Site U1365 | Site U1366 | Site U1367 | Site U1368 | Site U1369 | Site U1370
IODP Expedition 329:
Subseafloor Life in the South Pacific Gyre
Site U1371 Summary
PDF file is available for download.
Site U1371 (Scientific Prospectus Site SPG-12A) was
selected as a drilling target because (1) its microbial activities and cell
counts were expected to be characteristic of the upwelling region just south of
the gyre, and (2) its basement age renders it a reasonable location for testing
the extent of sediment-basement interaction in a moderately sedimented region
of 71.5 to 73 Ma basaltic basement.
The principal objectives at Site U1371 are (1) to document
the nature of life in moderately slowly accumulating sediments of great age (up
to 73 Ma), where the surface ocean is characterized by moderate mean
chlorophyll content (less than 3 mg/m3), (2) to determine the extent
to which basement age, thermal regime and chemical transport through the 73 Ma
basaltic basement affect microbial communities and biogeochemical processes in
the sediment and the extent to which chemical transport and microbial
activities in the sediment affect the alteration and habitability of the
basaltic basement, (3) to provide a much higher-activity standard of comparison
for the sites within the gyre (U1365-U1370), and (4) to test the extent to
which life in this sediment may be supplied with an electron donor (dissolved
hydrogen) by radiolysis of water.
Site U1371 is located in the South
Pacific Gyre at ~5300 m water depth. The coring site is located within
magnetic polarity Chron 32n.2n, so the crustal age may range from 71.5–
72.9 Ma. The sedimentary succession was recovered by
APC coring in Holes U1371D, U1371E and U1371F. Additional mudline cores were
recovered in Holes U1371B, U1371C, U1371G and U1371H. Altered basaltic
fragments were recovered from the basal core of Hole U1371F.
sediment at Site U1371 consists of approximately 130 m of diatom ooze and
pelagic clay, divided into two units based on their sharply contrasting
mineralogy. Unit I is clay bearing diatom ooze. It is 104-107 m thick. It
contains numerous ash layers and multiple thin hardgrounds. Unit II is a blend
of clay, zeolite and red-brown to yellow-brown semi-opaque iron-manganese
oxyhydroxides. Other minor sedimentary components at Site U1371 include quartz,
pyrite, manganese oxide/hydroxide, radiolaria, spicules, and silicoflagellates.
Bioturbation is a prominent feature of the sediment, causing diffuse boundaries
on most beds. Overall sediment thickness and composition appear to be broadly
uniform from hole to hole.
Microbial cell counts were above the minimum detection limit (MDL: ~103 cells cm-3) throughout much of the
Profiles of dissolved chemicals clearly indicate that most
of the sediment column is anoxic, with thin oxic zones at the top and bottom of
the column. Manganese is a prominent net electron acceptor throughout most of
the column. Dissolved oxygen nitrate concentrations decrease rapidly and are
below detection by a few mbsf. Below that depth, both are indistinguishable
from zero until the sediment/basalt interface is approached and they rise above
detection. Dissolved ammonium mirrors dissolved nitrate, rising to a maximum
value mid-column, and then decreasing down column. Dissolved
manganese strongly increases in
the uppermost 3 mbsf, exhibits four broad maxima and three local minima within
the column, and decreases again as the basaltic basement is approached. Redox potential broadly mirrors the dissolved manganese profile.
A wide range of microbiology experiments was initiated
shipboard. Experiments on major microbial processes and cultivations of viable
microbes were initiated with samples taken 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 microbiological experiments.