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doi:10.2204/iodp.proc.331.101.2011

Principal results

Lithostratigraphy

The geological setting for Expedition 331 is highly complex and dynamic, recording a constantly changing interplay of pelagic and volcaniclastic sedimentation, gravity, and seismically triggered reworking, volcanism, faulting, and hydrothermal alteration and mineralization. In such an environment lithostratigraphic correlation is difficult, with many, if not all, lithological units having limited areal extent. Indeed, the most distinctive lithological units seen in material recovered during Expedition 331 are inherently difficult to correlate between sites: coarse pumiceous gravel typically forms lenticular bodies, which commonly vary by orders of magnitude in thickness over distances of a few tens of meters; massive sulfide mineralization is by definition highly localized around discrete sites of hydrothermal venting; and anhydrite precipitation is strongly controlled by temperature and subsurface hydrology, both of which have been demonstrated during Expedition 331 to vary rapidly, both spatially and temporally, at Iheya North. Given this complexity, it is not feasible to attempt to correlate lithostratigraphy between the sites drilled during the expedition, other than in the grossest sense, although detailed geochronological, isotopic, mineralogical, micropaleontological, and geochemical postcruise work may allow more detailed correlation in the future.

Broadly speaking, the entire sequence drilled at Sites C0014, C0015, and C0017 may be considered to belong to the same sequence of mixed volcaniclastic and pelagic sediments, with the first site showing significant hydrothermal alteration of the sequence. Similarly, Units I–III at Site C0013 belong to this same sedimentary sequence, with an extremely strong hydrothermal overprint. The interval drilled at Site C0016 may reasonably be considered to represent volcanic basement underlying the sediments, so is older than these sequences. Unit IV at Site C0013 may also be equivalent to the Site C0016 rocks, although recovery issues in the deeper drilling at Site C0013 make it difficult to determine with any certainty that volcanic basement has been drilled at the site.

Biostratigraphy

The active hydrothermal discharging Site C0016 and the nearby Site C0013 did not have microfossils in any of the core catcher samples, even those from 0 to 10 mbsf. At Site C0014, which has a moderate temperature gradient of 3°C/m within the uppermost 50 mbsf, microfossils were found only within the uppermost 6.5 mbsf. Microfossils were present throughout the sediment at Site C0015 with two particularly foraminifer rich layers at ~3 mbsf. Recharge Site C0017 yielded microfossils from two depths: 9–28 mbsf, at <6°C, and 112 mbsf, at a measured temperature of 44°C. In all cases, major microfossils are modern assemblages of planktonic foraminifers. Hydrothermal activity is deleterious both to growth of benthic foraminifers and to the preservation of microfossils, especially coccoliths, which were found rarely and were invariably badly corroded. Some foraminifer tests in near-surface samples from Sites C0014 and C0017 were exquisitely replaced by micrometer- and submicrometer-scale pyrite, so accurately that species could be determined.

Iron oxides in near-surface samples from Site C0015 possess micrometer-diameter, filamentous organic coatings and need to be assessed using high-resolution imaging for the presence of bacterial microfossils.

Petrology

Interpretation of hydrothermal alteration and mineralization for Sites C0013–C0017 provides a broad overview of the nature, evolution, and architecture of the Iheya North hydrothermal system.

Site C0016 at NBC mound, despite very poor recovery overall, intersected massive sphalerite-rich sulfides, thus recovering for the first time, from beneath an active seafloor hydrothermal system, material that strongly resembles the black ore of the Kuroko deposits of Miocene age in Japan. Despite an obvious clastic protolith, 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. Additionally, the sulfide and sulfate paragenesis of the samples shows an evolving system, with early sphalerite mineralization overprinted by pyrite and then chalcopyrite, as temperature increased, before a second sphalerite mineralizing event, as temperatures cooled, and a final seawater influx, indicated by late coarsely crystalline anhydrite.

The underlying altered volcanic rocks recovered at Site C0016 show a similar evolution to the massive sulfide. The silicified volcanic rock from Core 331-C0016-1L shows similar sulfide paragenesis and, in all cases, anhydrite was among the last phases precipitated during alteration.

With increasing depth at Site C0016, the relative abundance of pyrite increases with respect to sphalerite, both on a local scale within the massive sulfide recovered in Core 331-C0016-1L and overall within the sequence. This variation is one that is observed in many volcanic-hosted massive sulfide (VHMS) mineral systems and is interpreted to be a function of increasing temperature with depth. The predominance in Hole C0016B of quartz-muscovite/illite alteration grading to quartz-chlorite alteration at depth is also consistent with the proximal quartz-white mica grading to chloritic alteration commonly recorded in the immediate footwall of ancient VHMS systems, including the Miocene Kuroko deposits of Japan.

