FLUID FLOW AND EVOLUTION OF GAS HYDRATE MOUNDS OF

JOETSU BASIN, EASTERN MARGIN OF JAPAN SEA: CONSTRAINTS FROM HIGH-RESOLUTION GEOPHYSICAL SURVEY BY AUV

Ryo Matsumoto∗, Mineo Hiromatsu

University of Tokyo Hongo Bunkyo-ku Tokyo 113-0033

JAPAN

Mikio Sato National Institute of Advanced Industrial Science and Technology

C-7 Tsukuba Ibaraki 305-8567 JAPAN

YK10-08 Shipboard Scientists

ABSTRACT Accumulation of massive gas hydrates in shallow sediments forms gas hydrate mounds, 200-500 in diameter, on the spur and knoll of the Joetsu basin in the eastern margin of Japan Sea. Acoustic survey by AUV-URASHIMA has revealed contrasting topographic features and subsurface structures between gases hydrate mounds and adjacent hydrate-free areas. MBES (Multi-Beam Echo Sounder) and SSS (Side Scan Sonar) have identified two types of mounds. Type A is a high relief mound with strong SSS-reflections and crater-like depression while Type B is a low relief conical mound with weak SSS-reflections. SBP (Sub-Bottom Profiler) has demonstrated three acoustic units in gas hydrate-free areas. They are, Unit-I: 5-10 m thick massive unit, Unit-II: 15-35 m thick evenly stratified unit, and Unit-III: 10-20 m+ thick massive to weakly stratified unit, in descending order. The acoustic units are correlated to the stratigraphic and lithological units observed on sediment cores. SBP of gas hydrate mounds appears as mud-volcano-like, acoustic transparent zone with high amplitude reflection cap. The high reflection cap extrudes from the seafloor to form high relief mound with depressions (Type A mound) or low relief mound with occasional thin sediment cover (Type B mound). Some mud-volcano-like structures with high reflection cap occur in Unit-II or III sediments below seafloor. The seafloor above the subsurface cap often swells to form low relief mound-like topography (Type C mound), representing the early stage of the formation of gas hydrate mound. Deep-seated gases migrate through gas chimneys and accumulate as solid gas hydrates above BGHS, whereas the hydrate formation is strictly limited by the amount of free waters. Excess gases continue to move up in gas chimney to form gas hydrate in shallower sediments, where the sediments contain enough amounts of free-waters due to downward supply of seawater. On the other hand, excess CH4 and seawater-derived SO4 precipitate carbonates through the anaerobic oxidation of methane (AOM) in shallow sediments. Subsurface high amplitude cap (Type C) is regarded as mineralization front of gas hydrate and carbonates, resulting in the formation of mixed gas hydrate and carbonate buildups. The buildup grows up and reaches to the seafloor (Types B), and then collapse and decay through dissolution, rifting and floating of gas hydrates (Type A).

Keywords: gas hydrate, carbonates, mounds, Japan Sea, Sub Bottom Profiler

∗ Corresponding author: Phone/Fax: +81-3-5841-4522 E-mail: ryo@eps.s.u-tokyo.ac.jp

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

INTRODUCTION Pockmarks and mounds, 50 to 500 m in diameter and 10 to 50 m deep and high, respectively, have been found on the crestal zone of NNE-SSW trending ridges, Umitaka spur and Joetsu knoll of the Joetsu basin, eastern margin of Japan Sea (Fig. 1). The Joetsu basin is located in the southern end of the tectonic mobile zone along the incipient convergent boundary between the eastward moving Amrian micro-plate, the eastern edge of the Eurasian plate, and the North American plate. Formation of a series of ridges, troughs and basins were initiated by tectonic inversion of the stress-field from extension to compression at around 2-3 Ma [1]. The Umitaka spur and Joetsu knoll are regarded as uplifted antiforms bounded by eastward dipping reverse faults. Piston and box coring has recovered visible gas hydrates in the form of layer, platy and nodular concentration or fracture filling veins in shallow sediments (<35 mbsf) and ROV has found massive gas hydrates exposed on the seafloor of the spur and knoll. A number of gigantic methane plumes, 600-700 m high, and numerous gas venting sites have been observed on and around the mounds in close association with bacterial mats and carbonate crusts and concretions [2] [3]. Seismic profiles have revealed a few 100 m to a few km wide, vertical seismic anomaly zones, called "gas chimney," in areas of pockmarks and gas hydrate mounds (Fig. 2). Gas chimneys are characterized by chaotic to transparent seismic facies, however the reflections of bedding planes around gas chimneys are traceable into the chimney, suggesting that the sediments in the

