Significance We document organic matter encapsulated in rock clasts from a oceanic serpentinite mud volcano above the Izu–Bonin–Mariana subduction zone (Pacific Ocean). Although we cannot pinpoint the exact origin of the organic matter, chemical analysis of the constituents resembles molecular signatures that could be produced by microbial life deep within or below the mud volcano. Considering the known temperature limit for life, 122 °C, and the subduction zone forearc geotherm where such mud volcanoes are located, we estimate that life could exist as deep as ∼10,000 m below the seafloor. This is considerably deeper than other active serpentinizing regions such as midocean ridges and could have provided sheltered ecosystems for life to survive the more violent phases of Earth’s history.

Abstract Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts expelled from mud volcanoes above the Izu–Bonin–Mariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni–Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122 °C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ∼10,000 m below the seafloor. This is deeper than the 122 °C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, Mid-Atlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth’s largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth’s history such as the late heavy bombardment and global mass extinctions.

Microbial life may be sustained within the lithosphere by mineral-mediated chemical reactions that provide usable energy resources (1). For example, redox-coupled reactions during serpentinization, the formation of serpentine [(Mg,Fe) 3 Si 2 O 5 (OH) 4 ] through mantle olivine [(Mg,Fe) 2 SiO 4 ] hydration, generate substantial amounts of H 2 (2). Although serpentinization leads to extreme pH conditions and limited nutrient and electron acceptor availability (1), microgenomic studies of serpentinization-fueled hydrothermal deep-sea vents and continental fluid seeps show evidence for microbial H 2 and CH 4 utilization (1, 3⇓–5). Furthermore, micrometer-sized organic matter has been detected in dredged seafloor serpentinites (6) and in subseafloor mixing zones between seawater and serpentinization-derived fluid (7). The former study suggests that serpentinization-fueled microbial communities may use solid electron acceptors, particularly ferric iron from magnetite (Fe 2 O 3 ) or other Fe(III)-bearing minerals, such as andradite garnets [Ca 3 Fe(III) 2 Si 3 O 12 ] (8). However, the architecture of potentially habitable domains within Earth’s hydrated mantle rocks remains largely unknown. Understanding the possible relationship between mineral reactions and biological activity requires identification of in situ signatures of the deep biosphere that allow us to plunge beneath the Earth’s surface to assess its extent and how mineral reactions may support or even form life. Serpentinite clasts recovered from the South Chamorro mud volcano [13°47′N, 146°00′E; Ocean Drilling Program (ODP) Leg 195] above the Izu–Bonin–Mariana (IBM) subduction zone (9) (Fig. 1) can potentially provide just such a window into the deep biosphere. The mud volcanoes source their serpentine from >20-km depth via deep-reaching forearc faults, where serpentinite gouges mix with slab-derived fluids before buoyantly rising toward the seafloor (10).

Fig. 1. Location of the South Chamorro serpentinite mud volcano and the Izu–Bonin–Mariana (IBM) subduction zone (modified from ref. 11). (A) Bathymetry map of the Mariana arc-basin system displaying the location of the South Chamorro Seamount (Leg 195) in relation to the volcanic islands such as Guam. Approximately 50 km south of the seamount the water depth exceeds 8 km highlighting the trench of the IBM subduction zone that runs approximately north to southwest. (B) Three-dimensional view of the South Chamorro Seamount, depicting the location of ODP Site 1200. The subducting Pacific slab beneath the serpentinite mud volcano is in ∼20-km depth.

