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Site U13691

Expedition 329 Scientists2

Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1369 (proposed Site SPG-10A) was selected as a drilling target because

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

  • Its basement age renders it a reasonable location for testing the extent of sediment-basement interaction in a thinly sedimented region of ~58 Ma basaltic basement.

The principal objectives at Site U1369 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 sediment-basement exchange and potential activities in the basaltic basement vary with basement age and hydrologic regime (from ridge crest to abyssal plain).

Site U1369 (5277 meters below sea level) is in the South Pacific Gyre within a region of abyssal hill topography trending northwest–southeast (335°) with relief ranging from 300 to 500 m (Fig. F1). Abyssal hill spacing is ~4–10 km with a very pronounced topography. No obvious seamounts are present. The closest previous drilling site is Deep Sea Drilling Project Leg 92 Site 597, 1350 nmi away.

Because of insufficient coverage of magnetic data, the age of the crust is estimated from extrapolated magnetic models and changes in spreading rate recorded by neighboring magnetic profiles. Our best estimate of the crustal age is ~58 Ma, which corresponds to magnetic polarity Chron 25r (57.2–58.4 Ma; Gradstein et al., 2004). Based on the age of the crust and regional tectonic history, the crust was accreted along the Pacific-Farallon spreading center at ~58 Ma. The calculated spreading rate from our magnetic survey suggests an intermediate- to fast-spreading ridge system with spreading half-rates of ~25–30 km/m.y. These slower spreading rates are consistent with the relatively rough abyssal hill topography.

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 shallow sediment (0–5.6 meters below seafloor [mbsf]) consists of homogeneous brown clay with yellowish brown circular mottles (D’Hondt et al., 2009). Smear slides are barren of microfossils and contain micronodules. Manganese nodules were found on the top of all 12 cores taken by the survey and in the uppermost meter of sediment. The nodules are uniformly ~8–10 cm in diameter.

D’Hondt et al. (2009) documented the presence of microbial cells and oxic respiration throughout the uppermost 5.6 m of sediment at Site U1369. 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 5.6 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 U1369. 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.329.107.2011

2 Expedition 329 Scientists’ addresses.

Publication: 13 December 2011
MS 329-107