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IODP Expedition 318:
Wilkes Land Glacial History
Week 5 Report (31 January-6 February 2010)
PDF file is available for download.
7 February 2010
Operations this week consisted of completing coring at Site
U1356 (WLRIS-07A), coring at Site U1357 (ADEL-01B), and starting coring at Site
Site U1356: RCB Cores U1356A-97R to -106R penetrated from
910.2 to 1006.4 mbsf and recovered 41.98 m (44%). Total core recovery for the
entire hole (0 to 1006.4 mbsf) was 350.13 m (35%). After coring was finished,
we began preparing the hole for logging and raised the drill bit to 103 mbsf.
While lowering the bit back down, the weather deteriorated to the point where
we had to abandon the hole and we started to retrieve the drill string. After
the drill string was recovered, the weather system had passed and the sea
conditions began to improve. After the beacon was recovered, we began the
transit to Site U1357 (ADEL-01B) at 0615 hr on 2 February.
Site U1357: Site U1357 (ADEL-01B) was the southernmost of
the shelf sites - all of which are prone to ice coverage. Shortly after
departing Site U1356, we began to encounter floating ice. The vessel speed
was frequently adjusted to match visibility and ice conditions as we proceeded
on a SE heading toward the site. We were able to negotiate through large concentrations
of ice with the benefit of a clear and sunny day providing excellent
visibility. The 218 nmi transit to Site U1357 took 25.7 hours at an average
speed of 8.5 nmi/hr. We were positioning over Site U1357 at 0800 hr on 3
February. After a short wait to evaluate the movement of proximal ice, we began
assembling the APC/XCB drilling assembly.
Coring at Hole U1357A started at 1445 hr on 3 February with
seafloor depth of 1014.8 mbsl. However, the extremely diffuse boundary between
bottom water and seafloor sediment precludes accurate definition of water depth
using a standard shipboard sonar system; most coring systems (including the
APC) typically also do not recover this boundary. APC Cores U1357A-1H to -21X
penetrated to 186.6 m and recovered 183.87 m (99%). The last APC core (Core
U1357A-20H) was an incomplete stroke. We then took a single XCB core from 185.6
to 186.6 mbsf and this bottomed out in diamict. Coring was interrupted for four
hours when an iceberg approached close to the ship. The bit cleared the
seafloor at 1120 hr on 4 February.
After offsetting the ship 50 m to the east, we started
coring in Hole U1357B at 1345 hr on 4 February. APC Cores U1357B-1H to -19H
penetrated to 170.7 mbsf and recovered 172.44 m (101%). The bit cleared the seafloor at 0340 hr on 5 February.
Hole U1357C, offset 25 m west of Hole U1357A, started coring
at 0645 hr on 5 February. APC Cores U1357C-1H to -11H penetrated to 103.8 mbsf
and recovered 110.7 m (107%). Once again, coring had to be suspended because of
the proximity of several icebergs. This time, however, the ice passed close
enough that we had to abandon the hole so we could offset the ship 2.8 km to
the SE. The bit cleared the seafloor at 1358 hrs on 5 February and we had to
wait until the next morning to retrieve the beacon.
Site U1358 (WLSHE-08A): The transit to our next site began
at 0900 hr on 6 February. We wanted to core at Site WLSHE-09A, but it was
decided from visual and radar observations that there was too much floating ice
in the vicinity for safe operations – especially considering the severe
weather predicted for late 7 February. We continued past WLSHE-9B to WLSHE-08A
– a location 22 nmi to the northeast of WLSHE-09A and closer to open
We were positioning over Site U1358 (WLSHE-08A) at 1730 hr
on 6 February. The 93 nmi transit was accomplished at an average speed of 10.9
nmi/hr. The drill string was lowered to the seafloor, but we had to wait to
start coring for 1.3 hours as an iceberg moved through the drilling area. We
tagged the seafloor with the bit at 512.0 mbrf and started RCB coring in Hole
U1358A at 2245 hr on 6 February.
Science results this week cover the final coring at Site U1356 (WLRIS-07A) as well as all three holes cored at Site U1357 (ADEL-01B).
Site U1356 (WLRIS-07): As mentioned last week, the primary
objective at this site was to recover a distal record of the first arrival of
glaciers to the eastern Wilkes Land margin by sampling sediments across a
regional unconformity in the seismic data, WL-U3, which is interpreted to
separate pre-glacial strata below from glacial strata above.
This week we described cores from 782 to 1006.4 mbsf. Three
additional Lithostratigraphic Units were identified; a total of ten units have
been identified. Unit VIII consists of bioturbated claystones (some of which
are calcareous) and laminated claystones, indicating hemipelagic sedimentation
with periodic bottom current sorting. Unit IX (879.7 to 948.8 mbsf) is
characterized by highly contorted claystones and coarse grained deposits
inferred to be emplaced by gravity flows. Unit X (948.8-1000.1 mbsf) is
characterized by bioturbated claystones with subordinate stratified siltstone
and sandstone, indicating hemipelagic sedimentation with minor influence of
bottom currents and gravity flows.
Samples from Cores U1356A-95R through -106R were examined
for palynomorphs, siliceous microfossils, and foraminifers. The abundances of
protoperidinioid dinocysts decline dramatically from Core U1356A- 94R downwards
and the dinocyst assemblages are dominated by in situ early middle Eocene
dinocysts; early Eocene dinocysts are dominant below Core U1356A-100R.
Sporomorphs are present in all samples investigated, with generally higher
abundances than further up in the succession. Sporomorph assemblages are
characterized by well-preserved Nothofagus pollen grains, bisaccates and
spores. No indication of reworking has been found. Radiolarians are present,
preserved both in silicified and pyritized form, however, no age diagnostic taxa
were detected. Foraminifers are dominated by agglutinated forms and
characterized by low diversity and abundances; the foraminifer samples also
yielded fish teeth. Diatoms occur as pyritized fragments and provide only
limited age information.
