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IODP Expedition 320:
Pacific Equatorial Age Transect 1
Site U1333 Summary
PDF file is available for download.
19 April 2009
holes were cored at Site U1333 (10°30.996'N, 138°25.159'W, 4853 m water depth).
At Site U1333, Eocene age seafloor basalt is overlain by ~183 m of pelagic
sediment, dominated by nannofossil and radiolarian ooze with varying amounts of
In Hole U1333A,
APC cored sediments were recovered from approximately 3 m below the mudline
(~4850 m water depth) to 95 m CSF-A (Core U1333A-10H). XCB coring advanced to 184.1
m DSF, through an approximately 60 m thick sequence of lowermost Oligocene
carbonate oozes and nannofossil-bearing Eocene sediments. Near the basal
section, Core U1333A-20X recovered a 30 cm long interval of lithified
carbonate. The following Core U1333A-21X contained a dolostone basalt breccia.
Core U1333A-22X recovered a 6 cm piece of basalt.
Coring in Hole
U1333B started 5 m shallower than Hole U1333A to recover the mudline and to
span the core gaps from the first hole. Core U1333B-1H recovered 7.73 m of
carbonate bearing ooze overlain by a few meters of clay. Since the cores
recovered from Hole U1333A showed that there were no significant porcellanite
or chert layers, we used the APC drill-over strategy in Hole U1333B to obtain
APC cores across and below the EO transition down to 162.7 m CSF-A. We then XCB
cored to basement and a total depth of 180.3 m CSF-A.
Hole U1333C was
designed to provide stratigraphic overlap and confirm stratigraphic correlations
made between Holes U1333A and U1333B. APC coring in Hole U1333C started 2.75
shallower than Hole U1333B and reached to 163.2 m CSF-A before we had to switch
to XCB coring. No downhole logging was conducted at Site U1333.
sediment column at Site U1333 has a strong resemblance to that of ODP Site 1218
(Lyle et al., 2002), but with notably more carbonate bearing sediments in the
Eocene portion. The ~183 m of pelagic sediments have been divided into four
major lithological units. Unit I is about 7 m thick and contains an alternating
sequence of clay, clayey radiolarian ooze, radiolarian clay, clayey nannofossil
ooze, and nannofossil ooze from the early Miocene period. Unit II is
approximately 112 m thick and composed of alternating very pale brown
nannofossil ooze and yellowish brown nannofossil ooze with radiolarians of
early Miocene to latest Eocene age. Unit III is approximately 60 m thick and
composed of Eocene biogenic sediments comprising clayey nannofossil ooze,
nannofossil radiolarian ooze, nannofossil ooze, radiolarian nannofossil ooze,
and porcellanite of latest Eocene to middle Eocene age (Unit III). Unit III is
divided into two subunits, based on the absence (subunit IIIa) or occurrence of
porcellanite (subunit IIIb). Porcellanite is a third lithology in Unit III
between approximately 168 and 174 m CSF-A. Unit IV is a ~3.3 thin unit of
lithified carbonate (partly dolostone) and dolomitized nannofossil ooze,
overlying basalt of Eocene age (Unit V).
major microfossil groups have been found in sediments from Site U1333, and
provide a consistent, coherent and high resolution biostratigraphic succession
from basement up to the top of lithological Unit II. Shipboard biostratigraphy
indicates that sediments recovered at Site U1333 span a near continuous
succession from around the lower Miocene boundary to the middle Eocene.
Radiolarians are common and well preserved in the Eocene succession but less
well preserved in the Oligocene sediments. A complete sequence of radiolarian
zones from RN2 down to RP14 (middle Eocene) was described. Initial assessment
of the radiolarian assemblages across the EO boundary interval indicates a
significant loss of diversity through this apparently complete succession.
Although a few species from the Eocene carry through to the Oligocene, only one
stratigraphic marker species (Lithocyclia angusta) first appears near the Eocene/Oligocene boundary. Calcareous
nannofossils are present and moderately to well preserved through most of the
succession, although there are some short barren intervals in the middle to
late Eocene. The succession spans a complete sequence of nannofossil zones from
the early Miocene zone NN1 to the middle Eocene zone NP15. The
Oligocene/Miocene boundary are bracketed by the base Sphenolithus
disbelemnos in Sample 320-U1333A-2H-5, 70
cm (16.20 m CSF-A) and the occurrence of rare Sphenolithus delphix in Sample 320-U1332A-2H-CC-PAL (9.57 m CSF-A).
