IODP Expedition 342:
Paleogene Newfoundland Sediment Drifts
Site U1408 Summary
PDF file is available for download.
Background and objectives
U1408 (proposed site SENR-21B; 41° 26.3'N, 49° 47.1'W) is a mid-depth site
(~3022 m water depth; ~2575 m paleodepth at 50 Ma, Tucholke and Vogt, 1979), in the shallow end of the
Expedition 342 Paleogene Newfoundland Sediment Drifts
depth transect. The site is positioned to capture a record of sedimentation
around 2 km shallower than the largely sub-carbonate compensation depth record
drilled at IODP Site U1403. The location, well above the average late Paleogene
carbonate compensation depth, should be sensitive
to both increases and decreases in carbonate burial, whether these reflect
variations in dissolution related to changes in the CCD, changes in carbonate
production, or variations in background non-carbonate sedimentation. The site
is also intended to sample the core of a small sediment drift for comparison
with the age structure and depositional processes of other drift deposits on Southeast
Newfoundland Ridge and J Anomaly Ridge.
The vessel arrived at Site U1408 (SENR-21B) after a 1.46 nmi
transit from Site U1407 in dynamic positioning mode, which took 1.75 hours at 0.83
nmi/hr. The vessel stabilized over Site U1408 at 2045
h (UTC-2.5h) on 11 July 2012. Cores U1408A-1H through 20H were retrieved using
non-magnetic core barrels and the FLEXIT core orientation tool. Core U1408A-14H
experienced the first partial stroke and the APC system was advanced by
recovery to the APC total depth of 182.9 m drilling depth below seafloor (DSF).
The XCB system was deployed for Cores U1408A-21X through 27X to the final depth
of 246.5 m DSF. The seafloor was cleared at 1045 h on 13 July 13, ending Hole
U1408A. Overall core recovery for Hole U1408A was 243.92 m for the 246.5 m
interval cored (99% recovery). The total time spent on Hole U1408A was 38.00
The vessel was offset 20 m to the east and Cores U1408B-1H through 18H were
retrieved using non-magnetic core barrels and the FLEXIT core orientation tool.
A 3-m long interval (25.7-28.7 m DSF) was drilled without recovery to optimize
coverage of coring gaps in Hole U1408A. Coring was slowed down when the FLEXIT
housing sheared at the overshot connection above the core barrel three times
during APC coring in Hole U1408B. Total advance including the drilled interval
with the APC system was 154.5 m. The XCB system was deployed for Cores
U1408B-19X through 26X to a final depth of 217.5 m DSF. The seafloor was
cleared at 1700 h on 14 July, ending Hole U1408B. The recovery for Hole U1408B
was 224.09 m over the 214.5 m cored (105% recovery). The total time spent on
Hole U1408B was 30.25 hours.
The vessel was offset 20 m to the south and Hole U1408C was spudded at 1905 h
on 14 July. Cores U1408C-1H through 19H (0-165.1 m DSF) were retrieved using
non-magnetic core barrels. A 3-m long interval (8.8-11.8 m DSF) was drilled
without recovery to optimize coverage of coring gaps in Hole U1408A. Core
orientation was performed with Cores U1408C-6H through 8H. Again, the FLEXIT
tool was thought responsible for mechanical trouble, including one mechanical
shear at the overshot and one mechanical shear of the APC shear pins. During
APC operations, an intermittent electrical fault developed and coring
operations were suspended for 6.5 h while the problem was fixed. The XCB system
was deployed for Cores U1408C-20X through 23X to a final depth of 187.5 m DSF.
The recovery for Hole U1408C was 181.52 m over the 184.5 m cored (98% recovery).
The total time spent on Hole U1408C was 38.25 hours. The drill string was
pulled to the surface and the drill floor was secured at 0715 h on 16 July
ending Hole U1408C and Site U1408. The overall recovery for Site U1408 was 106%.
The total time spent on Site U1408 was 106.5 hours or 4.4 days.
~250-m thick sedimentary succession recovered in Holes U1408A, U1408B, and
U1408C consists of pelagic deep-sea sediments of Pleistocene to late Paleocene
age, however, all stratigraphic epoch boundaries (Paleocene-Eocene,
Eocene-Oligocene, Oligocene-Miocene) are represented by hiatuses. The
succession is subdivided into four lithostratigraphic
units. Unit I is ~13 m thick and includes sediments of Pleistocene to Oligocene
age. These are composed of brown silty clay at the
top, followed by decimeter-scale alternations of brownish foraminiferal
nannofossil ooze and reddish-brown clay. These likely represent Pleistocene
glacial-interglacial cycles. Manganese nodules and dropstones are common.
The 10-m-thick Unit II consists of well-homogenized, silty clay with nannofossils
and nannofossil clay with silt of Oligocene age.
Sand-sized lithoclasts are found in the >63 μm size fraction from Section
U1408-4H-3 (NP23 nannofossil biozone, Oligocene)
upwards. Unit III is 202 m thick and displays a predominantly greenish gray
color, but also cyclical color changes between greenish gray to dark green and
very light-gray intervals on a decimeter-scale. Burrowing is mostly of moderate
intensity, in particular Zoophycos, Planolites and Chondrites burrows are common.