IBM Subduction Zone and Serpentinite Mud Volcanism The IBM subduction zone is a convergent plate margin ranging over ∼2,800 km from near Tokyo (Japan) to south of Guam (Mariana Islands; Fig. 1). The IBM is located along the eastern margin of the Philippine Sea Plate in the Western Pacific Ocean and formed due to the subduction of the Pacific Plate under the Philippine Sea Plate (11). The southern boundary is marked by the intersection of the IBM trench with the Palau–Kyushu Ridge at 11°N. The northern boundary is at 35°20′N close to southern Honshu, Japan (12). The eastern boundary extends along a deep-sea trench and ranges in depth from 3 km at the Ogasawara Plateau (trench entrance) to ∼11-km depth within the Challenger Deep—the deepest site in the world. The Mariana forearc is pervasively faulted by tectonic activity and only minor sediment accretion occurs along the margin (9, 13). As the Pacific Plate descends, oceanic upper mantle, oceanic crust, overlaying sediment, and water are transported into the forearc mantle. Some of this material is transferred from the subducting slab into the overlying mantle and oceanic plate, where large quantities of fluids rise through faults and fractures, carrying dissolved constituents from the subducting slab. These fluids can either vent as cold springs onto the seafloor (14) or hydrate and serpentinize the mantle wedge. The latter is supported by deep-sea drilling and geophysical measurements showing that at least part of the Mariana forearc mantle wedge is hydrated (15, 16). Within the deep-reaching forearc faults, serpentinite fault gouges mix with the rising slab-derived fluids. This mud–rock mixture buoyantly rises in conduits along fault planes until it extrudes onto the seafloor to form various kilometer-scale seamounts, that is, mud volcanoes, predominantly composed of serpentinite, situated on the outer forearc of the Mariana margin (e.g., ref. 17) (Fig. 1B). The mud volcanoes are located in a trench-parallel zone ∼30–100 km arcward of the trench axis and reach up to 50 km in diameter and over 2 km in height (17, 18). Unconsolidated flows of clay- to silt-sized serpentinite mud enclose up to boulder-sized rock clasts of variably serpentinized mantle peridotite and subordinately blueschist-facies fragments (18). The samples studied here are recovered from drill cores taken from the South Chamorro serpentine mud volcano (13°47′N, 146°00′E; Fig. 1B) drilled during ODP Leg 195 (19). The seamount is a partly collapsed, roughly conical structure ∼2-km high and ∼20-km wide with active serpentine/blueschist mud volcanism. The subducting slab beneath the serpentinite mud volcano is at ∼20-km depth (14, 18).

