OH2O [31]. Stable isotopic signatures of water in samples (dDH2O and d 18OH2O) provide excellent tracers for the origin of low-Cl fluids in seafloor discharges [11,32]. The dDH2O and d 18OH2O data for the interstitial fluids from the Oomine Ridge are presented in supplemental Table 1 and Fig. 2. The data are also plotted as CldDH2O and Cld 18OH2O diagrams in Fig. 3a and b, respectively. As shown in Figs. 2, 3a and b, the fluids exhibit D- and 18O-depletion in association with the Cl depletion. The interstitial fluids from the Oomine Ridge show the depletion of up to 4x in dDH2O, and up to 0.7x in d 18OH2O in comparison with ambient bottom seawater. These data strongly suggest that the fluids from the Oomine Ridge have little relation to gas hydrate dissolution. If we assume a simple two-component mixing of ambient seawater with fresh water of Cl=0 mmol/ kg, the estimated end members of dDH2O and d 18OH2O are 46F7x and 6.3F0.7x SMOW, respectively, as shown in Fig. 3. The estimated dDH2O and d 18OH2O values for hypothetical fresh water fall into the reported range of dDH2O from 30x to 50x SMOW and d 18OH2O from 4x to 8x SMOW of the groundwater at nearby land areas [33]. Land-derived groundwater could, there-
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fore, be considered as one of the possible sources of the present fresh water discharging from the Oomine Ridge cold seep site. Another possible process that may provide fresh water for the formation of such low-Cl, dDH2O, and d 18OH2O fluids is clay membrane ion filtration reaction. However, the efficiency of ion filtration has been generally understood to be negligible in any natural environment [34], and the fractionation has been reported to be up to 2.5x for dDH2O and 0.8x for d 18OH2O, respectively [35]. So far, it must be difficult to consider clay membrane ion filtration for the process to produce the chemical composition. The Oomine Ridge is located at the depth of ca. 2500 m and a distance of ca. 100 km from the Japan Islands (Fig. 1). Long-distance lateral transport of groundwater from the land area has been mentioned in explaining the extremely low-Cl interstitial fluids in the convergent margin ca. 70 km off the coast of Peru, as observed at ODP site 679 [36,37] and in the Barbados accretionary prism from the Orinoco fan, although the subsurface groundwater migration in convergent margins has not been evaluated in any detail. Recent works on submarine groundwater discharge in near-shore environments and on continental shelves indicate that submarine groundwater discharge can be a common phenomenon for marginal areas, where sufficient hydraulic head and permeability and a tight enough aquiclude permit it [38,39].
Fig. 3. Chloride vs. dDH2O (a) and chloride vs. d 18OH2O (b) plot for the Oomine Ridge samples, together with the regression line, giving each end-member composition of (a) 46F7x for dDH2O and (b) 6.3F0.7x for d 18OH2O (at 0 mmol/kg Cl).
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In this case, however, the Ghyben–Herzberg (freshwater–seawater) boundary forced offshore [40] cannot explain this long transport, but a significant unique way like a pipe through the fluids by siphon from the land to the deep seafloor can. We cannot specify the path of the lateral transport from the land any more, but other sources cannot explain the chemical and isotopic compositions of the interstitial water observed in the Oomine Ridge. Future seafloor drilling studies on the Oomine Ridge will provide direct evidence for this hypothesis and outline the plumbing regime in the Nankai accretionary wedge, which give answers to the following questions: What is the conduit that the fluids flow through and how long the fluids flow through the path from the sources? What mechanism might be for the fluid seepage in detail? 5.3. Calculations of fluid rate in sediments at the Oomine ridge Transport of dissolved species can take place both molecular diffusion and advection of interstitial water. The magnitude of the diffusive flux and the rate of advective flow provide constraints for models that can be estimated from the observed profiles of the conservative aqueous species, in this case, chloride ion concentration. D674C4, C5, D744C3, D752C2, and D754C1 interstitial fluid samples with low chloride concentrations exhibit depth profiles that are convex upward in sediments, as portrayed in Fig. 2a. The shape of Cl concentration profiles for these samples reflects upward migration of interstitial water through the sediment column. If steady-state condition is assumed for approximate calculations of the flow velocity, the concentration of dissolved chloride can be described by a partial differential equation [41]: D d2 C=dx2 þ vðdC=dxÞ ¼ 0 where C is the concentration of dissolved chloride (mmol/kg), x is depth measured positively downward from the sediment–water interface (cm), D is the diffusion coefficient of chloride in sediments (cm2 year1), and v is the vertical flow velocity of interstitial water relative to the sediment–water inter-
face, positive upward (cm year1). Diffusion coefficient for this modeling was taken from Ref. [42], 160 cm2 year1. From the calculations presented in this section, it will be assumed that D and v are constant with depth. In addition, bioturbation may have not been considered in this expression, and instead, no bioturbation at these sites will be assumed, except for the upper 12 cm of the D752C2. It is possible to fit the dissolved chloride data to an exponential curve of the form: C ¼ ðCl C0 Þ 1 eðv=DÞx þ C0 where C ¼ C0 at x ¼ 0 and CYCl as xYl The dissolved chloride data are fit by varying v and C l until the best least-squares fit is obtained, and examples of this curve fitting are shown in Fig. 2a. With this approach, C l is determined as 490, 520, 510, 520, and 520 mmol/kg, respectively, for the D674C4, C5, D744C3, D752C2, and D754C1 samples. These results reflect mixing of fresh water with marine interstitial fluids and/or seawater several times at a depth of greater than 30 cm, a core barrel length. Different vertical flow velocity of interstitial water is obtained: 200, 50, 50, 50, 40 cm year1, respectively, for the D674C4, C5, D744C3, D752C2, and D754C1 samples. The measured interstitial water chloride profile in sediment sequences shows horizontally heterogeneity for a distance of ca. 1000 m (Fig. 1c). Active fluid seepage seems to be a localized phenomenon, which may cause spatial variations in fluid flow. The modeled values for v range from 200 cm year1 at core D674C4 to 40 cm year1 at core D756C1. The positive values for the velocity indicate that the flow direction is upward. This upcoming flow is comparable to the previously reported values (b10 m year1) inside bacterial mats and significantly lower than the values (75–100 m year1) within bivalve beds in the eastern Nankai Trough accretionary wedge, inferred by modeling of subbottom temperature profiles [43]. Slower advection might allow longer time for diagenetic reactions in the vent conduits and carry more reductive compounds in the fluids, and consequently, might be favorable to the development of bacterial mats [43].
