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doi:10.2204/iodp.pr.331.2010

Site summaries

Site C0013

Operations, lithostratigraphy, minerals, and physical properties

Eight holes (C0013A–C0013H) were drilled at Site C0013, and core was recovered from all but Hole C0013A (Fig. F8). Hole C0013E was the deepest (54.5 mbsf) and was cased down to 40.2 mbsf and fixed with a corrosion cap (open outlet pipe) mounted on the guide base. During drilling and coring operations at Site C0013, we encountered many operational and sample-handling problems due to the unexpectedly high temperature gradient at the site and the presence of repeated hard layers that appear to behave as cap rocks, alternating with soft and sticky clay-rich layers. Porosity measurements on the core clearly document the repeated occurrence of low-porosity harder layers (e.g., 0–2, 7–10, and 20–30 mbsf). We observed at several depths that when we drilled through a hard cap rock into the softer intervening layers, subseafloor hydrothermal fluid began to upwell in the hole and exit to the seafloor, where it was imaged by the video camera mounted on the ROV. We surmise that fluids were flowing laterally, trapped beneath the cap rock. To tap this fluid, we used slotted, perforated casing pipe over the depth interval 21–39.8 mbsf in Hole C0013E (Fig. F9). Immediately after casing and capping this hole, we observed, in the ROV video image, strong hydrothermal fluid discharge from the casing pipe, which was hung in the guide base. Thermoseal temperature-sensitive strips on the corrosion cap outlet pipe and read in the ROV video imagery indicated that the discharging water was >250°C. When the JAMSTEC ship Natsushima visited Hole C0013E with its Hyper Dolphin ROV 2 days later, they found that flow from the casing pipe, through the outlet in the corrosion cap, had ceased. Instead, water that appeared blackish was now discharging directly from the hole beneath the guide base. This flow apparently came up through the annulus, between the wall of the hole and the outer surface of the casing pipe. The Hyper Dolphin lowered a probe 30 m into cased Hole C0013E and measured a temperature of only 4°C; the hole was plugged below that depth. The Hyper Dolphin also measured a temperature of 11°C in diffuse flow from uncased Hole C0013D, higher than the bottom water temperature in the area of 4.51° ± 0.17°C (1σ, n = 4, measured at Holes C0014C, C0014D, C0014F, and C0014G). On these same dives the Hyper Dolphin revisited the NBC vent about 170 m to the west-northwest, which we drilled as Site C0016, and found it to be venting at 310°C, compared with the highest temperature measured there in previous years of 311°C (Kawagucci et al., in press).

Four lithologic units were identified at Site C0013. The uppermost Unit I (0–4 mbsf) is hydrothermally altered mud containing crystalline pipes of elemental sulfur and sulfide grit. The ROV survey done at the site to choose actual hole locations shows that the seafloor is covered by a thin (<1 m) discontinuous layer of unaltered sediment, as has been documented in previous surveys over the past 10 y. The origin of the sulfide grit is uncertain, and two potential sources, collapsed sulfide mound fragments or in situ deposition by high-temperature hydrothermal fluids, have been proposed. It is apparent in any case that Unit I has been subjected to high temperatures and hosted sulfidic hydrothermal fluids, supporting the in situ formation scenario for at least some of the sulfide.

Unit II (4–14 mbsf) is hydrothermally altered mud with some heavily veined intervals and clastic units containing anhydrite breccia and fragments of metalliferous massive sulfide. Unit II is commonly cut by anhydrite veins, indicating that seawater infiltrated and was heated to >150°C. Unit II represents a possible hydrothermal reservoir or fluid migration path, consistent with our observation of upflow in Hole C0013E.

Unit III (14–26 mbsf) consists of hydrothermally altered mud, some layers of which contain abundant nodular anhydrite. Unit III was distinguished from Unit II by the presence of large whole anhydrite nodules and less abundant vein anhydrite. Unit III likewise experienced high temperatures (>150°C) and hosted hydrothermal flow, of solutions with a different chemical composition than those which infiltrated Unit II, as described below.