SI Raman Spectroscopy Raman spectra of organic matter were collected using a Kaiser HoloLab Series 5000 equipped with a diode-pumped solid-state laser (785 nm) (Utrecht University). Hyperspectral mapping was performed with a Horiba Scientific LabRam HR800 (Steinmann Institute, University of Bonn). Raman scattering was excited with a diode-pumped solid-state laser (784 nm) with less than 40 mW at the sample surface. A 100× objective with a numerical aperture of 0.9 was used resulting in a diffraction-limited lateral resolution of ∼1 µm. The confocal hole was set to 1,000 μm, resulting in a depth resolution of a few micrometers. The scattered Raman light was collected in a 180° backscattering geometry by an electron-multiplier charged coupled device detector after passing through a 100-μm (single measurements) or 200-µm (mapping) spectrometer entrance slit and being dispersed by a grating of 600 grooves per millimeter, yielding a spectral resolution of 1.7 and 2.3 cm−1, respectively, as determined from the width of Ne lines. The spectrometer was calibrated with the first-order Si Raman band at 520.7 cm−1 and Ne lines. The total acquisition time varied between 900 and 1,200 s for single measurements and 25 × 0.5 s for mapping. Normalization of Raman spectrum to the most intense epoxy band at 815 cm−1 shows no contribution of bands from the epoxy resin (Fig. S1). This is evident by the absence of epoxy bands (arrow in Fig. S1) between 950 and 1,000 cm−1 in the sample spectra. The false-color Raman image shown in Fig. 2G is generated from the spectra recorded for each pixel of the image by color-coding the ratio between the integrated intensity of an organic vibrational modes near 639 cm−1 and a serpentine mode at 690 cm−1 (Fig. S2). Warm colors reflect a high content of organic material within the analyzed volume, whereas blue colors mark regions with low or undetectable organic material. Spectra were collected over an area of 60 × 60 µm2 with a step size of 0.5 µm, resulting in 7,200 spectra. The resulting map is shown in Fig. S4. To obtain reduced or R(ω) intensities for future comparison that are independent from the instrument, the laboratory temperature, and the excitation wavenumber, ν e , and thus directly proportional to relative scattering activity, the Raman spectra from the mesh core were also corrected for (i) the instrument response function (white light correction); (ii) the excitation frequency dependence, that is, by the scattering factor, (ν e − ν)3, with ν being the wavenumber of the scattered light (intensities were measured in photons per second); and (iii) the temperature effect, that is, by the Bose–Einstein temperature factor, 1 − exp(−hνc/kT), with h, c, k, and T being the Planck constant, the speed of light, the Boltzmann constant, and the temperature, respectively. The corrected spectra were deconvoluted by least-squares fitting Gauss–Lorentz functions along with a linear background that was subtracted after the least-squares fit (Fig. S5 and Table S1). The error of the reported band positions is smaller than 0.1 cm−1. Raman evaluation of the serpentine polymorphs present in the rim and core structures was conducted by examining the water vibration region at ∼3,600 cm−1. Raman spectra taken, using the 785-nm laser, showed no evidence for the OH bands due to the low scattering efficiency of these bands with this laser. Therefore, the 532-nm coherent compass sapphire laser of a WITec alpha 300R was used for the OH band analysis of serpentine group minerals. The analysis was conducted with a 50× long working distance lens with a 0.55 numerical aperture in backscattering geometry to the sample. After the scattered Raman light passed through a pinhole of 20 µm, the light was dispersed on a grating of 1,800 grooves per millimeter, resulting in a spectral resolution of 1.1 cm−1 in the spectral region of interest, measured using a built-in calibration light source. Fig. S1. Epoxy-normalized Raman spectrum of sample E7H2-5 (28.70 mbsf). All spectra were normalized to highest intensity epoxy Raman mode (815 cm−1), showing that epoxy does not contribute to the observed Raman spectra of the identified organic molecules. Fig. S2. Representative Raman spectra of the mesh core and rim region taken from the area in which hyperspectral imaging was performed. Bands marked with an asterisk belong to lizardite/chrysotile. All other bands reflect complex organic material (see main text and Table S1). Fig. S3. Representative Raman spectra of OH-stretching modes, fingerprinting lizardite and chrysotile (e.g., ref. 64) typically found in the mesh rim and core regions, respectively, of the serpentinite clasts. Fig. S4. (A) Backscattered electron image showing the location of the Raman map. (B) Shown is the distribution of organics within the mesh core and rim. I org and I serp are the integrated intensities of the bands near 639 and 690 cm−1, respectively. Fig. S5. Raman spectrum as shown in Fig. S2 from the mesh core, but with reduced intensities R(ω) that were obtained by correcting the measured intensities for the instrumental response function, temperature effects, the excitation frequency dependence, and background (for more details, see Methods in the main text). Also shown is the deconvolution of the spectrum obtained from least-squares fitting individual Gauss–Lorentzian functions (gray curves) to the data. The red curve represents the sum curve. The residuals of the fitting procedure are also shown. Table S1. Band positions, band widths (FWHM), and relative intensities of Raman bands observed from the mesh core region

SI Opaque Mineral Grain Analysis Fig. S6 shows the distribution of opaque mineral grains (high backscattering intensity in Fig. S6A) within the serpentinites clasts. Several FIB-SEM sections were prepared across the mesh rim–core interfaces, where Fig. S6B shows a representative high angle annular dark field (HAADF) image highlighting three nanoparticles with high intensity (overview image in main-text Fig. 3C). The intensity in an HAADF image scales with atomic number, implying that the nanoparticles are atomically heavier than the surrounding serpentine grains. Energy-dispersive X-ray analysis (Fig. S6C) reveals that the nanoparticle exist exclusively of Ni and Fe in a ∼2:1 ratio. Together with chemical analysis conducted on the micrometer-scale in a scanning electron microscope, the microstructural association of the particles within the serpentinites, and the limited possibilities of potential mineral phases with Ni and Fe in a ∼2:1 ratio (65), we conclude that the nanoparticles are awaruite (Ni 2 Fe–Ni 3 Fe). Fig. S6. A is a backscattered electron (BSE) image taken in a scanning electron microscope showing the distribution of opaque minerals (high backscattering intensity). B is a high-angle annular dark-field (HAADF) image taken with transmission electron microscope in scanning mode. The corresponding EDX analysis of a nanosized awaruite grain is shown in C. The Cu Kβ peaks originates from the FIB section sample holder. The Mg Kα, Si Kα, and Ο Kα peaks are a minor contribution from the surrounding serpentine grains.