Discoasters are very rare in the basal assemblages, indicative of a eutrophic
environment, consistent with the paleolatitude of this site in the early middle
Eocene within the equatorial upwelling zone. Planktic foraminifers are
relatively abundant and well preserved from the lowest part of the Miocene to
the lower Oligocene. The Oligocene fauna is characterized by the common
occurrence of Catapsydrax spp., Dentoglobigerina
spp., and Paragloborotalia spp. In contrast the upper Eocene sediments contain
poorly preserved specimens or are barren of planktic foraminifers. Preservation
and abundance slightly increased in some intervals of the middle Eocene, which
is recognized by the presence of acarininids and clavigerinellids. The absence
of the genera Globigerinatheka
and Morozovella makes precise age
determination of individual samples problematic. The high abundance of Clavigerinella spp. has been linked to high-productivity
environments, consistent with the paleogeographic location of this site. Benthic foraminifers were almost
continuously present and indicate lower bathyal to abyssal depths. The
Oligocene fauna is characterized by calcareous hyaline forms, such as Nuttallides
umbonifer, Oridorsalis umbonatus and Cibicidoides mundulus. Nuttallides truempyi and Oridorsalis umbonatus often dominate the Eocene fauna. Benthic
foraminifers are present through most of the section apart from an interval in
the middle Eocene equivalent to radiolarian zone RP16. They indicate lower bathyal
to abyssal paleodepths. Diatoms have been observed throughout the column, but
will have to await analysis by specialists not onboard Expedition 320.
rates at Site U1333 are about 6 m/m.y. in the upper sediment column from the
early Miocene to the late Oligocene. In the early Oligocene linear
sedimentation rates increase to ~12 m/m.y. Between about 31 Ma (the earliest
Oligocene) and the earliest late Eocene they are about 4 m/m.y., increasing
slightly in the middle Eocene section (~39-45 Ma) to ~ 5 m/m.y..
results from measurements made along split-core sections and on small discrete samples
from Site U1333 provide a well-resolved magnetostratigraphy. Shipboard analyses
suggest that a useful magnetic signal is preserved in most APC cored intervals
after removal of the drilling-induced overprint by partial alternating-field
(AF) demagnetization at 20 mT. The overprint was nearly absent in those cores
collected in non-magnetic core barrels at Site U1333 whereas it was quite prominent
for cores recovered in the standard steel core barrels. Paleomagnetic
directions from discrete samples agree well with those from split cores,
confirming that AF demagnetization at 20 mT is generally sufficient to resolve
the primary paleomagnetic direction regardless of which type of core barrel was
used. The cleaned paleomagnetic data provide a series of distinct ~180¡
alternations in the declination and subtle changes in inclination, which when
combined with biostratigraphic age constraints, allow a continuous
magnetostratigraphy to be constructed that correlates well with the geomagnetic
polarity timescale. The magnetostratigraphic record extends from the base of
Chron C6n (19.722 Ma) at 1.7 m CSF-A in Hole U1333C down to the top of Chron
C20r (43.789 Ma) at 161.6 m CSF-A in Hole U1333C. Highlights include very
high-quality paleomagnetic data across Chrons C13r and C13n, which span the
latest Eocene and earliest Oligocene, and a newly recognized cryptochron within
results indicate that samples from the uppermost ~4 m of Site U1333 have modest
CaCO3 concentrations of 26% to 69%, with frequent variations between
58% and up to 93% in the interval between 4-35 m CSF-A. Carbonate
concentrations are consistently high (75.5% to 96%) from 35 m to 111 m CSF-A,
while in the Eocene (between 111 to 171 m CSF-A), CaCO3 concentrations
vary rapidly between less than 1% and 74%. The lowermost lithified carbonate
rocks between 173 and 180 m CSF-A have high CaCO3 concentrations
between 76% and 90%. Total organic carbon concentrations, as determined using
an acidification method, are generally very low or below the detection limit
(<0.1%, apart from samples in the top most 5 m, which reached ~ 0.17%).
Porewater alkalinity values are never elevated, but alkalinity and dissolved
strontium values are somewhat higher near the Eocene-Oligocene transition;
these are generally consistent with carbonate dissolution or recrystallization
processes. Dissolved silicates
increase with depth, with values always less than 1000 µM.
full physical property program was run on cores from Holes U1333A, U1333B, and
U1333C comprising whole-round multisensor core logger measurements of magnetic
susceptibility (MS), bulk density, P-wave velocity, non-contact resistivity,
natural gamma radiation, and measurements of color reflectance, followed by
discrete measurements of moisture and density properties, sound velocities and
thermal conductivity on cores from Hole U1333A only. All track data show
variability throughout the section, allowing a detailed correlation between
holes primarily using magnetic susceptibility and density (MS varies around
~24x10-5 SI in radiolarian ooze dominated sections, and ~3x10-5
SI in more carbonate-rich intervals). MS values gradually increase uphole.
Natural gamma (NGR) measurements are elevated by an order of magnitude in the
uppermost clays and show an increase near the lower Oligocene at around 115 m
CSF-A (from 5 to 8 counts per second). P-wave velocity shows a gradual increase
downhole, as we move from carbonate- to radiolarian-dominated successions.
P-wave velocity generally varies between 1490 and 1560 m/s depending on
lithology, with lower velocities corresponding more to carbonate-rich sections.