chimneys are not reworked, not derived from below but mostly of in-situ origin. Thus the gas chimneys capped by mounds are not "mud volcanoes" but effective conduits of fluid-flow conveying deep-seated gas and gas-containing waters. Regional BSRs widely occur nearly horizontally, though weak and discontinuous, in parallel to the bedding on and around the spur and knoll, whereas BSRs within gas chimneys are strong and uneven, demonstrating characteristic pull-up structures (Fig. 2). Distribution and occurrence of GSRs are likely to suggest two types of gas hydrate accumulation. One is a stratigraphic accumulation along the BSRs and the other is a structure accumulation within gas chimneys [4]. Data acquired by preceding seismic and acoustic surveys, piston coring, CTD, and ROV dives seem to imply that hard-capped mounds are the focus of gas hydrate accumulation and methane seepages. However, due to difficulty to penetrate down the mounds by piston and box corers, the mechanism and timing of shallow accumulation of gas hydrates, and the relation between two types of accumulation have not been well understood. AUV-URASHIMA DIVES We conducted high-resolution acoustic mapping of mounds and pockmarks of gas hydrate bearing field of the Joetsu basin by AUV-URASHIMA onboard R/V YOKOSUKA (JAMSTEC) in August 2010 (Fig. 3). AUV-URASHIMA is a 10.0-meter-long cigar-shaped vehicle with the weight of 7.5 tons, capable of autonomous cruising in waters as deep as 3500 m for about 300 km operated by fuel cell and 100km by lithium ion

Figure 1 Map showing the location of the Joetsu basin in the eastern margin of Japan Sea. Gas hydrate bearing Umitaka spur and Joetsu knoll (Fig. 5) are 30 to 40 km off Joetsu City, Niigata Prefecture.

Figure 2 Seismic profile showing mounds and pockmarks on the seafloor well developed gas chimney structure, and upwelling of BSR within gas chimney. BSRs occur at around 110 to 120 meter below sea floor (mbsf) (Data from [10]).

batteries. 2010 Dives of AUV-URASHIMA was operated at the cruising speed of 2.4 knots at approximately 70 to 120 meters above the seafloor with the line interval of 90 meters. AUV-URASHIMA is equipped with MBES (Multi- narrow Beam Echo Sounder) with 400 kHz, SSS (Side Scan Sonar) with 120kHz, and SBP (Sub Bottom Profiler) with a chirp of 1 to 6 kHz. Theoretical lateral resolution of MBES and SSS are 1.0 m and 3.0 m, respective, and vertical resolution of SBP is 0.5 m. Figure 4 shows the location and bathymetry of the Joetsu knoll and Umitaka spur, where a number of gas hydrate exposures (red stars) have been found by ROV dives on and around gas hydrate mounds. Small

red dots indicate the site of methane plumes detected for the last 2 years. Figure 5 illustrates the central part of the Joetsu knoll, in which red line in the center and four red rectangles cover the AUV survey areas. Red stars, blue stars, and black dots indicate gas hydrate exposure, gas venting sites, and methane plumes, respectively. Two mounds and a pockmark are included in survey area X, elongated mound in Y, and a low relief mound with gentle slope, 1 km x 0.5 km, on the track line Z. Figure 6A is an enlarged view of survey area X on the Joetsu knoll (Fig. 5), showing the bathymetry of circular depression of a pockmark, an elongated mound 300 m east of the pockmark, and an

Figure 3 AUV-URASHIMA on the R/V Yokosuka of JAMSTEC in Japan Sea.

Figure 5 Map showing the survey area of AUV-URASHIMA dives in the central part of the Joetsu knoll. Red star, blue star and black dots indicate gas hydrate exposure and gas venting sites as observed by ROV, and methane plume as detected by echo sounders.