Depth Limit for Microbial Life Within Subduction Zone Forearcs To test whether microbial life is a feasible source of the organic matter observed, we need to establish an estimate for the depth limit for life within the Mariana forearc. As microbial organisms can survive temperatures as high as 122 °C (32) and pressures into the gigapascal range (33), we estimated the potential depth limit for microbial life in this region using a one-dimensional steady-state heat conduction model (34) (SI Estimation of the Maximum Depth for the Current Temperature Limit for Life). In this calculation, heat is transferred in one direction without consideration of minor advective heat flow through the ascending mud or heat generated through the exothermic serpentinization reaction. Both of these processes are expected to only play a minor role. Particularly heat through serpentinization has been shown to be nearly negligible (35). Assuming no variations in temperature or heat flow, the basic equation of conductive heat transfer theory is a statement of conservation of energy and can be written as follows: T = T 0 + q 0 k y − ρ H 2 k y 2 , [1]where T and T 0 are the temperature at depth and at the ocean floor, y is the depth in meters below seafloor, q 0 is the surface heat flow (0.03 W⋅m−2), k is the thermal rock conductivity (2.9 W⋅m−1⋅K−1), ρ is the density (2,900 kg⋅m−3, partially serpentinized peridotite), and H is the current mean mantle heat generation rate due to radioactive decay (7.42 × 10−12 W⋅kg−1) (34). At relatively shallow forearc depths, H will only play a minor role. Rearranging the equation above to solve for y at a given temperature (i.e., the known temperature limit for life at 122 °C) results in the following: y ± = q o ± 2 T 0 H k ρ − 2 H k ρ T + q o 2 H ρ , [2]where y + is a nonphysical solution and thus disregarded. Surface heat flow values of the Mariana forearc were taken from measurements acquired during the Deep Sea Drilling Project Leg 60 (36). Thermal conductivity values of serpentinites are based on measurements taken during ODP 209 (37) and an average value of 2.9 W⋅m−1⋅K−1 is used. Using these values, the maximum depth for the 122 °C isotherm varies between ∼8,000 and 15,000 mbsf (Fig. 5C). These depth estimates are based on surface heat flow values of 0.03–0.04 W⋅m−2 that agree with the observed depression of isotherms in most forearc mantle wedges, even those of relatively hot origin such as the Cascadia subduction zone (38, 39). Moreover, our thermal calculations are in agreement with more complex geodynamic models (40, 41), confirming that the 122 °C isotherm is reached at ∼10,000 mbsf in forearcs. Hence, current serpentinization-fueled microbial life within subduction zone forearcs could be supported down to these depths and corresponding pressure (∼0.34 GPa; Fig. 5). In contrast, habitable zones in the vicinity of oceanic spreading centers are limited to the first hundred meters to few kilometers below the seafloor. The exact location of the 122 °C isotherm will likely vary in depth at and around the spreading center as a result of the ridge architecture, heat flux, and hydrothermal circulation (42, 43). The downhole temperature within the International Ocean Drilling Program (IODP) Hole U1309D at Atlantis Massif oceanic core complex (Mid-Atlantic Ridge) places the 122 °C upper temperature limit for microbial life at ∼1,000 mbsf (44). This is one order of magnitude less than compared with our estimated limit of life in the Mariana subduction zone forearc. As the serpentinite mud originates directly from the forearc mantle wedge (>20-km depth), the model indicates the potential for a biosphere deep within the forearc. This makes the Mariana serpentinite clasts a natural laboratory of prime interest when searching for habitable zones of life deep within the lithosphere. Fig. 5. Conceptual model of a deep biosphere environment within the IBM subduction zone forearc with limit for serpentinization-fueled microbial life estimated at 10,000 mbsf based on the known upper temperature limit for life (122 °C) (34) and our heat conduction model. A shows a cross-sectional sketch of the IBM forearc. Fluid release from the subducting plate results in partial forearc mantle serpentinization. Tectonic activity causes mud–rock mixture to rise buoyantly in conduits along fault planes until it protrudes onto the seafloor to form massive serpentinite mud volcanoes (up to 50-km diameter and >2 km above the surrounding seafloor). The sketch in B displays a conceptual serpentinization evolution model and the depth range for possible subsurface microbial colonization. C and D show results of the one-dimensional heat conduction model (SI Estimation of the Maximum Depth for the Current Temperature Limit for Life), where C shows the maximum depth as a function of surface heat flow at constant thermal conductivity (partially serpentinized peridotite) below which microbial life is theoretically possible. D displays the influence of surface heat flow and thermal conductivity, at an average depth of 12,000 mbsf, on the upper temperature limit for life.