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6. Conclusions (1)
(2)
(3)
(4)
(5)
Chemical and isotopic characteristics of interstitial waters from the Oomine Ridge in the Nankai Trough off Kumano indicate that fluids rich in CH4 and ACO2 and low in chloride, sulfate, dDH2O, and d 18OH2O are discharging from the sea bottom to the ocean. Based on the values for carbon isotopic composition of dissolved methane (less than 70x PDB) and the C2H6/CH4 ratio (less than 103), the hydrocarbons in the interstitial fluids from the Oomine Ridge are considered to be of microbial origin at a relatively shallow layer of sediment, not from the deeper sedimentary layer more than 300 mbsf. It is estimated that the hydrogen and oxygen isotopic compositions of the hypothetical fresh water (Cl =0) would be 46F7x and 6.3F0.7x SMOW, respectively, implying that land-derived groundwater could, therefore, be considered a possible source of the lowCl fluid. Distribution of chloride concentration illustrates the enormous variability in the depth profile of interstitial fluids. It is suggested that seawater and/or pore water seems to be added several times during the ascent of the low-Cl fluids. Results from steady-state models applied to the chloride distribution in the interstitial fluids yield the estimated values of fluid flow velocities in the range of 40–200 cm year1, comparable to the previously reported values (b10 m year1), supporting the postulate that bacterial mats may favor slow seepage.
Acknowledgments The Authors are grateful to the officers, crew, and scientist group, as well as the Shinkai 6500 operating team of JAMSTEC aboard R/V Yokosuka during the YK02-02 and YK03-03 cruises for their valuable collaboration. The reviewers E. Boyle, J.M. Gieskes, and J.B. Martin are thanked for valuable information and their constructive comments. This research was supported in part by a 21st Century Center of Excellence (COE) Program on bNeo-Science of
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Natural HistoryQ (Program Leader: Hisatake Okada) at Hokkaido University financed by the Ministry of Education, Culture, Sports, Science and Technology, Japan. Financial support has been obtained from MEXT Special Coordination Fund bArchaean ParkQ project.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.epsl.2004.09.007.
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[10] J. Ashi, S. Kuramoto, S. Morita, U. Tsunogai, S. Goto, S. Kojima, T. Okamoto, T. Ishimura, A. Ijiri, T. Toki, S. Kudo, S. Asai, M. Utsumi, Structure and cold seep of the Nankai accretionary prism off Kumano-Outline of the off Kumano survey during YK01-04 Leg 2 Cruise, JAMSTEC J. Deep Sea Res. 20 (2002) 1 – 8. [11] U. Tsunogai, N. Yoshida, T. Gamo, Carbon isotopic evidence of methane oxidation through sulfate reduction in sediment beneath cold seep vents on the seafloor at Nankai Trough, Marine Geology 187 (1–2) (2002) 145 – 160. [12] F.T. Manheim, Disposable syringe techniques for obtaining small quantities of pore water from unconsolidated sediments, J. Sediment. Petrol. 38 (1968) 666 – 668. [13] T. Miyajima, Y. Yamada, Y.T. Hanba, K. Yoshii, T. Koitabashi, E. Wada, Determining the stable-isotope ratio of total dissolved inorganic carbon in lake water by Gc/C/Irms, Limnology and Oceanography, 40 (5) (1995) 994 – 1000. [14] U. Tsunogai, H. Wakita, Precursory chemical-changes inground water—Kobe earthquake, Japan, Science 269 (5220) (1995) 61 – 63. [15] M.L. Coleman, T.J. Shepherd, J.J. Durham, J.E. Rouse, G.R. Moore, Reduction of water with zinc for hydrogen isotope analysis, Analytical Chemistry 54 (6) (1982) 993 – 995. [16] A. Ijiri, U. Tsunogai, T. Gamo, A simple method for oxygen18 determination of milligram quantities of water using NaHCO3 reagent, Rapid Communications in Mass Spectrometry 17 (13) (2003) 1472 – 1478. [17] M. Schoell, The hydrogen and carbon isotopic composition of methane from natural gases of various origins, Geochimica et Cosmochimica Acta 44 (5) (1980) 649 – 661. [18] M.J. Whiticar, A geochemical perspective of natural-gas and atmospheric methane, Organic Geochemistry 16 (1–3) (1990) 531 – 547. [19] E. Faber, W.J. Stahl, M.J. Whiticar, Distribution of bacterial and thermogenic hydrocarbon gases, in: R. Vially (Ed.), Bacterial Gas, Editions