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IODP Expedition 320:
Pacific Equatorial Age Transect 1
Week 3 Report (21-28 March 2009)
PDF file is available for download.
29 March 2009
We cored three
holes at Site U1332 (Site PEAT-2C). Hole U1332A provided well-recovered, high
quality APC cores from the mudline to 125.9 m CSF-A. Core U1332-14H encountered
chert after which we switched to XCB coring. XCB coring advanced to 152.4 m DSF,
through a ~10 m cherty interval with reduced recovery. In the basal section, XCB
Core U1332A-18X recovered a short, ~3.8 m long interval of barren very dense
and stiff clay above basalt. The basalt was reached at 152.4 m CSF-A, ~10 m
shallower than predicted from the seismic data. Hole U1332A cored 152.4 m and
recovered 145.6 m (96%).
A single logging
run with the natural gamma, density, and magnetic susceptibility tools obtained
good data from basement up to the bit at ~80 m DSF. The logging wireline parted
when attempting to get the tool string back into the drill pipe. Multiple
attempts to retrieve the tool string failed, so we isolated the logging tool in
Hole U1332A under 35 m of cement.
We started coring Hole
U1332B with the bit offset 5 m deeper than the seafloor depth established for
Hole U1332A. Hole U1332B cored to basement at 148.6 m CSF-A where we recovered
~2.4 m of dark brown sediment above several small pieces of basalt. Hole U1332B
cored a total of 148.6 m and recovered 141.8 m (95%).
Hole U13323C was designed to provide stratigraphic overlap, and confirm stratigraphic correlations made between the first two holes. Operations are nearing completion at Hole U1332C during the writing of this report.
Site U1332 was located
on 50 Ma oceanic crust so that we could core the interval between 50 and 48 Ma
in basal carbonate sediments above the CCD, the second oldest time slice of the
PEAT Program. Another objective was to obtain the second deepest paleo-depth
constraint for the Oligocene sediments above the CCD (~4 km during the
At Site U1332,
basalt is overlain by approximately 150 m of pelagic sediments that are divided
into four major lithologic units. Unit I is an ~17.7 m-thick, light yellowish
brown clay with radiolarians that grades downhole into a very dark grayish
brown zeolite clay at ~11 m. Unit II extends from ~17.7 m to 75 m and is
composed of Oligocene biogenic sediments. These sediments are dominated by
brown to white nannofossil ooze but radiolarian ooze is a secondary major
lithologic component in the upper 13 m of this unit. Unit III extends from ~75
m to 126 m and is composed of Eocene biogenic sediments. These sediments are
dominated by brown to very dark brown radiolarian ooze but nannofossil ooze is
a secondary major lithologic component between ~84 m and 118 m. Unit IV extends
from ~126 m to 150 m and is composed of dark brown radiolarian ooze,
porcellanite, and very dark brown to black clay. At the base of Hole U1332A, a
16 cm-long piece of calcite-veined basalt was recovered.
The recovered sediments span a near
continuous succession from around the Oligocene/Miocene boundary to the
early/middle Eocene boundary. Radiolarians are common and well preserved in the
Eocene succession but less well preserved in the Oligocene carbonates. There
appears to be a complete sequence of radiolarian zones from RN1 down to RP 14
(middle Eocene), just above the top of the cherty interval. Calcareous
nannofossils are present and moderately well preserved throughout the Oligocene
sequence of alternating nannofossil and radiolarian oozes. The top of the
Eocene-Oligocene transition interval is well constrained by the top occurrence
of Coccolithus formosus. Nannofossils
become very poorly preserved or absent through the late Eocene, equivalent to
nannofossil zones NP18-20. Middle to late Eocene sediments were recovered with
moderately to poorly preserved but generally common to abundant nannofossil
assemblages from nannofossil zones NP15 to NP17. The red-brown clays lying
directly on basement basalt contain a dissolution-modified assemblage of discoasters
that indicate zones NP13-14, close to the lower-middle Eocene boundary, and
close to the predicted basement age (48.4-50.7 Ma). Moderately well preserved
and relatively abundant planktic foraminiferal assemblages were recovered from
the Oligocene succession, equivalent to zones O1 to O6. All Eocene sediments
were barren of planktic foraminifera. Benthic foraminifers were sporadically
present and indicate lower bathyal to abyssal paleodepths.
