Introduction

Methane (CH4) is an energy resource and a greenhouse gas. Its accumulation in the subsurface and emission on the seafloor have long drawn research attention. On the other hand, the source and supply of hydrogen (H2) in the geosphere have also attracted growing interest because H2 supply is a critical factor for sustaining the deep biosphere and the production of microbial CH4. In the geosphere, molecular H2 is produced from the degradation of sedimentary organic matter by H-producing bacteria1, inorganically by hydrothermal alteration such as serpentinization2,3, and mechanochemically by the interaction between water and silicate rock fracturing4,5. H2 is also generated thermally with aromatization, condensation, and graphitization of sedimentary organic matter6,7,8. In an arc-trench system, marine sediments on the oceanic plate are transported deep into the crust and undergo metamorphic processes. The underthrust sediments below the detachment thrust (décollement) of an oceanic plate in an arc-trench system may serve as CH4 and H2 source rocks and play an essential role in the accumulation and emission of CH4 in the plate subduction zone. However, few studies have addressed these potential roles of the underthrust sediments so far.

The Nankai Trough subduction zone in southwest Japan is a typical example of the arc-trench system composed of inner and outer prisms covered by forearc sediments and the underthrust sediments subducting with the Philippine Sea plate9,10 (Fig. 1). The CH4 accumulations and emissions, such as mud volcanoes, methane plumes, and gas hydrates or bottom simulating reflectors (BSRs), occur widely in the forearc basin and accretionary prism of the Nankai Trough11,12,13,14 (Fig. 1). The total amount of CH4 in gas hydrate-bearing sediments in the eastern Nankai Trough has been estimated to be about 40 trillion cubic feet (1.1 × 1012m3), probably one of the world’s largest marine gas hydrate deposits15. Dissolved-in-water type gas deposits (dissolved gas deposits), composed mainly of CH4, are distributed in the coastal land of Tokai and southwestern Kyushu near the Nankai Trough, and CH4 seeps also occur in these areas16,17,18,19,20. These CH4 accumulations and emissions are thermogenic and/or microbial13,16,17,18,19,20,21,22,23,24. It is, however, unclear where and how much thermogenic CH4 is generated in the Nankai Trough subduction zone. In addition, although the active CH4 production by methanogens in the organic-poor environment requires abiogenic geological H2, the origin and source of geological H2 remain uncertain1. Understanding the spatial distribution and rate of CH4 and H2 generation in the subduction zone will clarify the geological context of active CH4 accumulations and emissions in an arc-trench system. In this regard, we focused on the underthrust sediments below the décollement of the subducting oceanic plate.

Fig. 1: Methane accumulations and emissions in the Nankai Trough subduction zone.
figure 1

a Location of the study area. b Distribution of mud volcanoes14, gas hydrates (BSRs)12, dissolved gas deposits16,18,20, and condensate/gas seeps17,18 in the Nankai Trough subduction zone. c Schematic geological section of the Nankai Trough off the Kii Peninsula10. The black line with triangles represents the trough axis of the Nankai Trough. The water-depth map is courtesy of the Hydrographic and Oceanographic Department of the Japan Coast Guard.

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In contrast to the considerable time required to form conventional oil and gas deposits, gas hydrates and mud volcanoes are formed within a geologically short time in regions where the CH4 flux is high. The convergence rate of the Philippine Sea Plates with the southwest Japan Arc is about 50–60 mm yr−1 (year)10,25,26,27, a much faster rate than the subsidence rate of the sedimentary basin. The subsidence rate is related to the heating rate, which can affect the thermal decomposition rate of sedimentary organic matter. The maturity level of the thermogenic CH4 and H2 generation in the subduction zone can be estimated based on residual gases in shales and metapelites from subaerial accretionary prisms. From the heating and subsidence rates of the accretionary prisms and the underthrust sediments, we have predicted the spatial distributions and rates of thermogenic CH4 and H2 generation in the Nankai Trough subduction zone. The expulsion and migration of thermogenic CH4 and H2 have been discussed, considering the active seismogenesis and their solubility in the deep fluids. The present study shows that the generation rate of thermogenic CH4 in the underthrusut sediments is much higher than in the accretionary prisms, and the thermogenic CH4 in the Nankai Trough subduction zone is mainly derived from the underthrust sediments. Following the thermogenic CH4 generation, the thermogenic H2 is generated in the underthrust sediments during the metagenesis to metamorphism in the deep subduction zone. The thermogenic H2 derived from the underthrust sediments can be another potential geological source of H2 for microbial CH4 production in the Nankai Trough. The sustainable supply of thermogenic CH4 and H2 from the underthrust sediments associated with the oceanic plate subduction can contribute to the accumulation and emission of CH4 represented by the formation of large-scale gas hydrates in the Nankai Trough.