Figure 4 Map showing the location and bathymetry of study area. Gas hydrate outcrops (red star) as detected by ROV dives and methane plume sites (red dots) observed by echo sounder.

irregularly shaped low hill to the south. Thin red lines indicate the track lines of the AUV-URASHIMA. ROV dives observed scattered carbonate concretions on the eastern slope of the pockmark and wide distribution of bacterial mats and carbonate crusts atop the high relief mound to the east, whereas the south hill did not show any indications of methane seeps and carbonates crusts. High-resolution bathymetry of mounds and pockmark are shown in Figures 6B and 6C. The pockmark image is nearly identical to the bathymetry in Figure 6A, which was acquired by MBES-SEABAT at the bottom of R/V NATSUSHIMA (JAMSTEC). The topography of the mound to the east (Fig. 6B), however, is totally different from the image obtained by R/V NATSUSHIMA. The mound is characterized by 8- m-deep central depression, called crater, surrounded by ring-shaped circular ridge with highly reflective, rough surface. Rough

topography and highly reflective surface of the mound is consistent with the observation by ROV, which identified 1 to 3 meter-high rock-towers and mud mounds covered by broken debris and carbonate crusts, collapse structures such as steep slopes, vertical walls and fresh exposures of gas hydrates, and dense colony of bacterial mats. Low relief mound to the south (Fig. 6C) has also 6 to 7 meter deep crater, however, the mound seems to retain the original dome-shaped conical form with low reflective, smooth surface. This mound is likely to represent the early stage of gas hydrate mounds, whereas the rough and high relief mound is considered as matured and evolved one due to collapse of hard-ground and subaqueous erosion and chemical dissolution of gas hydrates. Based on the topographic features, two types of mounds are identified. Evolved, high reflective mound is referred as Type A and low relief mound, Type B mound.

Figure 6 High-resolution bathymetry of the pockmark and mounds in the area X of the Joetsu knoll (Fig. 5) as revealed by MBES (multi-beam echo-sounder) of AUV-URASHIMA.

Figure 7A and B SBP (sub-bottom profile) showing Type A and Type B hydrate mounds in the area X on the Joetsu knoll (Fig. 5). Type A mound is characterized by surface gas hydrates, methane plumes, methane-induced carbonates, and bacterial mat, whereas Type B by occasional bacterial mat and some carbonates. Evenly bedded units develop for the interval approximately between 10 to 30 mbsf. The lithologic column, MD179-3304, taken by the giant piston corer Calypso of R/V Marion Dufresne (Matsumoto et al., 2011, this volume), is well consistent with the acoustic features of the SBP profiles.

SUB BOTTOM PROFILES Type A and B mounds and Stratigraphic Units SBP is a quick and efficient tool to delineate high-resolution sub-bottom structures, but the penetration depth is usually 30 to 80 mbsf at most depending of the lithology, due to low intensity and high frequency source of chirp (1 to 6kHz). Figure 7A shows the SBP images of the host

sediments and gas hydrate mounds acquired along the track line S to N in Figure 6. The host sediments are observed to be composed of evenly bedded units, while the high amplitude reflectors at about 10 msec (TWT) and 50 msec (TWT) divide the sedimentary sequence into the acoustic Unit-I, Unit-II and Unit-III. Unit-III has

Figures 8A and B SBP showing Type A and Type C hydrate mounds in the area D on the Joetsu knoll (Fig. 5). Type A of the SBP seems to be eroded and collapsed. Type C mound is not identified by the topography on the seafloor but by subsurface acoustic features.

not been well imaged because of poor penetration of chirp down to this level. Lithologic column MD179-3304 on the left (Fig. 7A) was recovered by giant piston corer Calypso of the R/V Marion Dufresne in June 2010 (Matsumoto et al., 2011, this volume). The column is divided into three lithologic parts, 1: massive bioturbated mud (0 to 5 mbsf), 2: interbedded, dark gray thinly laminated mud with light colored massive bioturbated mud (5 to 30 mbsf), and 3: massive silt and mud with frequent intercalation of ash beds (30 mbsf+). The lithologic boundary 1/2 and 2/3 are correlated to the high amplitude reflectors at around 13 msec and 30 msec (TWT). If this is the case, upper and lower high amplitude events correspond the depth of 5 mbsf and 30 mbsf, and dated as 18 ka and 100 ka, respectively. Gas hydrate mounds appear as seafloor manifestation of mud-volcano-like, acoustic transparent zone, called gas chimney (Fig. 7A). Methane plumes, gas hydrate exposures, carbonate crust and bacterial mat were recognized on Type A but only a small bacterial mat on Type B. Highly reflective zone in the uppermost part of gas chimneys represents hard materials such as gas hydrate and carbonate concretions. Type A mound extrude high above the seafloor, while Type B is partly covered by the sediments. Acoustic blanking of gas chimneys may not indicate totally gas charged units, but probably reflects the blocking effects of surface and shallow hard-