Technip, Paris, 1992, pp. 63 – 74. [20] J. Boulegue, L. Aquilina, A. Mariotti, T. Gamo, H. Sakai, Chemistry of fluids in the KAIKO-NANKAI area (Nankai accretionary prism), in: Proc. Int. Conf. Fluids in Subduction Zones and Related Process, 1990. [21] T. Masuzawa, N. Handa, H. Kitagawa, M. Kusakabe, Sulfate reduction using methane in sediments beneath a bathyal bcold seepQ giant clam community off Hatsushima-Island, Sagami Bay, Japan, Earth and Planetary Science Letters 110 (1-4) (1992) 39 – 50. [22] J.M. Gieskes, T. Gamo, M. Kastner, Major and minor element geochemistry of interstitial waters of site 808, Nankai Trough: an overview, Proceedings of the Ocean Drilling Program. Scientific Results 131 (1993) 387 – 396. [23] M. Kastner, H. Elderfield, W.J. Jenkins, J.M. Gieskes, T. Gamo, Geochemical and isotopic evidence for fluid flow in the western Nankai subduction zone, Japan, Proceedings of the Ocean Drilling Program. Scientific Results 131 (1993) 397 – 413. [24] J.B. Martin, M. Kastner, Chemical and isotopic evidence for sources of fluids in a mud volcano field seaward of the Barbados accretionary wedge, Journal of Geophysical Research 101 (B9) (1996) 20325 – 20345.
[25] P.D. Jenden, J. Gieskes, Cheimal and isotopic composition of interstitial water from Deep Sea Drilling Project Sites 533 and 534, Initial Reports DSDP 76 (1983) 453 – 461. [26] R. Hesse, J. Lebel, J. Gieskes, Interstitial water chemistry of gas-hydrate-bearing sections on the Middle America Trench slope, Deep-Sea Drilling Project Leg 84, in: Initial Reports DSDP 84 (1985) 727 – 737. [27] W.E. Harrison, R. Hesse, J. Gieskes, Relationship between sedimentary facies and interstitial water chemistry of slope, trench, and Cocos plate sites from the Middle America Trench transect, active margin off Guatemala, Deep Sea Drilling Project Leg 67, Initial Reports DSDP 67 (1982) 603 – 614. [28] R. Hesse, S.K. Frape, P.K. Egeberg, R. Matsumoto, Stable isotope studies (Cl, O, and H) of interstitial waters from site 997, Blake Ridge gas hydrate field, West Atlantic, Proceedings of the Ocean Drilling Program Scientific Results 164 (2000) 129 – 137. [29] Y. Aoki, T. Tamano, S. Kato, Detailed structure of the Nankai Trough from migrated seismic sections, American Association of Petroleum Geologists Memoir 34 (1983) 309 – 322. [30] J. Ashi, J. Segawa, X. Le Pichon, S. Lallemant, K. Kobayashi, M. Hattori, S. Mazzotti, K. Aoike, Distribution of cold seepage at the Ryuyo Canyon off Tokai: the 1995 KAIKOTokai dShinkai 2000T dives JAMSTEC J. Deep-Sea Research 12 (1996) 159 – 166. [31] K.A. Kvenvolden, M. Kastner, Gas hydrates of the Peruvian outer continental margin, in: E. Suess, R. von Huene (Eds.), Proc. ODP Sci. Results 112 (1990) 517 – 526. [32] A. Dahlmann, G.J. de Lange, Fluid-sediment interactions at Eastern Mediterranean mud volcanoes: a stable isotope study from ODP Leg 160, Earth and Planetary Science Letters 212 (3-4) (2003) 377 – 391. [33] C. Mizota, M. Kusakabe, Spatial-distribution of dD-dO18 values of surface and shallow groundwaters from Japan, South-Korea and East China, Geochemical Journal 28 (5) (1994) 387 – 410. [34] J.B. Martin, M. Kastner, P.K. Egeberg, Origins of saline fluids at convergent margins, in: B. Taylor, J. Natland (Eds.), Active Margins and Marginal Basins of the Western Pacific Geophysical Monograph, vol. 88, American Geophysical Union, Washington, DC, 1995, pp. 219 – 239. [35] T.B. Coplen, B.B. Hanshaw, Unfiltration by a compacted clay membrane—I. Oxygen and hydrogen isotopic fractionation, Geochimica et Cosmochimica Acta 37 (1973) 2295 – 2310. [36] M. Kastner, H. Elderfield, J.B. Martin, E. Suess, K.A. Kvenvolden, R.E. Garrison, Diagenesis and interstitial water chemistry at the Peruvian continental margin—major constituents and strontium isotopes, Proceedings of the Ocean Drilling Program Scientific Results 112 (1990) 413 – 440. [37] M. Kastner, H. Elderfield, J.B. Martin, Fluids in convergent margins—What do we know about their composition, origin, role in diagenesis and importance for oceanic chemical fluxes? Philosophical Transactions of the Royal Society of London, Series A: Mathematical and Physical Sciences 335 (1638) (1991) 243 – 259. [38] T.M. Church, An underground route for the water cycle, Nature 380 (6575) (1996) 579 – 580.