Results and discussion

Organic matter in the underthrust sediments

The oceanic lithosphere ages of the subducting Philippine Sea Plate slab in southwest Japan are estimated to be 15–30 Myr28. Underthrust sediments consist of pelagic sediments that have been transported by the movement of the oceanic plate and marine and terrestrial sediments deposited in the trench. The pelagic sediments of the Philippine Sea Plate are composed of claystone, radiolarian and diatomaceous siliceous mudstones, and nannofossil calcareous mudstones29,30. Rock-Eval® analyses for the Ocean Drilling Program (ODP)/Deep-Sea Drilling Project (DSDP)/Integrated Ocean Drilling Program (IODP) sediment samples equivalent to the subducting section in the Nankai Trough showed that the Cretaceous to Quaternary sediments deposited far from the continents were richer in marine organic matter (Type II kerogen)30. Conversely, those deposited in the ocean-continent subduction zone showed higher terrestrial organic matter content (Type III kerogen). Frequent earthquakes in the subduction zone trigger the remobilization of surficial sediments along the landward slope of the trough, resulting in marine and terrestrial sediment accumulation31,32. In the Nankai Trough, the sediment sections on the Philippine Sea Plate are relatively thick (more than 1000 m), and the upper and younger parts of sedimentary layers were accreted landward to form the accretionary prism with plate subduction9,10. The underthrust sediments subducted beneath the accretionary prism are the lower and older sedimentary sections, for which information is lacking since few drilling operations have penetrated through all the sedimentary layers above the basaltic basement.

The total organic carbon (TOC) concentrations of Neogene sediments from IODP Expedition 333 drill sites (C0011 and C0012) close to the Nankai Trough are low, ranging from 0.2 to 0.5 wt%33. Based on the results of ocean drilling programs (ODP, DSDP, and IODP) in the Pacific Ocean, the average TOC concentration of the underthrust sediments of the Philippine Sea Plate is 0.5 wt%30. Although this TOC concentration may be somewhat higher than the average TOC concentrations of shale (0.44 wt%, n = 28) and metapelites (0.33 wt%, n = 41) from the old accretionary prisms in the Shikoku region (Supplementary Table 1), a TOC of 0.5 wt% is reasonable when considering thermal decomposition during diagenesis to metamorphism. The present study assumes an initial TOC concentration of 0.5 wt% to estimate the thermogenic CH4 and H2 generation in the accretionary prism and underthrust sediments of the Nankai Trough.

Heating rate in the accretionary prism and underthrust sediments

Since the difference in the heating rate of accretionary prisms and underthrust sediments directly affects the generation rate of thermogenic CH4 and H2, the difference in their heating rates in the subduction zone was first examined. The heating rate of sediments in the Nankai Trough subduction zone can be estimated based on the thermal structure, movement of the accretionary prism, and the convergence rate of the Philippine Sea Plate. The Kumano-nada region, offshore of the Kii Peninsula, is one of the sections in the Nankai Trough where the subduction history of the Philippine Sea Plate, the formation of accretionary prism, and thermal structure, respectively, have been well investigated9,10 (Fig. 2). In the subduction zone 25–100 km landward from the trough axis, the heat flow is nearly constant at 50–65 mW m–2 34,35. The depth-temperature relationship below 200 °C in the Kumano-nada basin is obtained from the long-term borehole monitoring system installed at Site C0002 during the IODP Expedition 33236, extrapolated to the subsurface temperature distribution in the Kumano-nada region (Fig. 2 and Supplementary Fig. 1). Isotherms above 200 °C (Fig. 2) were from the modeling results of the thermal structure around the Philippine Sea Plate (Supplementary Figs. 2, 3)37,38.

Fig. 2: Subsurface isotherms and annual subduction and heating rates of sediments in the Nankai Trough subduction zone.
figure 2

a Location of the geological section with IODP Site C000236. b Subsurface isotherms superimposed on the schematic geological section of the Kumano-nada region off the Kii Peninsula. The boundary between the outer and the inner prisms is the Kumano basin edge fault zone (KBEFZ)76. c The annual subduction and heating rates of the inner accretionary prism. d The annual subduction and heating rates of the underthrust sediments. The water-depth map is courtesy of the Hydrographic and Oceanographic Department of the Japan Coast Guard.

