| IODP
Expedition 306: |
North Atlantic Climate 2
Expedition
306 Scientific Party |
| Introduction |
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Figure
1. Map of Expedition 306 site locations in the
North Atlantic.
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The primary objective of Expedition 306 was developing high-resolution
late Neogene–Quaternary climate proxy records in the
North Atlantic and putting these into a PAC (Paleointensity
Assisted Chronology) framework. This PAC is constructed from
combination of geomagnetic paleointensity, stable isotope,
and detrital layer stratigraphies. Sites drilled during Expedition
306 are located north of the Azores near the Mid-Atlantic
Ridge (Sites U1312 and U1313 (reoccupation of ODP Sites 608
and 607, respectively), on the middle of the Gardar Drift
(Site U1314), and a CORK emplacement at Site U1315 with associated
downhole logging at nearby ODP Hole 642E (Figure
1). Several possible locations for drilling on Eirik
Drift in Labrador Sea had to be abandoned due to poor weather.
The primary logging objectives of Expedition 306 were Sites
U1313 and one of 2 Eirik Drift sites. The goal at the two
sites was to provide corrected depth-scale information from
core-log integration to account for core expansion and overall
quality control of the spliced record. Due to high sedimentation
rates at most of the Expedition 306 sites, a secondary goal
was to look at millennial-scale changes that would be identifiable
in Formation MircoScanner (FMS) data and high-resolution
Multi-sensor Gamma ray tool (MGT) data. When the Eirik Drift
sites were abandoned, Site U1314 was chosen but, again, due
to poor weather, no logging was possible.
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Logging Operations |
Ultimately, only Site U1313 was logged as part of Expedition
306. Downhole logging operations were carried out after completing
coring of Hole U1313B to a depth of 302 mbsf (3727 meters
below rig floor (mbrf)) and displaced with sepiolite mud.
The drill pipe was raised to 65.3 mbsf (3489.6 mbrf) prior
to logging. During logging operations, the sea state was
fairly calm with a typical heave of 2m or less. The initial
plan was to use two tool string configurations, the triple
combo” with an additional Multi-sensor Gamma-ray Tool
(MGT) and the Formation Micro Scanner (FMS)–sonic.
However, shortly after deploying the triple combo-MGT,
power problems forced us to bring the tool string back on
deck for examination. It was determined that the MGT tool
was leaking and had caused damage to the telemetry cartridge
below. The MGT was removed from the tool string and a new
telemetry cartridge was installed on the tool string. Following
the repairs, the triple combo was deployed successfully to
the bottom of the borehole at 300 mbsf (3725.3 mbrf). This
leak in the housing of the MGT caused significant delays
and there was no time available for any additional toolstrings.
As part of a CORK emplacement project at Site U1315 that
deployed a long term (5 years) bottom water temperature monitoring
experiment, we did have the unique opportunity to reoccupy
and log ODP 642E again after 20 years. The plan was to use
two tool string configurations, the triple combination (triple
combo) with an additional General Purpose Inclinometer Tool
(GPIT) and the FMS–sonic. The Lamont Borehole Research
Group’s (LDEO-BRG) Temperature, Acceleration, and
Pressure (TAP) tool was deployed with triple combo and we
logged down slowly stopping every 5-10 m over the upper 100
m and then logged continuously at 1800 ft/hr down to total
depth of 588 mbsf. While collecting the downhole temperature
data, we also logged down with the triple combo tool string.
At 588 mbsf, we reached an impassable obstruction and stopped
the downhole logging. We then logged the hole up into casing
to a depth of 335 mbsf. After the triple combo, the FMS-sonic
tool string was also deployed to ~580 mbsf after again reaching
the same hole obstruction as before. The second pass of the
FMS-Sonic was only able to reach a total depth of ~440 mbsf
before reaching an obstruction. So, a shortened second run
was made from that depth into casing until 310 mbsf.
Logging Results
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Figure
2. Comparison of core and log physical properties
from Hole U1313B. (A.) Bulk density for the interval
80 to 300 mbsf. (B.) Gamma ray radiation for the
interval 0 to 300 mbsf. cps = counts per second;
gAPI = American Petroleum Institute gamma ray units.
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| Figure
3. Detailed comparisons of core and log physical
properties from upper 70 mbsf of Hole U1313B. (A) Core
and log gamma-ray. (B) Core “L”-(~CaCO3)
and log gamma-ray. |
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Figure
4. Linear correlation of log gamma-ray (interval
0-225 mbsf) from Hole U1313B and benthic oxygen isotope
stack over last 5.4 Ma (Lisiecki and Raymo, 2005).
Both data sets are shown with scales inverted so that
warm interglacials/low gamma-ray (ie low Th =low clay)
intervals are shown as prominent peaks. The correlation
has only two tiepoints at 0 and 5.2 Ma with no stretching
or squeezing of log data depths.