Unit IV consists of volcanic breccia with clasts of various volcanic lithotypes. Both monomictic and polymictic breccias were observed, and all were matrix supported and silicified. Other volcanic components within the breccia include clasts of tube pumice and vesicular and flow-banded lavas. The occurrence of various volcaniclastic components such as lavas, vesiculated glasses, breccias, and blocks of polymictic breccia is consistent with breccia formation by mass wasting of a mixture of volcaniclastic units, initially deposited further upslope from Site C0013.

We observed the strongest discharge of hydrothermal fluid after using the BHI system for the first time to drill the interval 26–35.2 mbsf. Beneath that depth, to the total depth of 54.5 mbsf in Hole C0013E, our total recovery was 25 cm of rock. Drilling records indicate that the material we failed to recover became soft again below 35.2 mbsf, suggesting that additional porous and permeable formations exist below Unit IV.

Interstitial water chemistry

Twenty-six whole-round samples were processed for chemical analyses of interstitial water at Site C0013, from five different holes (C0013B and C0013D–C0013G). Chloride, the major anion in seawater, is nearly constant with depth in Holes C0013B and C0013D at the concentration in seawater. In Hole C0013E, however, it varies widely, from 34% lower than seawater to 12% higher in the deepest sample from 17 mbsf. The major cation in seawater, Na, generally follows Cl, as expected from charge balance constraints. Na/Cl and Na/Br are higher than in seawater in Hole C0013B, probably because of expulsion of Na from ion exchange sites in clay minerals by other cations, but these ratios decrease systematically with depth to values lower than seawater in the other two holes, showing that Na is being removed from the seawater-derived pore water into alteration minerals over this interval and at greater depth.

Mg increases with depth in Holes C0013B and C0013D, probably because of ion exchange with clays as for Na, but decreases to only 7 mM in Hole C0013E. Ca increases irregularly with depth in all three holes, to concentrations nearly five times that in seawater. Although some of this increase is typical for hydrothermal solutions, as Ca is leached from the host rock, much of the increase at Site C0013 undoubtedly results from dissolution of the anhydrite that is distributed abundantly throughout much of the core during the 3–25 h that elapsed between the arrival of the core on deck and squeezing of the sediment to separate pore water. Dissolution of anhydrite is implied by the drastic increases in sulfate. By contrast, sulfate decreases with depth in Hole C0013E to less than half the concentration in seawater, as would be expected from anhydrite precipitation within the hydrothermal system with increasing temperature.

The deepest sample obtained from this site, from Hole C0013E at 17 mbsf, is in many ways a typical high-temperature subseafloor hydrothermal fluid. Relative to seawater, it is slightly briny at 623 mM Cl; it has highly elevated Ca, K, Rb, and Cs and greatly depleted Mg and sulfate; and it has lost Na to the altered rocks.

Microbiology

The abundance of microbial cells was evaluated by a fluorescent microscopic analysis using SYBR Green I as a fluorochrome dye. Total cell counts obtained from core samples at Site C0013 are generally lower than the detection limit of our onboard counting method (1 × 10⁵ cells/cm³ sediment), and the lowest among all coring site in this expedition. Microbial abundance in the sediment of the Iheya North hydrothermal field is known to be relatively low (<2 × 10⁷ cells/mL sediment) (K.T. Ishibashi et al., unpubl. data), but the cell abundances we observed are even lower than those previously found. Cultivation tests were conducted for Thermococcales (e.g., Thermococcus spp.) and Aquificales (e.g., Persephonella spp.) and thermophilic Epsilonproteobacteria (e.g., Nitratiruptor spp.) using core from Site C0013 and different recipes for media at various temperatures. No growth of these organisms was identified in the onboard experiments.

