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doi:10.2204/iodp.sp.336.2010

Introduction

The upper ~500 m of igneous ocean crust is fractured and permeable, harboring the largest hydrologically active aquifer on Earth. Most of the oceanic crust is hydrologically active (at least 60%; 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 degree to which microbes "take seed," colonize, alter, and evolve in subsurface rock is not known. Sizable fractions of ocean crust remain uncovered by sediments for thousands to millions of years on the flanks of mid-ocean ridges (MORs) before being blanketed in the abyssal plains of the ocean and eventually subducted at trenches, and these basement outcrops serve as conduits for fluid flow. Laboratory studies, field examinations, and in situ field colonization and alteration experiments have shown that microbes are abundantly present and play active roles 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 biological communities may be harbored in crust older than 100 Ma (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 10–20 m.y. of crustal age and thereafter slows or ceases.

The principal science objectives for Expedition 336 address fundamental microbiological questions concerning the nature of the subseafloor deep biosphere in an oceanic hydrological, geological, and biogeochemical context. First, we plan 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 will 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. Second, we intend to study the biogeography and dispersal of microbial life in subseafloor sediments.

The primary operational goal for Expedition 336 is to install multilevel subseafloor borehole observatories ("CORKs") for long-term coupled microbiological, geochemical, and hydrological experiments. Installation of these CORKs will enable us to monitor conditions and study processes in situ after drilling-induced disturbance and contamination of the borehole environment have dissipated.

Our specific operational goals are to

1. Drill a basement hole to ~565 meters below seafloor (mbsf) at Site NP-1, core the bottom ~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 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 at 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. Core the thin sediment cover using the advanced piston corer (APC) in a single hole at four sites (proposed Sites NP-1 [64 m] and NP-2 [85 m], Hole 395A [93 m], and Ocean Drilling Program [ODP] Site 1074 [64 m]).

Operations during Expedition 336 will include installation of the initial observatory experiments and will lay the foundation for subsequent long-term monitoring, experimentation, and observations using remotely operated vehicle (ROV) or submersible dive expeditions. CORKs will be used at perturbation and monitoring points for single- and cross-hole experiments and will include the recently developed, novel in situ microbiological experimentation system Flow-through Osmo Colonization Systems (FLOCS; Orcutt et al., 2010, in press; also see www.darkenergybiosphere.org/).

Expedition 336 will also include an enhanced education and outreach program intended to facilitate and communicate excitement about scientific drilling and exploration to a broad audience, build educational curricula, and create media products (photographic, sound, video, and web based) that will help achieve critical outreach goals.

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