grounds to reduce penetration of chirp signals. Type A and Type C mounds SBP image of Figure 8A depicts the NNE-SSW trending topographic high in the area Y on the Joetsu knoll (Fig. 5), in which a number of methane plume sites and gas hydrate exposures had been detected. The topographic high is observed to be composed of two gas hydrate mounds. Type A mound occupies the southern-end of the high, while the northern-end is underlain by subsurface mound (Fig. 8A). Type A mound in the area Y is the most active zone of methane venting and gas hydrate accumulation, exhibiting massive gas hydrate on collapsed valleys and caves associated with active gas venting. Various depression structures and collapse breccias have been observed by ROV dives [5]. The active gas chimney capped by hard gas hydrate mound has extruded from the Holocene sedimentary cover to the seafloor. Low hill at the northern-end of the topographic high had not been recognized as gas hydrate mound, but SBP has clearly demonstrated that subsurface gas chimney, which is capped by high reflective, hard-ground, is likely to push up the seafloor. We call the subsurface, hidden structure in the area Y as Type C gas hydrate mound. Approximately 8-meter thick Holocene sediments cover the Type C. Dotted high reflective parts in

Figure 9 Type C mound on the Joetsu knoll (Fig. 5). The mound appears as low and gentle swell on the crest of the knoll without any diagnostic features of methane seeps, but identified as hidden, subsurface hydrate mound.

the Holocene unit may represent carbonate concretions and nodular gas hydrates (Fig. 8A). Another low relief, low reflective hill, 1.0 km x 0.6 km, on the track line Z (Figs. 5 and 9) shows characteristic features of Type C gas hydrate mound. Hard-cap of gas chimney has developed in the upper part of Unit-2 forming a convex-up structure to form dome-up swell on the seafloor. Possible accumulation of gas hydrate and carbonates has been indicated by SBP in the area of low reflective, low relief hill on the Joetsu knoll. We can identify a number of a low relief mound without any diagnostic indications of gas venting and gas hydrate accumulation. They may possibly be Type C mounds associated with shallow gas hydrate accumulation. LOW VELOCITY ANOMALY AND EXTENSIVE DISTRIBUTION OF FREE-GAS P-wave velocity of hemipelagic sediments is usually 1500 to 1600 m/sec whereas the velocity of gas-hydrate-bearing sediments is expected to be much higher than the values due to high velocity of gas hydrate (~3.0 km/sec). Therefore, the high velocity anomaly has been often regarded as diagnostic parameter or even used to estimate the amount of gas hydrate (Sh) in sediments. Pull-up structure of BSR in gas chimney observed in the Umitaka spur (Figs. 2 and 10) is explained in terms of velocity contrast between gas-hydrate-

bearing gas chimney and host sediments. Difference in the two-way-transit-time between gas chimney (160 msec) and host sediments (220 msec) implies that the P-wave in gas chimney is 40% faster than the host sediments. P-wave velocity of the host sediments can be estimated from the (1) observed depth of BSR (Fig. 10), (2) experimentally determined, equilibrium depth of the base of gas hydrate stability (BGHS) in sea-water [6], and (3) observed thermal gradients of the study area [5]. Figure 10 illustrates the stability condition (green line) and thermal gradients (red and black line). According to large number of heat flow measurements, thermal gradients are given to be 106 mK/m on the spur and knoll and 96 mK/m in the trough region. Then, the depth to BGHS is estimated to be 115 mbsf on the spur and knoll and 135mbsf in the trough (Fig. 10). On the basis of these parameters, P-wave velocity of the host sediments is calculated to be 1000 to 1150 m/sec on the spur and knoll and 1170 to 1230 m/sec in the trough sediments. It is really surprising to have such low velocities for the sediments above the BSR, because the sediments are expected to contain some amount of gas hydrate as documented by BSRs. Some of the gas chimneys exhibit flat lying BSR at the same level of the host sediments, suggesting low velocity even in gas chimneys. Low velocity anomalies in gas-hydrate-bearing gas chimneys and host sediments are to be ascribed to extensive

