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IODP Expedition 323:
Bering Sea Paleoceanography
Week 5 Report (3-9 August 2009)
PDF file is available for download.
9 August 2009
Operations
Week 5 began while APC coring in Hole U1341C at Core
U1341C-6H at a depth of 2203.9 m DRF and continued through Core U1341C-17H to a
depth of 2297.9 m DRF using non-magnetic coring assemblies and with the FLEXIT
orientation tool installed. Standard steel core barrels were then installed and coring continued
through Core U1341C-27H. Final cored depth was 230 m DSF. Overall core recovery
for the APC system was 106.2%. The drill string was pulled back to the rig
floor and the BHA secured and the vessel was put into cruise mode at 0024 hours
on August 3rd officially ending Site U1341.
After an 87 nautical mile transit from Site U1341 to Site
U1342 (BOW-15A) averaging 10.9 knots, the speed was reduced, and thrusters and
hydrophones were lowered. We then initiated dynamic positioning over Site U1342
(BOW-15B) at 0830 hr (UTC-11h) on August 3rd. Hole U1342A was
spudded at 1320 hours on August 3rd recovering 1.8 m of core
establing an official seafloor depth of 829.7 m DRF. APC coring continued
through Core U1342A-8H to a depth of 49.3 mbrf using non-magnetic coring
assemblies and with the FLEXIT orientation tool installed. Coring with the APC system was suspended
after two successive short, incomplete strokes of the core barrel and apparent
refusal. Three meters of hole were then drilled with the APC/XCB bit and the
XCB system was deployed and a short core (1 m) was drilled to identify the
material at APC refusal. Basalt was recovered and Hole U1342A was terminated.
Overall recovery for Hole U1342A using the APC coring system was 115.5% with
56.93 m recovered. Recovery for the XCB core was 0.46 m or 46.0%. The top drive
was then set back and the drill string was pulled out of the hole and cleared
the seafloor at 2220 hr on August 3rd.
Hole U1342B officially began when the drill string cleared the seafloor at 2220 hr on August 3rd. The vessel was offset 20 m east of Hole U1342A. Hole U1342B
was spudded at 2315 hr on August 3rd. The barrel contained 5.33 m of
core and an official seafloor depth was established at 830.4 m DRF. APC coring
continued through Core U1342B-5H to a depth of 43.3 m DRF using non-magnetic
coring assemblies. The top drive was then set back and the drill string was
pulled out of the hole and cleared the seafloor at 0150 hr on August 4th.
Hole U1342C commenced at 0150 hr on August 4th.
The vessel was moved 20 m east of Hole U1342B, the top drive was picked up and
the drill string was spaced out placing the bit at 828.0 m DRF. Hole U1342C was
spudded at 0235 hours on August 4th and the barrel recovered 7.2 m
of core establishing an official seafloor depth of 830.3 m DRF. APC coring
continued through Core U1342C-6H to refusal at a depth of 875.7 m DRF using
non-magnetic coring assemblies and with the FLEXIT orientation tool installed.
Overall recovery for Hole U1342C using the APC coring system was 103.7% with
47.06 m recovered.
Hole U1342D commenced at 0800 hr on August 4th.
The vessel was moved 20 m east of Hole U1342C and the top drive was picked up
and the drill string was spaced out placing the bit at 826.2 m DRF. The first
APC barrel was pressured up and fired and the barrel recovered 6.0 m of core
and an official seafloor depth was established at 829.7 m DRF. Hole U1342D was spudded at 0830 hours
on August 4th. APC coring continued through Core U1342D-5H to a
depth of 873.7 m DRF using non-magnetic coring assemblies and with the FLEXIT
orientation tool installed. On reaching refusal, the center bit was dropped and
the hole was drilled 18.6 m to determine if there were sediments under the
layer of hard rock (basalt). After a drilling break was observed the XCB coring
system was deployed and used successfully to core an additional 65.1 m into the
formation. Overall recovery for Hole U1342D using the APC coring system was
102.9% with 45.29 m recovered. Recovery for the XCB core was 41.1 m or 63.1% recovery. The drill string
was pulled back to the rig floor and the BHA secured and the vessel was put
into cruise mode at 1715 hr on August 5th officially ending Site
U1342.
