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doi:10.2204/iodp.proc.331.101.2011

Preliminary scientific assessment

Artificial hydrothermal vents created

A total of 24 holes were drilled during Expedition 331, of which 21 recovered core. We drilled 708 m and recovered 312 m of core from 560 m attempted, yielding an overall recovery of 56%. This compares highly favorably with the only other attempt to drill a submarine felsic-hosted hydrothermal system during ODP Leg 193, which recorded an overall core recovery of 10.7% at the PACMANUS site in the Eastern Manus Basin, Papua New Guinea (Shipboard Scientific Party, 2002). Holes C0013E and C0014G were cased with stainless steel pipe and capped with stainless steel corrosion caps with open outlets, and Hole C0016B was capped but not cased above a 3 m insertion pipe. Four artificial hydrothermal vents were created by our drilling operations, in Holes C0013E, C0014G, C0016A, and C0016B, in which hydrothermal fluid formerly trapped in the subseafloor ascended up the hole and exited into the ocean (Fig. F9). All hydrothermal vent emissions were confirmed to be >240°C using thermoseal strips taped to the outlet pipe of the corrosion cap. These newly created hydrothermal vents will serve as windows into the subseafloor and any associated microbial communities entrained into them in postdrilling, long-term monitoring studies of fluid composition and flow and of in situ microbial colonization.

Subseafloor hydrothermal alteration, fluid flow, and reservoirs within the defined hydrogeologic structure

Drilling and coring operations during Expedition 331 provide insight into the hydrothermal flow regime at Iheya North Knoll. Although the thermal gradient was known to be high at Site C0016 at NBC mound, Sites C0013 and C0014 have steeper thermal gradients than we expected.

Site C0013 is located 160 m east of the vigorous high-temperature vents and mounds. On our third coring attempt in Hole C0013C, the core liner melted, indicating a temperature >82°C at 12.5 mbsf. Immediately on penetrating a cap rock just a few meters below the seafloor, hot hydrothermal fluid began to flow from Hole C0013E. Drilling to our maximum depth of 54.5 mbsf, we penetrated several hard, low-porosity layers that could function as a cap rock and found thick, porous sediment hydrothermally altered at high temperature between the harder, less permeable layers. The interstitial water showed large changes in composition both laterally and vertically over short distances, suggesting chaotic lateral flow in permeable horizons separated by impermeable barriers. The lithostratigraphy, physical properties of the sediment and rock, and interstitial water chemistry thus all provide insight into the hydrothermal flow regime at Site C0013. These results are, in turn, broadly consistent with the seismic reflection structure, which shows strong reflectors connecting several of the sites at very shallow depths or at depths greater than the drilled interval (Fig. F5).

Site C0014 is located ~450 east of the high-temperature vents and mounds. Even here we found that the temperature exceeds 210°C at only 50 mbsf. The temperature gradient is roughly linear from 0 to 47 mbsf, increasing from the bottom water temperature of 4.5° to 145°C over that depth range, but it deviates greatly from this line at 0–9 and 47–50 mbsf, where it is clearly affected by high-temperature fluid pooling or lateral flow. Interstitial water chemistry demonstrates vertical stratification, from water of seawater chloride values at 0–25 mbsf to a vapor at 29–38 mbsf and a brine from 48 mbsf to the deepest sample at 114 mbsf. Within the upper sediments, which consist of pelagic sediments and pumiceous gravel, there is considerable lateral variability in the intensity of microbial sulfate reduction utilizing organic matter between the four holes that are only meters apart; clearly there is a functionally prosperous and metabolically diverse subseafloor biosphere here. At 20 mbsf, where the temperature is ~70°C, and to the bottom of the deepest Hole C0014G at 137 mbsf, where a linear fit would have the temperature exceed 400°C, the sediment and rock we recovered is intensely hydrothermally altered. As at Site C0013, chaotic flow must be occurring through permeable formations and along fault structures, and is likely separated vertically by impermeable beds that behave as cap rocks. Again as at Site C0013, the seismic reflection structure at Site C0014 is generally compatible with the lithostratigraphic and hydrogeologic architecture we infer from drilling.

Site C0016 is located on and immediately adjacent to the NBC high-temperature hydrothermal mound and vents. We did not recover sufficiently continuous core at this site to determine a detailed lithostratigraphy. We did recover both massive and stockwork sulfide ores, as discussed below.

Site C0017 is located 1550 m east of the high-temperature vents of the Iheya North hydrothermal field, in an area of low heat flow. The overall temperature profile is exponential and concave upward, consistent with downwelling of cold water, implying that this is an area of recharge to the hydrothermal system. We reached a maximum temperature of 90° ± 5°C at the bottom of the deepest Hole C0017D at 151 mbsf. Deviations from a smooth temperature profile indicate the presence of a discrete zone of cold water recharge, consistent with interstitial water chemistry and the presence of a highly oxidized layer at 26–35 mbsf that supports a microbial community. We found only a small amount of hydrothermally altered sediment deep in Hole C0017D.