Figure 10 (Right) Estimation of the depth of BGHS (base of gas hydrate stability) on the spur (~900 mbsl) and basin floor (~1000 mbsl). (Left) BSR occurs at 200 to 230 ms (TWT) on the spur, knoll and in the trough, whereas it appears at around 140 to 180 ms (TWT) within gas chimney due to sharp pull-up structure.

distribution of free-gas in the sediments in gas hydrate stability zone [3]. EVOLUTION OF DEEP-SEATED GAS Gas hydrates of the Joetsu basin is predominated by methane with trace amount of sulfide (Lu et al., 2011, this volume). Extremely high C1/ (C2+C3) ratio is explained as the result of drying effect during the migration of long distance [7], not indicating microbial source. δ13C of methane has been used as a diagnostic parameter to discuss the origin of natural gas. δ13C of methane from gas hydrates of the Joetsu basin ranges between -29 to -41‰VPDB, typical thermogenic methane [8,9]. Methane plumes and gas seeps are also plotted within the range of thermogenic. However, headspace gas extracted from the sediments of piston cores is depleted in 13C, widely ranging from -56 to -101‰VPDB. Figure 11 illustrates the δ13C values of methane from gas hydrate (red bar) and headspace gas of PC sediments (brown bar). The diagram also shows headspace gas taken from deep cores (blue square) recovered from the northern part of the Umitaka spur [10]. The δ13C of deep-seated gas is nearly constant at -30 to -34‰VPDB below 1000 mbsf while the value is getting more depleted above 1000 mbsf probably due to the intermixing with microbial methane, which is believed to be -60 to -100‰VPDB. Depth profile of deep-seated methane well explains the range of headspace gas in shallow sediments. Upward migration of deep-seated thermogenic gas has two different pathways to shallow levels. One is a dispersive flow through the sediments, in which thermogenic gas should be intermixed with in-situ microbial gas. Assuming a simple mixing of microbial (-100‰) and thermogenic (-30‰), 50% contribution of thermogenic will make the mixed gas of -65‰, common headspace gas from shallow sediments. Sediments close to gas chimneys are characterized by higher contribution of thermogenic methane, while sediment gases far away from chimneys are predominated by microbial. Another conduit of gas migration is a gas chimney. Deep-seated gas migrates as focused fluid flow through fractures, faults or cavities within gas chimneys. Thermogenic gas has little chance to mix with microbial gas and reaches to gas hydrate stability zone and sea floor without changing ?13C value.

EVOLUTION OF GAS HYDRATE MOUND Within Gas Chimneys Formation and collapse of gas hydrate mounds are schematically illustrated in Figures 7B and 8B. Deep-seated thermogenic gas migrates upward through gas chimneys without mixing with microbial, without changing δ13C. Themogenic gas migrates upward through gas chimney and precipitates thermogenic gas hydrates above the BGHS at 110 to 120 mbsf. Probably because of extremely high flux of methane, interstitial waters should be consumed by the formation of gas hydrate, and the sediments become deficient in waters. Excess gas should continue to move up in water-deficient "dry sediments" with some nodular and disseminated gas hydrates. Due to downward supply of seawater, sediments in shallow levels should contain enough amounts of free-waters, which enable the formation of massive gas hydrate in the upper part of gas chimneys. Dissolved methane in fluid should react with sulfate from seawaters to increase carbonate alkalinity, and eventually precipitates methane-induced carbonates. Thus the water-fed shallow sediments with numerous fractures provide the space for the precipitation of nodular and massive gas hydrate and carbonate concretions.

Figure 11 Depth profile of the d13C of methane of headspace gases extracted from the deep sediment cores, METI "Sado-oki-nansei" Stratigraphic Drilling in the northern part of the Umitaka spur in 2003. Yellow and red bars indicate d13C of headspace gas extracted from shallow sediments (< 40 mbsf) and gas hydrates and methane bubbles corrected on the seafloor (Matsumoto et al., 2009; Hachikubo et al., 2011, this volume).