After a 292 nautical mile transit from Site U1342 to Site
U1343 (GAT-4C) averaging 11.2 knots, the speed was reduced, and thrusters and
hydrophones were lowered. We then initiated dynamic positioning over Site U1343
(GAT-4C) at 2008 hours (UTC-11h) on August 6th. The top drive was
picked up and the drill string was spaced out placing the bit at 1953.4 m DRF
or 5 m above the ÒcorrectedÓ PDR depth of 1958.4 m DRF. The first APC barrel
was pressured up and fired and the barrel recovered approximately traces of mud
line in the core catcher. After calling the first core empty, the bit was
repositioned at 1958.4 m DRF and a 5.5 m core was recovered and an official
seafloor depth was established at 1962.4 m DRF. Hole U1343A was spudded at 0305
hr on August 7th. APC coring continued through Core U1343A-22H to a
depth of 201.5 m DRF using non-magnetic coring assemblies. Coring with the APC
system was suspended by plan with no recorded drill-over. Overall recovery for
Hole U1342A using the APC coring system was 101.2% with 203.86 m recovered. The
top drive was then set back and the drill string was pulled out of the hole and
cleared the seafloor at 2100 hr on August 7th ending Hole U1343A.
Hole U1343B officially began when the bit cleared the
seafloor at 2100 hr on August 7th. The vessel was offset 30 m north of the prospectus site
position. The top drive was picked up and the drill string was spaced out
placing the bit at 1959.9 m DRF. The first APC barrel was pressured up and
fired and the barrel recovered 7.0 m of core and an official seafloor depth was
established at 1950.9 m DRF. Hole U1343B was spudded at 2240 hr on August 7th.
A total of 4 APC cores were recovered for high resolution microbiological
sampling to a depth of 43.3 m. Average core recovery for the APC on Hole U1340B
was 103.5% with 44.8 m CSF. Hole U1343B ended on clearing the seafloor at 0115
hr on August 8th.
Hole U1343C began when the drill string cleared the seafloor
at 0115 hr on August 8th. The ship was offset 30 m west from the prospectus site position. The top
drive was picked up and the drill string was spaced out placing the bit at
1961.9 m DRF. The APC was deployed and Hole U1343C was spudded at 0305 hr on
August 8th. APC coring continued through Core U1343C-26H to a depth
of 234.2 m DRF using non-magnetic coring assemblies. Overall recovery for Hole
U1343C using the APC coring system was 98.65% with 231.04 meters recovered. The
top drive was then set back and the drill string was pulled out of the hole and
cleared the seafloor at 0010 hr on August 9th ending Hole U1343C.
Science Results
Week five marks the start of the second half of the
Expedition and a time when scientists and the USIO staff start planning for a
successful completion of the cruise. It is both a time for reflection and
excitement as expedition scientists begin to evaluate their findings, discuss
and plan for post-expedition sampling and research, and focus on the work
ahead.
Two meetings were held during the week to present and
discuss the preliminary scientific results from Sites U1341 and U1342, as well
as an additional meeting to present the scientific objectives and operations
plan for the Bering Gateway Sites GAT-4C (U1343) and GAT-3C (U1344).
During the week, we finished describing and analyzing
sediments from Hole U1341C (original Site BOW-14C) and completed coring four
holes at the last site on the Bowers Ridge (U1342, original Site BOW-15A).
Holes U1342A, U1342B and U1342C were cored to only 53.3, 43.3, and 45.4 m DSF
because we unexpectedly encountered basaltic rock at the bottom of Hole U1342A.
The recovered sediment section for each of the first three holes was 56.93 m
CSF (115.5%), 44.83 m CSF (104%), and 47.06 m CSF (103.7%), respectively. Hole
U1342D was APC cored to 44.0 m DSF (recovery 45.29 m CSF or 102.9%) and then
XCB cored to recover the underlying volcanoclastic rocks down to a total depth
of 127.7 m DSF.
We arrived at the Site U1343 (GAT-4C) on Friday August 7th
at night and by Sunday night we had completed APC coring of Holes U1343A
through U1334C.
Sedimentology
Site U1341
The sediment recovered from Hole U1341C was similar to the
material in Holes U1341A and U1341B, consisting mainly of dark greenish gray to
dark olive to olive diatom ooze, diatom silt and diatom clay. Hole U1341C
extended to Core U1341C-27H and therefore covered only the upper part of the
sequence recovered in Hole U1341B. Sediment that contained variable amounts of
sponge spicules, and calcareous microfossils (nannofossils, foraminifers) were
recovered in the upper 120 m (CSF-A). Soft sediment deformation due to slumping was largely confined to Cores
U1341C-2H and 10H. Laminated intervals were found in Cores U1341C-1H, 2H,
10H-13H and 25H. Authigenic carbonate was found as an accessory component at
the bottom of the hole. Dropstones occurred occasionally throughout the record.
Sediment bioturbation was slight to moderate, and absent in laminated
intervals. Drilling disturbance was mostly negligible.