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Detection of low-chloride fluids beneath a cold seep field on the Nankai accretionary wedge off Kumano, south of Japan T. Tokia,b,*, U. Tsunogaia, T. Gamoa,b, S. Kuramotoc, J. Ashid a
Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan b Marine Inorganic Chemistry Group, Department of Chemical Oceanography, Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164-8639, Japan c Center for Deep Earth Exploration, Japan Marine Science and Technology Center, 2-15, Natsushima-cho, Yokosuka 237-0061, Japan d Department of Ocean Floor GeoScience, Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164-8639, Japan Received 4 November 2003; received in revised form 28 June 2004; accepted 3 September 2004 Available online 26 October 2004 Editor: E. Boyle
Abstract Chemical and isotopic characteristics were determined for interstitial waters extracted from surface sediments in and around dense biological communities on the seafloor of the Nankai accretionary prism off Kumano, south of Japan. We found the following unique features when compared with usual interstitial water samples of normal seafloor in those of samples from bacterial mats on the Oomine Ridge, one of the outer ridge in the Nankai accretionary prism: (1) significant depletion of chloride concentration (maximum 10% depletion from bottom seawater), (2) high concentrations of CH4 and ACO2 (more than 660 Amol/kg and 60 mmol/kg, respectively), (3) sulfate depletion (more than 90% depletion compared to bottom seawater), and (4) dDH2O and d 18OH2O depletion [more than 4x and 0.7x depletion, respectively, compared to standard mean ocean water (SMOW)]. The highest CH4 value among these samples was comparable to the highest value so far reported at one of the most active seep areas in the Nankai Trough, suggesting that these sites should also be regarded as one of the most active seep sites in the Nankai Trough. The chemical compositions of the samples taken from the Oomine Ridge strongly suggest that the fluid originates not from normal sediment–seawater interaction at the sediment surface of hemipelagic environments, but from active seepage of fluids that are rich in CH4 and ACO2, depleted in Cl and SO42, and low in dDH2O and d 18OH2O compared to normal seawater. Values for the carbon isotopic composition (d 13CCH4) of the dissolved methane in the interstitial fluid [less than 70x PeeDee Belemnite (PDB)] and for the C2H6/CH4 ratio (less than 103) suggest that the methane originates from microbial production in a relatively shallow layer of sediment, not from the deep sedimentary layer of higher temperature than 60 8C at the depth of more than 300 m below the seafloor. The Cl=0 mmol/kg extrapolated end-member dDH2O and d 18OH2O values of low-chloride fluids were 46F7x and 6.3F0.7x SMOW, respectively, suggesting that land-derived groundwater could be one of the possible sources for the low-Cl fluids. Depth profiles of chloride concentrations of
* Corresponding author. Marine Inorganic Chemistry Group, Department of Chemical Oceanography, Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164-8639, Japan. Tel.: +81 3 5351 6453; fax: +81 3 5351 6452. E-mail address: [email protected] (T. Toki). 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.09.007
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interstitial fluids show the heterogeneity of end members and upward fluid flow velocities suggest that active fluid seepage on the Oomine Ridge seems to be a localized phenomenon. Assuming steady-state emission of fluid from the cold seep vent, upward fluid flow velocities from the seeping vent are estimated to be 40–200 cm year1, comparable to the previously reported values within the bacterial mats in the Nankai Trough accretionary wedge. Development of bacterial mat might favor slower advection, which might allow longer time for diagenetic reactions in the vent conduits, and consequently, carry more reductive compounds in the fluids. D 2004 Elsevier B.V. All rights reserved. Keywords: Nankai accretionary prism; microbial methane; groundwater; localized seepage; fluid flow velocity
1. Introduction Since benthic communities based on chemosynthesis were first discovered at the Galapagos Spreading Center in 1977, chemosynthetic communities have been reported from hydrothermal vent fields along mid-ocean ridges around the world (e.g., Ref. [1]). They are dependent on the primary production by chemoautosynthetic bacteria that oxidize chemically reduced components, such as H2S and CH4. Similar chemosynthetic communities, free of the direct influences of heat and materials from hot magmas, have been found along passive margins (e.g., the Gulf of Mexico [2]) and along active continental margins (e.g., Oregon subduction zone [3]). These observations have demonstrated discharge of low-temperature fluids (cold seeps), containing H2S and/or CH4, and upward migration of fluids from deeper zones of the margins [4]. Chemical and isotopic compositions of the expelled fluids at the seafloor are controlled by two sets of processes: deep reservoir conditions and secondary processes during ascent. Chemically reactive constituents should record physicochemical changes prior to seepage from the seafloor, whereas chemically inert species could provide information about their sources. Circulation of interstitial fluids within accretionary wedges has been identified as an important process controlling the earthquake stress cycle [5]. It is expected that the chemical and isotopic compositions of dissolved constituents in the seeping fluids should contain imprints of their subsurface history and thus provide useful information on the structure of accretionary wedges, especially the flow paths of the fluids. We are going to provide answers to the following questions: Where do the fluids come
from? How are the fields of seepage distributed on the seafloor? In Nankai Trough, numerous chemosynthetic communities have been found on the accretionary prism, indicating the existence of cold seeps and possible fluid migration from deep zones of the accretionary prism [6]. We collected interstitial fluids from 27 push cores (up to 30 cm long) both inside and outside of dense biological communities on the seafloor of the Nankai accretionary prism off Kumano and analyzed them for CH 4 , ACO 2 , d 13 C CH4 , d 13CCO2, chloride, sulfate, dDH2O, and d 18OH2O. The purpose of the present study is to elucidate the origin of cold seep fluids, as well as quantify fluid velocities from seepage vents on the seafloor of the Nankai accretionary wedge. On this basis, we discuss the distribution of seepage at the seafloor.