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The active subduction of the Philippine Sea Plate with formations of the forearc basin and new accretionary prism began about 6.0 Myr ago9,10. The subduction zone in its present configuration was formed about 2.2 Myr ago39,40. Based on sediment accretion models in the Nankai Trough41,42, we estimated the lateral velocity of the accretionary prism. The thickness of the accretionary prism in the Kumano-nada region gradually increases landward. Considering the total volume of the outer prism and its initiation 2.2 Myr ago, the average annual volume of sediment accretion for one km length of the trough is estimated at approximately 53.2 × 103 m3 yr−1 km−1 (Supplementary Figs. 4, 5). The lateral velocity of the accretionary prism gradually decreases with increased thickness and distance from the trough. At its thickest, the current lateral velocity of the prism is approximately 6.0 mm yr−1 (Supplementary Fig. 5). The vertical subsidence rate of the accretionary prism can be estimated from the burial rate of the Kumano forearc basin (about 1.0 mm yr−1) and the taper angle (7.5°) of the subducting plate. The average subduction rate of the Philippine Sea Plate along the Nankai Trough was set at 50 mm yr−1 25,27,43. As a result, the movement velocities of the deepest part of the inner prism and the underthrust sediments were estimated at approximately 6.3 mm yr−1 and 50.2 mm yr−1, respectively (Fig. 2 and Supplementary Fig. 5). The heating rates of sedimentary rocks are mainly related to the vertical velocity of movement and the temperature gradient. Considering the subsurface temperature distribution in the Kumano-nada region, the heating rates of the accretionary prism and the underthrust sediments are estimated to be approximately 30 °C Myr−1 and 125 °C Myr−1, respectively (Fig. 2).

Timing of thermogenic CH4 and H2 generation in the subduction zone

The thermogenic CH4 and H2 generation in the subduction zone was predicted based on the concentration changes of residual gases in shales and metapelites with increasing diagenetic and metamorphic temperatures. Although the residual gas concentration in pelitic rocks has an upper limit because of the limited capacity to retain gaseous components, the relative residual gas concentration depends roughly on the total gas composition generated during diagenesis and metamorphism. Shales and metapelites were resampled from the same sampling site of the old accretionary prism8 (Supplementary Fig. 6) and analyzed for residual gas using an improved analytical method. A small peak of O2 relative to the N2 peak in the chromatogram shows negligible air contamination during analysis (Supplementary Fig. 7). The CH4, C2H6 (ethane), CO2, N2, and H2 were detected in shales and metapelites from the old accretionary prism (Supplementary Table 1). The relationship between vitrinite reflectance (VR) and the maximum heating temperature of the samples was determined using Easy%RoV44, assuming the heating rate of 30 °C Myr−1, the heating rate for the inner accretionary prism in the Nankai Trough subduction zone (Fig. 2).

The concentration changes of residual C2H6, CH4, and H2 with increasing diagenetic and metamorphic temperatures are shown in Fig. 3, with some typical gas chromatograms of residual gas. Residual C2H6 attains peak concentration at around VR = 2.0% and almost disappears at the maturity level of VR = 2.5%. The residual CH4 concentration rises at about 130° to 150 °C (VR = 0.7%) and reaches a peak (1500–1700 μL g TOC−1) at about 220° to 230 °C (VR = 2.5%), after which it decreases probably due to the formation of graphitic carbonaceous material45,46. The residual H2 concentration rises at around 150° to 200 °C (VR = 1.0–2.0%) and shows a higher concentration in the pelitic rocks at metagenesis to metamorphism (VR > 4.0%). In metapelites that experienced higher temperatures of more than 300 °C, the volume concentration of H2 exceeds that of CH4 (Fig. 3). The residual CO2 concentration decreases drastically from several hundred to less than 10 μL g TOC−1 at VR < 2.0 % (Supplementary Table 1), possibly due to the decarboxylation of sedimentary organic matter and expulsion of CO2 with porosity reduction. Since some CO2 is adsorbed on the mineral surface during pulverization, the residual CO2 concentration measured is less than the total CO2 retained in shale rocks8. The concentration of residual N2 ranges mostly from 100 to 1000 μL g TOC−1 and does not show any systematic change with increasing temperature (Supplementary Table 1). The origin of residual N2 is currently unknown. The residual C2H6, CH4, and H2 in the shales and metapelites are mainly derived from the thermal decomposition of sedimentary organic matter8.

Fig. 3: Concentration changes with temperature for residual CH4, C2H6, and H2 in shales and metapelites.
figure 3

The relationship between VR and the maximum paleo-temperature was computed using Easy%RoV44, assuming a heating rate of 30 °C Myr−1. Maximum temperatures attained by metapelites were estimated from metamorphic mineral assemblages. Some typical gas chromatograms for the residual gases show changes in the relative abundances of H2, N2, CH4, and C2H6 with increasing the maximum temperature. A part of the dataset is from Suzuki et al.8.

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Thermogenic CH4 is mainly generated by the thermal cracking of hydrocarbons and kerogen during the thermal condensation of macromolecular intermediates and kerogen. Therefore, the amount of thermogenic CH4 generation is related to the expulsion of hydrocarbons. In the case of accretionary prisms and underthrust sediments, the average TOC concentration is estimated to be so low (0.5 wt%) compared to ordinary petroleum source rocks, suggesting minimal expulsion of hydrocarbons because of their poor saturation in the pore space. Hence, from the viewpoint of thermal decomposition, thermogenic CH4 generation is regarded as proceeding almost in a closed system. In closed-system pyrolysis experiments of sedimentary organic matter, thermogenic CH4 concentration gradually increases with increasing thermal maturation from VR = 0.7–4.0%, and C2H6 disappears at around VR = 2.5%47,48, a pattern consistent with the relationships between maturation stages (VR values) and residual CH4 and C2H6 concentration changes in the old accretionary prism (Fig. 3). The maturation level of the oil generation zone is generally variable due to the difference in kerogen type. However, the timing of wet and dry gas generations is not so dependent on the initial kerogen type because mature kerogens are characterized by similar chemical structures dominated by C-C bonds49. We assumed the C2H6 and CH4 generation zones were at VR < 2.5% and VR = 1.5 to 4.0%, respectively, considering the closed-system pyrolysis experiments and the change of residual gas concentrations in the old accretionary prism.

