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IODP Expedition 321:
Pacific Equatorial Age Transect 2
Site U1338 Summary
PDF file is available for download.
21 June 2009
(PEAT-8D, 2° 30.469'N, 117° 58.178' W, 4200 m water depth) was sited to collect
an 183 Ma segment of the PEAT equatorial megasplice, and is located on ~18 Ma
crust just north of the Galapagos Fracture Zone, 324 nautical miles (600 km)
southeast of Site U1337. A seamount (3.7 km water depth) with surrounding moat
is found ~25 km to the NNW of Site U1338, at the downslope end of the survey
area. Originally a site (PEAT-8C) was chosen about 10 km from the seamount.
However, an alternate PEAT-8D was selected and drilled uphill and further away
from the seamount to avoid possible turbidites as were found near seamounts on
Exp. 320 Sites U1331 and U1335. The recovered sediment column at Site U1338
represents a nearly complete and continuous early Miocene to Holocene sedimentary section.
holes were cored at Site U1338. At Hole U1338A, APC cores were taken from the
seafloor to 221.2 m DSF (U1338A-1H to 24H) using non-magnetic core barrels with
the Flexit core orientation installed. Flexit and steel core barrels were used
from Core 25H-26H. In addition, five successful APCT3 temperature measurements
were taken with Cores 5H, 7H, 9H, 11H, and 13H. XCB coring continued with Cores
27X through 44X. A small piece of basement was recovered in the core catcher of
Hole U1337B, APC cores were taken from the seafloor to 188.1 m DSF (U1338A-1H
to 20H) except for a short drilled interval of 2.5 m (from 235.6-238.1 m DSF)
to adjust the core breaks. Non-magnetic core barrels and Flexit core
orientation system were used through Core 20H. Flexit and steel core barrels
continued through Core 42H to a depth of 387.4 m DSF. Coring continued with
three XCB Cores 43X through 45X to a depth of 416.1 m DSF. The basement
contact was recovered in Core 45X. Three logging strings (triple combo,
vertical seismic imager (VSI), and FMS-sonic) were deployed in Hole U1338B.
U1338C was cored to recover sections that were missing from Holes U1338A and
U1338B. APC cores were taken from the seafloor to 189.8 m DSF (U1338A-1H to
21H) using non-magnetic core barrels and the Flexit core orientation system.
Flexit and steel core barrels were used through Core 44H to a depth of 396.9 m
DSF. Coring continued through Core 47H to a total depth of 414.4 m DSF setting
a new all time depth record for the APC.
U1338D was primarily planned to recover a few "instructional" cores to be used
during the upcoming expedition. Three APC cores were cut to a depth of 23.9 m
At Site U1338,
approximately 415 m of nannofossil ooze and chalk with varying concentrations
of diatoms and radiolarians overlie early Miocene seafloor basalt, and are
divided into three lithologic units. Pleistocene through middle Pliocene sediments
of Unit I are characterized by multicolored (various hues of white, brown,
green, and gray) nannofossil ooze, diatom nannofossil ooze, and radiolarian
nannofossil ooze that alternate on decimeter- to meter-scale. Light green and
light gray nannofossil ooze with occasional darker intervals with abundant
siliceous microfossils, notably diatoms, comprise the upper Miocene to middle
Pliocene Unit II. Decimeter-, meter-, and tens of meter-scale color
alternations in Units I and II are associated with variations in lithology and
physical properties. Some of these color changes, as well as common mm- and
cm-scale color banding, are not associated with compositional changes and
likely reflect variations in sediment redox state. White, pale yellow, light
greenish gray, and very pale brown nannofossil oozes and chalks dominate Unit
III of lower to upper Miocene, although slightly darker green and gray
intervals with larger amounts of siliceous microfossils remain present. Lower
Miocene seafloor basalt (Unit IV) was recovered at the base of the sedimentary
All major microfossil groups have
been found in the ~415 m thick succession of Holocene to lower Miocene sediment
bulge recovered from Site U1338. The calcareous nannofossils at Site U1338 are in general moderately preserved, but there are some intervals in which the preservation is good or poor. Nannofossil Zones NN4 to NN21 are present, indicating an apparently complete sequence. Planktic
foraminifers vary from rare to abundant, with moderate to good preservation
throughout most of the succession, but are absent or rare in a short interval
in the late Miocene. Planktic foraminifer Zones PT1b (late
Pleistocene) to M2 (early Miocene) are documented, with the exception of Zones PL4, M12, and M6. The radiolarian stratigraphy spans
the interval from the uppermost part of Zone RN16-17 (late Pleistocene) to
uppermost part of RN3 (early Miocene). The radiolarian assemblages show good to
moderate preservation, except in the lowermost portion (early Miocene), which is
barren of radiolarians. The high resolution
diatom stratigraphy spans the interval from theFragilariopsis (Pseudoeunotia) doliolus Zone (late Pleistocene) to the lowermost part of the Craspedodiscus elegans Zone (early Miocene). The diatom
assemblage is generally well to moderately preserved throughout the recovered
section; however, there are several intervals in which valve preservation
becomes moderate to poor. The nannofossil, foraminiferal, radiolarian, and diatom datums and zonal schemes generally agree, with some inconsistencies. Benthic foraminifers occur continuously throughout the
succession recovered in Hole U1338A, and show generally good preservation. The
overall assemblage composition indicates lower bathyal to abyssal paleodepths.
