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Site U13681

Expedition 329 Scientists2

Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1368 (proposed Site SPG-6A) was selected as a drilling target because

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

  • Its basement age renders it a reasonable location for testing the extent of sediment-basement interaction, basalt alteration, and openness to flow in a thinly sedimented region of ~13.5 Ma basaltic basement.

The principal objectives at Site U1368 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 U1368 (~3740 meters below sea level) is in the South Pacific Gyre within a region of primarily seamount topography with possible evidence of original abyssal hill seafloor fabric oriented nearly north–south (Fig. F1). The site is bordered to the east and west with what appear to be large (500 m high; 6 km wide) seamounts preferentially perched on top of abyssal hills. The closest previous drilling site is Deep Sea Drilling Project Leg 92 Site 598, 550 nmi away.

Site U1368 is within magnetic polarity Chron 5ABn, so the original crustal age ranges from 13.4 to 13.6 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 ~13.5 Ma. The calculated spreading rate from the KNOX-02RR magnetic survey suggests the crust was accreted at an ultrafast spreading ridge with spreading half-rates of ~80–85 km/m.y. (D’Hondt et al., 2010; D’Hondt et al., 2011).

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 cored sediment grades from dark yellowish brown clay to yellowish brown foraminifer-bearing clayey nannofossil ooze. The core is mottled (D’Hondt et al., 2009).

D’Hondt et al. (2009) documented the presence of microbial cells and oxic respiration throughout the uppermost 2.6 m of sediment at Site U1368. 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 2.6 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 U1368. 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.329.106.2011

2 Expedition 329 Scientists’ addresses.

Publication: 13 December 2011
MS 329-106