Bulk density and grain density show a marked decrease around 112 m CSF-A
(~1.704 to 1.313 g/cm3 in bulk density) and occurs where carbonate
content decreases rapidly. Porosity values are generally high in the
radiolarian rich sediments (80%), and decrease within the carbonate-rich
section (~60%). Thermal conductivity measurements show increased values in
carbonate rich intervals, and range from around 0.8 W/(K m) in lithological
Unit I to 1.2-1.3 W/(K m) in lithological Unit II.
correlation indicated that a composite section was recovered down to a depth of
~130 m CSF in the upper Eocene, equivalent to a composite depth of ~150 m CCSF.
For Site U1333, a growth factor of 15% is estimated from the ratio between the
CCSF and CSF (formerly mcd and mbsf) depth scales. Stratigraphic correlation
with ODP Site 1218 suggest a complete stratigraphic section in the Oligocene to
uppermost Eocene interval.
formation temperature measurements were conducted in Hole U1333B with the APCT3
tool. These temperature measurements, when combined with the thermal
conductivity values obtained from the cores, indicate that Site U1333 has a
heat flow of about 43 mW/m2, and a thermal gradient of 38¡C/km.
carbonate fluctuations in middle Eocene sediments
at Site U1333 was designed to capture a time period when the carbonate
compensation depth (CCD) was slightly deeper, within the middle Eocene interval
that showed prominent fluctuations of carbonate content (Lyle et al., 2005).
This interval occurs during the cooling that took place after the Eocene
Climatic Optimum ("EECO", Zachos et al. 2001a) and before the
Eocene-Oligocene transition (e.g., Coxall et al., 2005). Unlike ODP Site
1218, Site U1333 sediments show carbonate concentrations of over 75% in this
interval at a deeper water depth, and apparently co-eval with the CCD cycles
described by Lyle et al., 2005. The basal lithological Unit IV recovered
partially lithified carbonates.
Eocene Climatic Optimum (MECO), Eocene/Oligocene and Oligocene/Miocene transitions and depth transects
U1333 forms the third oldest and deepest component of the PEAT depth transect,
and can be directly compared with ODP Site 1218, which will allow the study of
critical intervals (such as the Eocene-Oligocene transition, see Coxall
et al., 2005) and variations of the equatorial CCD. Site U1333 is estimated to
have been approximately 3.8 km deep during the Eocene-Oligocene
transition, approximately 1 km shallower than today, and 200 m shallower at
that time than Site U1332. Sediments do not change carbonate content as rapidly
as at the deeper and older Sites U1332 and U1333. They appear to contain
Eocene-Oligocene transition sediments that are suitable for
palaeoceanographic studies using carbonate based geochemical proxies, and thus
are an improvement over ODP Site 1218. Of note Site U1333 also contains high
carbonate content bearing sediments around the Middle Eocene Climatic Optimum
event (Bohaty and Zachos, 2003; Bohaty et al., 2009), allowing a detailed study
of the sequence of events linking carbonate preservation cycles (Lyle et al.,
2005) with climatic oscillations.
U1333 also recovered carbonate bearing sediments across the
Oligocene-Miocene transition (e.g., Zachos et al., 2001b), adding
important data to the study of this time interval in the context of the PEAT
Oligocene-Miocene depth transect.
3) An age transect of seafloor basalt
U1333 recovered what appear to be fresh fragments of seafloor basalt, aged
between 45 and 46 Ma. This material will, when combined with other PEAT basalt
samples, provide important sample material for the study of seawater alteration
Bohaty, S. M., and J. C. Zachos (2003), Significant Southern
Ocean warming event in the late middle Eocene, Geology, 31(11), 1017-1020.
Bohaty, S.M., Zachos, J.C., Florindo, F., and M. L. Delaney,
in press 2009, Coupled greenhouse warming and deep-sea acidification in the
middle Eocene, Paleoceanography. doi:10.1029/2008PA001676
Coxall, H.K., P. A. Wilson, H. Pälike,
C. H. Lear, and J. Backman, 2005. Rapid stepwise onset of antarctic glaciation and deeper calcite compensation in the paciÞc ocean. Nature, 433(7021):5357.
Lyle, M., P. A. Wilson, T. R. Janecek et al., 2002. Init. Rep., Proc. Ocean Drill. Prog. 199, Ocean Drilling Program, College Station, TX. doi:10.2973/odp.proc.ir.199.2002
Lyle, M.W., Olivarez Lyle, A.,
Backman, J., and Tripati, A., 2005, Biogenic sedimentation in the Eocene
equatorial Pacific: the stuttering greenhouse and Eocene carbonate compensation
depth, in Lyle, M., Wilson, P., Janecek, T.R., and Firth, J., eds., Proceedings
of the Ocean Drilling Program, Scientific Results, Leg 199: College Station TX,
Ocean Drilling Program. doi:10.2973/odp.proc.sr.199.219.2005
Zachos, J., Pagani, M., Sloan, L., Thomas, E. & Billups, K., 2001a. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science 292, 685693.
Zachos, J.C., Shackleton, N.J.,
Revenaugh, J.S., Pälike, H., and Flower, B.P., 2001b. Climate response to
orbital forcing across the OligoceneMiocene boundary. Science, 292(5515):274278.