2. Geological setting The Nankai Trough is an active plate margin, where the Philippine Sea plate subducts beneath the Eurasian plate (Fig. 1). The Nankai accretionary prism forms on the landward slope of the Nankai Trough margin, and is associated with great earthquakes along the subduction zone. In order to reveal the mechanism of the earthquakes, dive surveys and dense topographic surveys have been conducted from geological, geophysical, geochemical, and biological points of view along the Nankai Trough margin [7–9]. Multichannel seismic reflection surveys have clearly imaged splay faults branching upward from the plate–boundary interface in the area. The faults break through the accretionary prism to reach the seafloor just seaward of the outer ridge, such as the
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Fig. 1. (a) A topographic map of the Nankai Trough off Kumano area, together with the locations of Site 808 (star), and the Ryuyo Canyon (triangle). Contour interval is 100 m. The box represents the outline of Fig. 1b. (b) A bathymetry map with the specific positions of the Oomine Ridge site and the Oodai Ridge site (circle) and the reference sites (dots). Contour interval is 50 m. The box represents the outline of Fig. 1c. (c) A topographic map of the Oomine Ridge together with sampling sites of cores, which show two groupings: the dots symbols denote samples characterized by CH4 enrichment and Cl depletion and the triangle ones denote samples characterized by slight CH4 enrichment and little Cl depletion, in comparison with normal seawater. Contour interval is 50 m. The shaded area represents the area where we observed many bacterial mats scattered at intervals of several hundred meters at water depths ranging from 2500 to 2550 m.
Oomine Ridge and the Oodai Ridge as shown in Fig. 1 [7]. On the Oomine Ridge in particular, recent submersible exploration has revealed the distribution of chemosynthetic communities (shell fragments, living tube worms, and bacterial mats) around the seafloor scarp of the splay faults [8,10]. The thermal gradient inside of a chemosynthetic community observed in the Oomine Ridge showed anomalously
high heat flow, suggesting the existence of active fluid discharge at this site [9].
3. Sampling Fig. 1 shows the sediment sampling locations. During the cruise YK02-02 (5/30–6/17 2002) and
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YK03-03 (4/7–5/1 2003) of R/V Yokosuka (JAMSTEC), push core samples were collected by the manned submersible bShinkai 6500Q (JAMSTEC). Supplemental Table 1 summarizes brief descriptions of the core samples collected in this study. In a laboratory on board the ship, core samples were treated for the extraction of interstitial fluids, which were distributed to sample bottles within 5 h after their recovery with an effort to avoid evaporation, degassing, and contamination. Details about sampling strategy and handling are given in Tsunogai et al. [11]. In brief, the water over the sediment–water interface of a corer was first taken by siphon, and then the interstitial water was extracted from the subsampled sediment using the procedures and equipment described by Manheim [12]. About 2 cm3 of the retrieved interstitial fluid was immediately transferred to a 3-cm3 serum glass vial, poisoned with HgCl2, and capped with a gray butyl rubber septum as a preservative for subsequent shore-based analysis of the concentration and stable isotopic composition of dissolved CH4, C2H6, and ACO2. The remaining fluids were preserved in polypropylene bottles for the measurement of major anion concentrations and dD and d 18O values of water. All the samples were refrigerated in the dark and kept at 4 8C until analysis.
