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doi:10.2204/iodp.proc.329.112.2011

Introduction

This expedition to the South Pacific Gyre, identified as Cruise KNOX-02RR of the R/V Roger Revelle, was the first stage in a multistage program to explore the sediments that have accumulated extraordinarily slowly over tens of millions of years. KNOX-02RR departed Apia, Samoa, 17 December 2006 and ported in Dunedin, New Zealand, 27 January 2007. South Pacific Gyre sediments are farther from continents and productive oceanic zones than any other site on Earth. This exploration provided an unequaled opportunity to understand the nature of life in the most oxidized and food-limited sediments of Earth’s ocean (D’Hondt et al., 2006). These sediments are relatively rich in cosmic debris and provide a direct terrestrial analog for slowly accumulating energy-poor sedimentary environments on other planetary bodies (such as subseafloor Europa and wet subsurface sediments of Mars). The expedition investigated the extent and nature of habitability and life in this ancient and energy-starved subsurface environment, including its extent of metabolic independence from the surface photosynthetic world.

This region contains the clearest seawater in the world (Morel et al., 2007). Its organic flux to the seafloor is lower (Jahnke, 1996), sedimentation is slower, and concentrations of cosmic debris are higher than anywhere else in the ocean. Preliminary studies indicate that water radiolysis is likely to be a significant source of electron donors in these sediments (D’Hondt et al., 2006). Biomass and rates of metabolic activities are likely to be very low, and subseafloor communities and activities are likely to be very different from all previously explored subseafloor sedimentary communities, which are principally fueled by organic matter from the surface world.

This expedition had three fundamental objectives:

• To survey broad characteristics of subseafloor communities and habitats in this region in order to refine the planning and objectives of drilling by Integrated Ocean Drilling Program (IODP) Expedition 329;

• To document the metabolic activities, genetic composition, and biomass of prokaryotic communities in shallow subseafloor (≤8 meters below seafloor [mbsf]) sediments with very low total activity; and

• To quantify the extent to which those communities may be supplied with harvestable energy by water radiolysis, a process independent of the surface photosynthetic world.

The expedition met these objectives by

1. Coring the upper sediment column at several sites along two transects in the region of the South Pacific Gyre;

2. Undertaking extensive microbiological, biogeochemical, geophysical, and geological analysis of the cores; and

3. Accurately surveying the geophysical (seismic, magnetic, bathymetric, and geothermal) characteristics of each site on both transects (Fig. F1).

Coring and geophysical surveys were undertaken during the expedition. Microbiological, biogeochemical, geological, and geophysical analyses were initiated during the cruise and continued long after the expedition ended in Dunedin, New Zealand.

Our results addressed several questions:

• Are the communities in mid-gyre subseafloor sediments uniquely structured?

• Do they contain previously unknown organisms?

• What are their principal sources of biologically utilized energy?

• Do their principal activities and composition vary with water depth, sediment composition, and other properties that co-vary with distance from ridge crest?

These questions can be framed as a series of hypotheses to be tested. For example, D’Hondt et al. (unpubl. data; www.nsf.gov/awardsearch/showAward.do?AwardNumber=0527167) hypothesized the following:

• Net metabolic activities are low, and oxygen (O₂) is the principal net terminal electron acceptor in the subseafloor ecosystem of mid-gyre sediments. Consequently, the diversity of anaerobic activities will be far less than in previously examined subseafloor sediments.

• The composition of the subseafloor sedimentary community in this province will be distinctly different from the communities observed in the higher activity anaerobic subseafloor ecosystems that have been examined to date.

• Current estimates of active subseafloor biomass are at least an order of magnitude too high because the abundance of active cells in this habitat is at least two orders of magnitude lower than current estimates of average cell concentrations in deep-sea sediments.

• Because organic flux to the mid-gyre seafloor is so low, radiolysis of water is a relatively significant source of microbially harvestable energy in this subseafloor environment.

The results of the KNOX-02RR survey cruise significantly advanced understanding of the subsurface world by testing the extent to which distinct oceanographic and geologic provinces contain distinct shallow subseafloor (≤8 mbsf) communities. Survey results also set the stage for documenting the extent to which life in these low-activity sediments depends on the surface photosynthetic world, and the extent, if any, to which it is metabolically independent of the surface world (e.g., Blair et al., 2007; Jørgensen and D’Hondt, 2006). These results place firm constraints on estimates of total subseafloor biomass.

To successfully undertake the broad range of research required by our objectives, our proponent group consisted of scientists from different institutions with very different areas of expertise (University of Rhode Island [URI], University of North Carolina at Chapel Hill [UNCCH], University of North Carolina at Wilmington [UNCW], University of Michigan [UM], Caltech, University of Southern California [USC], and Max Planck Institute [MPI] for Marine Microbiology [Bremen]). As described in our original proposal, the US-based scientists were funded by US National Science Foundation and the US National Aeronautics and Space Administration, whereas the MPI scientists were funded by the Max Planck Society for the Advancement of Science and the German National Science Foundation (DFG) IODP.

KNOX-02RR shipboard and postcruise studies fall into five general areas: geophysical survey, sedimentology, biogeochemistry, activity experiments, and microbiology. The following sections describe the principal objectives and protocols of each area of study.

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