Although onboard microbiological measurements and experiments were quite limited and did not provide any conclusive result, it is evident that no numerically prosperous microbial communities are present in the subseafloor at Site C0013. Except for the upper several meters below seafloor of altered sediment, the subseafloor environments are at higher temperatures (>150°C) than microbes can survive. However, it is still possible that shorebased work will yield evidence for functionally active hyperthermophilic microbial communities at Site C0013.

Site C0014

Operations, lithostratigraphy, minerals, and physical properties

Seven holes were drilled at Site C0014 (Holes C0014A–C0014G) (Fig. F10). Hole C0014G was the deepest (136.7 mbsf) and was cased down to 117.8 mbsf and fixed with a corrosion cap (open outlet pipe) mounted on the guide base. As at Site C0013, we encountered repeated hard layers that may behave as cap rock, and we saw discharge from the holes, beyond what may be expelled drilling fluid only, after penetrating these layers (35–44.5 mbsf in Hole C0014B, 25.5–35 mbsf in Hole C0014E, and 37.7–47.2 mbsf and 89.2–93.7 mbsf in Hole C0014G). We again cored multiple low-porosity layers (e.g., in Holes C0014B, C0014E, and C0014G). Based on the low recovery and the lithology of the cores, we infer lateral hydrothermal flow at depths of 35–45 and 90–95 mbsf at Site C0014 (Fig. F10). We therefore installed slotted, perforated casing pipe at 29.8–49.2, 78.3–97.8, and 107.5–117.2 mbsf in Hole C0014G (Fig. F9). After casing and capping, we saw in the ROV-mounted video diffuse hydrothermal fluid discharge, not from the corrosion cap outlet but from the seafloor, through the annulus, the space between the wall of the hole, and the casing pipe. The temperature of the diffusing fluids was found to be >240°C based on exposure of thermoseal strips mounted on the corrosion cap outlet pipe and observed by ROV.

We measured temperature at Site C0014 using the APCT-3 temperature shoe on the HPCS core barrel for lower temperatures (0°–55°C) and thermoseal temperature-sensitive strips for higher temperatures (75°–250°C). The temperature-depth profile at Site C0014 is shown in Figure F11. Temperature increases nearly linearly at 3°C/m to 145° ± 5°C at 47 mbsf and then increases abruptly to >210°C at 50 mbsf, below a hard layer near that depth. Two main lithologic and sediment types were identified at Site C0014: volcanic breccias, often silicified, and hemipelagic hydrothermally altered mud. The dominant depositional process is pelagic sedimentation, although mass wasting and debris flows likely lead to high rates of resedimentation of both hemipelagic and volcaniclastic sediments. Thus the bulk of the sediment volume encountered at the surface is likely to have been resedimented. Furthermore, the bulk of the rock volume cored has been hydrothermally altered. Differing styles of hydrothermal alteration form the basis for the division of Site C0014 sediments and lithologies into lithostratigraphic units.

Unit I (0 to <18 mbsf) is a succession of woody pumice breccias and hemipelagic ooze 12 to 16 m thick that is little altered by hydrothermal activity. It includes a diversity of foraminifers dominated by warm forms of the planktonic Neogloboquadrina pachyderma, some of them pyritized. Minor coccoliths were present, dominated by Emiliania huxleyi, Gephyrocapsa oceanica, and Reticulofenestra asanoi. Unit II consists of partially consolidated hydrothermally altered mud with quartz and muscovite as major phases and minor kaolinite and illite/montmorillonite, along with hydrothermally altered pumice breccias. Clasts of pumice have been devitrified to soft clay that has retained the primary woody texture. Unit II is 12 to 15 m thick and extends to a depth of 27 to 30 mbsf. Unit III is defined by the occurrence of consolidated and often cemented volcanic sediments as lithoclasts within breccias or horizons interbedded with hemapalgic and hydrothermally altered mud. Mg chlorite becomes an important alteration phase in Unit III, regardless of lithology. Anhydrite is present below 57 mbsf in Hole C0014G, mostly in millimeter-scale irregular veinlets with halite, but in much lower abundance than was seen at Site C0013. Pyrite occurs in trace amounts as fine-grained disseminations and very rare coarser veins throughout the sequence. Unit III is 128 m thick in Hole C0014G, extending to 136.7 mbsf, and has a gradational contact with Unit II.