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Logging Highlights: Hole U1313B
The successful deployment of the triple combo tool string
at U1313B provided complete coverage of the 300m section
and provided very good physical property and lithologic information
for density, porosity, natural gamma-ray, resistivity and
photoelectric effect. Corresponding core physical property
measurements were very consistent with in situ downhole data
(Figure 2).
Due to the high detrital content of the core, we were able
to use the downhole natural gamma measurements recorded through
the cased portion of the hole (0-70 mbsf) for detailed stratigraphic
correlation (Figure 3). While the signal was attenuated by
4-5X, the correlation was critical for correlation in this
critical Quaternary section at this locations. Also of special
note is the dramatically consistent linear correlation of
downhole natural gamma-ray (upper 225 mbsf) with the recent
Lisiecki and Raymo (2005) benthic oxygen isotope record over
the last 5.4 Ma (Figure 4). The consistency of downhole data
with both core data and age models will allow mapping of
spliced core record to actual depth resulting in more accurate
sedimentation rate calculations as well as more detailed
age/depth models.
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Figure
5. Comparison of old and new physical properties
from Hole 642E.
Caliper (in), Porosity (%), Gamma-ray and Thorium (gAPI = American Petroleum
Institute gamma ray units). All new data is shown in blue and old data in red.
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Figure
6. Detailed resistivity profile over an interval
from 530 to 550 mbsf at Hole 642E showing a pattern
of basalt flows with lower resistivity at the top
and increasing towards the bottom. An enlarged portion
of a FMS image showing volcaniclastic (basaltic vitric
tuff) interval beneath one flow and at the top of
the next between 546 to 548 mbsf. |
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Figure
7. Detailed resistivity profile over an interval
from 530 to 550 mbsf at Hole 642E showing the pattern
of basalt flows with a lower resistivity at the top
and increasing towards the bottom. A blowup of a
FMS image showing fine-grained basalt interval between
542 to 545 mbsf.
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Logging highlights: Hole 642E
A) Log(New)-Log(Old) Comparisons
An important part of re-visiting this ODP legacy site is
an evaluation of hole conditions after 20 years. The rotary
bit size used for coring this site was 9.75 in. The original
caliper log is plotted against 2 calipers from the FMS tool
(Figure 5). As can be seen, the original
caliper (density tool) was not very reliable showing a much
larger than bit size hole for almost the entire length of
the cored interval. Most of the intervals with hole sizes
larger than 12 in the new FMS caliper logs correspond to
high porosity-low resistivity zones.
A comparison of porosity logs shows a very good correlation
downhole. The overall variability of porosity is much larger
(10 to 95%) than the original measurements (15 to 70%)(Figure
5) and is attributed to a perhaps more sensitive porosity
sonde.
Measured total gamma-ray data from the old and new logs
at 642E are generally close overall. Density logs (not shown)
from both studies also appear to be reliable between the
two data sets with most values ranging between 2 and 3 g/cm3.
B) FMS/Sonic Logging
FMS imaging of the hole yielded good results and will allow
easy correlation to existing core data and filling in the
gaps (~60% of the formation). Examples from the volcaniclastic
and fine-grained basalt intervals are shown in Figures
6 and 7,
respectively.
The fine-scale (cm) resistivity data will allow high-resolution
studies of fracture density of basalts and porosity within
the sequence. Combined with new shear wave data from the
Sonic tool, it should be possible to construct more reliable
permeability estimates as well as revised synthetic seismograms
that may yield better depth-velocity correlations.
C) Temperature Log at ODP Hole 642E
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| Figure 8. Temperature log profile versus depth
at Hole 642E collected using TAP tool. |
A temperature log (Figure 8) was
obtained at Hole 642E using the L-DEO-BRG TAP tool. This
tool logs at a rate of 1 Hz, has a precision of 5 mK and
an accuracy of 1 K. The temperature was logged on the way
down. The TAP tool was held off bottom for a few minutes
and indicates a bottom water temperature of approximately
0.2° C. The 10 m of the borehole has a very steep gradient
(~2500 °C/km). Below this section the borehole has a
relatively low gradient of approximately 22° C/km. The
borehole is cased to a depth 390 mbsf. At a depth of approximately
500 mbsf a positive temperature excursion may indicate inflow.
The temperature log as a whole indicates significant fluid
discharge that may be as much as 10’s of meters per
year. This excursion may correlate with a high permeability
zone indicated in the other logs.
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Sean Higgins : Logging Staff Scientist, Borehole
Research Group, Lamont-Doherty Earth Observatory of Columbia
University, PO Box 1000, 61 Route 9W, Palisades NY 10964,
USA
Email: Sean Higgins
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Additional Expedition-Related
Publications:
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