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doi:10.2204/iodp.sp.329.2010

Background

Geological setting

The proposed drilling sites span nearly the entire width of the Pacific plate in the Southern Hemisphere between 20° and 45°S (Fig. F2). This ocean crust was accreted along at least four different plate boundaries (e.g., Pacific/Phoenix, Pacific/Antarctic, Pacific/Farallon, and Pacific/Nazca). Crustal ages range from ~100 Ma (Chron 34n) at proposed Site SPG-1A to ~6 Ma (Chron 3An.1n) at proposed Site SPG-7A. Calculated spreading rates range from slow–intermediate (<20 km/m.y. half-rate) to ultrafast (>80 km/m.y., half-rate).

The site locations cover a relatively wide range of crustal ages, spreading rates, and tectonic/volcanic environments. The depth and crustal age of each coring site correlates well with the predicted depth versus age curve (Stein and Stein, 1994), which suggests the sites are located on representative crust. Calculated spreading rates at each site are somewhat biased toward fast and ultrafast spreading rates (28–95 km/m.y., half-rate). Surprisingly, the 95 km/m.y. value is one of the fastest spreading half-rates measured globally. The abyssal hill fabric is relatively well defined for most coring sites. However, off-axis volcanism at proposed Site SPG-5A and possibly proposed Site SPG-6A masked the original seafloor fabric. Sediment thickness ranges from <3 to 122–130 m and generally increases west and south of our survey area. This sediment thickness trend is consistent with greater sediment cover on older crust and on crust located farther away from the center of the gyre. The notable exception to this trend is along the northern transect on crust accreted along the Pacific-Farallon spreading system and older than ~30 m.y. Sediment at each of the sites generally appears as pelagic drape, with some localized mass wasting deposits. Seismic images also reveal areas of bottom current activity occasionally resulting in localized scouring of all sediment above volcanic basement (e.g., proposed Site SPG-5A).

Microbiological setting

The sedimentary communities and activities of South Pacific Gyre sediment are unlike those in any sediment previously explored by IODP or ODP (D'Hondt et al., 2009). Cell concentrations in the shallow sediment (0–8 meters below seafloor [mbsf]) of proposed Sites SPG-1A–SPG-11B are orders of magnitude lower than concentrations in previously examined sediment of equivalent depth (Kallmeyer et al., 2009) (Fig. F3). Dissolved O₂ penetrates extremely deeply (Fig. F4A) (Fischer et al., 2009). Organic-fueled respiration is much lower in this sediment than in any previously examined deep-sea sediment (D'Hondt et al., 2009).

These results demonstrate that at least in the shallow sediment of the South Pacific Gyre (proposed Sites SPG-1A–SPG-11B) (1) net metabolic activities are low and oxygen (O₂) is the principal net terminal electron acceptor and (2) biomass is substantially different than in any previously examined deep-sea sediment. In contrast, at proposed Site SPG-12A on the southern edge of the gyre, where sea-surface chlorophyll content is much higher, cell concentrations and dissolved chemical concentrations resemble those of ODP Site 1231 (on the northeastern edge of the gyre) (Figs. F3, F4), where most of the subseafloor pore water is anoxic and the community may be principally supported by oxidation of organic matter coupled to reduction of Mn(IV), Fe(II), and NO₃^– migrating up from the underlying basaltic aquifer (Shipboard Scientific Party, 2003a; D'Hondt et al., 2004). These results suggest that biomass and microbial activity in subseafloor sediment vary predictably with sea-surface chlorophyll content.

Seismic studies/Site survey data

From 18 December 2006 to 27 January 2007, Cruise KNOX-02RR, aboard the R/V Revelle, surveyed all 11 drilling sites (Fig. F1). Sediment was geophysically imaged and cored at all 11 sites. Proposed Sites SPG-1A–SPG-11B are in the central portion of the South Pacific Gyre. Proposed Site SPG-12A is below higher productivity water at the gyre's southern edge. There is no proposed Site SPG-8.

All of the sites are at a crossing point of two track lines or on a single track line immediately adjacent to a crossing point. The multibeam and seismic results are provided for each site (see "Site summaries").

Geophysical data collected at each site include SIMRAD EM120 swath map bathymetry and Knudsen digitally recorded 3.5 kHz seismic reflection and multichannel seismic reflection. Data were collected at 4.5–6 kt with continuous Global Positioning System navigation and include at least one set of intersecting track lines. Following each geophysical survey, sediment was recovered using gravity, piston, and multicores. At sites with water depths deeper than 4 km, the sediment is principally abyssal clay capped by manganese nodules. At sites in shallower water (proposed Sites SPG-6A and SPG-7A), the sediment is clayey nannofossil ooze.

All proposed sites, supported by seismic, navigation, and bathymetric data as presented in this report, have been classified as "1Aa" by the Site Survey Panel. The 1Aa classification indicates that all required data are in the data bank and have been reviewed by the SSP and that they image the target adequately and there are no scientific concerns of drill site location and penetration.

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