Hydrogen atoms and radicals produced during oil and wet gas generation are consumed immediately by secondary reactions, e.g., the hydrogenation of alkenes7. The formation of molecular H2 proceeds during the carbonization and graphitization of carbonaceous material in the metagenetic and metamorphic stages. The generation of thermogenic H2 from kerogen at a high temperature, following CH4 generation, has been confirmed in laboratory pyrolysis experiments6,7. We assumed that the thermogenic H2 generation zone starts from VR = 2.5%, corresponding to the maturity level of wet gas disappearance. The progress of the organic reaction under high pressure is generally retarded compared to that under low pressure50,51. Since metamorphism in the Nankai Trough proceeds under relatively higher pressures than other subduction zones52, the timing of thermogenic H2 generation may be retarded compared to the old accretionary prism. In the present study, the thermogenic CH4 and H2 generation zones have been estimated to range from VR = 1.5 to 4.5% and VR > 2.5%, respectively. Temperatures in the inner accretionary prism and the underthrust sediments corresponding to these VR values are different because of different heating rates. According to the Easy%RoV, the VR values of 1.5%, 2.5%, and 4.5% for the inner accretionary prism with a lower heating rate of 30 °C Myr−1 correspond to 194°, 222°, and 268 °C, respectively. On the other hand, the same VR values for the underthrust sediments with a higher heating rate of 125 °C Myr−1 are equivalent to 204°, 233°, and 279 °C, respectively, which are about 10 °C higher than in the inner accretionary prism. The thermogenic CH4 and H2 generation zones in the Nankai Trough subduction zone are shown in a schematic geological section of the Kumano-nada region off the Kii Peninsula (Fig. 4).

Fig. 4: Thermogenic CH4 and H2 generation zones in the Nankai Trough subduction zone.
figure 4

The thermogenic CH4 and H2 generation zones within the accretionary prism and underthrust sediments are indicated in the schematic geological section of the Kumano-nada region off the Kii Peninsula. Temperatures in the deepest part of the inner prism are not high enough to generate CH4 + H2. The subducting underthrust sediments pass through the CH4 + H2 generation zone and sustainably generate CH4 and H2.

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Generation rate of thermogenic CH4 and H2 in the subduction zone

To clarify the respective contribution of the underthrust sediments and accretionary prisms to the thermogenic CH4 generation in the subduction zone, their annual CH4 generation rates were estimated and compared. Although the sediments in the subduction zone contain some terrestrial organic matter, we assumed the same Type II (marine) kerogen and TOC concentration (0.5 wt%) for comparison convenience. According to pyrolysis experiments of kerogen in a closed system, the ultimate yield of CH4 from Type II kerogen was approximately 300 mg CH4 g TOC−1 47,48. Therefore, the annual generation rate of CH4 can be estimated from the TOC concentration, the apparent sediment density (2.6 g cm−3), and the movement velocity of the underthrust sediments and the inner prism. The underthrust sediments of about one km thick are moving through the CH4 generation zone at a subduction rate of about 50.2 mm yr−1 in the Kumano-nada region (Fig. 2). Under these conditions, the annual generation rate of CH4 in the underthrust sediments subducting along one km length of the Nankai Trough was estimated to be approximately 2.7 × 105 m3 yr−1 km−1. In the case of the accretionary prism, only the sediments at the deepest part of the inner prism are within the CH4 generation zone (Fig. 4). The old accretionary prism acts as a static backstop and comprises highly mature sediments and metamorphic, volcanic, and plutonic rocks10,43, suggesting a low thermogenic CH4 and H2 generation.