Interstitial water chemistry

A total of 75 whole-round samples were processed at Site C0014 from four different holes. Within the upper ~30 m of sediment, these holes exhibit considerable spatial variation in the concentrations of those chemical species that are most influenced by organic matter diagenetic reactions.

The most sulfate-depleted interval, at 38 mbsf, also has the highest NH₄ and notable depletions in Cl and possibly Br. High NH₄ and low Cl typically result from high-temperature phase separation into a brine and a gas-rich, Cl-depleted vapor, as has happened at high-temperature vents in the Iheya North field (Kawagucci et al., 2010; Takai and Nakamura, 2010). Surprisingly, NH₄ in Hole C0014G is also inversely correlated with sulfate, as would result from the anoxic ammonia-oxidizing sulfate-reducing microbial metabolism recently predicted by thermodynamic calculations of microbial energy metabolisms (Schrum et al., 2009).

Below 50 mbsf in Hole C0014G, Cl and Br are enriched relative to seawater as they would be in a brine. Sodium, the major cation in seawater, generally follows the distribution of Cl, but it is often slightly depleted in hydrothermal solutions because of removal into alteration minerals, particularly albite. In contrast to Na, K is enriched relative to seawater at depth at this and all the other sites we drilled. The initial depth (~17.7 mbsf) at which K increases is slightly deeper in Hole C0014G than in Holes C0014B and C0014D, consistent with other observations of hydrothermal fluid influence. This difference in depth implies that the thick pumice deposit in Hole C0014G acted as a barrier for the remixing of the phase-separated and phase-segregated hydrothermal fluids and kept them spatially stratified, with gas-rich fluid at shallower depths and brine-rich fluid at deeper depths, probably because of the difference in their relative density or buoyancy.

Microbiology

Maximum cell number was found just below the seafloor in Hole C0014B (1.8 × 10⁷ cells/cm³ at 0.33 mbsf), Hole C0014G (3.3 × 10⁷ cells/cm³ at 0.27 mbsf), and Hole C0014D (5.6 × 10⁸ cells/cm³ at 0.23 mbsf). Cell numbers generally decrease with increasing depth, although there are some secondary peaks in biomass within shallower intervals at Holes C0014B and C0014D. Numerically abundant microbial communities at these shallow depths are not regarded as deep hot biosphere.

Microbial biomass at Site C00014 is generally similar to that found at Sites C0015 and C0017 and in a gravity core taken previously at Site C0014 (K.T. Ishibashi et al. unpubl. data). By contrast, the maximum microbial cell count in Hole C0014D is the highest we saw during Expedition 331. This densest microbial population was obtained from the CH₄-enriched, sulfate-depleted sediments from just beneath the sulfate-enriched pumice layer. A second peak in microbial population in Hole C0014D was found in the sulfate-depleted sediments at 8.7 mbsf. All the microbial population peaks occur within CH₄-enriched and sulfate-depleted layers, strongly suggesting the occurrence of functionally active AMO communities within the shallow subsurface.

Microbial populations in the deeper and hotter zones were numerically less abundant (<1 × 10⁵ cells/cm³). Overall, however, the microbial population at Site C0014 is more abundant than that at Site C0013.

Cultivation tests for Thermococcales (e.g., Thermococcus spp.), Aquificales (e.g., Persephonella spp.), and thermophilic Epsilonproteobacteria (e.g., Nitratiruptor spp.) produced no positive enrichment. Enrichment experiments for marine iron-oxidizing bacteria (FeOB) such as the Zetaproteobacteria (e.g., Mariprofundus ferrooxydans), however, showed limited growth within the top two meters of sediment, to be further investigated during shore-based research.

