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IODP Expedition 320:
Pacific Equatorial Age Transect 1
Week 7 Report (20-26 April 2009)
PDF file is available for download.
26 April 2009
This week we finished coring in Hole U1335B, made
the transit to Site U1336 (PEAT-5C) and cored Holes U1336A and U1336B.
Site U1335, Hole U1335B
Coring at Hole
U1335B started at 1955 hr on 18 April with the bit 5 m deeper than at Hole
U1335A. APC cores U1335B-1H to 41H were taken from 0 to 378.2 m DSF and
recovered 392.7 m (104%). This is the second deepest APC penetration in the
history of ocean drilling. Twelve core barrels had to be drilled over to
release them from the formation and Cores 37H to 41H did not fully stroke and
were advanced by recovery. XCB Cores 42X to 46X were taken from 378.2 to 417.5
m and recovered 36.0 m (92%). Coring was terminated when basement was reached.
Originally we had planned three holes at Site U1335. However, after
recovering a nice Miocene section with nearly complete recovery and overlap
between the two holes, we decided not to core a third hole at Site U1335. In
addition, the presence of turbidites and a thick interval with very weak
magnetic signal contributed to this decision. We decided to use our remaining
time to core at Site U1336 (PEAT-5C) to provide the second PEAT Expedition more
information for optimizing their operations plan. We pulled the drill
string clear of the seafloor at 1215 hr and departed for Site U1336 at 2145 hr
on 21 April.
Site U1336, Hole U1336A
After an 185 nmi
transit (17.8 hrs; 10.4 nmi/hr), the ship arrived at Hole U1336A at 1545 hr on
22 April. Coring started in Hole U1336A at 0135 hr on 23 April. APC Cores
U1336A-1H to 21H were taken from 0 to 184.8 m and recovered 190.2 m (103%).
Several cores only partially stroked and were advanced by recovery; one
encountered an inferred ~0.15 m thick chert layer at ~124 m. Cores U1336A-22X
to 35X were taken from 184.8 to 302.9 m and recovered 69.0 m (58%). We stopped
coring before reaching the basement objective because of the decreasing rates
of penetration (as low as ~4 m/hr for Cores 34X and 35X), the relatively low
recovery, and the possibility of obtaining a stratigraphically complete Miocene
section by coring in a second hole.
Site U1336, Hole U1336B
We started coring
in Hole U1336B at 0540 hr on 25 April with the bit 5 m shallower than in Hole
U1336A. So far, Cores U1336B-1H to 9H have been taken from 0 to 77.8 m and
recovered 70.6 m (91%). We will stop coring no later than 0400 hr and start the
transit to Honolulu, Hawaii by 1600 hr on 26 April.
Sites U1335 and
U1336 focus on the paleoceanographic events from the early Oligocene into the
early and middle Miocene, and both provide a depth transect for the early and
middle Miocene together with Site PEAT-7C.
Site U1335 targets
the late Oligocene through Miocene. At the end of the Oligocene there is a
significant multi-million year long rise in the oxygen isotope record, which is
closely followed by a relatively short, sharp increase in oxygen isotope values
that has been interpreted as a major glacial episode ("Mi-1") and correlated to
a pronounced drop in sea level. This event is very close to the
Oligocene/Miocene boundary, and has now been astronomically age calibrated in
several ocean basins. Although there are clear periodic isotopic signals
indicating major changes in ice volume, ocean temperatures and/or ocean
structure, this biostratigraphic boundary has always been somewhat of an
enigma. Unlike the major changes in the isotopic stratigraphy, the
biostratigraphies of planktonic microfossils show very little change at all
across this boundary. In fact it is one of the most difficult epoch boundaries
to pick using solely the microfossil biostratigraphies.
(PEAT-5C) targeted the Oligocene and was located on lower Oligocene crust. This
interval of time is noted for its markedly heavy benthic oxygen isotopes and
its relatively deep CCD. There was probably ice on Antarctica during this
interval, but not the large ice sheets to be found there later in the
mid-Miocene. There is no compelling evidence for ice sheets in the Northern
Hemisphere during the Oligocene and early Miocene. Thus, there was apparently a
relatively low global ice volume, relatively cold bottom waters, a relatively
cold South Pole, and a relatively warm North Pole. This scenario of a "one cold
pole" world has given rise to speculation on the impact of inter-hemispheric
temperature imbalance on pole to equator temperature gradients and on the
symmetry of the global wind systems. Site U1336 will focus on the
paleoceanographic events in the late Oligocene and into the early and middle
Miocene, including the climatically significant Oligocene-Miocene
transition and its recovery.
Site U1335: Core description, imaging, and color reflectance
logging has been completed for Site U1335. About 420 m of pelagic sediments are
divided into two major lithologic units. Unit I, spanning the late Miocene
through the Pliocene-Pleistocene,
is composed of biogenic oozes with varying amounts of nannofossils, diatoms,
radiolarian and foraminifers. Unit II is composed of nannofossil ooze and chalk
with many thin beds of nannofossil foraminifer ooze of late Oliogene to late
Middle Miocene age. The foraminifer oozes overlie sharp erosional boundaries
and tend to fine upwards. Occasionally the foraminifer ooze beds contain a
laminated interval. Two distinct intervals of light greenish gray occur in Unit
II. The greenish intervals correspond to extremely low magnetic susceptibility
and the overall loss of sediment magnetic properties.
