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Site U13651

Expedition 329 Scientists2

Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1365 (proposed Site SPG-1A) was selected as a drilling target because

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

  • Its basement age rendered it a reasonable location for testing the extent of alteration and openness to flow in a thinly sedimented region of ancient (~100 Ma) basaltic basement, and

  • Previous drilling of nearby Deep Sea Drilling Project (DSDP) Leg 91 Sites 595 and 596 provided important preliminary information about the sediment and basalt to be drilled.

The principal objectives at Site U1365 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 (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 subseafloor microbial communities may be supplied with electron donors by water radiolysis, a process independent of the surface photosynthetic world; and

  • To determine how basement habitats, potential activities, and, if measurable, communities vary with crust age and hydrologic regime (from ridge crest to abyssal plain).

Secondary objectives are

  • To constrain regional tectonic models by documenting basement age and

  • To test global biogeochemical models by documenting the nitrate composition of the glacial ocean.

Site U1365 (~5695 meters below sea level [mbsl]) is in the western portion of the South Pacific Gyre within a region of abyssal hill topography trending roughly east–west (085°), with relief ranging from 150 to 200 m (Fig. F1) (D’Hondt et al., 2010; D’Hondt et al., 2011). The abyssal hill spacing is ~5–8 km with a relatively subdued fabric that has been smoothed by sedimentation. Three modest seamounts with a maximum local relief of 400 m border the region. The largest seamount is ~5 km west of the coring site. The closest previous drilling site is Site 596, <1 nmi away.

The site is located within magnetic polarity Chron 34n, so the crustal age could range from 84 to 124.6 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 ~100 Ma at ultrafast spreading rates (~90 km/m.y., half rate).

Lithologies, biostratigraphy, and many geophysical characteristics of the target site were characterized by earlier studies of nearby Sites 595 and 596 (Menard, Natland, Jordan, Orcutt, et al., 1987) and by the 2006/2007 KNOX-02RR survey expedition (D’Hondt et al., 2011) (Figs. F1, F2, F3, F4, F5, F6). Drilling of Site 595 placed the sediment/basement interface at 70 meters below seafloor (mbsf) (Menard, Natland, Jordan, Orcutt, et al., 1987). Fish-tooth biostratigraphy indicates that the sediment of Site 596 was deposited from the Cretaceous to the present (Shipboard Scientific Party, 1987). Iridium chemostratigraphy places the Cretaceous/Paleogene boundary at ~20 mbsf (Zhou and Kyte, 1992). Chert and porcellanite beds occur approximately midway through the sediment column, and metalliferous clay occurs above the basement (Shipboard Scientific Party, 1987). The sediment is capped by manganese nodules (D’Hondt et al., 2009). Manganese oxide and cosmic debris occur throughout the uppermost 8 m of sediment (D’Hondt et al., 2009).

D’Hondt et al. (2009) documented the presence of microbial cells and oxic respiration throughout the uppermost 8 m of sediment at Site U1365. Cell concentrations are approximately three orders of magnitude lower than at similar depths in previously drilled marine sediment of other regions. Net respiration is similarly much lower than at previously drilled sites. From extrapolation of dissolved oxygen content in the uppermost 8 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 U1365. 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.329.103.2011

2Expedition 329 Scientists’ addresses.

Publication: 13 December 2011
MS 329-103