Fluorescent microspheres were used to check for contamination in cores taken with HPCS, and perfluromethylcyclohexane (PFC) was used as a tracer for contamination in all cores. Fluorescent microspheres were not detected in most of the core samples from Site C0014, even within exterior parts of the core. High numbers of microspheres were detected in pumice layers, however, such as those sampled at 6.67 mbsf in Hole C0014D and 7.84 mbsf in Hole C0014G. Contamination by drilling fluids and/or by core disturbance would be difficult to avoid for such layers because of their high porosity and heavy fragmentation. Because such samples are important for hydrogeological controls on formation of subseafloor microbial communities, such samples need to be studied carefully in future research.

Site C0015

We spent only 10 h at Site C0015. Three holes were drilled there, to a maximum depth of only 9.4 mbsf, but only two recovered core (Fig. F12).

Despite our shallow penetration, we recovered several different lithologies: volcanic breccias, siliciclastic sands, and hemipelagic mud. The dominant depositional process is pelagic sedimentation, although resedimented volcanic lithoclasts make up the bulk of the sediment encountered. Siliciclastic sedimentation is evidenced in the deposition of texturally mature sands, and most intervals in the core are very porous. Despite the short interval cored, these holes provide good geological control and useful insight into the prealteration sedimentary architecture at the other Expedition 331 sites.

Interstitial water at Site C0015 is generally indistinguishable from seawater. Methane and hydrogen are very low and provide no evidence for hydrothermal input. Nevertheless, the microbial population is more abundant (4.8 × 10⁶ to 1.2 × 10⁷ cells/cm³ sediment) than at those sites that exhibit hydrothermal flow in the subseafloor. Over the entire depth interval cored (0–9.4 mbsf), the mud, sand, and pumiceous gravel show weak oxidation, expressed as a yellow to brown discoloration of the mud and orange to brown iron oxide staining on some pumice fragments. Scanning electron microscope (SEM) imaging of orange-brown botryoidal aggregates of Fe-Si oxyhydroxides shows a filamentous structure typical of oxyhydroxides. Subseafloor microbial communities at Site C0015 may thus be sustained by both organotrophic and Fe-oxidizing chemolithotrophic production. Indeed, enrichment experiments conducted at Site C0015, selecting for the growth of marine FeOB, were successful for most samples to ~9 mbsf, exhibiting cellular morphologies common for known iron-oxidizers. FeOB communities are commonly found at low-temperature hydrothermal systems (Rassa et al., 2009), and further exploration of samples collected at Site C0015 will enhance our understanding of FeOB biodiversity.

