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IODP Expedition 320:
Pacific Equatorial Age Transect 1
Week 6 Report (13-19 April 2009)
PDF file is available for download.
19 April 2009
This week we finished operations at
Site U1334, made the transit to Site U1335 (PEAT-6C), finished coring in Hole
U1335A, and started coring operations in Hole U1335B.
After coring in
the three holes at Site U1334 was finished, logging Hole U1334C was our final
planned operation. After preparing the hole, we raised the bit to 95 m and
rigged up for logging. Unfortunately, while lowering the first tool string down
the pipe, the logging winch failed and we had to stop logging. Preparations are
underway to repair this at the end of the expedition in Honolulu. Once the
wireline, logging tool string and drill string were recovered, we began the transit for Site U1335 (PEAT-6C) at 2200 hr on 13 April.
Site U1335, Hole U1335A
375 nmi voyage to Site U1335 (PEAT-6C) took 42.3 hr at an average of 8.9
nmi/hr. Once the ship was stabilized over the new site at 1630 hr on 15 April,
we assembled the drill string, lowered it to the seafloor, and spudded Hole
U1335A at 0355 hr on 16 April. Cores U1335A-1H to 36H were taken from 0 to
341.4 m and recovered 354.7 m (104%). We then switched to XCB coring and took
Cores 37X to 45X from 341.4 to 421.1 m and recovered 67.9 m (85%). We retrieved Core U1335A-45X
after taking ~1 hr to advance only 3 m. When retrieving this core, it became
stuck in the drill string and we had to drop another core barrel on top of it
to free it. When Core 45X was recovered, it contained hard limestone and the
XCB bit was very worn down, so we decided it was best to stop coring in this
hole. The basement depth is deeper than anticipated, and suggests higher
seismic velocities in the sediment column from Site U1335 than those from the nearest ODP Site 574.
During this week we continued
analyzing cores from Site U1334 and started processing cores from Hole U1335A.
Site U1334: Core description, imaging and color reflectance
logging has been completed for Holes U1334B and U1334C with only modest
refinement to the lithologic description provided in last week's report.
Site U1335: Hole U1335A has been described down
through Core 17H. The uppermost cores of Pliocene-Pliestocene age contain
foraminifer-rich biogenic oozes with cyclic variations in the relative
contributions of nannofossils, radiolarians, diatoms, and clays. The cores of
late and middle Miocene age are nannofossil oozes. A downhole color change
takes place in these sediments from white to yellow to light greenish gray, similar to that seen at Site U1334. Core description and other lab measurements
Site U1335: Initial biostratigraphy indicates that
sediments recovered at Site U1335 span a succession from the Pliocene-Pleistocene
to the late Oligocene (calcareous nannofossil zone NP25, younger than 26.8 Ma),
including a very expanded sequence of middle and early Miocene nannofossil ooze
and chalk. The top 56 m recovered a late Miocene (~9 Ma) to Pleistocene
succession. Initial biostratigraphic assessment suggests the presence of at
least one major hiatuses through the basal late Miocene (~9 to 11 Ma;
nannofossil zones NN8 to NN9; planktonic foraminfera zone M12). Below we
recovered a thick succession (~290 m) of middle and early Miocene nannofossil
oozes. Late Oligocene carbonates were recovered in the basal ~70 m of the hole.
At the resolution of initial biostratigraphy the hole recovered complete
Miocene-Pliocene, early-middle Miocene and Oligocene-Miocene boundaries.
Calcareous nannofossil preservation is moderate throughout, with some more
etched assemblages in the Pliocene-Pliestocene. Planktonic foraminifers are
moderately preserved throughout, but are absent around the Oligocene-Miocene
boundary. Radiolarians are moderately well preserved throughout. Benthic
foraminifers are present through most of the section and indicate lower bathyal
to abyssal paleodepths.
Site U1335: Paleomagnetism measurements have been completed
down through Core U1335A-8H.
The resulting magnetostratigraphic record extends from the Brunhes (Chron C1n;
0-0.78 Ma) down into Chron C5r
(11-12 Ma). From Core U1335A-5H through 8H, the paleomagnetic
quality varies on the scale of several meters, with poor quality intervals likely
resulting from more extreme reduction diagenesis. Magnetic susceptibility
values are higher and strongly cyclical again from Core U1335A-13X downwards.
We reprocessed data from previous
sites to remove intervals with drilling disturbance and end of core section
effects (new automated feature in database query). A problem with discrete
P-wave x-axis velocity data was identified and fixed by software adjustments
and corrections to previous data. We are currently conducting full physical
property program on Holes U1335A cores.
Site U1334: Using whole-round magnetic susceptibility and gamma
ray density measurements, Holes U1334A, U1334B, and U1334C have been spliced to
form a continuous section to ~338.8 m core composite depth (CCSF), with
interruptions between 222 and 245 m CCSF caused by coring gaps and lack of
correlation due to very weak susceptibility signals within the grayish
green-colored sediment intervals. Sedimentation rates between the basement and
the lower Oligocene section are ~8 m/m.y., increase in the lower Oligocene to
24 m/m.y., and then decrease throughout the Oligocene and Miocene to 4 m/m.y.
Site U1335: Magnetic susceptibility and gamma ray attenuation
(GRA) density data for the first 25 cores reveal a pronounced cyclic
sedimentary pattern through most of the cored interval that can be used to
guide coring offsets in the next two holes to retrieve a complete stratigraphic
Sediment geochemistry this week
continued work on calcium carbonate analyses from Site U1334 and on organic
carbon by acidification from Sites U1332, U1333, and U1334. At Site U1334,
carbonate contents are low in the upper 20 m but increases up to >80% by 50
m; high values (>60-80%) continue to ~250 m where it returns to lower values
(15-85%) in the basal section. Organic carbon was determined on samples that
were acidified to remove calcium carbonate prior to combustion. At Sites U1332-U1334, organic carbon
content are very low (mostly <0.1% with a few peaks up to ~0.28%). Site
U1334 interstitial water analyses continued, including a high-resolution
porewater transect across Cores U1334C-13H to Core 22H. We used 61 rhizon
samples (one per core section) to span an interval with a pronounced dissolved iron
peak and light grayish green oozes with low magnetic susceptibility. This
high-resolution sampling is allowing a more detailed look at the
characteristics of the dissolved manganese and iron peaks, resulting from inferred suboxic diagenesis of organic carbon.
Technical Support and HSE Activities
The shipboard labs
and staff continued to process remaining cores from Hole U1334C. Ongoing
activities include inventory updates and communicating to shore the supplies
required for the next expedition as well as storeroom organization. A second
test of the seismic source for Vertical Seismic Profiles was conducted prior to
the planned experiment during logging in Hole U1334C the test was
We continue to
work on outstanding action items reported in the TAMU Environmental Health and
Safety Audit Review of the laboratories. A fire and boat drill was held on
April 13 for the entire ship's complement; some USIO staff and scientists
practiced lowering the lifeboats and some were shown use of emergency signaling
flares and smoke signals.