SI Estimation of the Maximum Depth for the Current Temperature Limit for Life A sketch of the one-dimensional steady-state heat conduction model to estimate the maximum depth for the current temperature limit for life in subduction zone forearcs is found in Fig. S7. Fig. S7. Sketch of the one-dimensional steady state heat conduction model (modified after ref. 34).

Sustaining Microbial Life Within Subduction Zone Forearcs To sustain deep microbial life within a solid rock framework requires energy resources that can either migrate or be produced close to areas suitable for colonization. There is little to no chemical benefit for microbes to interact directly with serpentine minerals; thus, other life-supporting energy-generating pathways need to be present. Microgenomic studies show evidence for microbial H 2 and CH 4 utilization in serpentinizing systems with known microbial colonization. Indeed, several studies (e.g., ref. 5) indicate that Archaea found up to 20 mbsf within the South Chamorro mud volcano are fueled by deeply derived CH 4 -enriched fluids. Experiments suggest that H 2 produced during low-temperature serpentinization (<200 °C) (45) could react with a carbon source to form CH 4 on catalytic mineral surfaces. Investigations of naturally occurring abiogenic CH 4 indicate that abiotic hydrocarbon synthesis can potentially take place at temperatures as low as ∼120 °C (46). However, there are contrasting experimental results concerning the formation and synthesis kinetics of CH 4 production at (very) low temperatures (47⇓–49). In hydrothermal experiments, awaruite has been identified as a possible CH 4 production catalyst (50). The awaruite grains observed here are nanosized (Fig. 3C) and, therefore, have a high surface area-to-volume ratio, which should enhance their catalytic activity (51). Thus, nanosized alloys could have facilitated CH 4 production over geologically relevant timescales below the upper temperature limit for life (122 °C). In near-surface serpentinizing systems, hydrothermal fluids can mix with, for example, seawater, resulting in disequilibria that may provide the energy and substrates needed to support chemolithoautotrophic life (7). In contrast, a slowly ascending serpentinite mud along deep-reaching forearc faults may allow the system to remain much closer to equilibrium, regulating the activities of H 2 , CH 4 , CO 2 , and the Fe(II)/Fe(III) ratio in the solids and fluids limiting energy sources. Nielsen et al. (25), however, documented the occurrence of rodingite within the Mariana serpentinite mud volcanoes derived from in situ alteration within the forearc mantle. These rocks suggest hydrothermal interactions between mafic and ultramafic units and together with simultaneous serpentinization may be the source of fluids that can produce disequilibrium environments. An additional source of externally derived fluids could come directly from the subducting slab and would thus be in disequilibrium with the overriding forearc wedge. These fluids are most likely different in composition compared with serpentinizing systems at midocean ridges or passive margins. Kelemen and Manning (52) recently reevaluated the global carbon flux through subduction zones and estimated that several million tons of carbon per year could be released from subducting slabs into the overriding forearc wedge. Therefore, these fluids could provide the carbon source needed for abiotic hydrocarbon synthesis or even directly contribute organic molecules to the forearc and thus the serpentinite clasts. If the organic matter reported here was sourced from the subduction slab alone, we would expect the same level of maturation and thus evidence for the same functional groups. However, we observe the absence of specific Raman bands [N(C–C)] in clasts from different depths (Fig. 2F). Various studies have shown that subduction zone dehydration reactions across a range of temperatures (200–600 °C) and pressures release nitrogen that is able to enter the overriding forearc directly or travel with the expelled fluids upward through the subduction zone channel (53⇓–55). Moreover, recent thermodynamic calculations of the nitrogen speciation in aqueous fluids under upper mantle conditions suggest that the oxidized mantle wedge of subduction zones favors nitrogen over ammonium (NH 4 +), promoting outgassing rather than mineral trapping of nitrogen (56). Thus, the actions of mud volcanoes as conduits for slab-derived fluids may provide missing resources that would otherwise be limiting factors for life in the forearc mantle or contribute nitrogen to the abiogenic synthesis of more complex organic compounds. To rigorously evaluate the energy sources for microbial life within the subduction zone forearc, better constraints are needed for the fluid influxes from the subduction zone and how these slab-derived fluids interact with the forearc. Increasing sophisticated fluid speciation models coupled to fluid–rock interaction simulations and experiments at high pressures and temperatures (56, 57) will provide critical insights into this problem.