Unit IV was recovered only in Hole U1408A and consists principally of whitish and
pinkish-brown nannofossil chalk. This unit spans parts of the early
Eocene and the late Paleocene. Biostratigraphy constrains the Paleocene/Eocene
boundary close to the base of Section U1408A-26X-2; however, the
Paleocene-Eocene Thermal Maximum was not recovered due to an unconformity.
foraminifers and benthic foraminifers are present through most of the Pleistocene
to upper Paleocene succession recovered at Site U1408. Radiolarians are only
present in a short interval of the middle Eocene and upper Paleocene. Thin
Pleistocene, upper Miocene and lower Oligocene intervals overlie a middle
Eocene through upper Paleocene succession with significant hiatuses between the
Oligocene and middle Eocene (~7 my) and middle and lower Eocene (~3 my), and a
minor hiatus or condensed interval around the Paleocene/Eocene boundary.
Sedimentation rates are high (~1.49-3.14 cm/ky)
through the middle Eocene, low through the lower Eocene (around 0.08 cm/ky),
and relatively high through the upper Paleocene (~1.27 cm/ky).
results reveal a series of normal and reversed magnetozones
between Cores U1408A-4H and 25X (~29-220 m core depth below seafloor [CSF-A]),
between Cores U1408B-5H and 18H (~30-154 m CSF-A), and between Cores U1408C-5H
and 19H (~35-165 m CSF-A). These magnetostratigraphies can be correlated between
all three holes and consist of lower Chron C17n.3n (~ 38.3 Ma) through upper Chron
C20r (~43.4 Ma). The geomagnetic field transitions from C18n.1n to C18n.1r to
C18n.2n are recorded in exceptional detail in all three holes at Site U1408. Chron C20r
is continuously recognized down to the bottom of
the APC-cored interval and continues into the XCB portion where we recognize
the C20r/C21n chron boundary (45.724 Ma). The magnetochronology suggests that
sedimentation rates varied between ~1.5 and 3.14 cm/ky across the Middle Eocene.
Sedimentation rates were higher before the Middle Eocene Climatic Optimum
(MECO, ~41.5 Ma) than after it.
properties at Site U1408 largely reflect compaction and calcium carbonate
content. Bulk density globally increases downhole from 1.40 to 1.95 g/cm3
but an abrupt step down to ~ 1.4 g/cm3
occurs at the transition between lithostratigraphic
Units I and II. Grain density averages 2.77 g/cm3
in Hole U1408A. Overall in Hole U1408A, water content and porosity show a
decreasing trend downhole (from 50% to 25% and from
75% to 50%, respectively). P-wave velocity increases progressively downhole
from 1500 to 1800 m/s. Color reflectance parameters a* and b* show higher
values in lithostratigraphic Units I, II and IV and display cyclicity
in lithostratigraphic Unit III (a* averages
0.5 and b* averages -0.35). Magnetic susceptibility drops from ~60 to 10 IU at
12 m CSF-A and also cycles regularly around an average value of ~14 IU to the
bottom of the sediment column. NGR and L* show a relatively low variability and
average 25 cps and 55 respectively. Throughout lithostratigraphic
Unit III, variations in magnetic susceptibility, NGR and L* correlate with
calcium carbonate content (with peaks at ~95, 155 and 230 m CSF-A).
clear signal in magnetic susceptibility resulted in an accurate splice of the
three holes at Site U1408. The expression of orbital (most likely precession)
cycles between 35 and 220 m core composite depth below seafloor (CCSF) was a
particular aid to the correlation. The overall trends and patterns between
holes are very similar amongst the three holes. The splice for the upper 150 m
CCSF of the section is quite robust and could be used for high-resolution
sampling. However low variability in magnetic susceptibility between 70 and 80
m CCSF and between 150 and 226 m CCSF creates intervals with a tentative
splice. Our correlation results in a growth rate of 13%.
The downhole patterns of alkalinity, manganese and iron
suggest two zones of organic matter degradation within the recovered sequence.
Very high concentrations of manganese in the upper 30 m CSF-A coupled with iron
concentrations of 0 μm suggest ongoing oxic to suboxic diagenesis driven
by microbial reduction of manganese
oxides. Below 30 m CSF-A the reduction of iron oxides is evident from the rapid
increase in iron concentrations. The broad peak in iron between 40 and 90 m
CSF-A suggests a locus of organic matter consumption within this depth. Increasing
alkalinity concentrations beginning at 50 m CSF-A corroborate this
interpretation. Relatively high carbonate contents throughout the recovered
sequence in Hole U1408A are consistent with the relatively shallow paleodepth
of the site throughout the Eocene. Carbonate concentrations are roughly 40 wt%
in lithostratigraphic Unit I and fall to 0-10 wt% in lithostratigraphic Unit
II, which is consistent with low carbonate levels observed in other Oligocene
sequences recovered during Expedition 342. In the expanded middle Eocene
sequence represented by Unit III, carbonate content ranges from 40 to 50 wt%,
with a few peaks up to 80 to 90 wt%. Carbonate content increases to 90 wt% in
Unit IV, which corresponds to lower Eocene sediments. The downhole
increase in carbonate content at the lithostratigraphic
Unit III/IV boundary (225 m CSF-A) is typical of the early to late Eocene
boundary in Expedition 342 sediment cores. It is possible that the change in
carbonate is linked to a change in production, preservation, or dilution by
other sedimentary components.