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Site U13661

Expedition 329 Scientists2

Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1366 (proposed Site SPG-2A) was selected as a drilling target because

  • Its microbial activities and cell counts were expected to be characteristic of a setting midway between the western gyre edge and the gyre center and

  • Its basement age rendered it a reasonable location for testing the extent of basalt alteration and openness to flow in a thinly sedimented region of ancient (~95 Ma) basaltic basement.

The principal objectives at Site U1366 were

  • To document the habitats, metabolic activities, genetic composition, and biomass of microbial communities in subseafloor sediment with very low total activity;

  • To test how oceanographic factors (such as surface ocean productivity, sedimentation rate, and distance from shore) control variation in sedimentary habitats, activities, and communities from gyre center to gyre margin;

  • To quantify the extent to which these sedimentary microbial communities may be supplied with electron donors by water radiolysis; and

  • To assess how basement habitats and potential activities vary with basement age and sediment thickness (from ridge crest to abyssal plain).

Site U1366 (~5129 meters below sea level) is in the western portion of the South Pacific Gyre within a region of abyssal hill topography trending roughly northeast–southwest (065°) (D’Hondt et al., 2010) (Fig. F1). Two populations of abyssal hill topography are present. The larger hills’ relief ranges from 300 to 400 m with a spacing of ~20 km. The smaller hills are superimposed on the larger abyssal hills and have a relief of ~50–100 m and a spacing of ~5–6 km. Several small seamounts (2 km wide; 300 m high) are scattered throughout the region. The largest seamount is ~3–4 km south of the coring site. The closest previous drilling site is Deep Sea Drilling Project (DSDP) Leg 91 Site 596, 500 nmi away.

Site U1366 is within magnetic polarity Chron 34n, so the crustal age could range from 84 to 124.6 Ma (Gradstein et al., 2004). Based on tectonic reconstruction of the region by Larson et al. (2002), the crust was accreted along the Pacific-Phoenix spreading center at ~95 Ma at ultrafast spreading rates (~90 km/m.y., half rate).

Many geological and geophysical characteristics of the target site were characterized by the 2006/2007 KNOX-02RR survey expedition (D’Hondt et al., 2011) (Figs. F1, F2, F3, F4, F5, F6). The sediment is brown clay capped by manganese nodules (D’Hondt et al., 2009). Manganese oxide and cosmic debris occur throughout the upper 8.2 m of sediment (D’Hondt et al., 2009).

D’Hondt et al. (2009) documented the presence of microbial cells and oxic respiration throughout the uppermost 8.2 m of sediment at Site U1366. Cell concentrations were approximately three orders of magnitude lower than at similar depths in previously drilled marine sediment of other regions. Net respiration was similarly much lower than at previously drilled sites. From extrapolation of dissolved oxygen content in the uppermost 8 m of sediment, Fischer et al. (2009) predicted that dissolved oxygen penetrates the entire sediment column, from seafloor to basement.

1 Expedition 329 Scientists, 2011. Site U1366. In D’Hondt, S., Inagaki, F., Alvarez Zarikian, C.A., and the Expedition 329 Scientists, Proc. IODP, 329: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.329.104.2011

2 Expedition 329 Scientists’ addresses.

Publication: 13 December 2011
MS 329-104