Implications Although the origin of the organic matter cannot be unequivocally identified, we suggest, based on the similarities with molecular signatures of bacteria-derived biopolymers, that the organic matter may represent remnants of microbial life within or even below the mud volcanoes. Our simple model supports this hypothesis, showing that the temperature window for life could extend deep into the forearc. Hence, the identification of complex organic matter recovered from depths of up to 110.07 mbsf may be evidence for life in an oceanic serpentinite-hosted rock formation from depths likely far exceeding the drill core depth, where serpentinite-supported life has not yet been documented. Thus far, evidence for microbial communities within the Mariana mud volcanoes has only been indirectly detected in fluid samples no deeper than 20 mbsf (5, 24). There are a variety of examples indicating that microbial life can colonize shallow serpentinization-fueled environments and use abiogenically produced H 2 and CH 4 (1, 3⇓–5), but microbial life within the deep subsurface, for example, deep within the Mariana forearc, with no connection to the Earth’s surface, may have little resemblance with presently known serpentinization-fueled ecosystems. In any case, if life is present in the subduction zone forearc, it has far-reaching implications as recent studies suggest that environments resembling those both within and below the Mariana serpentinite mud volcanoes were already present on the early Earth (58, 59). Thus, even if modern-type subduction was not fully established in the Hadean and Archean, Mariana forearc-like deep subsurface environments may have allowed early forms of life to thrive, despite violent phases such as the so-called Late Heavy Bombardment, a period of intensive meteorite bombardment around 3.9 Ga (60). Even if only a small amount of the global forearc mantle hosts microbial life, fluctuations in the total subduction zone length (61) could have significant consequences for the deep carbon budget. During these fluctuations, fluid flow through subduction zone forearc regions, visible in the form of serpentinite mud volcanism, are a crucial connection between the deep biosphere and surface world, influencing geochemical fluxes throughout Earth’s history. Only if we keep exploring the windows into the deep subsurface, such as the serpentinite clasts presented here, will we be able to establish a full budget of Earth’s deep carbon and the potential for a subsurface biosphere.

