Site U1365 | Site U1366 | Site U1367 | Site U1368 | Site U1370 | Site U1371
IODP Expedition 329:
Subseafloor Life in the South Pacific Gyre
Site U1369 Summary
PDF file is available for download.
Site U1369 (Scientific Prospectus Site SPG-10A) was
selected as a drilling target because (1) its microbial activities and cell
counts were expected to be characteristic of midway between gyre center and the
southern gyre edge, and (2) 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 are (1) to document
the habitats, metabolic activities, genetic composition and biomass of
microbial communities in subseafloor sediment with very low total activity, (2)
to test how oceanographic factors control variation in sedimentary habitats,
activities and communities from gyre center to gyre margin, (3) to quantify the
extent to which these sedimentary communities may be supplied with electron
donors by water radiolysis, and (4) 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).
U1369 is located in the South Pacific Gyre at 5277 m water depth. Basement age
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 and corresponds to magnetic polarity Chron 25r (57.2 – 58.4 Ma)
(Gradstein et al., 2004). The complete sedimentary succession was
recovered by the APC coring system in Holes U1369B, U1369C and U1369E. Basalt fragments were recovered from the basal cores of these holes.
The sediment at
Site U1369 consists of approximately 16 m of zeolitic metalliferous clay. The
sediment is massive in texture and contains visible burrows throughout. The
principal components of the clay are phillipsite, red-brown to yellow-brown
semi-opaque oxide (RSO), and clay. Manganese nodules were recovered from the
sediment/water interface and from deep in the sediment column. Micro- and nannofossils are almost completely absent. The
sediment/basalt interface consists of vitric sand overlying altered basalt. Sediment thickness and composition are fairly uniform from hole to hole.
Microbial cell counts decline rapidly within 2 mbsf and
remain low for the remainder of the sediment column.
Total nitrogen and total organic carbon decrease rapidly to ~3 mbsf and are then extremely low for the remainder of the sediment column.
Dissolved oxygen, dissolved nitrate, dissolved phosphate
and dissolved inorganic carbon are present deep in the sediment. Dissolved
hydrogen concentration is consistently low throughout the column.
A wide range of microbiology experiments was initiated
shipboard. Experiments on major microbial processes and cultivations of viable
microbes were initiated at selected depths ranging from near the sediment/water
interface to the sediment/basalt interface. Subsamples were routinely taken
from all of the distinct lithologic units for post-cruise molecular assays and
Gradstein, F.M., Ogg, J., Smith, A. and 35 others, 2004, A Geologic Time Scale 2004. Cambridge University Press, 500 p.