4. Analysis 4.1. Concentration and carbon isotopic composition of hydrocarbons and RCO2 The concentrations and the carbon isotopic analyses of hydrocarbons (CH4 and C2H6) in interstitial fluid samples were performed using isotope-ratio-monitoring-gas chromatography/mass spectrometry system [11]. A portion of the headspace gas of each vial was transferred into a gas loop using a gas-tight needle syringe and injected into a gas chromatograph (Hewlett-Packard 6890 equipped with a 25-m-long 32 Am ID PoraPLOT-Q capillary column). Separated hydrocarbon compounds were quantitatively oxidized to CO2 using an oxidizing furnace equipped with a copper oxide wire and platinum wire as a catalyst (T=960 8C). Each CO2 gas was transferred by carrier gas via a splitting system into the ion source of a high-precision mass
spectrometer (FINNIGAN MAT 252). A working standard gas containing ca. 875 ppm CH4 and ca. 44 ppm C2H6 in nitrogen, made from the NIST RM 8560 (IAEA NGS2) standard, was used for calculation of the hydrocarbon contents. Analytical precision of the concentration determination (1 r value for 10 determinations) was estimated to be 6.5% for CH4 and C2H6. The detection limit of the isotope analysis was 200 pmol with isotope ratio standard deviation of 0.3x. The analytical blank being less than 1 pmol was negligibly small. Isotope ratios (13C/12C) are reported in the common dnotation relative to the PeeDee Belemnite (PDB) standard: d 13CCH4={((13CCH4/12CCH4)sample/(13CCH4/12CCH4)PDB) 1}1000. The concentrations of ACO2 in interstitial fluid samples were measured by the above-mentioned isotope-ratio-monitoring GC/MS system in a similar way as that described by Miyajima et al. [13]. The precision of the concentration determination was estimated to be 7%. 4.2. Major components, dD and d 18O of water The Cl and SO42 concentrations in the interstitial fluid samples were measured with an ion chromatograph [14]. Analytical precision was estimated to be 0.4% for these components by five repeated measurements of the same sample, and accuracy was estimated to be 3% from the measurements of IAPSO standard ocean water. For measurement of hydrogen isotopic composition, the interstitial water was reduced to hydrogen using zinc [15]. The obtained hydrogen was introduced into dual inlet mass spectrometer (Finnigan MAT 252). Analytical precision was estimated to be within 1x. Results are reported in the usual delta notation relative to standard mean ocean water (SMOW): dD={(D/H)sample/(D/H)SMOW1}1000x. The oxygen isotopic compositions of water were analyzed repeatedly three times with a MAT252 continuous flow isotope ratio mass spectrometer
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following the technique described by Ijiri et al. [16]. The interstitial water was introduced into a glass vial together with solid NaHCO3 for H2O–CO2 equilibration. The oxygen isotopic composition of CO2 in the gas phase in the vial was analyzed by using the abovementioned isotope-ratio-monitoring GC/MS system. Analytical precision was estimated to be within 0.1x. Results are given relative to SMOW: d 18O={(18O/16O)sample/(18O/16O)SMOW1}1000x.
5. Results and discussion 5.1. Chemical and isotopic characteristics of the seeping fluids Analytical results of chemical and isotopic compositions are shown in supplemental Table 1. Fig. 2 shows vertical profiles of chemical and isotopic compositions of interstitial waters in the 27 sediment cores as well as of the overlying bottom waters. Among the 27 cores sampled in this study, a remarkable feature in the vertical chemical distribution was observed for the following samples: D674C4, C5, D744C1, C3, D752C2, D754C1, and D756C1, all retrieved from the Oomine Ridge. They were characterized by CH4 and ACO2 enrichment and Cl and SO42 depletion compared with the other samples. The CH4 value of 667 Amol/kg observed for the D674C5 samples (supplemental Table 1) is the highest so far reported in the Nankai Trough, suggesting that these sites on the Oomine Ridge should be regarded as one of the most active seep sites in the Nankai Trough. It is noteworthy that the minimum chloride concentration was as low as 490 mmol/kg (ca. 10% depletion compared to bottom seawater). Such anomalies in the chemical composition of the samples from the Oomine Ridge strongly suggest that the fluid originates not from normal sediment– seawater interaction at the sediment surface of hemipelagic environments, but from active seepage of fluids that are enriched in CH4 and ACO2 and depleted in Cl and SO42 relative to normal seawater. It is known that isotopic compositions of methane and C2H6/CH4 (C2/C1) ratios are useful in distinguishing between microbially produced methane and thermogenically produced methane [17,18]. The Oomine Ridge fluids are characterized by 12C-