Site C0016

Operational observation, lithostratigraphy, minerals, and physical properties

Two holes were drilled at Site C0016, but core was recovered from only one of them (Fig. F13). Hole C0016A was drilled with the BHI system, without a guide base, directly into the top of the 30 m high NBC hydrothermal mound that was thickly covered with Galatheid crabs and was discharging fluid at temperatures as high as 311°C. We spudded the hole at the crest of the mound within 1–2 m of a vent that was steadily and continuously phase separating, which made it look as if it was flaming. The drill bit penetrated easily and quickly. We noticed almost immediately that the drill string was canted over by 10°, which made the pipe move erratically as it rotated and caused it to enlarge the top of the hole. That the drill pipe was listing 10° from vertical was later attributed by the drillers to the effect of the Kuroshio Current. The designated 18 m of pipe was lowered within minutes at the rig floor, although we determined later that only about 15 m actually went into the hole; the rest just added to the bowing of the drill string in the water column. When 18 m of pipe had been lowered (with ~15 m of penetration), the drillers attempted to pull out of the hole. Two small steam explosions were seen around the pipe in the ROV video image, the flow of hot water and "black smoke" increased dramatically, and then the end of the pipe snapped off. The steam explosions were probably triggered by pressure transients induced by movement of the pipe in the hole in a fluid that was already on the verge of boiling. The drillers recorded a series of brief, sharp incidents of overpull that ended abruptly with the breaking of the pipe. A short section fell onto the mound and was recovered by the ROV, but the bit and most of the BHA stayed in the hole. The most likely explanation is that the pipe became bent in the hole, largely because of the stress put on it by rotating 10° from the vertical, and then broke when it was pulled out vertically. For the pipe to bend, though, it must have encountered some hard material within the mound, most of which appeared to be very soft. According to the drillers, it is not likely that either the high temperature or the small steam explosions contributed to the break. The major cause appears to have been the Kuroshio Current, which caused the drill string to buckle and so deviate 10° from vertical. The increase in black smoker flow that we observed with the ROV was almost certainly the result of our enlarging the orifice at depth, possibly by poking a larger hole into a cap rock that was inhibiting flow. This black smoker flow was still vigorous 1 day later but had diminished somewhat and become a clear, dark fluid 5 days after drilling. When first seen >10 y ago, the NBC vent site was emitting a clear, dark, single-phase fluid, which subsequently evolved to a boiling fluid. The fluid emitted since our drilling appears to resemble this earlier fluid, and so may have a different composition from that described by Kawagucci et al. (2010).

Hole C0016B was again drilled with the BHI system, this time with a guide base. It was positioned at the foot of the NBC mound, immediately adjacent to it and 20 m west of the NBC vent and Hole C0016A. It penetrated to 45 mbsf in three runs of 9, 18, and 18 m. Each run recovered several large pieces of core, but the total recovery was only 2.095 m (Fig. F13), nearly all of it hard rock. The only evidence for the actual depths of the recovered rock thus comes from the drilling records. Hole C0016B was not cased, but it was fitted with a corrosion cap with 3 m of 5.5 inch pipe hanging beneath and extending 0.4 m into the seafloor. ROV video images showed vigorous black smoker discharge from the corrosion cap outlet immediately after its deployment. This hydrothermal emission began only after the third coring run, which penetrated 27–45 mbsf, and was probably derived from a depth below 38 (Fig. F13).

The overall core recovery was only 4.7% from Hole C0016B, consisting mostly of hard rock. The first core comprises two pieces of massive sulfide totaling 61 cm, separated by 3 cm and underlain by another 15 cm of highly silicified and mineralized volcanic material altered to illite/muscovite clay, which probably came from 6–9 mbsf. The massive sulfide ore contains 40%–60% sphalerite (ZnS), 10%–40% pyrite (FeS₂), and a few percent each of galena (PbS) and chalcopyrite (CuFeS₂). It closely resembles the classic "black ore" of the Kuroko deposits of Miocene age in Japan and represents the first time such ore has been recovered from an active hydrothermal system on the seafloor. The second core consists of three pieces of three different lithologies, totaling 31 cm, from 9–27 mbsf: two pieces of altered, silicified, mineralized volcanic rock, above and below a 12 cm piece of very coarsely crystalline, snow white anhydrite with a sphalerite-pyrite vein running along one side. The third coring attempt recovered 99.5 cm of quartz-chlorite altered volcanic rock with abundant stockwork veining filled with quartz, chlorite, pyrite, and late anhydrite. The rocks from all three cores show a consistent paragenesis, from early formed sphalerite, to pyrite and galena, to chalcopyrite with increasing temperature, followed by a second generation of sphalerite with decreasing temperature. The last mineral to precipitate was anhydrite, as seawater penetrated into the waning system. Pyrite increases relative to sphalerite, and illite/muscovite alteration gives way to chlorite-quartz, with increasing depth and temperature. The textures and relationships seen in thin section for the massive sulfide require that a significant proportion of the sulfide mineralization occurred via subseafloor precipitation, with at least some sphalerite precipitating into void space in the rock.

