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Site U13671

Expedition 329 Scientists2

Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1367 (proposed Site SPG-4A) 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 ~33.5 Ma basaltic basement.

The principal objectives at Site U1367 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 determine how basement habitats, potential activities, and, if measurable, communities vary with basement age and hydrologic regime (from ridge crest to abyssal plain).

Site U1367 (~4288 meters below sea level [mbsl]) is in the South Pacific Gyre within a region of abyssal hill topography trending slightly west of north (355°) with relief ranging from 100 to 150 m (Fig. F1). Abyssal hill spacing is ~10–15 km with a relatively subdued fabric and broad abyssal valleys. Several seamounts are present with relatively low relief (<300 m) and are typically located on abyssal hill ridges. The closest seamount to the coring site is ~8 km west of the coring site. The closest previous drilling site is Deep Sea Drilling Project (DSDP) Leg 92 Site 597, 600 nmi away.

The coring site is within magnetic polarity Chron 13n, so the crustal age is 33.3–33.7 Ma (Gradstein et al., 2004). Based on the age of the crust and regional tectonic history (Tebbens and Cande, 1997), the crust was accreted along the Pacific-Farallon spreading center at ~33.5 Ma. The calculated spreading rate from our magnetic survey suggests the crust was accreted at a fast to ultrafast spreading ridge with spreading half rates of ~65–70 km/m.y.

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 homogeneous brown clay capped by manganese nodules (D’Hondt et al., 2009).

D’Hondt et al. (2009) documented the presence of microbial cells and oxic respiration throughout the uppermost 7.2 m of sediment at Site U1367. 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 7 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 U1367. 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.329.105.2011

2 Expedition 329 Scientists’ addresses.

Publication: 13 December 2011
MS 329-105