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Site U13701

Expedition 329 Scientists2

Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1370 (proposed Site SPG-11B) was selected as a drilling target because

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

  • Its basement age (~75 Ma) renders it a reasonable location for quantifying the extent to which relatively old and deeply buried sedimentary microbial communities may be sustained by hydrogen from water radiolysis, and

  • Its basement age also makes it a reasonable location for testing the extent of sediment-basement interaction in a moderately sedimented region of relatively old basaltic basement (D’Hondt et al., 2010).

The principal objectives at Site U1370 are

  • 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 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 U1370 (5074 meters below sea level) is in the South Pacific Gyre within a region of abyssal hill topography trending northeast–southwest (065°) with relief ranging from 50 to 100 m (Fig. F1). Abyssal hill spacing is ~5–8 km with a relatively subdued fabric that has been smoothed by sedimentation. Two seamounts (500 m high; 6 km wide) are at the eastern limit of the survey area. The largest seamount is ~10 km east of the coring site. The closest previous drilling site is Ocean Drilling Program Leg 181 Site 1123, 810 nmi away.

The coring site is within magnetic polarity Chron 33n, so the crustal age may range from 73.6 to 79.5 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 ~75 Ma.

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 brown clay with frequent round mottles of very pale brown (D’Hondt et al., 2009). Smear slides are barren of microfossils. Manganese nodules occur at the sediment/water interface.

D’Hondt et al. (2009) documented the presence of microbial cells and oxic respiration throughout the uppermost 3 m of sediment at Site U1370. 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 3 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 U1370. 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.329.108.2011

2 Expedition 329 Scientists’ addresses.

Publication: 13 December 2011
MS 329-108