In the inner prism, it can be assumed that about two km thick sediment layer above the décollement is moving landward through the CH4 generation zone at a rate of 6.3 mm yr−1 (Fig. 4). Like the underthrust sediments if the ultimate amount of thermogenic CH4 was generated within the two km thick sediment layer moving at 6.3 mm yr−1, the annual CH4 generation rate in the accretionary prism along one km length of the Nankai Trough is estimated at approximately 6.9 × 104 m3 yr−1 km−1. However, the highest temperature of the CH4 generation zone in the inner prism is about 225 °C (VR = 2.5%), and only a limited part has intersected the CH4 generation zone. Since the area of the inner prism in the CH4 generation zone is less than half of the whole thermogenic CH4 generation zone (ca. 195‒270 °C), the generation level of thermogenic CH4 in the inner prism is less than 50 % of the ultimate level. Therefore, the annual generation rate of CH4 in the inner prism along one km of the Nankai Trough is estimated to be less than 3.5 × 104 m3 yr−1 km−1, much lower than the underthrust sediments. The higher generation rate of thermogenic CH4 in the underthrust sediments is attributed to sufficient thermal maturation and a faster subduction rate. The volumetric concentration of residual H2 in highly mature shales and metapelites is similar to residual CH4, suggesting that the thermogenic H2 generation rate is comparable to that of thermogenic CH4 (Fig. 3). Thermogenic CH4 and H2 are being generated in the underthrust sediments for at least the past 2.2 Myr, according to the evolutionary history of the subduction of the Philippine Sea Plate9,10,39,40,43. The total thermogenic CH4 generation during the last 2.2 Myr reaches approximately 5.9 × 1011 m3 per one km length of the Nankai Trough. The movement velocity of the inner prism indicates that its deepest part arrived at the CH4 generation zone about 2.0 Myr ago. Since then, continued thermogenic CH4 generation in the inner prism has gradually increased over time. However, the inner prism of the Nankai Trough has not yet reached the thermogenic H2 generation zone.

Spatial distribution of thermogenic CH4 and H2 generation zone

The spatial distribution of the thermogenic CH4 and H2 generation zones of the underthrust sediments and the accumulation and emission of CH4 in the Nankai Trough is shown in Fig. 5. The heating rate of the underthrust sediments is assumed to be the same as the Kumano-nada region at 125 °C Myr‒1. Therefore, the CH4 and H2 generation zones in Fig. 5 were set at about 205° to 280 °C and >235 °C, respectively. Gas hydrates and BSRs are located away from the thermogenic CH4 and H2 generation zones and distributed widely trough-ward. In contrast, mud volcanoes in the Kumano-nada region off the Kii Peninsula and the Hyuga-nada off Nichinan tend to be distributed landward and overlap with the CH4 and H2 generation zones (Fig. 5). The δ13C values of CH4 from mud volcanoes in the Kumano-nada region from −20 to −40‰23,24 suggest much contribution of thermogenic CH4. The dissolved gas deposits are distributed in the coastal area from Miyazaki to Nichinan in the southeastern Kyushu region and Yaizu and Kawane in the Tokai region (Fig. 5). The δ13C value of CH4 from dissolved gas deposits and gas seeps in the southeastern Kyushu and the Tokai region ranges from –68‰ to –37‰ and –34‰ to –33‰, respectively16,17,18,19,20. Those from Nichinan and the Tokai region near the thermogenic CH4 generation zone tend to have higher δ13C values16,17,18, suggesting a contribution of thermogenic CH4. The mud volcanoes and dissolved gas deposits comprising thermogenic CH4 are located above or near the thermogenic CH4 generation zone in the underthrust sediments, suggesting a causal relationship. The δ13C value of CH4 from gas hydrates in the subduction zone of the Nankai Trough varies from –70‰ to –40‰, and like water-dissolved gas deposits, those near the CH4 generation zone tend to have higher δ13C values22,23,24. The BSRs widely distributed in the Nankai Trough are probably indicative of gas hydrates12,15. Gas hydrate samples, however, have only been recovered from a limited number of BSRs. Widely distributed gas hydrates possibly contain mainly microbial CH4, considering the lower δ13C values of CH4 in available gas hydrates13,22.

Fig. 5: The spatial distributions of the thermogenic CH4 and H2 generation zones in the Nankai Trough.
figure 5

The red contours show the temperatures of the upper surface of the subducting Philippine Sea Plate in the Kyushu, Shikoku, and Kii Peninsula regions37 and the Tokai region38, respectively. The blue contours show the depths of the upper boundary of the Philippine Sea Plate77,78,79. The water-depth map is courtesy of the Hydrographic and Oceanographic Department of the Japan Coast Guard.

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Expulsion and migration of thermogenic CH4 and H2

The thermogenic CH4 and H2 generated in the underthrust sediments may cause overpressure in and around the décollement of the subducting plate, as suggested by Raimbourg et al.30. The present earthquake rupture area at a depth of 10–20 km along the décollement10,53,54 almost corresponds to the thermogenic CH4 and H2 generation zones in the underthrust sediments (Fig. 4). The expulsion of thermogenic gas from the source rocks presumably occurs intermittently due to the micro and macro fracturing of rocks and releases the overpressure associated with seismogenesis. The expulsion of thermogenic CH4 and H2 most likely occurs mainly in their generation zone corresponding to the earthquake rupture area. Many studies on fluids and fluid inclusions in the accretionary prisms and underthrust sediments show active fluid migration in the Nankai Trough subduction zone41,55,56. Slab-dehydrated fluids from the oceanic plate contribute to subaerial hot spring emanations in the subduction zone57,58 as well as fluids expelled in the Kumano forearc basin and Nankai Trough accretionary prisms59,60,61. In addition to slab-dehydrated fluids, fluids expelled from the underthrust sediments due to porosity reduction and clay/silica mineral dehydration migrate widely in the accretionary prisms through décollement megathrust and splay faults41,57. The active deep fluids may dissolve CH4 and H2 expelled from the source rocks, thus promoting their secondary migration in the Nankai Trough subduction zone.