Microbiology

Cultivation tests for Thermococcales (e.g., Thermococcus spp.), Aquificales (e.g., Persephonella spp.), and thermophilic Epsilonproteobacteria (e.g., Nitratiruptor spp.) produced no positive enrichment from any of the samples from Hole C0016B. We did detect PFC contamination on the surfaces of several core samples from Hole C0016B, from PFC added to the drilling fluid.

Site C0017

Operational observation, lithostratigraphy, minerals, and physical properties

Hole C0017D was the deepest (150.7 mbsf) of four holes drilled at Site C0017 and is located ~1500 m east of the high-temperature vents. Based on its low heat flow it was inferred to be a location of probable recharge of the hydrothermal system. We observed no evidence for discharge of water from any of these holes, but the concave-upward temperature profile we measured (Fig. F14) can be fit with an exponential function (r² = 0.92) and is consistent with overall downwelling, with kinks that suggest lateral flow that may be influenced by the variable lithologies we cored.

We measured temperature successfully at seven depths in Holes C0017B, C0017C, and C0017D over the interval from 18.3 to 150.7 mbsf; combined with the ocean bottom water temperature at the site of 4.87° ± 0.54°C (1σ, n = 8), our profile is defined by eight points. Six of the downhole measurements were made using the APCT-3 temperature shoe on the HPCS. The seventh and deepest measurement, at 150.7 mbsf, was made in triplicate using three identical thermoseal temperature-sensitive strips with chemically impregnated beads for 75°, 80°, 85°, 90°, and 95°C taped to the bottom outer surface of the plastic core liner. These beads were darkened and thus exposed at maximum temperatures of 85°, 90°, and 90°C on the three strips, which we are reporting as 90° ± 5°C. The APCT-3 shoe recorded a temperature in excess of its maximum range of 55°C for this core (but did not burn up; we recovered the entire temperature record). Down to ~50 mbsf, where we encountered a hard layer that is probably pumice (no core was recovered), temperature remains low, <15°C (Fig. F14). Beneath this hard layer it jumps up to 25°–39°C at 69–85 mbsf and then appears to level off for a short interval, reaching only 44°C at 112 mbsf before increasing nearly exponentially to 90°C at the bottom of Hole C0017D at 150.7 mbsf.

Coring at Site C0017 recovered three different lithologies: homogeneous hemipelagic mud (Unit I), pumiceous sediment (Unit II), and volcaniclastic-pumiceous breccia and mixed sand with erosional bases (Unit III). None of these showed obvious evidence of hydrothermal alteration, except weak alteration to clay within the deepest part of Hole C0017D. These three units together resemble the lithologies within Unit I defined at Sites C0014 and C0015. At Site C0017, the volcanic lithologies are relatively more abundant from 18 to 70 mbsf, such that the total section drilled could be interpreted as two layers of hemipelagic mud separated from each other by a pumiceous horizon. The thicknesses of mud and pumice beds at Site C0017 are significantly greater than at Site C0014, consistent with its location downslope.

Different lithologies of widely varying porosity and permeability alternate within the upper 37 mbsf at Site C0017. In particular, the pumiceous gravel/clast layers (Unit II) and the volcaniclastic-pumiceous sand layers (Unit III) are likely to be much more permeable than the clay layers (Unit I). As such, they represent potential pathways for recharge of cold bottom seawater into the hydrothermal system. One such example is the prominent oxidized interval from 26 to 35 mbsf in Hole C0017C. These observations and inferences are consistent with the low thermal gradient in the shallow subseafloor and the concave-upward temperature profile that increases exponentially with depth. Similar lithologies occur over the depth interval 60–100 mbsf, with probable high porosity and permeability and a lower thermal gradient, suggesting the possibility of lateral flow within this deeper interval. No core was recovered between 37 and 60 mbsf, but the drilling record indicates the presence of a hard layer at ~50 mbsf that could represent a hydrologic barrier between upper and lower permeable zones. At greater depth, the only hydrothermal alteration evident at Site C0017 was seen in the deepest core recovered, from 140–150 mbsf, where more permeable pumiceous grit horizons are altered to pale gray clay. These grits are intercalated with apparently unaltered indurated dark gray calcareous clay, again suggesting that the coarser sediment layers acted as flow paths. As noted above, the temperature at this depth is 90° ± 5°C.