At Site C0013, a likely location of recent high-temperature discharge, sediments exhibit kaolinite-muscovite alteration and contain variable thicknesses of sulfide-rich material to ~5 mbsf. Native sulfur is also abundant near-surface, which, together with the abundance of kaolinite, suggests acidic fluids, probably caused by oxidation of H₂S dissolved in hydrothermal fluid or released from the fluid during decompression. Below ~5 mbsf, Mg chlorite and anhydrite are the dominant alteration phases to ~26 mbsf, at which depth a hard sequence of volcanic breccias altered to quartz and Mg chlorite with scattered quartz veins was intersected; we interpret this sequence to represent volcanic basement. This lithology extends to the bottom of the deepest Hole C0013E at the site, at 54.5 mbsf.

The transition from kaolinite-muscovite-rich to chlorite-rich rocks with increasing depth at Site C0013 is similar to the gradation from paragonitized to chloritized rocks that was documented for the basement underneath the TAG hydrothermal mound at the Mid-Atlantic Ridge (26°N) (Humphris et al., 1995). The difference in mineralogy between Iheya North Knoll and the Mid-Atlantic Ridge can be attributed to the lack of iron and the abundance of potassium available within the felsic volcanic rocks in the Okinawa Trough when compared with the mafic volcanic rocks of the mid-ocean-ridge setting. Although it is difficult to estimate alteration temperatures from clay and phyllosilicate mineralogy, transitions seen at Site C0013 are consistent with temperatures of ~150°–200°C for the uppermost kaolinite-bearing sequence and ~220°–300°C for the Mg chlorite–rich sediments and rocks below ~5 mbsf.

The latest alteration phase seen at Site C0013 is coarse-grained anhydrite in veins, which overprint both kaolinite-muscovite and Mg chlorite alteration between ~4 and ~10 mbsf. This anhydrite likely precipitated when downwelling seawater mixed with upwelling hydrothermal fluid as the system waned, at a temperature of ~150°C.

Hydrothermal alteration at Site C0014 shows a change, with increasing depth, from illite/montmorillonite-bearing to Mg chlorite–bearing rocks at ~25–30 mbsf. This alteration is similar to the change from kaolinite-muscovite to Mg chlorite alteration observed at Site C0013. Mg chlorite implies alteration temperatures of ~220°–300°C, which is broadly consistent with the current temperature data at Site C0014.

When compared to Site C0013, the relative predominance of illite/montmorillonite, rather than kaolinite, at Site C0014 and the absence of native sulfur at Site C0014 imply higher fluid pH for the altered sequence at Site C0014, consistent with an absence of the high-temperature H₂S-rich fluids that form sulfide chimneys in the Okinawa Trough.

Significantly, transitions in alteration mineralogy occur at much greater depths at Site C0014 than at the more proximal Site C0013, consistent with the lower temperature gradient observed at the more distal site. Anhydrite is present at depths >57 mbsf in Hole C0014G, mostly in millimeter-scale irregular veinlets with halite but in much lower abundance than was seen at Site C0013. Primary volcanosedimentary mineralogy is also partially preserved throughout the altered sequence at Site C0014, implying a significantly lower fluid flux than that at Site C0013. Pyrite occurs only at extremely low abundances, mostly as fine-grained disseminations and very rare millimeter-scale veinlets throughout the sequence. These observations are consistent with the interpretation that Site C0013 represents a site of recent high-temperature discharge, whereas Site C0014 is a “background” location within the hydrothermal system at Iheya North.

At Site C0017, there is little evidence of hydrothermal alteration, other than a zone from ~26 to 35 mbsf that shows weak to moderate oxidation within the otherwise reduced sedimentary sequence. Geochemical results indicate that this zone of oxidation is the result of an influx of seawater at this depth, potentially representing recharge flow into the Iheya North hydrothermal system.

Hydrothermal alteration is absent in the shallow (maximum depth = 9.4 mbsf) cored interval at Site C0015. With the exception of the interval from 3.2 to 5.07 mbsf in Hole C0015B, fine-grained sediment, sand, and pumiceous gravel recovered from the site show weak oxidation. The interval from 3.2 to 5.07 mbsf in Hole C0015B comprises reduced olive-green mud with abundant biogenic fragments. This interval shows no evidence of oxidation. As is expected, very fine grained framboidal pyrite, likely to have been formed by biogenic processes, is found in trace quantities within this interval.