Following experiments on the solubility of CH4 in pure water and salt water over a wide range of temperatures and pressures from 0 to 350 °C and 1 to 260 MPa62,63,64, Duan and Mao65 proposed a thermodynamic model for estimating CH4 solubility in pure water and salt water from 0 to 300 °C and 1 to 200 Mpa. Zhu et al.66 extrapolated the solubility of H2 in pure water and salt water from low temperature (100 °C) and low pressure (50 Mpa) experiments67,68 to 150 °C and 110 Mpa using the particle interaction theory. This theoretical model allows the prediction of the solubility of H2 in deep fluids in the subduction zone. The solubility of CH4 and H2 in pure water from 0 to 150 °C and 1 to 110 Mpa are shown in Fig. 6. The 110 Mpa nearly corresponds to the overburden pressures at a burial depth of 4.5 km36,52. At 60 °C, the solubility of CH4 and H2 in pure water generally increases with increasing temperature and pressure (Fig. 6). However, the increase in solubility of CH4 with increasing temperature and pressure is not as prominent as that of H2. Since the thermogenic CH4 generation from the source rocks (TOC = 0.5 wt%) has been assumed to be 300 mg g TOC−1 (1.9 × 10−2 mol g TOC−1), the ultimate generation of thermogenic CH4 is approximately 9.5 × 10−5 mol g Rock−1. The solubility of CH4 in water at 300 °C and 200 Mpa, roughly corresponding to the thermogenic CH4 generation zone (10 to 12 km depth), is estimated to be 2.8 × 10−3mol g H2O−1 64. According to the extrapolated porosity curve in the Nankai Trough36, the sediment porosity at 10 km depth is about 0.75 %. Therefore, in the source rock (density=2.6 g cm−3) with a porosity of 0.75 % at the CH4 generation zone, approximately 8.1 × 10−6 mol g Rock−1 of CH4 dissolves in water. This is less than 9 % of the ultimate generation of thermogenic CH4, and much of the thermogenic CH4 can behave as free gas. Although the TOC concentration of sedimentary rocks in the Nankai Trough is assumed to be much lower than in oil and gas fields, much of the thermogenic CH4 can still behave as free gas in the deep subduction zone. In oil and gas fields, the main driving force for gas migration from the source rocks to the reservoir is thought to be buoyancy69, possibly the same for the migration of thermogenic CH4 and H2 in the subduction zones. However, because of the smaller radius and higher diffusion rate, molecular H2 is more widely dispersed than CH4. Since the solubility of H2 seems to be higher than CH4 (Fig. 6), more H2 than CH4 possibly migrates as a dissolved species in deep fluids. Although the solubility of H2 in deep fluids under high pressures and temperatures is not fully clarified, the extrapolated curves in Fig. 6 suggest that more H2 is dissolved in the deep fluids than CH4. Understanding the migration of the thermogenic H2 in the subduction zone is a future research topic.

Fig. 6: Extrapolated solubilities of CH4 and H2 in pure water under high temperatures and pressures.
figure 6

The solubilities of CH4 and H2 from Duan et al.64 and Zhu et al.66, respectively.

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Geological sources of H2 in the Nankai Trough

The generation, expulsion, and possible migration of CH4 and H2 in the Nankai Trough subduction zone are summarized in Fig. 7, where accumulation and emission of thermogenic and microbial CH4 are also shown. The mud volcanoes and dissolved gas deposits are distributed nearly above the thermogenic CH4 generation zone in the underthrust sediments and accretionary prisms (Fig. 5), suggesting the upward migration of free CH4 by buoyancy. The formation of mud volcanos occurs intermittently and quickly. It seems unlikely that the eruption of CH4 to form mud volcanoes occurs by direct CH4 supply from the deep subduction zone. A type of CH4 chamber might have developed below the mud volcanoes from which CH4 is then expelled (Fig. 7). The fracturing and deformation of the CH4 chamber due to earthquakes and tectonics can induce CH4 release to form mud volcanoes and CH4 plumes. In the shallow subsurface, where temperatures are below 60 °C, the solubility of CH4 in water does not change much with a decrease in temperature and pressure (Fig. 6). Therefore, in an aquifer unsaturated with CH4 in the shallow subsurface, the free CH4 migrating upward can be dissolved and trapped by the aquifer to form dissolved gas deposits (Fig. 7).

