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doi:10.2204/iodp.sp.329.2010

Coring-drilling strategy

The objectives of this proposal require an IODP riserless drilling expedition. The sediment and basalt that are necessary to meet our objectives cannot be collected without a drilling platform capable of recovering cores from as deep as 171 mbsf in 3170–5700 mbsl water depth. The duration of the proposed expedition is 65 days (Tables T1, T2, T3).

Our general strategy will be to core the entire sediment column multiple times at seven sites and to core the upper basement at three sites. The sites collectively underlie the full range of surface-ocean productivity conditions present in the South Pacific Gyre, ranging from the extremely low productivity conditions of the gyre center (proposed Sites SPG-6A and SPG-7A) to the moderately high (for open ocean) productivity at the southern edge of the gyre (proposed Site SPG-12A at the northern edge of the Antarctic Convergence) (Figs. F1, F2). This series of sites is composed of two transects (Fig. F1), with the first transect centered at ~26°S, beneath the heart of the South Pacific Gyre, and the second transect centered at ~42°S in the southern portion of the gyre.

The sites in the northern sequence have been continuously far from shore and beneath the low-productivity gyre waters for many tens of millions of years (Figs. F1, F2). They provide an ideal opportunity to meet our first objective (to document the nature of life in subseafloor sediment with very low biomass and very low rates of activity). In combination with the southern transect, the northern transect is also necessary to meet our second objective (to determine how subseafloor sedimentary activities and communities vary from gyre center to gyre margin).

Proposed Sites SPG-1A–SPG-11B are necessary for our third objective (to quantify the extent to which subseafloor communities in organic-poor sediment are sustained by H₂ from radiolysis of water). Proposed Sites SPG-1A and SPG-11B are particularly crucial for this objective because their sediment columns are thick enough that their dissolved He-4 (alpha particle) concentrations and fluxes will be measurable.

The sites in the second transect have been in the southern portion of the present gyre (proposed Sites SPG-9A–SPG-11B) or south of the gyre (proposed Site SPG-12A) for tens of millions of years. Particularly at proposed Site SPG-12A, chlorophyll-a concentrations and primary productivity are much higher than at all of the sites in the northern transect (Figs. F1, F2). This transect is necessary to meet our second objective (to document how subseafloor sedimentary activities and communities vary from gyre center to gyre margin). Because proposed Site SPG-12A provides an anoxic standard of comparison for the other sites, it is also crucial for documenting the potential uniqueness (or ubiquity) of the communities and activities that persist in the low-activity, low-biomass sediment beneath the gyre center.

The northern sequence of sites (proposed Sites SPG-1A–SPG-7A) is placed on crust of steadily increasing age from east to west (Fig. F2). They range in age from 7 to as much as 125 Ma (proposed Site SPG-1A). Crust age of the southern sites ranges from 39 to 73 Ma. Their water depths generally follow the classic curve (Parsons and Sclater, 1977) of increasing water depth with increasing basement age (Fig. F2). These sites are necessary to meet our fourth objective (to document the evolution of basalt hydrology and its implications for metabolic habitability and microbial communities in ocean crust under very thin sediment cover).

We propose to drill three or four sediment holes with the advanced piston corer (APC) at each site. We also propose to drill and log a single hole into the upper basement at three select sites (to at least 100 m below the sediment/basalt interface). In the first sediment hole, we will sample for microbiological and geochemical analyses at regular intervals in each core. This sampling will include O₂ measurements immediately after core recovery. This record will be immediately used to (1) define broad downhole microbiological and biogeochemical trends, (2) identify horizons of special interest (including, but not limited to, the sediment/basalt interface, redox interfaces, metalliferous layers), and (3) identify diagnostic properties of those horizons. Cores from the second hole will be used for (1) key biogeochemical and physical property measurements that rely on optodes or electrodes inserted into whole cores (e.g., dissolved O₂ concentrations and electrical resistivity) and (2) close sampling of key horizons identified from the first hole. The third hole will be used for high-resolution sampling of special horizons identified in the first and second holes. The fourth hole will provide a continuous record of sediment appropriate for high-resolution paleoceanographic sampling. Where circumstances only allow coring of three holes, the cores from the final hole will generally be retained as a continuous record; they may be used for high-resolution electrode or optode analyses. This multihole strategy is necessary because (1) microbiological and biogeochemical studies are time- and labor-intensive and (2) microbiological and biogeochemical analyses require immediate sampling, whereas optode measurements require cores to thermally equilibrate for several hours before analysis (Fischer et al., 2009). Shipboard core flow will be arranged to maintain maximal sample integrity for microbiological and biogeochemical analyses, as successfully executed during Leg 201.

This drilling strategy will require the shipboard scientists to rapidly generate data from each sediment hole in order to guide sampling at the next hole. Key redox species, particularly dissolved O₂ (but also including NO₃^–, Fe(II), Mn(II), and SO₄^2– to the extent possible) will be immediately measured from samples taken from freshly cut ends of cores as they arrive on the catwalk. Cut-off syringe samples will be taken on the catwalk from cut core ends for contamination tracing (perflourocarbon), cell counts, and volatile hydrocarbons (methane, etc.). The gas chromatograph for perfluorocarbon analysis will be operated continuously, so recovered cores can be immediately assayed for potential microbial contamination (Smith et al., 2000a, 2000b; House et al., 2003). Rapid turnaround on these assays (<10 min/sample) will allow us to focus resources on uncontaminated samples while working in a microbiologically relevant time frame.

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