Geochemistry

Site C0016, drilled directly into and immediately adjacent to a major high-temperature hydrothermal mound of the Iheya North hydrothermal field, NBC mound, yielded no samples suitable for interstitial water or headspace gas analyses. Such samples were recovered from Sites C0013 and C0014, drilled, respectively, ~160 and 450 m east of the high-temperature vents, and they clearly show the chemical signatures of high-temperature fluid-rock interaction.

Depth profiles for CH₄ and H₂ show different patterns at Site C0013: methane is most enriched at 10–12 mbsf, whereas H₂ increases with depth. The peak in methane combined with a consistently high ratio of methane to ethane suggests an input of hydrothermal fluid that already contains unusually high concentrations of microbially produced methane. The CH₄ maximum occurs just beneath a cap rock layer and is consistent with the observed flow of hydrothermal fluid up the hole. The logarithmic increase in hydrogen with depth suggests a deep source for this gas, as well as a relatively shallow sink at ~10 mbsf that is probably microbial consumption. Total sulfur content of the sediment varies from 1% to 66% and is highest within the uppermost 10 mbsf. Total organic carbon and total nitrogen are generally both quite low, <0.2 and <0.02 wt%, respectively, although there are a few high values that cannot be attributed to melting of the plastic core liners. These liners melted at depths as shallow as 12.5 mbsf, indicating that the temperature at this depth well exceeded the 82°C found to be the deformation temperature in tests done shipboard. It appears that hydrothermal alteration and possibly also microbial activity remineralize sedimentary organic carbon relatively quickly.

Site C0013 interstitial water is heavily affected by the abundant anhydrite in the core, which partially dissolved prior to separation of the pore water, greatly increasing Ca and sulfate in solution; the increased Ca, in turn, displaced K and Mg from ion exchange sites in clay minerals in the sediment. Chloride varies from 34% lower than the seawater value to 12% higher in the deepest pore water sample from the site, from Hole C0013E at 17 mbsf, almost certainly because of phase separation. This deepest sample is a relatively pure hydrothermal fluid, containing only 7 mM Mg and 13 mM sulfate; a Na/Cl ratio 10% lower than that in seawater, indicating uptake of Na into alteration minerals; and elevated Cl (623 mM), Ca (48 mM), K (81 mM, among the highest values ever measured in a seafloor hydrothermal system), alkalinity (22 mM), and ammonium (1.5 mM).

At Site C0014, concentrations of dissolved sulfate and methane are inversely correlated, especially above 30 mbsf. Variable (Hole C0014B) and low (Hole C0014G) methane concentrations in the upper section indicate zone(s) of anaerobic methanotrophy at the sulfate–methane transitions. The observed high methane to ethane ratios further suggest that methane at this site is mainly biogenic. Temperatures within the deeper horizons, which exceed 55°C at only 16 mbsf and 210°C at 50 mbsf, would seem to preclude in situ methanogenesis, however. Biogenic methane could be migrating downward from the biologically active zone, or the observed methane at depth (Hole C0013G) could be a component of the subseafloor hydrothermal fluids.

Interstitial water at Site C0014 shows large variations in composition both laterally and vertically as a result of hydrothermal input. At 0–10 mbsf, it closely resembles seawater, except for sulfate, alkalinity, and the nutrient species, which are affected by microbial sulfate reduction and show large lateral variation from hole to hole. Below 10 mbsf, it deviates sharply from seawater, with generally deceasing sulfate, Mg, and Na/Cl and increasing Ca, K, and Si. Chloride varies from that of seawater in the uppermost 27 mbsf to a vapor at 27–40 mbsf and a brine at 40–114 mbsf. These large lateral and vertical variations display the role of permeable and less permeable layers in the sediment in permitting and isolating lateral flow of hydrothermally influenced waters.

Interstitial water at Site C0015 is generally indistinguishable from seawater. Methane and hydrogen are very low and provide no evidence for hydrothermal input.

At hydrothermal recharge Site C0017, 1550 m east of the hydrothermal vents, 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. As at Sites C0013 and C0014, interstitial water at Site C0017 shows the effects of lateral flow of water through permeable layers, but at this site it is unaltered seawater, through a layer of oxidized pumice at 26–35 mbsf and possibly through deeper layers as well. As at Site C0014, pore water within the uppermost sediments, at 0–15 mbsf, is affected by microbial reduction of sulfate-utilizing organic matter. At greater depth, concentrations generally return to those in seawater because of lateral recharge, probably from an outcrop of pumice to the east. Chloride increases slightly with greater depth along with Ca and alkalinity, and K decreases and then increases again to nearly the concentration in seawater in the deepest sample collected, from 141 mbsf.