Fig. 7: The generation, expulsion, and possible migration of CH4 and H2 in the Nankai Trough subduction zone.
figure 7

Thermogenic CH4 and low-temperature H2 generation zones in the accretionary prism are not indicated. The thermogenic CH4 and H2 generation rate in the underthrust sediments below the décollement is higher than in the accretionary prism because of sufficient thermal maturation and a faster subduction rate. The thermogenic CH4 and H2 generation zones of underthrust sediments overlap with the earthquake rupture area, suggesting the expulsion of thermogenic CH4 and H2 associated with earthquakes. The mud volcanoes and dissolved-in-water type gas deposits with thermogenic CH4 are distributed nearly above the thermogenic CH4 generation zones of the underthrust sediments and accretionary prism. Thermogenic CH4 expelled from the source rocks in the deep subduction zone migrates upward to form mud volcanoes and dissolved gas deposits. Gas hydrates holding biogenic CH4 are also widely distributed in the Nankai Trough. In organic-poor sedimentary environments, microbial CH4 production requires abiogenic geological H2 such as low-temperature H2 from the thermal decomposition of immature organic matter, hydrolysis H2 from serpentinization and hydrothermal rock alterations, mechanochemical H2 from the interaction of silicate rock fracturing and water, and thermogenic H2 from the thermal alteration of sedimentary organic matter undergoing metagenesis to metamorphism. The migration of the slab dehydrated fluids possibly plays an essential role in transporting H2 from various sources to the subseafloor biosphere.

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Several sources of H2 that might contribute to microbial CH4 production and sustain the subseafloor biosphere have been proposed. These include H2 generated by the thermal decomposition of organic matter under relatively low temperatures (<120 °C), H2 released during hydrothermal rock alteration and serpentinization of oceanic crust, and mechanochemical H2 generated by the interaction of water with silicate rock fracturing1. The open-system pyrolysis of kerogen shows that molecular H2 is liberated from kerogen by thermal cracking of hetero-bonds, demethylation, aromatization, and condensation6. Since the heating rate of the underthrust sediments is higher, the generation rate of such low-temperature thermogenic H2 is expected to be higher than in the accretionary prisms (Fig. 7). However, hydrous pyrolysis of kerogen under the closed system shows that hydrogen radicals generated in the hydrocarbon generation zone are consumed quickly by hydrogen-requiring reactions such as hydrogenation of unsaturated compounds7. The hydrocarbon generation in organic-poor sedimentary rocks can be regarded as proceeding in a nearly closed system. Hence, a sufficient amount of molecular H2 might not be generated in the hydrocarbon generation zone (<120 °C). Seismological analysis has detected slab dehydration and seismic velocity decrease near the surface of the oceanic crust beneath the Shikoku region, suggesting the slab serpentinization by the water released from the slab70. Low seismic velocity zones, possibly indicating serpentinization, have also been detected near the surface of the oceanic crust beneath the inner accretionary prism off the Kii Peninsula71. The serpentinization of oceanic crust beneath the inner accretionary prisms is thought to be due to the invasion of seawater through the fault system extending to the mantle72. The hydrothermal rock alteration and serpentinization, possibly ongoing in the subduction zone, can be a potential source of hydrolysis H2 (Fig. 7). In the tectonically active subduction zone, the mechanochemical H2 generated by the interaction of water and silicate rock fracturing can be expected as one of the potential H2 sources. The active fractures and faults in and around the décollement and earthquake rupture area are likely principal sites of mechanochemical H2 generation4,5 (Fig. 7). The thermogenic H2 generated under relatively high temperatures (>220 °C) discussed in the present study can be another potential geological source contributing to microbial CH4 production in the subseafloor biosphere (Fig. 7). The expulsion of the thermogenic H2 from the underthrust sediments is likely to occur associated with seismogenesis. Therefore, the mechanochemical H2 generated in the earthquake rupture area would behave with the thermogenic H2 generated in the underthrust sediments.

The H2 from the sources described above has the potential to contribute to sustaining the subseafloor biosphere and the CH4 production by hydrogenotrophic methanogens. The low-temperature thermogenic H2 generated in the shallow sedimentary rocks is comparatively close to the subseafloor biosphere. However, a sufficient amount of low-temperature H2 generation seems unlikely compared to the thermogenic H2 generation in the deep subduction zone. Other geological sources of H2 are located deep in the subduction zone, and long-distance transportation of H2 is required to support the subseafloor biosphere. The migration of slab-dehydrated fluids is active in the Nankai Trough subduction zone, and such deep fluids may play an essential role in the long-distance transportation of H2. Compared to gas phase migration, migration of low-solubility gas by solution would be inefficient and require large volumes of fluids. The efficient transport of H2 by gas phase migration might occur under a limited amount of fluid in the deep subduction zone. However, our knowledge of the solubility and the behavior of H2 under high pressures and temperatures is currently limited. In addition, the generation rates of H2 derived from the low-temperature thermal decomposition of organic matter, serpentinization of oceanic crust, and silicate rock fracturing are currently unknown. If the generation rates of H2 from these geological sources are clarified, comparing them with that of the thermogenic H2 from the underthrust sediments estimated in this paper, the relative contributions of each source of H2 for sustaining the subseafloor biosphere will be evident. Moreover, it will deepen our understanding of the geosphere-biosphere interaction leading to the active microbial CH4 production and the formation of large-scale gas hydrates in the oceanic plate subduction zone.