As was observed within Unit I at Site C0014, abundant foraminifers and coccoliths dominated by Emiliania huxleyi, Gephyrocapsa oceanica, and Reticulofenestra asanoi were identified within the upper 28 mbsf. Biostratigraphy thus supports the equivalence of Units I through III at Site C0017 with Unit I at Sites C0014 and C0015.

Interstitial water chemistry

A total of 21 whole-round samples were processed at Site C0017 from four different holes. No samples were obtained between 30 and 63 mbsf. At 0–15 mbsf, the interstitial water profiles show the clear effect of microbial sulfate reduction utilizing organic matter in the sediment: sulfate decreases slightly and alkalinity, ammonium, and phosphate increase with depth from 0 to 15 mbsf. From 15 to 30 mbsf, however, all of these chemical species reverse direction and return to their concentrations in seawater, indicating a source of unaltered seawater at ~30 mbsf. This source can be identified as the oxidized layer at 26–35 mbsf discussed above, which is almost certainly a permeable layer through which seawater is flowing laterally, recharging the hydrothermal system 1500 m to the west. This layer presumably outcrops a short distance to the east.

Except for sulfate, the major ions are present over the depth interval 0–30 mbsf at their seawater concentrations. Below 64 mbsf, however, K decreases to a low of 6.5 mM at 106 mbsf before rebounding again to the seawater value in the deepest sample from 141 mbsf. Over this same interval, Ca increases more or less steadily to 18 mM and alkalinity to 6 mM. Changes in other ions are small or nonexistent. The causes of these changes are not obvious. The increase in Ca and alkalinity may result in part from dissolution of CaCO₃, but this compound should become less soluble with rising temperature, which increases from 25° to 90°C between 69 and 151 mbsf. K is typically taken up from seawater into clay minerals and zeolites at temperatures below ~150°C, but the abrupt rebound in K to the seawater value that occurs between 131 and 141 mbsf is hard to explain except by another permeable layer providing relatively unaltered seawater to that depth. It would have to do so without affecting the relatively more reactive species Ca and alkalinity.

Concentrations of methane and hydrogen are low and show no evidence for significant input from either hydrothermal processes or a prosperous anaerobic microbial community. Methane increases slightly near the bottom of the deepest Hole C0017D, probably from microbial methanogenesis at greater depth.

Microbiology

Maximum cell number was found within the mud of Unit I at 6.36 mbsf (3.2 × 10⁷ cells/cm³) and a relatively abundant microbial population (2.4 × 10⁷ to 3.2 × 10⁷ cells/cm³) was identified above 10.8 mbsf. At greater depths, cell number generally decreased, although we found a substantial population at 20–40 mbsf, within the layer of lateral recharge identified by the oxidized sediment and the return of the interstitial water to seawater composition. This microbial community may thus be energized by iron oxidation under oxidative chemical conditions. Enrichment for FeOB in this zone of lateral recharge showed growth under microaerophilic conditions only. These are the deepest enrichment of putative FeOB to date, though caution is advised for the potentially contaminated samples.

Cultivation tests produced no positive enrichment for any samples from Site C0017, besides those enrichments conducted for the FeOB. Contamination tests found no fluorescent microspheres in most samples even within exterior parts of core at Site C0017. Contamination evaluated by PFC introduced into the drilling fluid was low even for unconsolidated pumice samples, and even at the surfaces of cores consisting mainly of pumice. We did find low levels of PFC in the interior of one core from Hole C0017D taken by ESCS, but even there the concentration of PFC was much lower than in the drilling mud.

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