Microbiology

We did not obtain immediate evidence of an active deep hot biosphere during Expedition 331. We did, however, firmly establish the hydrogeologic basis for a variety of subseafloor microbial habitats associated with a high-temperature hydrothermal system. We also conducted the first tests for contamination by drilling fluid using PFC as a tracer with a variety of drilling and coring systems. We found that, in general, these systems on the Chikyu can collect core that is free of contamination and therefore useful for microbial studies.

Two of the five sites we drilled, Sites C0015 and C0017, hosted relatively abundant subseafloor microbial populations, but these depended on buried sedimentary organic matter. Even at these two sites, the microbial cell abundance was much lower than those found previously at Ocean Drilling Program (ODP)/IODP sites along continental margins (Parkes et al., 1994, 2000; D’Hondt et al., 2004), even though the Iheya North hydrothermal field is located in a backarc basin along a continental margin. These low cell abundances presumably result from the relatively low total organic carbon in the sediments, generally <0.1 wt% at Site C0015 and <0.5 wt% at Site C0017. The abundant occurrence of very permeable layers of pumice and volcaniclastic sediments and the consequent recharge of seawater into the hydrothermal system at Site C0017 generates a locally highly oxic subseafloor environment. This environment promotes the formation of oxidative oligotrophic and/or chemolithoautotrophic microbial communities, rather than obligately anaerobic communities including fermentors, methanogens, and sulfate reducers. Methane concentrations at Sites C0015 and C0017 are generally low, and microbial methanogenesis is quite unlikely. Instead, we found layers enriched in iron oxide minerals at these two sites, which were probably produced by aerobic to anaerobic subseafloor microbial communities sustained by oligotrophic and chemolithoautotrophic iron oxidizers such as Zetaproteobacteria (Emerson et al., 2007). Successful enrichments of putative FeOB from these two sites will allow us to investigate the relationships of these subseafloor microbial communities.

The other three sites we drilled, Site C0016 at an active hydrothermal mound and Sites C0013 and C0014 about 160 and 450 m to the east, all had very high temperatures at very shallow depths, limiting microbes to the uppermost part of the sediment column, if they were present at all.

Physical properties and downhole temperature measurements

Physical properties from the MSCL system as well as discrete measurements of density and porosity, thermal conductivity, formation factor, P-wave velocity, and in situ temperature were made at the five sites drilled during Expedition 331. Sediment was cored at Sites C0013, C0014, C0015, and C0017. The major sediment types are pelagic and hemipelagic mud and volcaniclastic deposits, variably altered by hydrothermal processes including silicification and precipitation of anhydrite. At Site C0016 only rocks were recovered, including massive sulfide and sulfate and altered and mineralized volcanic breccia and flows.

At Site C0013, there is evidence from the density, thermal conductivity, and P-wave velocities within the uppermost 12 mbsf for hard layers rich in anhydrite, interbedded with softer layers. A hard volcanic layer was penetrated at ~22 mbsf. At Site C0014, physical property results are similar in the surface portions of all seven holes, and density and porosity are consistent with pumiceous sediments. At greater depths at Site C0014, sediments become more consolidated, as evidenced by higher thermal conductivity, lower porosity, and higher formation factor, but no clear evidence for a hard basement layer is seen until possibly in our deepest sample, from 128 mbsf. Temperature was measured four times in four different holes at Site C0014 using the APCT3 shoe, yielding values between 16° and 22°C at 4–9 mbsf and ~55°C at 16 mbsf. Thermoseal strips were used three times in Hole C0014G and measured >120°C at 38 mbsf, 145°C at 47 mbsf, and >210°C at 50 mbsf. At Site C0015, physical properties are relatively constant to the maximum depth of 9 mbsf. There is a slight increase in bulk density and a corresponding decrease in thermal conductivity toward the bottom of the hole that coincide with the presence of sandy layers. At Site C0017, the uppermost 20 mbsf exhibits fairly uniform physical properties consistent with the presence of clay; between 20 and 40 mbsf the density and porosity reflect a higher percentage of pumice in the sediments. The in situ temperature at 37 mbsf is 6°C. Between 70 and 90 mbsf at Site C0017, thermal conductivity and bulk density increase along with pumice content, whereas temperature increases from 25° to 40°C. In the lower portion of Hole C0017D, at 95–148 mbsf, thermal conductivity, bulk density, and formation factor all increase with depth, consistent with the presence of increasingly indurated clay. At the maximum depth drilled of 151 mbsf, the temperature is 90° ± 5°C, as measured in triplicate with thermoseal strips.

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