Methods Sample Preparation. Drilling during ODP Leg 195 was executed with the support of inorganic drilling mud (mainly sepiolite) and seawater. Immediately after the core was recovered, a 6-in. length of whole-round core was cut and refrigerated. Samples (diameter, 4.25 cm) were removed from the whole-round core using a piston core sampler. These subcores were loaded into a Manheim squeezer for the analysis of physical properties (62). The applied axial pressure (6.3 MPa) under drained conditions is insufficient to cause pore collapse within individual clasts. From the “squeeze cakes,” 1/4 rounds were extracted for further onshore analysis and stored in a nonsterile fashion. Serpentinite clasts, hundreds of micrometers to a few millimeters in size, were extracted from these mud-pellets rounds by dissolution in distilled water and hand picking under a binocular microscope. Clasts were mounted in 1-in. round sections using epoxy resin and polished to expose the internal structures. The samples were not subjected to a vacuum impregnation step to avoid the penetration of epoxy into open spaces. For Raman spectroscopic details of the epoxy, SI Raman Spectroscopy. To avoid laboratory contamination before microstructural and microchemical investigations, the samples were treated with 5% sodium hypochlorite and repolished using a diamond paste and concentrated ethanol to expose fresh sample surfaces. Subsequently, samples were again treated with 5% sodium hypochlorite. ToF-SIMS. Element distribution maps were obtained using an ION-TOF ToF-SIMS IV instrument at the Smithsonian Institution’s National Museum of Natural History (Washington, DC). The 25-kV 69Ga+ primary ion column was operated in a low-current bunched mode with a cycle time of 45 μs, allowing for a mass resolution (full width half-maximum) of 4,600 at nominal mass 61 u (C 2 H 5 S)+. To remove surface contamination, an area four times larger than the analytical field of view was sputter-cleaned with a 3-keV Ar+ ion beam before the actual measurement. FIB-SEM and Transmission Electron Microscopy. Scanning electron microscopy (SEM) investigations on Pt-coated specimens were carried out in a JEOL JCM-6000. A FEI Nova Nanolab FIB-SEM was used to acquire nanotomography volumes (voxel size, 2.5 × 2.5 × 10 nm) and to extract electron transparent foils for transmission electron microscopy (TEM). FIB-SEM nanotomography was carried out at an acceleration voltage of 2 kV and a beam current of 0.21 nA. The nanotomography volumes were visualized and analyzed using Avizo 9. TEM investigations were executed in a FEI Tecnai 20F operated at 200 kV equipped with a high-angle annular dark field (HAADF) detector and an energy-dispersive X-ray (EDX) spectroscopy system. Raman Spectroscopy. Raman spectra of organic matter were collected using a near-infrared (784/785 nm) laser of a Horiba Scientific LabRam HR800 and a Kaiser HoloLab Series 5000. Hyperspectral Raman mapping of the organic matter distribution was executed with a lateral resolution of ∼1 µm and a spectral resolution of 2.3 cm−1. Analysis of the “fingerprinting” serpentine OH bands was performed using a 532-nm laser of a WITec alpha 300R. Further information about instrument settings and Raman spectra analysis is found in SI Raman Spectroscopy.

Acknowledgments We thank T. Ludwig for discussion. O.P. was supported by Netherlands Organization for Scientific Research Veni Grant 863.13.006, H.E.K. by the European Union Fellowship PIOF-GA-2012-328731, T.Z. by the German Science Foundation (ZA285/5), and Y.L. by the Utrecht University Sustainability Program. For I.P.S., Ocean Drilling Program (ODP) Leg 195 participation and post-cruise research were funded by Joint Oceanographic Institutes/US Science Advisory Committee (Schlanger Fellowship) and the UK International Ocean Discovery Program (IODP) Program via the Natural Environment Research Council (NE/M007782/1).

Footnotes Author contributions: O.P. designed research; O.P., H.E.K., T.G., Y.L., S.P., I.P.S., D.R., and T.Z. performed research; T.G., Y.L., D.R., and T.Z. contributed analytic tools; O.P., H.E.K., T.G., Y.L., S.P., D.R., and T.Z. analyzed data; and O.P., H.E.K., and T.Z. wrote the paper.

The authors declare no conflict of interest.

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