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doi:10.2204/iodp.proc.329.109.2011

Site U1371¹

Expedition 329 Scientists²

Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1371 (proposed Site SPG-12A) was selected as a drilling target because

• Its microbial activities and cell counts were expected to be characteristic of the upwelling zone just south of the southern gyre edge and

• Its basement age (~75 Ma) renders it a reasonable location for testing the extent of sediment-basement interaction in a moderately sedimented region of relatively old basaltic basement.

The principal objectives at Site U1371 were

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

• To test how oceanographic factors control variation in sedimentary habitats, activities, and communities from gyre center to just outside the gyre margin;

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

• To determine how sediment-basement exchange and potential activities in the basaltic basement vary with basement age and hydrologic regime (from ridge crest to abyssal plain).

Site U1371 (5301 meters below sea level [mbsl]) is in the South Pacific Gyre within a region of abyssal hill topography trending northeast–southwest (050°) with relief ranging from 50 to 100 m (Fig. F1). Abyssal hill spacing is ~5–8 km with very subdued fabric that has been smoothed by sedimentation. No obvious seamounts are observed, but a small conical feature ~200 m high is located ~4 km south-southeast of the coring site. The closest previous drilling site is Deep Sea Drilling Project Leg 29 Site 276, 800 nmi away.

The coring site is within magnetic polarity Chron 32n.2n, so the crustal age may range from 71.5 to 72.9 Ma (Gradstein et al., 2004). Based on a tectonic reconstruction of the region by Larson et al. (2002), the crust was accreted along the Pacific-Phoenix spreading center at ~73 Ma.

Many geological and geophysical characteristics of the target site were characterized by the Cruise KNOX-02RR site survey (D’Hondt et al., 2011) (Figs. F1, F2, F3, F4, F5, F6). The shallow sediment (0–5 meters below seafloor [mbsf]) consists of grayish brown to light yellowish brown siliceous ooze, with frequent burrows and bioturbation (D’Hondt et al., 2009). Smear slides contain abundant diatoms and some sponge spicules. D’Hondt et al. (2009) documented the presence of microbial cells throughout the uppermost 5 m of sediment at Site U1371. Dissolved oxygen was undetectable below ~1 mbsf (Fischer et al., 2009), and dissolved nitrate was undetectable below ~2.5 mbsf (D’Hondt et al., 2009). Cell concentrations were approximately two orders of magnitude higher at 5 mbsf than at similar depths in the South Pacific Gyre sites. Preliminary molecular study of archaeal 16S rRNA genes in three clone libraries (0.6, 1.5, and 2.1 mbsf) suggested the presence of Marine Crenarchaeota Group 1 (MG-1) and other non-MG-1 archaeal groups in shallow sediment (Durbin and Teske, 2010).

¹ Expedition 329 Scientists, 2011. Site U1371. 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.329.109.2011

² Expedition 329 Scientists’ addresses.

Publication: 13 December 2011
MS 329-109

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