The natural remanent magnetizations reveal a
very clear record of geomagnetic field reversals that extend from the Brunhes
Chron (0-0.78 Ma) to the top of Chron C2Ar (3.596 Ma) between 0 and 11 m and
from the base of Chron C6An.1r (20.439 Ma) to the top of Chron C20n (42.536 Ma)
between 14 and 125 m. Site U1332 has so far yielded excellent palaeomagnetic
results, that will achieve many of the objectives set out for a detailed
intercalibration of bio-, magneto, and other stratigraphies.
Interstitial water analyses indicate
dissolved silica concentrations generally increase downhole except for a
distinct minimum at ~80 m CSF-A, possibly related to lithological changes at
the E/O boundary. Phosphate concentrations decrease with depth, in the upper 30
m. Increasing chloride in the upper 30 m might possibly be related to elevated
glacial salinities. Boron and Lithium concentrations show a rapid decrease at
~40 m CSF-A a zone that also exhibits minima in alkalinity, calcium and
magnesium concentrations in the interstitial water, and high (up to 90%) bulk
sediment CaCO3 content. A 30 m interval in Hole U1332C was sampled
at high resolution (2 per 1.5 m core section) using Rhizon samplers to
investigate geochemical gradients defined by pore water data from interstitial
water whole rounds.
Whole round and split core sections from all
three sites were analyzed on the track systems for magnetic susceptibility,
bulk density, P-wave velocity, non-contact resistivity, natural gamma
radiation, and color reflectance. Discrete measurements of moisture and density
properties, sonic velocity and thermal conductivity were conducted on split
core sections from. The track data allow a detailed correlation between
different holes primarily using magnetic susceptibility and density. MS varies
from ~25x10-5 SI in radiolarian ooze dominated sections to ~5x10-5
SI in more carbonate-rich intervals. Natural gamma measurements are elevated by
an order of magnitude in the uppermost clay layer. Velocities are low and show
little variation except for relative highs occurring around lithological
boundaries. Bulk density and grain density show a marked increase in
carbonate-rich sections. Porosity values are generally high in the radiolarian
rich sediments (80-90%), and decrease within the carbonate-rich section (~70%).
At present, we have developed a composite
depth section down to the top of the chert interval, near 150 m CCSF-A depth.
This will be used to construct a splice representing a complete stratigraphic section
to at least 150 m CCSF-A depth based primarily on hole-to-hole correlation of
magnetic susceptibility data, which are currently being refined with GRA bulk
density and other data.
Downhole natural gamma, density, and
magnetic susceptibility logs provide important constraints on the poorly
recovered lithologies below and between chert horizons. The logging data
documents the occurrence of two thin chert horizons around 126 and 130 m WSF
and an approximately 14 m thick interval of increased magnetic susceptibility,
reduced conductivity, and enhanced density and photo-electric factor that
appears to be the dark and dense clays and zeolitic clays above basement,
rather than carbonate.
Eight formation temperature measurements
(APCT-3) were conducted in Holes U1332B and U1332C. Three of these yielded good
data and will be used together with the core thermal conductivity measurements
to derived estimates of heat flow at this site. Tool movement caused by ship
heave (up to 3 m) negatively impacted the other measurements.
Technical Support and HSE Activities
The focus of the
technical staff has been supporting core/sample processing through the labs and
maintaining the associated instruments and controlling software. Ongoing
projects included the organization of the storerooms and inventory updates for
A fire and boat drill was held 21 March for the entire ship's complement.
Eye wash stations were installed in
the thin section lab, the two hazardous stores lockers, chemistry lab, and
downhole lab. First aid boxes were distributed throughout the labs.