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enriched CH4 (d 13CCH4b70x PDB) and a depleted C2/C1 ratio (less than 103) as shown in supplemental Table 1. It can, therefore, be deduced that most of the hydrocarbon gases at these sites on the Oomine Ridge are generated via microbial methanogenesis, not via thermal degradation of organic matter (e.g., Ref. [17]). The thermal decomposition of organic material occurs at temperatures more than 60 8C [19] and the thermal gradient in the Oomine Ridge has been estimated to be about 60–70 8C/km as background and up to about 200 8C/km within the bacterial mat [9]. The methane seemed to be added from the shallow sedimentary layer during the ascent of cold seep fluids, not from the deep sedimentary layer of higher temperature than 60 8C at the depth of more than 300 m below the seafloor (mbsf). 5.2. Origin of low-Cl fluids Previous studies of the Nankai Trough have widely noted the Cl depleted interstitial fluids present there [11,20–23]. At Site 808 during Leg 131 of the Ocean Drilling Program (ODP) in the Nankai accretionary wedge off Muroto (Fig. 1a), low-Cl fluids were collected at the depths from 620 to 1150 mbsf and were regarded to be the result of clay mineral dehydration [22,23]. On the other hand, Tsunogai et al. [11] explained the low-Cl fluids seeping onto the seafloor of the biological community area at the Ryuyo Canyon in the Nankai Trough, 30 km off the coast of Japan Islands (Fig. 1a), as the result of incorporation of land-derived groundwater. In view of recent findings on methane-rich seeping fluids on the seafloor in several convergent environments, gas hydrate dissociation and upward fluid migration are proposed as important sources for methane-rich cold seep fluids (e.g., Refs. [4,24–28]. In the Nankai Trough, gas hydrates are widespread as shown by the findings of bottom-simulating reflectors (BSRs) delineating the base of the gas hydrate stability zone [29]. The destabilization of gas hydrate is a highly probable source for the interstitial fluids at the Nankai Trough [30]. Dissociation of gas hydrate is known to produce a low-Cl fluid whose dDH2O and d 18OH2O should be higher than those of seawater because gas hydrates not only exclude Cl from their lattice structure but also preferentially incorporate the heavier isotopes of O and H in their cage water. The
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Fig. 2. Vertical profiles of (a) chloride, (b) sulfate, (c) methane concentration, (d) d 13CCH4, (e) dDH2O, and (f) d 18OH2O of interstitial fluid samples. Each symbol denotes the classification of sampling location: the Oomine Ridge site (solid diamond); open diamond; solid circle; solid triangle; open square; cross; open triangle; solid square; open circle), the Oodai Ridge site (plus: D676C1, C2, C3, C6, D746C1, C2, and D747C1), and the reference sites (dot: D675C1, C3, D680C3, D683C3, C4, D684C1, C2, C3, D685C1, C4, and D753C1). The exponential curves, a dashed line for D674C4 and a dotted line for D674C5, respectively, represent the best exponential fit to the dissolved chloride data, together with (a).
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values for dissociated hydrate samples corrected for any impurities from the surrounding interstitial water have been reported to be +22x for dDH2O and +2.4x for d 18OH2O [31]. Stable isotopic signatures of water in samples (dDH2O and d 18OH2O) provide excellent tracers for the origin of low-Cl fluids in seafloor discharges [11,32]. The dDH2O and d 18OH2O data for the interstitial fluids from the Oomine Ridge are presented in supplemental Table 1 and Fig. 2. The data are also plotted as CldDH2O and Cld 18OH2O diagrams in Fig. 3a and b, respectively. As shown in Figs. 2, 3a and b, the fluids exhibit D- and 18O-depletion in association with the Cl depletion. The interstitial fluids from the Oomine Ridge show the depletion of up to 4x in dDH2O, and up to 0.7x in d 18OH2O in comparison with ambient bottom seawater. These data strongly suggest that the fluids from the Oomine Ridge have little relation to gas hydrate dissolution. If we assume a simple two-component mixing of ambient seawater with fresh water of Cl=0 mmol/ kg, the estimated end members of dDH2O and d 18OH2O are 46F7x and 6.3F0.7x SMOW, respectively, as shown in Fig. 3. The estimated dDH2O and d 18OH2O values for hypothetical fresh water fall into the reported range of dDH2O from 30x to 50x SMOW and d 18OH2O from 4x to 8x SMOW of the groundwater at nearby land areas [33]. Land-derived groundwater could, there-
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fore, be considered as one of the possible sources of the present fresh water discharging from the Oomine Ridge cold seep site. Another possible process that may provide fresh water for the formation of such low-Cl, dDH2O, and d 18OH2O fluids is clay membrane ion filtration reaction. However, the efficiency of ion filtration has been generally understood to be negligible in any natural environment [34], and the fractionation has been reported to be up to 2.5x for dDH2O and 0.8x for d 18OH2O, respectively [35]. So far, it must be difficult to consider clay membrane ion filtration for the process to produce the chemical composition. The Oomine Ridge is located at the depth of ca. 2500 m and a distance of ca. 100 km from the Japan Islands (Fig. 1). Long-distance lateral transport of groundwater from the land area has been mentioned in explaining the extremely low-Cl interstitial fluids in the convergent margin ca. 70 km off the coast of Peru, as observed at ODP site 679 [36,37] and in the Barbados accretionary prism from the Orinoco fan, although the subsurface groundwater migration in convergent margins has not been evaluated in any detail. Recent works on submarine groundwater discharge in near-shore environments and on continental shelves indicate that submarine groundwater discharge can be a common phenomenon for marginal areas, where sufficient hydraulic head and permeability and a tight enough aquiclude permit it [38,39].
Fig. 3. Chloride vs. dDH2O (a) and chloride vs. d 18OH2O (b) plot for the Oomine Ridge samples, together with the regression line, giving each end-member composition of (a) 46F7x for dDH2O and (b) 6.3F0.7x for d 18OH2O (at 0 mmol/kg Cl).