Marked variations in downcore abundance and species distribution reflect major
changes in global climate linked to fluctuations in ice volume and
re-organization of Pacific Ocean circulation during the Neogene.
Stratigraphic correlation provided
a complete spliced record to a depth of approximately 260 m CCSF-A. Several
gaps were seen between 280 and 360 m CCSF-A. Comparison of GRA density records
with well logging density data suggests that no more than a meter of section
was lost in any of the gaps. Correlation between the holes was broken again
several times between 435 m CCSF-A and basement at 460 m CCSF-A. Growth factor
for the correlation was 1.11. The linear sedimentation rate decreases from
approximately 29 m/m.y. in the Miocene to 13 m/m.y. in the
measurements were conducted on archive half sections of 26 APC cores from Hole
U1338A, 42 APC cores from Hole U1338B, and 47 APC cores from Hole 1338C. The
Flexit core orientation tool was deployed in conjunction with all APC cores
except for the deepest 3 cores of U1338C, and we conclude that the Flexit
orientation data are generally reliable. Measurements of natural remanent
magnetization (NRM) indicate moderate magnetization intensities (on the order
of 10-3 A/m) for depth intervals of 0-50 m CSF-A, 280-225 m CSF-A, and 295-395
m CSF-A. Polarity reversal sequences of these intervals are provisionally
correlated to the Brunhes to the upper part of the Gilbert Chron (0 to ~4 Ma),
Chron C4An to C5n (~9 to 11 Ma), and Chron C5r to C5Br (~12 to 16 Ma) of the
GPTS, respectively. Except for these intervals, remanence intensities after
alternating-field (AF) demagnetization of 20 mT are reduced to values close to
magnetometer noise level in the shipboard environment (~1x10-5 A/m).
Magnetization directions are dispersed and not interpretable there.
Sedimentation rates increase downcore from ~12 m/m.y. at the top to ~30 m/m.y.
near the bottom.
Physical properties measurements on
whole-round sections and samples from split cores display a variation strongly
dependent on the relative abundance of biosiliceous and calcareous sediment
components at Site U1338. As at Site U1337, the general pattern is that
intervals enriched in siliceous microfossils and clay display darker colors,
lower grain density and bulk density, and higher porosity, magnetic
susceptibility (MS), and natural gamma radiation (NGR). The variation of
velocity is more complex in that it is dependent on both the wet bulk density
and the sediment rigidity. These parameters vary independently with the
variation in abundance of biosiliceous and calcareous components. The physical
properties at Site U1338 also display cyclicity on multiple scales, a decimeter
to meter scale and a scale with a spacing on the order of 10's of meters.
Lithologic Unit I
at Site U1338 is characterized by low wet bulk density that decreases from 1.4
g/cm3 near the seafloor to 1.2 g/cm3 at the base of the
unit, as a result of an increasing abundance of radiolarians and diatoms with
depth. The grain density in Unit I and Unit II displays a greater variability
than deeper at the site as a result of the greater variability in the abundance
of biosiliceous and calcareous components. The average grain density for Units
I and II is relatively low, 2.59 g/cm3. The NGR signal at Site U1338
is characterized by a near seafloor peak that is somewhat lower than those
recorded at the other PEAT drill sites, but it extends deeper and is marked by
a double peak. Spectral reflectance measurements show that Unit I is characterized by lower L* and higher a* and b* values in the upper 25 m of Unit I. Below 25 m CSF-A, the sediment becomes lighter colored (L* increases) and more bluish green (a* and b* decrease).