Coupled precipitation of gas hydrate and carbonates is considered to have started within the Unit III, because a number of Type C mounds occur in Unit III as an initial stage of the evolution of gas hydrate and carbonate buildups. Considering the strong pull-up structure in gas chimneys, upper part of gas chimneys above BGHS is expected to contain significant amounts of gas hydrates. Therefore, the thickness of gas hydrate deposits in gas chimneys is probably between 50 to 100 meters. Type C mounds and buildups should grow-up and extrude from the seafloor to evolve to Type B mound, which, in turn, should be eroded, collapsed and decay due to chemical dissolution, rifting, and floatation of gas hydrate, and changed to Type A. Some of the Type A mounds have large depressions deeper than the seafloor. This may be the initial stage of large pockmark formation. In Host Sediments Deep-seated gas migrates through the host sediments, in which microbial methane generation is taking place. Deep thermogenic gas is mixed with in-situ microbial methane to form stratigraphic accumulation along and above the BGHS. δ13C of ascending mixed gas is increasingly depleted in 13C. Methane flux in host sediments should be much lower than gas chimneys as evidenced by the depth of SMI (sulfate-methane interface), then, mixed-gas or microbial gas hydrate occurs sporadically, and gas hydrate saturation (Sh) is generally low. Gas hydrates mostly occur as stratigraphic accumulation at or above BGHS at 100 to 110 mbsf. SUMMARY High-resolution acoustic survey by AUV-URASHIMA has revealed three types of gas hydrate mounds, Type A, B, and C formed atop gas chimney structures. Type C represents an initial stage of the formation of gas hydrate and carbonate buildups, and Type B is an evolved stage on the seafloor. Type A shows collapse and decay with large depressions and gas hydrate exposures. Type C occurs in lithologic Unit-III, approximately 100ka or older, while Type A extrude from the Holocene units. The thickness of gas hydrate deposits in the upper part of gas chimneys is probably between 50 to 100 meters.

ACKNOWLEDGEMENTS We thank the Captain and crews of R/V YOKOSUKA and operation team for AUV-URASHIMA for their tremendous efforts to make the expedition YK10-08 successful and fruitful. We also thank JOGMEC (Japan Oil, Gas and Metals National Corporation) for the permission to use unpublished data from the METI Stratigraphic Well off Sado island. This study has been supported by the cooperative research project between the University of Tokyo and PETROBRAS, collaboration with MH21 consortium, and the Grant-in-Aid from the Japan Society for the Promotion of Science, KAKENHI to RM, 19204049. REFERENCES [1] Sato, H., The relationship between late Cenozoic tectonic events and stress-field and basin development in northeast Japan. Journal of Geophysical Research, 1994, 99, 22261-22274. [2] Matsumoto, R., Methane plumes over a marine gas hydrate system in the eastern margin of Japan Sea: A possible mechanism for the transportation of subsurface methane to shallow waters. Proceedings of the 5th International Conference on Gas Hydrates, 2005,Trondheim, 749-754. [3] Matsumoto, R., et al., Formation and collapse of gas hydrate deposits in high methane flux area of the Joetsu basin, eastern margin of Japan Sea. Journal of Geography, 2009,118, 43-71. [4] Milkov, A. V. and Sassen, R., Economic geology of offshore gas hydrates accumulations and provinces. Marine and Petroleum Geology, 2002, 19, 1-11. [5] Machiyama, H., Kinoshita, M., Takeuchi, R., Matsumoto, R., Yamano, M., Hamamoto, H., Hiromatsu, M., Satoh, M., and Komatsubara, J., Heat flow distribution around the Joetsu gas hydrate field, western Joetsu basin, eastern margin of Japan Sea. Journal of Geography, 2009, 118, 986-1007. [6] Dickens, G. R. and Quinby-Hunt, M. S., Methane hydrates stability in seawater. Geophysical Research Letter, 1994, 21, 2115-2118. [7] Lorenson, T. D., and Collett, T. S., Gas content and composition of gas hydrate from sediments of the southeastern North American continental margin. Proc. O.D.P., 164, 13-28., 2000. [8] Bernard, B.B., Brooks, J. M., Sackertt, W. M., Natural gas seepage in the Gulf of Mexico. Earth Planet. Sci. Lett, 1976, 31, 48-54.

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