Methods

Shales and metapelites from old accretionary prisms

Shales and metapelites that experienced a paleo-temperature ranging from 100 to 600 °C were collected from the Shimanto belt and the Sanbagawa metamorphic belt in the Kochi district (Supplementary Fig. 6). Rock samples used in the present study were resampled from the same sampling sites as our previous study8, and the geological background has been described therein. The Sanbagawa metamorphic belt is divided into the Ooboke and Besshi Nappes based on lithology and metamorphic age. The protoliths of the Sanbagawa metamorphic belt are Triassic to Jurassic and metamorphosed during the Late Cretaceous73. The chlorite, garnet, albite-biotite, and oligoclase-biotite metamorphic zones occur along the sampling traverse of the Asemi-gawa River. The metamorphic temperature of metapelites increases gradually from the chlorite zone (300 °C) to the oligoclase-biotite zone (610 °C). The Shimanto belt in the Shikoku region is divided by the Aki Tectonic Line into a northern and a southern belt. The northern belt comprises the Lower Cretaceous Shinjogawa and the Upper Cretaceous Aki groups. The southern belt comprises the Eocene to the Lower Oligocene Muroto Peninsula Group and the Upper Oligocene to the Lower Miocene Nabae Groups. Shale samples were collected from outcrops of Aki and the Muroto Peninsula Groups distributed along the eastern coast of Tosa Bay. All the shale and metamorphic rocks were deposited in the marine environment with a variable contribution of terrestrial organic matter. There may be some differences in the organic type among the samples, but this would not have much influence on the geochemical characteristics of sedimentary organic matter at the highly mature stage49. The paleo-maximum temperature of shales and metapelites was estimated by vitrinite reflectance (VR) and metamorphic mineral assemblages.

Pulverization and residual gas recovery

Residual gas is released from shale fragments during pulverization. Shale fragments were pulverized using a P-6 planetary ball mill and a tungsten carbide mill pot with needle valves for gas transfer lines (Fritsch GmbH, Idar-Oberstein, Germany). The inner volume of the mill pot is 69.6 mL. Fragments of shale and metapelite of 5–7 mm were pulverized in the tungsten carbide mill pot under an ultra-high purity helium (He) atmosphere of 0.3 MPa. Cleaning of the mill pot by ultra-high purity He was repeated three times before the pulverization. Residual gas released in the mill pot was directly transferred to the gas sampling loop of the gas chromatograph through the stainless transfer line. The temperature of the tungsten carbide mill pot during pulverization was below 45 °C. The grain size of the rock powder after the pulverization was measured by a laser diffraction-scattering method using a grain-size analyzer (LA-920: Horiba, Kyoto, Japan) and was generally from 3–7 µm.

Residual gas, elemental analysis, and vitrinite reflectance

The composition of the residual gas in the rock fragments was measured by gas chromatography (GC) using an instrument (7890 A: Agilent, Santa Clara, CA, USA) equipped with a pulsed discharge He ionization detector (PDHID) and a micropacked column containing ShinCarbon-ST 80/100 (2.0 m × 1.0 mm i.d.; Shinwa Co., Nagoya, Japan) (Supplementary Fig. 7). The oven temperature of the GC was programmed to 40 °C for 3 min, increased to 300 °C at a rate of 15 °C min−1, and then held at 300 °C for 15 min. Ultra-high purity He was used as the carrier gas. A constant amount of gas directly transferred from the tungsten carbide mill pot was introduced into the GC column using a 50 μL sampling loop. Compounds were identified and quantified by comparing the retention times with those of reference standards in a gas mixture containing CH4 (495 ppmv), C2H4 (494 ppmv), C2H6 (495 ppmv), and CO2 (480 ppmv) (Taiyo Nippon Sanso Group, Co., Kawasaki, Japan) and the ionization coefficients by Wentworth et al.74. The detailed analytical procedure is described in Saito et al.75. Total organic carbon (TOC) and total nitrogen (TN) contents were determined using an EA 3000 elemental analyzer (Euro Vector Co., Milan, Italy). The pulverized shale or metapelite was weighed and placed in a silver capsule with drops of 1 N HCl to remove carbonates. The carbonate-free sample was dried at 120 °C for 2 hrs and analyzed by the elemental analyzer. The mean VR of randomly oriented vitrinite grains was measured using a reflection microscope (Eclipse LV100ND; Nikon Corp., Japan) equipped with a stabilized halogen light source and photonic multichannel analyzer (PMA12; Hamamatsu Photonics K. K., Japan). The VR value was measured for a spot diameter of 20 μm at a wavelength of 542.8 nm by comparison with the standard values of polished glasses with VR values of 0.55, 0.79, 1.08, and 1.53%.