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IODP Expedition 320:
Pacific Equatorial Age Transect 1
Week 4 Report (29 March - 4 April 2009)
PDF file is available for download.
5 April 2009
Three holes were cored at Site U1333. Hole
U1333A provided high quality and recovery APC cored sediments from
approximately 3 m below the mudline (~4850 mbsl) to 95 m (Core
U1333A-10H). Since we had to drill-over this core barrel to recover it, we switched to XCB coring.
XCB coring advanced to 184.1 m, through an approximately 60 m thick sequence of
lowermost Oligocene carbonate oozes and nannofossil-bearing
Eocene sediments. In the basal section, Core U1333A-20X recovered a 30 cm
long interval of lithified limestone, followed by Core U1333A-21X that
contained a limestone basalt breccia.
Core U1333A-22X recovered a 6 cm piece of
basalt. No downhole logging was conducted at Site U1333.
Coring in Hole U1332B was started 5 m shallower than U1333A to
recover the mudline and to span the core gaps from the first hole.
Core U1333B-1H recovered 7.73 m of carbonate bearing ooze overlain by a few meters of clay. Since the cores recovered from Hole U1333A showed that
there was no significant porcellanite or chert layers, we used the APC drill-over strategy in Hole U133B to obtain APC cores across and below the
EoceneOligocene transition down to 162.7 m. We then
XCB cored to basement and a total depth of 180.3 m.
Hole U1333C was designed to provide stratigraphic overlap and confirm stratigraphic correlations made between Holes U1333A
and U1333B. APC coring in Hole U1333C
started 2.75 m shallower than Hole U1333B and reached
to 163.2 m before we had to switch to XCB coring.
During this week, we cored three holes at
Site U1333 (PEAT-3C). Site U1333 is located over seafloor basement that was
estimated to be 46 Ma targeting basal sediments within the equatorial zone. Our
objective is to recover middle and late Eocene sediments, as well as Oligocene
and younger sediment as part of the PEAT depth transect. These sediments will
provide information about changes in the
temperature and structure of the near-surface ocean to investigate fluctuating
accumulation rates of siliceous ooze and periods of highly fluctuating
carbonate compensation depths (CCDs).
U1333 recovered a carbonate-bearing interval between the earliest Oligocene at around 112 m up to
the earliest Miocene interval below the top few meters of clay, and contributes
towards achieving this important depth-transect objective. Site U1333 also documents high sedimentation rates of ~ 1.5 cm/ky during the early
Oligocene, and an increasing influx of
diatoms within the strikingly white sediments above the EoceneOligocene
transition. The middle Eocene sediments contain more carbonate
sediments/rocks as compared with those in the previous two Sites U1331
At Site U1333, seafloor basalt is overlain
by approximately 183 m of pelagic sediments that are divided into four major
lithologic units. Unit I is about 10 m thick and composed of a dark brown
radiolarian clay overlying an alternating
sequence of dark yellowish brown radiolarian clay and yellowish brown
nannofossil ooze. Unit II is approximately 107 m thick and is composed of
Oligocene homogenous white to very pale brown nannofossil ooze. Unit III is
approximately 60 m thick and is Eocene in age and composed an alternating
sequence of dark brown radiolarian ooze, radiolarite, brown
nannofossil ooze and chalk. Porcellanite also occurs in is Unit III between
approximately 168 and 174 m. Unit IV is an approximately 4-m-thick
brown micritic chalk that overlies inter-collated micritic limestone and basalt.
sediments span a near continuous succession from around the lower Miocene
boundary to the middle Eocene. Radiolarians
are common and well preserved in the Eocene succession but less well-preserved
in the Oligocene sediments. There appears to be a complete sequence of
radiolarian zones from RN1 down to RP 14 (middle Eocene). Calcareous
nannofossils are present and moderately to well preserved through most of the
succession, although there are some short barren intervals around in the
middle to late Eocene boundary. The succession appears to span a complete
sequence of nannofossil zones from the early Miocene zone NN1 to the middle
Eocene zone NP15. Planktic foraminifers are relatively abundant and well
preserved from the lowest part of the Miocene to the lower Oligocene; however
the upper Eocene sediments contain poorly preserved specimens or are
barren. Preservation and abundance slightly increased in some
intervals of the middle Eocene, which is recognized by the presence of
acarininids and clavigerinellids. Benthic foraminifers were almost continuously
present and indicate lower bathyal to abyssal depths.
Paleomagnetic results from Holes U1333 A and
U1333B have so far provided an interpretable magnetostratigraphy that extends
downhole from the top of
Chron C6An.1n (20.040 Ma) to the top of Chron C13r reversal (33.705 > Ma) at
~111.4 m using the APC cores. Cores from Holes U1333B and U1333C are being analyzed at present, and are likely to extend this coverage.
analyses focused on samples from Hole U1333A, as well as continuing work on
samples from the first two sites. Samples from the upper part of Hole U1333A
have relatively high calcium carbonate contents. The interstitial water
geochemistry shows relatively constant chloride values, and initial sulfate
results indicate limited sulfate reduction. Alkalinity values are never elevated.
However, alkalinity and dissolved strontium values rise slightly near the
EoceneOligocene transition, which is generally
consistent with carbonate dissolution/recrystallization processes. Dissolved
silicates increase with depth, with values always <1000 µM.
A full physical
property program was run on all cores from Site U1333 comprising whole-round
multi-sensor core logger measurements of magnetic susceptibility, bulk density,
P-wave velocity, natural gamma radiation, and measurements of color
reflectance, followed by discrete measurements of moisture and density
properties, sound velocities and thermal conductivity. Magnetic susceptibility
varies around ~24x10-5 SI in radiolarian ooze dominated sections and
~3x10-5 SI in more carbonate-rich intervals. Natural
gamma measurements are elevated in the uppermost clays and also increase near
the lower Oligocene at around 115 m. Velocity
gradually increases downhole (from carbonate to more radiolarian-dominated
sediments). Velocity varies between 1490 and 1560 m/s with lower velocities in
the more carbonate-rich intervals. Bulk density and grain
density show a marked decrease around 112 m, which
also corresponds to a decrease in carbonate content. Porosity values are
generally high in the radiolarian rich sediments (80%) and are lower in the carbonate-rich section (~60%).
Site U1333 magnetic susceptibility and density
data proved most useful for correlating between holes. Features in the magnetic
susceptibility data are well aligned between the three holes down to a ~160 m core
composite depth below seafloor (CCSF-A), although the section below ~140 m
CCSF-A is difficult to correlate. A splice representing a complete
stratigraphic section extends to below the Eocene/Oligocene
boundary to at least 160 m CCSF-A.
temperature measurements were conducted in Hole U1333B with the APCT-3 tool. Analyses of the previous sites data indicate that Site U1332 has a heat flow of ~67 mW/m2 (and a thermal gradient of
74°C/km), similar to heat flow values from nearby ODP Sites 1218 and 1219.
In contrast, Site U1331 had a heat flow of about 10 mW/m2
(and a thermal gradient of 13.4°C/km), but is within
the range of the lower values in the global heat flow data set for the eastern
Technical Support and HSE Activities
The shipboard labs
continued to be busy processing cores and samples from Holes U1332 and U1333.
Efforts continued to be intensely focused on efforts to maintain, troubleshoot,
and improve lab instruments, acquisition software, procedures, and database.
Ongoing projects included the organization of the storerooms and inventory
updates for the labs. A fire and boat drill was held on March 28 for the entire
ship's complement and USIO staff and scientists were shown how to lower the lifeboats.