Site U1365 | Site U1366 | Site U1367 | Site U1368 | Site U1369 | Site U1371
IODP Expedition 329:
Subseafloor Life in the South Pacific Gyre
Site U1370 Summary
PDF file is available for download.
Site U1370 (Site SPG-11A in the site survey cruise) 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 moderately
sedimented region of 74 to 80-Ma basaltic basement.
The principal objectives at Site U1370 are (1) to document
the habitats, metabolic activities, genetic composition and biomass of
microbial communities in subseafloor sediment with 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 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).
U1370 is located in the South Pacific Gyre at 5074 m water depth. The coring
site is located within magnetic polarity Chron 33n, so the crustal age may
range from 73.6 – 79.5 Ma. The sedimentary succession was recovered by
APC coring in Holes U1370D, U1370E and U1370F. Altered basaltic fragments were
recovered from the basal cores of Holes U1370D and U1370F.
sediment at Site U1370 is approximately 70-m thick. The dominant lithology is
dark brown zeolitic metalliferous pelagic clay. The principal components of the
clay are red-brown to yellow-brown semi-opaque oxide (RSO), phillipsite, and
smectite. Unit I lies between the sediment/water interface and the top of a
nannofossil ooze (Unit II), at approximately 61 mbsf. Unit II is a relatively
short (30 to 290 cm) pale yellow interval predominantly composed of
coccolithophores, with trace phillipsite and clay. Unit III is a thin clay
interval, containing 88% RSO and 12% clay; it directly overlays the basaltic
basement. Although volcanic glass is locally abundant (~43%) in Unit I, its
overall abundance is only 7% and it is completely absent in Units II and III. A
large, fragmented manganese nodule was recovered in Hole U1370D at 10 mbsf and
fragments of a manganese-encrusted hardground were recovered in Hole U1370F at
sediment structure at Site U1370 is massive, although occasional laminations
and thin beds are visible in the lower half of Unit I. Planolites (horizontal) burrows are faintly visible in most of the clay and Trichichnus
(vertical) burrows blend the upper and lower contacts of the nannofossil ooze
and the overlying and underlying clay. Sediment thickness and composition are
uniform from hole to hole.
The nannofossil ooze was deposited during early Paleocene
foraminiferal zone P1. Its occurrence in this deep-sea clay sequence is
attributed to deepening of the calcite compensation depth and lysocline during
the interval of decreased planktic carbonate precipitation that followed the
end-Cretaceous mass extinction.
Microbial cell counts were above the minimum detection limit throughout much of the sediment column.
dissolved oxygen and nitrate profiles at Site U1370 are strikingly different from the profiles at previous sites.
Dissolved oxygen concentration decreases sharply in the first several meters
below seafloor and then more gradually with depth. The rate of increase in dissolved nitrate concentration is higher than at
previous sites, suggesting that organic nitrogen oxidation in the sediment is
greater here than at those sites. The changes
in dissolved oxygen and nitrate throughout the upper sediment column are
attributed to oxygen consuming organic oxidation by sedimentary microbes.
Dissolved potassium concentration declines nearly linearly
with depth in the sediment, indicating a sink for dissolved potassium in the
underlying basaltic basement. This sink is inferred to be basalt alteration
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 postcruise
molecular assays and microbiological experiments.