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IODP Expedition 323:
Bering Sea Paleoceanography
Week 6 Report (9-15 August 2009)
PDF file is available for download.
15 August 2009
Week 6 of IODP Expedition 323 focused on coring and logging
operations at Site U1343 (originally GAT-4C) in the northern Bering Sea region.
The week began while coring Hole U1343D, 30 m south from the prospectus site
position, at 1964.7 m DRF. The hole was quickly terminated after the first APC
core did not penetrate as planned based on stratigraphic correlation data.
Overall recovery for Hole U1343D was 100.0% with 8.5 m recovered.
Hole U1343E was started without any offset from Hole U1343D.
APC coring continued down to Core U1343E-41H when the core was pulled with a
ruptured core liner. Core U1343E-41H had to be hydraulically pressed out of the
core barrel. This coincided with reaching the BSR (bottom simulating
reflector). The sediment recovered was noticeably drier and harder at this
depth. We continued coring the hole using a combination of the XCB and APC
coring systems through Core U1343E-83X when we reached the total depth of 744.3
m DSF. Once the coring tools were secured the hole was prepared for wireline
logging. Logging in Hole U1343E was conducted successfully using the triple
combo and FMS-sonic tools reaching the total hole depth of 745 mbsf.
During the last part of the week we completed the 139
nautical mile transit from Site U1343 to Site U1344 (GAT-3C) averaging 10.3
nmi/hr. APC coring of Hole U1344A commenced at 0940 hr on 14 August at 3183 m
DRF or 5 m above the "corrected" PDR depth of 3188 m DRF. APC coring continued
through Core U1344A-26H to a depth of 245.6 m DRF using non-magnetic coring
assemblies and with the FLEXIT orientation tool installed. Coring with the APC
system was suspended after severely bending the steel core barrel on Core
U1344A-27H. The XCB system was then deployed and coring continued through Core
U1344A-35X (330.8 m DRF) ending week 6 at midnight on 15 August.
During this week, we completed coring and logging operations
at Site U1343 and started coring our first hole at Site U1344 in the northern
Bering Sea gateway region. Five holes, U1343A through U1343E were cored at the
first site to total depths of 201.05 (U1343A), 35.5 (U1343B), 234.2 (U1343C),
8.5 (U1343D), and 744.3 (U1343E) m DSF. Recovery at these holes was 204.03 m
(101%), 34.56 (97%), 231.04 (99%), 8.53 m (100%), and 700.27 (94%),
respectively. Hole U1343B was a dedicated hole for microbiological sampling.
At Site U1344 we continue to APC/XCB core Hole U1344A. The
total depth as of Sunday 16 August 2009 at 16:30 hr local time is 474.4 m DSF
with a recovery of 417.03 m CSF (88%).
On Saturday 15 August 2009, we held the science meeting for
Site U1343 to present and discuss our results.
The sediments at Hole U1343E are mostly diatom-rich clayey
silt, diatom-rich silt and diatom silt with occasional diatom ooze. Laminated
intervals in the upper 130 m can be correlated to the other holes at this site.
There are sandy layers and sandy mottles in the core, and the sediment is coarser and more siliciclastic rich
than at any of the previous sites. Authigenic carbonate-containing layers are
observed from Core U1343E-5H and become more common below 500 m. An increase in
the occurrence of shells is observed throughout the same interval. The sediments
are slightly to moderately bioturbated throughout, with frequent cracks and
voids in the sediment from gas expansion as well.
At Site U1344, we have split, scanned, imaged and described all
archive sections from Cores U1344A-1H through -32X. The sediments are similar
to those observed at Site U1343, but are notably richer in siliciclastics. New
features of the sediment at this site include thin sandy layers and worm tubes.