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In this case, however, the Ghyben–Herzberg (freshwater–seawater) boundary forced offshore [40] cannot explain this long transport, but a significant unique way like a pipe through the fluids by siphon from the land to the deep seafloor can. We cannot specify the path of the lateral transport from the land any more, but other sources cannot explain the chemical and isotopic compositions of the interstitial water observed in the Oomine Ridge. Future seafloor drilling studies on the Oomine Ridge will provide direct evidence for this hypothesis and outline the plumbing regime in the Nankai accretionary wedge, which give answers to the following questions: What is the conduit that the fluids flow through and how long the fluids flow through the path from the sources? What mechanism might be for the fluid seepage in detail? 5.3. Calculations of fluid rate in sediments at the Oomine ridge Transport of dissolved species can take place both molecular diffusion and advection of interstitial water. The magnitude of the diffusive flux and the rate of advective flow provide constraints for models that can be estimated from the observed profiles of the conservative aqueous species, in this case, chloride ion concentration. D674C4, C5, D744C3, D752C2, and D754C1 interstitial fluid samples with low chloride concentrations exhibit depth profiles that are convex upward in sediments, as portrayed in Fig. 2a. The shape of Cl concentration profiles for these samples reflects upward migration of interstitial water through the sediment column. If steady-state condition is assumed for approximate calculations of the flow velocity, the concentration of dissolved chloride can be described by a partial differential equation [41]: D d2 C=dx2 þ vðdC=dxÞ ¼ 0 where C is the concentration of dissolved chloride (mmol/kg), x is depth measured positively downward from the sediment–water interface (cm), D is the diffusion coefficient of chloride in sediments (cm2 year1), and v is the vertical flow velocity of interstitial water relative to the sediment–water inter-
face, positive upward (cm year1). Diffusion coefficient for this modeling was taken from Ref. [42], 160 cm2 year1. From the calculations presented in this section, it will be assumed that D and v are constant with depth. In addition, bioturbation may have not been considered in this expression, and instead, no bioturbation at these sites will be assumed, except for the upper 12 cm of the D752C2. It is possible to fit the dissolved chloride data to an exponential curve of the form: C ¼ ðCl C0 Þ 1 eðv=DÞx þ C0 where C ¼ C0 at x ¼ 0 and CYCl as xYl The dissolved chloride data are fit by varying v and C l until the best least-squares fit is obtained, and examples of this curve fitting are shown in Fig. 2a. With this approach, C l is determined as 490, 520, 510, 520, and 520 mmol/kg, respectively, for the D674C4, C5, D744C3, D752C2, and D754C1 samples. These results reflect mixing of fresh water with marine interstitial fluids and/or seawater several times at a depth of greater than 30 cm, a core barrel length. Different vertical flow velocity of interstitial water is obtained: 200, 50, 50, 50, 40 cm year1, respectively, for the D674C4, C5, D744C3, D752C2, and D754C1 samples. The measured interstitial water chloride profile in sediment sequences shows horizontally heterogeneity for a distance of ca. 1000 m (Fig. 1c). Active fluid seepage seems to be a localized phenomenon, which may cause spatial variations in fluid flow. The modeled values for v range from 200 cm year1 at core D674C4 to 40 cm year1 at core D756C1. The positive values for the velocity indicate that the flow direction is upward. This upcoming flow is comparable to the previously reported values (b10 m year1) inside bacterial mats and significantly lower than the values (75–100 m year1) within bivalve beds in the eastern Nankai Trough accretionary wedge, inferred by modeling of subbottom temperature profiles [43]. Slower advection might allow longer time for diagenetic reactions in the vent conduits and carry more reductive compounds in the fluids, and consequently, might be favorable to the development of bacterial mats [43].
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6. Conclusions (1)
(2)
(3)
(4)
(5)
Chemical and isotopic characteristics of interstitial waters from the Oomine Ridge in the Nankai Trough off Kumano indicate that fluids rich in CH4 and ACO2 and low in chloride, sulfate, dDH2O, and d 18OH2O are discharging from the sea bottom to the ocean. Based on the values for carbon isotopic composition of dissolved methane (less than 70x PDB) and the C2H6/CH4 ratio (less than 103), the hydrocarbons in the interstitial fluids from the Oomine Ridge are considered to be of microbial origin at a relatively shallow layer of sediment, not from the deeper sedimentary layer more than 300 mbsf. It is estimated that the hydrogen and oxygen isotopic compositions of the hypothetical fresh water (Cl =0) would be 46F7x and 6.3F0.7x SMOW, respectively, implying that land-derived groundwater could, therefore, be considered a possible source of the lowCl fluid. Distribution of chloride concentration illustrates the enormous variability in the depth profile of interstitial fluids. It is suggested that seawater and/or pore water seems to be added several times during the ascent of the low-Cl fluids. Results from steady-state models applied to the chloride distribution in the interstitial fluids yield the estimated values of fluid flow velocities in the range of 40–200 cm year1, comparable to the previously reported values (b10 m year1), supporting the postulate that bacterial mats may favor slow seepage.
Acknowledgments The Authors are grateful to the officers, crew, and scientist group, as well as the Shinkai 6500 operating team of JAMSTEC aboard R/V Yokosuka during the YK02-02 and YK03-03 cruises for their valuable collaboration. The reviewers E. Boyle, J.M. Gieskes, and J.B. Martin are thanked for valuable information and their constructive comments. This research was supported in part by a 21st Century Center of Excellence (COE) Program on bNeo-Science of
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Natural HistoryQ (Program Leader: Hisatake Okada) at Hokkaido University financed by the Ministry of Education, Culture, Sports, Science and Technology, Japan. Financial support has been obtained from MEXT Special Coordination Fund bArchaean ParkQ project.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.epsl.2004.09.007.
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