Unit II is
characterized by increasing wet bulk density with depth, down to approximately
175 m CSF-A. Below this depth, an increase in the abundance of siliceous
microfossils produces a broad density minimum. The MS and NGR signals are low
in Unit II, down to the depth at which the biosiliceous material increases in
abundance. The interval of the broad density minimum is characterized by higher
MS values that are roughly equal to those in the upper 25 m of Unit I. Unit II
is lighter colored than Unit I (higher L*) and more blue (lower b*).
Unit III at Site
U1338 is characterized by a higher and more uniform carbonate content, and as a
result more uniform physical properties. Wet bulk density increases from
roughly 1.5 g/cm3 at the top of Unit III to 1.7 g/cm3 at
the base of the unit. Grain density varies over a narrower range in Unit III
than it does in Units I and II and displays an average, 2.64 g/cm3,
nearer to that of calcite. Velocity, which through much of Units I and II is
close to the velocity of water, displays a regular increase in Unit III, from
~1620 m/s at the top to ~1820 m/s near the base of the unit. The velocity
gradient increases near the base of Unit III accompanying the transition from
nannofossil ooze to chalk. MS is low from the boundary between Units II and
III, at ~245 m CSF-A, to 300 m CSF-A. Below 300 m CSF-A, susceptibility again
increase to values comparable to those in the upper part of Unit I. The variability
of the NGR is lower in Unit III than in Unit II and remains uniformly low
throughout the unit. Overall, Unit III is the lightest colored (highest L*
values) unit at Site 1338. The transition from greenish gray to pale yellow is
marked at ~385 m CSF-A by a shift to higher values of both a* and b*.
Downhole logging of Hole U1338B
began after the end of APC/XCB coring to a total depth of 416.1 m DSF. Three
tool strings were deployed in Hole U1338B a modified triple combo (that did not
include a neutron porosity measurement), a FMS-Sonic combination, and a VSI
seismic tool with a SGT-N gamma ray sonde. The modified triple combo and
FMS-Sonic tool strings took downhole measurements of natural gamma ray
radioactivity, bulk density, electrical resistivity, elastic wave velocity, and
borehole resistivity images in the depth interval 125-413 m WSF (wireline depth
below seafloor). The VSI seismic tool string measured seismic waveforms in a
vertical seismic profile experiment that covered the depth interval 189.5-414.5
m WSF. Measurement depths were adjusted to match across different logging
runs, obtaining a wireline matched below seafloor (WMSF) depth scale.
The downhole log measurements were
used to define three logging units: Unit I (139-244 m WMSF) and Unit II
(244-380 m WMSF) have average densities of ~1.45 and ~1.6 g/cm3,
respectively, that do not show any trend with depth, while in Unit III (from
380 m WMSF) density increases with depth reaching 1.7 g/cm3 at the
base of the hole. Resistivity and P-wave velocity follow a pattern similar to
that of density throughout the logged interval, suggesting that the major
control on these physical properties are variations in sediment porosity. Both
resistivity and density measurements show a small-scale peak at 280 m WMSF.
This peak at 280 m WMSF is clearly visible in the borehole resistivity images
as a high-resistivity layer 16 cm thick, and it corresponds to a chert layer
that has only been recovered as rubble in the cores. The natural gamma ray
measurements are low throughout (~4 degrees API), but do show a pronounced high
at the seafloor due to a local increase in uranium concentration.
In the VSP experiment, the arrival
time of a seismic pulse was measured from the sea surface at 14 stations.
Together with the travel time to the seafloor, the VSP measurements are the
basis for a travel time-depth conversion that allows seismic reflectors to be
correlated to stratigraphic events. Downhole temperature measurements and
thermal conductivities of core samples were combined to estimate a geothermal
gradient of 34.4°C/km and a heat flow of 33.6 mW/m2 at Site U1338.
of 118 interstitial water (IW) samples were collected from Holes U1338A and
U1338B, 43 using the whole-round squeezing approach and 75 by Rhizon sampling. The chloride ion concentration (not corrected for Br- contribution) varies
slightly with depth and is generally within the range of 555 to 565 mM.