The micropaleontology group analyzed all core catcher samples
from the Site U1343, plus an additional 3 extra PAL samples at Cores U1343A-8H,
-10H and -12H in order to examine interglacial periods. A composite age depth
model was constructed based on datums from all major groups of fossils (diatom,
radiolarian, dinoflagellates, calcareous nannofossils, planktic foraminifera
and silicoflagellates), yielding an age estimate of 2-2.4 Ma for the lowermost
samples. The average sedimentation rate is ~26 cm/ky for the upper 320 m, and
increases to ~56 cm/ky in the lower 300 m.
The preservation of all fossil groups is generally good to
moderate. Calcareous planktic fossils show barren intervals from 250 m to the
end of the record, probably related to dissolution and diagenetic
recrystallization. Abundances of all groups increase in the upper 300 m, where
all records show distinct larger amplitude variations.
Micropaleontological analysis of samples from the Site U1344
began by the end of the week. The latest age estimate is 250-430 ky and 300 ky
for the bottom of Core U1344A-13H based in radiolarian and diatom datums,
Paleomagnetic measurements were made on the rest of the XCB
cores from Hole U1342D and from Site U1343. At Site U1343, we identified the
Brunhes/Matuyama boundary at the Holes U1343A, U1343C and U1343E, and both the
top and bottom of the Jaramillo subchron and the top of the Cobb Mountain
subchron at the Hole 1343E.
Catwalk samples for interstitial water (IW) chemistry, bulk
sediment geochemistry, and microbial cell counts were collected from cores
retrieved from Holes U1343A, U1343B and U1343E. We sampled for microbiology
(i.e. cell abundance, RNA/DNA analysis, amino acid composition and community
structure) at high-resolution at Hole U1343B, a microbiology-dedicated hole and
in Hole U1343A and U1343E with lower resolution. In Hole U1343E we retrieved
microbiology samples from 700 m CSF-A. Contamination testing with liquid
tracers (PFT) confirmed that these samples were not contaminated with drilling
fluid although they were taken with XCB. IW samples were taken both on the
Catwalk and in the microbiology-dedicated hole. We determined concentrations of
pore water alkalinity, dissolved inorganic carbon, sulfate, ammonium, sulfide,
major ions (i.e., Ca, Na, K) and minor ions (i.e., Fe, Mn). Analysis of solid phase
fractions included total nitrogen, carbon and sulfur. Site U1343 has the
highest microbial respiration activity among all sites studied so far on the
expedition, and there is clear evidence in the IW chemistry for microbial
activity even below 400 m CSF-A. Interestingly, DIC and PO43-
are both produced and consumed at this site
Downhole logging operations in Hole U1343E were conducted
over 19 hrs starting on midday, 12 August. Two tool strings were deployed: the
triple combo (natural gamma ray, porosity, density, resistivity) and the
FMS-Sonic string (electrical images and sonic velocity). Operations proceeded
smoothly, and all tool strings reached the bottom of the hole with only minimal
fill between passes. The caliper logs shows that the hole was in good condition
and that all data should be of good quality, allowing reliable core/log and
seismic/log integration. All logging data were processed on site and transmitted
to LDEO for quality control. Preliminary data were distributed to the science
party for integration with the ongoing core measurement and description.
The composite depth scale and splice at Site U1343 was constructed from 0.0 to 779.18 m CCSF-A. The splice consists of one complete and continuous interval from the mudline to 270.47 m CCSF-A, and also includes appended cores from Hole U1343E (with unknown gaps) from 270.47 to 779.18 m CCSF-A.
Technical Support and HSE Activities
During this week, the technical staff was fully engaged
processing cores from Site U1343 and U1344, assisting scientists with their
instruments, data processing, and solving equipment issues as they arise.
Decking was installed in the forward section of the Science
Pallet Stores's over head, providing an additional ~410 cubic feet of science
supply storage. Including the overhead storage installed last week, we have
increased science storage capacity by ~730 during this expedition. Part of this
space has been used to store an emergency supply of D-tubes equivalent to 450
meters of recovery.
The weekly fire and boat drill was held as scheduled. There
are no HSE issues to report.