These results, taken together, provide a more detailed look at the large-scale hydrogeology of the Iheya North Knoll hydrothermal system than has typically been available for subseafloor hydrothermal systems elsewhere, sketched in Figure F16. The hydrological regime at Iheya North Knoll is characterized by large-scale hydrothermal alteration, deposition, and fluid migration within permeable rocks and sediments hosted by the Iheya North Knoll volcanic complex. Prior to Expedition 331, we knew little about the spatial and temporal scales and patterns of hydrothermal circulation in subseafloor hydrothermal systems, particularly those in subduction zone settings such as volcanic arcs and backarc spreading centers. Postcruise studies of data and samples recovered during Expedition 331, along with future field investigations that will benefit from the cased and capped holes we left, will further our understanding of these important hydrothermal systems.

Stratification of hydrothermal fluid by subseafloor phase separation and segregation

Interstitial water collected during Expedition 331 demonstrates that the Iheya North Knoll hydrothermal field is chemically stratified with respect to chloride concentration. Pore water near the seafloor has the chloride concentration of seawater. It is underlain by a thin and possibly discontinuous vapor-rich layer that has lower chloride than seawater at 5 mbsf and deeper at Site C0013 and at 29–38 mbsf at Site C0014. Beneath the vapor-rich layer lies a chloride-enriched brine, at least to the maximum depth of the holes drilled at these two sites. The recharge Site C0017 has no statistically convincing indication of a low-chloride layer. Previous surveys of the Iheya North hydrothermal vents have always found that the discharging fluids are low in chloride and vapor rich (Kawagucci et al., 2011; Takai and Nakamura, 2010). It has been a puzzle where the complementary brines may reside. Several theoretical calculations have predicted that the greater densities of brines cause them to sink to greater depths within subseafloor hydrothermal reservoirs (Fontaine and Wilcock, 2006). Until now there has been no observational evidence to support this hypothesis. Expedition 331 provides tentative evidence of subseafloor stratification of hydrothermal fluids that have phase separated. More evidence is needed, and we will monitor this process in the future using the hydrothermal fluids discharging from the artificial vents.

Is a subvent biosphere present?

So far, the shipboard analyses and experiments have provided little evidence for the existence of a hot subvent biosphere beneath the Iheya North hydrothermal field, though cultivation from a colder, diffusely venting site and a site of lateral recharge provided evidence for a subvent iron-oxidizing microbial community. Prior to this expedition, it was hypothesized that subseafloor mixing between hydrothermal fluids and recharging seawater was sustained by a fine-scale network of narrow hydrothermal fluid flow paths within the subseafloor along the eastern flank of the hydrothermal system. The hydrothermal features we intersected by drilling, including the permeable reservoirs and flow paths, appear to be larger in scale that we had envisioned. Most of the sediment and rock we cored at Sites C0013, C0014, and C0016 have been exposed to much higher temperatures than the microbiologically habitable temperature range (<150°C). Smaller and more limited zones that are habitable by microbes were sampled, however, as well as one large one, the hydrothermal recharge zone at Site C0017. Recharge zones are, of course, an essential part of all hydrothermal systems. Future shore-based, more detailed investigations by a team of international and interdisciplinary scientists will provide multiple lines of evidence for the functionally active, metabolically diverse subseafloor microbial communities that live within the environments of the Iheya North hydrothermal system.

Actively forming Kuroko deposit in the subseafloor environment of the Iheya North field

Much of the sediment and rock we cored at Sites C0013, C0014, and C0016 was intensely hydrothermally altered, and mineralized. Exceptional among the disseminated and vein, stockwork-type sulfide we recovered is the massive sphalerite-rich ore we cored in Hole C0016B. This marks the first time this type of massive sulfide, which closely resembles the Kuroko black ore, has been recovered from an active deep-sea hydrothermal system. We recovered only 2.1 m of core from 45 m of penetration in this hole, which makes reconstruction of the lithostratigraphy impossible, but the recovered core includes a wide diversity of lithologies that are typically associated with VHMS mineralization. In addition to the sphalerite-rich black ore, it includes a boulder-sized, coarsely crystalline piece of anhydrite veined by sphalerite-pyrite, as well as pyrite veined and altered volcanic rock. Detailed studies of these lithologies, in the context of all that is known about the active hydrothermal system at Iheya North Knoll, will provide direct evidence of how VHMS mineral deposits in general, and the Kuroko ores in particular, are formed.

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