Alkalinity increases slightly downhole from around 2.7 mM at the sediment/water
interface to peak slightly above 4 mM at 140 m CSF-A. A monstrous dissolved manganese
peak of 150 µM at 10 m CSF-A is captured by the high resolution IW
sampling and is remarkably similar to that observed at Site U1337. These peaks
are more than 10 times greater than the highest dissolved manganese
concentrations encountered on Expedition 320. Lithium concentrations decrease
from ~26 µM at the surface to a minimum of ~3 µM around 250 m CSF-A before increasing sharply with depth to seawater values at the base of the section. The interstitial water strontium profile is a mirror image to that of lithium except the decrease from the peak of 400 µM at 200 m CSF-A is punctuated by a sharp
drop of >100 µM between ~260 and 290 m CSF-A. The lithium and strontium
profiles indicate seawater circulation in the basement as their values tend
toward seawater values near the basement.
CaCO3 concentrations range between 26 and 88% with
substantial variability in the upper 273.31 m CCSF-A, corresponding to the
alternation between calcite and opal production in the upper two lithological
units. Below 273.31 m CCSF-A (lithological Unit III), calcium carbonate
contents become generally high and stable between 66 and 91% compared with the
upper part of the stratigraphic column. In the upper ~230 m CCSF-A, the TOC content
is generally high and variable ranging between 0.09 and 0.46%, whereas below
~230 m CCSF-A the TOC content is less than 0.09%. The downhole TOC variability
is most likely related to lithological changes with higher TOC being found in
the more biosiliceous intervals.
interstitial water and bulk sediment samples reflect large variations in the
sediment composition resulting from shifts in carbonate versus opal primary
production. The large scale redox state and diagenetic processes of the
sediment column are related to the overall changes in sediment composition. The
interstitial water chemistry points to seawater circulation in the basement
while the basement itself appears to exert little influence on the geochemistry
of the sediments and interstitial waters.
Color changes; lithology and redox state
Smear slide analyses and visual core descriptions
show that many of the decimeter-, meter-, and tens of meter-scale color
variations in Units I and II to some extent relate to changes in lithology. We
suspect, however, that some of these color variations, notably the transitions
between pale green and pale yellow lithologies are controlled by sediment redox
state, similar to those recorded at Sites U1331 through U1337 and earlier work
in the Equatorial Pacific Ocean (e.g., Lyle, 1983).
susceptibility is relatively low in the light gray and light brown intervals in
Unit I and for most of Unit II. A significant decrease in the intensity of the
magnetic signal in Unit II suggests dissolution of magnetite resulting from
intensified microbial Fe reduction. In the lower part of Unit III, a sharp
downcore transition from green to yellow is not associated with any other
lithological change, does not occur at the same stratigraphic level between
holes and thus should not be considered as an equivalent time horizon. While
pore water Fe concentrations reach 6 to 7 µmol/liter in the green interval, Fe is absent below the transition to yellow and brown. Although some of this
signal may be affected by seawater contamination during XCB drilling in Hole
U1338A, all available information suggests that the lowermost color change
represents a redox front.
Occurrence of diatom-rich layers
II at Site U1338 is mainly composed of nannofossil ooze with relatively high
abundances of biosiliceous components, notably diatoms. The relative abundance
of diatoms is lower than that at Site U1337 and the record lacks laminated
diatom ooze intervals ("diatom mats") such as observed at Site U1337. However,
cm- to sometimes 12 m thick diatom nannofossil ooze layers containing
abundant specimens of the diatom Thalassiothrix spp. are occasionally interbedded with nannofossil ooze (e.g., ~126.2127.1
m CSF-A and ~231.8234.3 m CSF-A in Hole U1138A, and ~127.3128.0 m
CSF-A and ~233.8234.8 m CSF-A in Hole U1338C). Units II and III also
contain significant amounts of pyrite, particularly in diatom-rich intervals in
Unit II (e.g., U1338B-14H, 19H21H, 26H, 28H29H, 32H41H).
In addition, the middle part of Unit III contains thin intervals of abundant
pyrite-filled siliceous microfossils (e.g., Core U1338B-33H-4, 5866 cm
and U1338B-35H-5, 7682 cm). These diatom rich layers, pyrite nodule
occurrences, and pyrite-rich siliceous microfossil layers in Units II and III
are associated with high TOC content, suggesting a relation between the
abundance of diatoms in the sediments, sediment redox-state and the production
or preservation of organic carbon.
Lyle, M., 1983.
The Brown-Green Color Transition in Marine Sediments: A Marker of the
Fe(III)-Fe(II) Redox Boundary. Limnology and Oceanography, 28(5), 1026-1033.