Site U1342
Coring at Site U1342 recovered both soft sediment and
basement lithologies. The upper
soft sediment consists of alternating intervals of beautifully laminated diatom
ooze and silty clay with occasional pennate diatom mats, planktonic foraminifer-rich
laminae and intervals with large benthic foraminifers and calcareous
nannofossils. A total of 50 laminated intervals were described in Holes U1342A,
U1342C, and U1342D. Laminations are mainly parallel, but there are also rare
cross laminations. The laminated intervals have sharp basal contacts with the
silty clay and gradational, bioturbated tops. There are occasional ash layers
and pods of ash that may be bioturbated thin ash layers. Gravel-sized,
matrix-supported clasts are present throughout. Bioturbation tends to be
moderate.
Below the diatom-rich sequence is very dark grey sandy silt
to silty sand that contains mostly volcanogenic lithic sand with variable
amounts of glauconite-bearing pellets, metamorphic rock fragments, quartz and
feldspar, sponge spicules, and volcanic glass. In general, the sediment
structure is chaotic due to bioturbation, soft-sediment deformation, and
drilling disturbances. The grains are moderately well sorted and subangular to
moderately rounded. The bottom section of the core through this lithology is
soupy and represents flow-in during drilling.
The basement sequence comprises two main lithologies. The
upper part is less than one meter thick consisting of vesicular and porphyritic
basalt. Below this are interbedded gray, green, and red volcanogenic sandstone,
siltstone and breccia. Most bed contacts are parallel but vague. Grains in the
sand and some of the breccia beds are polymictic comprising a range of
scoriaceous (black and red in color) and non-vesicular basalt fragments. There
are also monomictic breccias with irregular shaped mafic clasts. Most beds are
moderately to well sorted and some sandstone beds have dewatering structures.
Hole U1343A
Sediments in Hole U1343A are predominantly gray diatom-rich
silty clay with green foraminifer-bearing diatom ooze, occasional ash layers,
gravel size clasts, and pyrite mottles. Abundant gas expansion cracks occur
throughout the sedimentary section.
Biostratigraphy
Site U1342
An age-depth model was constructed from diatom, radiolarian,
dinoflagellate and calcareous nannofossil biostratigraphic markers giving a
composite age of <1.5 Ma and a sedimentation rate of 3 cm/k.y. for the
recovered section. Poor preservation of smaller siliceous and organic-walled
microfossils (silicoflagellates, diatoms and dinoflagellates) and intervals
barren in calcareous nannofossils suggests that winnowing is active at this
site. Diatom assemblages also revealed reworking at the uppermost layers and at
the basement of all holes at this site.
Site U1343
By the end of the week, analyses began of core catcher
samples from this gateway location. Biostratigraphic markers (radiolarians,
silicoflagellates, diatoms) have so far provided an estimated age between 0.74
and 0.9 Ma at the base of Core U1343C-20H.
Paleomagnetism
Site U1342
We have done paleomagnetic measurements on APC core section
halves of cores from Holes U1342A, U1342C, and U1342D except for the XCB cores.
We have identified the Brunhes/Matuyama boundary, and both the top and base boundaries
of Jaramillo and Cobb mountain subchrons. We collected 13 mini-core specimens
from the basement volcanic rocks to reconstruct the paleolatitude where the
rocks initially formed. We are planning to measure the rock samples during the transit to Yokohama.
Geochemistry and Microbiology
Site U1342
We collected catwalk samples for interstitial water
chemistry, bulk sediment geochemistry, and microbial cell counts. We sampled
for microbiology, interstitial water and solid phase at high-resolution at Hole
U1342B, a microbiology-dedicated hole. Samples taken for shore-based
microbiology analysis include total cell abundance, RNA, and amino acids. For
IW samples taken both on the catwalk and in the microbiology-dedicated hole, we
determined concentrations of alkalinity, dissolved inorganic carbon, sulfate,
ammonium, sulfide, major (i.e., Ca, Na, K) and minors (i.e., Fe, Mn) elements.
Analysis of solid phase fractions included total nitrogen, carbon and sulfur.
Site U1342 has the lowest microbial respiration among all sites.
Technical Support and HSE Activities
During this week, the technical staff was fully engaged
processing cores from Sites U1341, U1342 and U1343, assisting scientists with
instruments and data processing. Staff are troubleshooting problems as they
arise and continue to fine tune science and software systems, including
addressing issues with the unexpected recovery of hard rock core.
On a time available basis, work continues on the
reorganization of the ship's storerooms. Decking was installed in the forward
end of the staging area's overhead providing an additional ~320 cubic feet of
science supply storage.
The weekly fire and boat drill was held as scheduled. The
captain provided additional training for the technical staff in the operation
of the ship's lifeboats. There are no HSE issues to report.
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