Site U1357 (ADEL-01B): This site was designed to obtain an
ultra-high resolution Holocene record by triple coring an ~200 m-thick
sedimentary drift deposit overlying an inferred glacial diamict. Data from
these samples will help assess the forcing factors (solar, ocean-atmosphere
interaction, and volcanic) responsible for climate change over the past 10,000
years and their influence on climate and oceanographic processes in the
Southern Ocean. Development of high-quality, high-resolution Holocene
climate records from the East Antarctic margin is a key step toward
understanding the circum-Antarctic and Subantarctic response to climate forcing
and addressing similarities, differences, and links to the global record as
well as nearby ultra high resolution ice core records. These data will help us
evaluate the response of East Antarctic sea ice as well as the Ice Sheet (EAIS)
and margin to global warming.
Three holes were cored at Site U1357. Hole U1357A recovered
186.6 meters of diatomaceous ooze and penetrated the underlying last glacial
diamict. The lowermost 10 meters contains increasing amounts of terrigenous
debris towards the base, mostly silt, with some sand and gravel near the base
of the hole. We used the APC for all cores except for Core U1357A-21X, which recovered
12 cm of the underlying diamict. Recovery was generally greater than 100%,
mainly because of gas-induced expansion. Cores from Hole U1357A were split and
described after running through the whole-round track systems. Discrete samples
(10 cm3) for foraminifera were taken at 10 cm intervals to prevent
dissolution that had been observed in previous Adelie drift cores. Hole U1357B
is located 50 meters east of the first hole and recovered 172.4 m of core.
Gas-induced core expansion was reduced by immediately drilling small holes in
the plastic core liner. Although this core did not recover the basal diamict,
it likely overlaps the section from Hole U1357A down to at least 170 mbsf.
After processing the core sections from Hole U1357B through the whole-round
track systems, they were not split but sealed in shrink-wrap with oxygen
absorbers within 24 to 36 hours. Hole U1357C was cored 25 meters west of Hole
U1357A and 110.7 meters of diatomaceous ooze was recovered before the hole was
abandoned due to approaching icebergs. A large number of whole rounds samples
were taken from the first two cores in Hole U1357C for age dating as well as
geochemical and microbiological studies. Less frequent whole-round samples for
dating and microbiologic plugs were taken in the deeper cores from Hole U1357C.
The whole-round sections were treated similar to those from Hole U1357B.
Three Lithostratigraphic Units were identified in Hole
U1357A. The uppermost 176 m consists of diatom ooze. This overlies a
~10-m-thick unit of clay-bearing diatom ooze. The base of the succession
consists of carbonate-cemented diamictite with clasts of various lithologies.
Routine core catcher paleontology samples were not taken
from the laminated diatom oozes to preserve a more complete stratigraphy for
postcruise investigation. Samples were collected at section breaks whenever
core expansion pushed material out of the core liner. Samples taken from Cores
U1357A-1H to -9H were examined for siliceous microfossils, foraminifers, and
palynomorphs. All section break samples yielded abundant diatoms exhibiting a
diverse Holocene assemblage, with trace to rare silicoflagellates and
radiolarians. Samples from discrete light and dark lamina indicate that the
dark lamina generally contain a more diverse assemblage. Sieved splits contain
benthic and planktonic foraminifers as well as fish skeletal debris. These
samples also yielded rich, highly diverse palynomorph associations with very
good preservation; these are dominated by protoperidinioid dinoflagellate cysts
and zooplankton remains.
Paleomagnetic measurements on Hole U1357A archive halves
suggests that we can remove the drill string overprint by demagnetization to
15-20 mT and efforts are underway to remove data influenced by deformation and
gas-induced voids. The data might contain an unprecedented Holocene secular
variation record although several issues must be addressed prior any final
interpretation. Lack of sufficient susceptibility is complicating our ability
to normalize the intensity data to obtain a relative paleointensity record.
Samples for geochemical analyses of carbonate, C, N, and S
were taken at a frequency of ~2 samples/core from Hole U1357A. A subset of
these samples will also be analyzed for major and minor elements. Sixty
microbiology samples were taken from Hole U1357C - the majority are whole-round
samples <20 mbsf. Direct cell counts will be conducted to estimate viable
biomass. Forty-five interstitial water samples were also taken from the upper
20 mbsf to complement the microbiologic studies. The pore water from these
samples is being analyzed for salinity, alkalinity, anions, cations, nutrients,
and main and trace elements.
The physical properties measurements of whole-round sections
are highly impacted by both the extreme concentration of diatoms in the
sediments as well as pervasive gas expansion. Therefore, density and velocity
data are of very poor quality (or lacking) and magnetic susceptibility is at or
below the noise level. Stratigraphic correlation between the three holes at
Site U1357 is problematic due to the nature of the data available shipboard. A
composite section may not be possible with these shipboard data and may have to
be resolved with shorebased data.
Discrete moisture and density analyses of are of special
importance for this site. Physical properties generally reflect the nature of
these Holocene sediments and show typical density, porosity, and moisture
content trends with depth characteristic for diatom ooze from the seafloor to
~160 mbsf. At that depth, porosity, moisture content, and void ratio exhibit a
sudden shift to slightly higher values until the bottom of the hole.
Technical Support and HSE Activities
This week, the technical staff supported the processing and
data collection for the final 14 RCB cores from Site U1356 and 51 APC cores
from three holes at Site U1357 that recovered 467 m of sediment in under 48
hrs. Core expansion as well as intensive catwalk sampling in the third hole
required an all hands effort. A fire and boat drill was held on 2 February for
the entire ship's complement.