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The uppermost ~500 m of basaltic ocean crust is fractured and permeable, harboring the largest aquifer on Earth. The oceanic crust is hydrologically active beneath at least 60% of the seafloor (Fisher, 2005), with a fluid flux through the crust that rivals global riverine input to the oceans (Wheat et al., 2003). Solutes and colloids (including microbes) circulate actively through the crustal aquifer, but the extent to which microbes colonize, alter, and evolve in subsurface rock is not known. A large fraction of ocean crust remains uncovered by sediments for millions of years on the flanks of mid-ocean ridges before being blanketed in the abyssal plains of the ocean and eventually subducted at trenches. These basement outcrops serve as “breathing holes” through which seawater can ventilate the ocean crust. Fluid flow in the crust is focused in specific areas where permeability is high—usually at the contacts of lava flows or in brecciated zones. In these intervals, the extent of rock alteration is increased, suggesting elevated intensity of seawater–rock interaction. It is well known that the geochemical changes associated with basalt alteration in the uppermost oceanic crust play an important role in setting ocean chemistry. It is unknown, however, what role microorganisms play in mediating this seawater–ocean crust exchange.

Laboratory studies, field examinations, and in situ field colonization and alteration experiments have shown that microbes are abundantly present and play an active role in rock alteration of exposed outcrops at the seafloor at low temperatures (e.g., Wirsen et al., 1993, 1998; Eberhard et al., 1995; Rogers et al., 2003; Edwards et al., 2003a, 2003b). In the subseafloor, the extent of direct participation in alteration by extant communities is not as clear. Abundant petrographic observations show that crust older than 100 Ma may harbor biological communities (e.g., Fisk et al., 1998). However, studies suggest that young subseafloor ocean crust may be the most redox active—and thereby the most likely to support active biological communities. Furnes et al. (2001) compared the degree of alteration in ocean crust aged 0–110 Ma. These data suggest that the majority of alteration features are established early and change little thereafter. Bach and Edwards (2003) compiled data concerning the oxidation state of the upper ocean crust. These data also suggest that oxidative ocean crust alteration occurs during the first 10 m.y. of ocean crust evolution. We therefore expect that hydrologically active, young ridge-flank crust is undergoing progressive oxidative alteration. If microorganisms metabolize the energy associated with the oxidation of FeO in the basalt, a sizeable microbial biomass may be supported by these oxidative alteration reactions (Bach and Edwards, 2003).

The principal science objective of Integrated Ocean Drilling Program (IODP) Expedition 336 was to address fundamental microbiological questions concerning the nature of the subseafloor deep biosphere in oceanic hydrological, geological, and bio-geochemical contexts. Primarily, we planned to study the nature of subseafloor microbiological communities in young igneous ocean crust in order to understand the role of these communities in ocean crust alteration and their ecology in hydrological and biogeochemical contexts. Specifically, we wanted to test the hypothesis that microbes play an active role in ocean crust alteration, while also exploring broad-based ecological questions such as how hydrological structure and geochemistry influence microbial community structures. We also intended to study the biogeography and dispersal of microbial life in subseafloor sediments.

The primary operational goal of Expedition 336 was the installation of subseafloor borehole observatories (CORKs) for long-term coupled microbiological, geochemical, and hydrological experiments. The study site for Expedition 336 is North Pond, a sediment pond on the western flank of the Mid-Atlantic Ridge, which is underlain by hydrologically active upper oceanic crust (Figs. F1, F2). The observatories will enable us to monitor conditions and study processes in situ after the drilling-induced disturbance and contamination of the borehole environment have dissipated. Sampling for microbiological and geochemical studies was conducted on basement and sediment cores retrieved from the CORKed holes and their immediate vicinities.

Our specific operational goals were to

  1. Drill a basement hole to ~565 meters below seafloor (mbsf) at prospectus Site NP-1, core the bottommost ~200 m of the basaltic crust, conduct downhole hydrologic (packer) tests and wireline logging, and install a multilevel CORK to conduct experiments in the deeper portions of the upper basement hydrological environment;
  2. Drill a basement hole to ~175 mbsf at prospectus Site NP-2, core ~70 m of the basaltic crust, conduct downhole hydrologic (packer) tests and wireline logging, and install a single-level CORK to conduct experiments in the uppermost basement hydrological environment;
  3. Recover the existing CORK in Deep Sea Drilling Project (DSDP) Hole 395A, conduct downhole wireline logging, and install a multilevel CORK to conduct experiments in the deeper portions of the upper basement hydrological environment; and
  4. Recover the thin sediment covers of prospectus Sites NP-1 (64 m) and NP-2 (85 m), Hole 395A (93 m), and Ocean Drilling Program (ODP) Site 1074 (64 m) by drilling a single hole at each site with the advanced piston corer (APC).

The primary focus of operations during Expedition 336 was installation of the initial CORK experiments and preparation of the boreholes for subsequent long-term monitoring, experimentation, and observations using remotely operated vehicle (ROV) or submersible dive expeditions. CORKs are being used in perturbation and monitoring points for single- and cross-hole experiments using a recently developed novel in situ microbiological experimentation system, Flow-through Osmo Colonization System (FLOCS) (Orcutt et al., 2010, 2011a; also see​resources/​toolbox.html). An overview of the design, construction, and deployment of these CORKs is presented in a paper in this volume (Edwards et al., 2012).

Expedition 336 also included an enhanced education and outreach program intended to facilitate and communicate the excitement of scientific drilling and exploration to a broad audience, develop educational curricula, and generate products such as photographs, audio/​visual media, and web-based materials to help achieve critical outreach goals.