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IODP Expedition 321:
Pacific Equatorial Age Transect 2
Week 2 Report (10-16 May 2009)
PDF file is available for download.
18 May 2009
The transit to
Site U1336 (PEAT-5C) began at 1636 hr 9 May after completing the transfer of
the dynamic positioning system vendor from the JR to a small boat that was
standing by at the Honolulu Harbor outer sea buoy. The transit to the first
Expedition 321 drill site continued with speed varying considerably due to
fluctuating currents and eddies surrounding the Hawaiian Islands. Average transit speeds ranged from 9.9 to 10.7 kt with 135 turns on both shafts. On the 3rd
day of transit our speed further reduced to 9.0 to 9.9 kt due to the combined
effects of the opposing Northern Equatorial Current and winds of 25-30 kt off
the forward quarter. One propulsion motor remained off-line as field coil
replacement continued. On the morning of 11 May, after several days of
discussions with the science team, the decision was made to divert our course
from Site U1336 (PEAT-5C) to Site U1337 (PEAT-7C). The decision was driven by
several issues including (1) slower than anticipated transit speeds to date,
(2) time spent conducting automated station keeping (ASK) sea trials took
longer than planned, (3) addition of VSI (Versatile Seismic Imager) to the
suite of wireline logging tools to be deployed at Site U1337 (PEAT-7C), (4)
basement projections deepened by 50 m (for all holes) due to findings of
Expedition 320, and (5) scientific trade-offs in general between continued
coring at Site U1336 (PEAT-5C) versus doing a more complete program at Site
U1338 (PEAT-8D). During the transit, permission was requested to change the
drilling plan exchanging prospectus site PEAT-8C for a new site PEAT-8D located
nearby. The target drill site was moved to avoid turbidites from a nearby sea
mount after detailed review of the seismic data. The proposed new site has a
slightly more compressed geologic package and has a slightly shallower water
depth. Transit speeds continued to be slower than anticipated ranging from 9.0
to 9.9 kt until we began getting a boost from the Equatorial Counter Current on
the 16th of May. At the conclusion of this weeks report we were
making ~10.5 kt with the expectation that this will continue to increase over
the next few days. Estimated time of arrival at Site U1337 (PEAT-7C) is early
morning 19 May 2009.
Scientists began practicing core flow using Expedition 320T
cores at the beginning of the week. The rest of the week was spent describing
cores from Hole U1336B and preparing site summary reports. A total of 20 cores (320-U1336B-1H through 20H) were split and described.
Site U1336, ~300 m of pelagic sediments are divided into three major lithologic
units. The sediments are composed mainly of nannofossil oozes, nannofossil
chalks, and chert. The early to middle Miocene sedimentary sequence of Unit I
contains more radiolarians, clay, foraminifers, and diatoms relative to the
early Miocene to early Oligocene sediments below about 70 m CSF-A. Subtle
changes in the relative proportions of these minor components produce
meter-scale dark-light color cycles and two diatom rich layers. Numerous
rounded fragments of pumice occur throughout this unit. The oxidation-reduction
reactions responsible for the observed vivid colors and pore water chemistry
changes are likely fueled by enhanced availability of organic carbon relative
to overlying and underlying sediments. Thin chert layers were encountered near
124 m in Unit II but only broken fragments were recovered. More abundant
chert layers are common in the lower third of the recovered sequence.
Calcareous micro- and nannofossils occur throughout the 300
m thick sediment sequence recovered at Site U1336, showing an age span from
late middle Miocene at the seafloor to early Oligocene at the terminal depth.
The compositions of planktic foraminiferal assemblages are biased by
dissolution causing low abundances dominated by deeper dwelling forms.
Radiolarians are restricted to the upper 170 m of the sediment column and
diatoms to the uppermost 60 m. Diatoms are poorly preserved whereas
radiolarians show good to moderate preservation. These are distributed unevenly
over the ca. 20 m.y. long cored interval, with about 80% concentrated to the
Miocene section (12-23 Ma). Benthic foraminifers are characterized by diverse
assemblages from lower bathyal to abyssal paleodepths. Preservation is moderate
in the ooze interval of the upper ca 170-180 m, and deteriorates in the chalk
and chert interval below that depth. The general abundance of benthic foraminifers is low.
All cores from Hole U1336B were processed for thermal
conductivity and moisture and density and split-core velocity. The results generally match those of
Hole U1336A. Notable differences are grain density values that average ~0.1
g/cm3 less than those of Hole U1336A and x-direction velocities that
average ~50 m/s higher than Hole U1336A velocities. The overall characteristics of Site U1336 are higher
variability of all physical properties in the lithologically more diverse Unit
1, a relatively uniform Unit II, and significant increases in bulk density (to
~1.9 g/cm3) and velocity (to 2100 m/s) in Unit III.
Paleomagnetic data were collected
for Hole U1336B and correlated with the results from Hole U1336A. A continuous magnetostratigraphy for
the middle-early Miocene was constructed from the base of polarity zone C5r to
Chron C6n for the upper 90 m CCSF (composite core depth below sea floor). Below this depth, the magnetization intensity of the
sediments decreases below analytical noise level. The loss of magnetization
intensity correlates with the color change from brown to green sediments.
Correlation and splicing of data from Holes U1336A and
U1336B resulted in a continuous spliced section to a depth of 138.50 m CCSF.
Magnetic susceptibility was used for the correlation until a depth of
approximately 94 m CCSF. Density (GRA) was sufficient for the final 40 m.
Overlapping disturbed intervals in both holes, as well as low signal strength
precluded deeper meaningful correlations.
The geochemists have prepared samples for conducting
analyses upon arrival at Site U1337 (PEAT-7C). Samples from Hole U1336B have
been freeze dried and powdered ready for weighing for CaCO3, TOC
(total organic carbon), bulk sediment and bulk carbonate ICP-AES analyses.
The downhole measurements group finalized the wireline logging program planned for Site U1337 (PEAT-7C). The current
plan includes a VSI to acquire a vertical seismic profile. The VSI is being currently tested for deployment. Early in the week the scientists received an introduction to downhole logging techniques and tools.
Technical Support and HSE Activities
Technical staff prepared the labs to process cores from Hole
U1336B. Training was conducted for the core splitter for the technical staff.
Sample classes were given by the curator to scientific party and technical
staff. Core flow was introduced to all staff. The core reefer was re-arranged
to accommodate the latest boxed cores. Physical counts were conducted for items
on the shopping list. Air conditioning in the underway lab was repaired and the
navigation system (Winfrog) is now functioning.