Site U1336: Hole U1336A has
been described down through Core U1336A-10H. The uppermost cores of late
Miocene age contain nannofossil oozes with varying amounts of foraminifers,
diatoms, radiolarians and clay. Lower in the section, nearly homogeneous
nannofossil oozes dominate.
Sediment color shifts from very pale brown in through pale yellow to
light greenish gray. The transition from very pale yellow to light greenish
gray occurs as a sharp boundary in Core U1336A-10H.
Site U1336: Initial biostratigraphy indicates that 300 m of
sediments recovered at Site U1336 span a complete succession from the middle
Miocene to the early Oligocene. Calcareous nannofossil preservation is moderate
to good in the middle Miocene, moderate in the lower Miocene and moderate to
poor, with evidence of recrystallization and overgrowth, in the upper and lower
Oligocene. A complete sequence of nannofossil zones are recorded from the lower
Oligocene zone NP22 through to the middle Miocene zone NN6. Planktic
foraminifera are moderate to well preserved in the middle and lower Miocene but
become moderate to poorly preserved in the Oligocene. The succession spans planktic foraminiferal zones O1 through M9b.
Radiolaria are moderately preserved in the middle Miocene, becoming
moderate to poorly preserved in the lower Miocene and are absent from the
Oligocene sediments, except for limited assemblages recovered from the topmost
Oligocene zone RP22. Radiolarian biostratigraphy is complete from the top
Oligocene zone RP22 through to the middle Miocene zone RN5. Benthic
foraminifera are present through most of the section and indicate lower bathyal
to abyssal paleo-depths; as with the other microfossil groups, preservation
becomes moderate to poor in the Oligocene sediments.
Site U1335: Paleomagnetism results were obtained from the 77
APC cores recovered at Site U1335. Sediments above ~70 m generally record the paleomagnetic field reliably, providing a magnetostratigraphic record that extends from the Brunhes (Chron C1n; 0-0.78
Ma) down to Chron C5r.1r (~11.0 Ma). Sediments from 110-200 m have also
kept relatively good record of Chrons C5AAn down to C6n (~13.2-18.7 Ma). In other intervals, the
magnetic remanence is extremely weak and the paleomagnetic directions variably,
prohibiting further magnetostratigraphic interpretation. The loss of the
primary paleomagnetic signal is attributed to reduction diagenesis.
Site U1335: We finished processing discrete samples from Hole
U1335A and collecting magnetic susceptibility (MS), velocity, natural gamma,
and density data from Hole U1335B. All track data are variable throughout the
section, allowing a detailed correlation between holes. Magnetic susceptibility
varies between 5 to 20 x10-5 SI in carbonate-dominated sections and
~29 x10-5 SI in more radiolarian-rich intervals; MS values are
extremely low between 140 and 205 m. Natural gamma measurements are elevated by
an order of magnitude in the uppermost sediments. Velocity shows a constant and
gradual downhole increase from 1500 to 1540 m/s to ~350 m and then increase
steadily to 1750 m/s at the base of the section. Wet bulk density increases
gradually from 1.6 to 1.8 g/cm3 though this carbonate-dominated
succession, and grain density remains fairly constant around 2.7 g/cm3.
Porosity shows a downhole decrease from 80% to 50% at the base of the hole.
Site U1336: Initiated full physical property program on Site
Site U1335: Using whole-round magnetic susceptibility and gamma
ray density measurements, Holes U1335A and U1335B have been spliced to form a
continuous section to the lowermost Miocene at ~398 m core composite depth
(CCSF), with only one interruption at 165.15 m CCSF caused by flow-in at the
bottom half of Core U1335A-16H. The section below 398 m CCSF was mostly
XCB-cored, lacked clearly identifiable features, and had to be appended for
this reason. Sedimentation rates decreased throughout the recovered section
from 25 m/m.y. in the late Oligocene to 19 m/m.y in the early to middle
Miocene, and remained at 6 m./m.y. throughout the late Miocene to Recent.
Site U1336: Site U1336: Magnetic susceptibility (MS) and gamma
ray attenuation (GRA) density data for the first 15 cores of Hole U1336A reveal
a pronounced cyclic signal that can are being used to guide coring offsets in
the second hole to obtain the most complete stratigraphy possible.
Sediment geochemistry this week continued work on
calcium carbonate analyses from Site U1335 and on organic carbon by
acidification from Sites U1334 and U1335. Calcium carbonate contents in U1335
are generally >80% deeper than 65 m, and more variable toward lower values
in the upper 65 m. Interstitial water analyses from Site U1335 show alternating
peaks of dissolved Mn and Fe associated with distinct sediment color changes
that are inferred to be a result of suboxic diagenesis of organic carbon.
Interstitial waters also exhibit strong Sr enrichment and Li depletion at
Technical Support and HSE Activities
During the week of
April 18 the shipboard labs were busy processing cores and samples as coring
was completed at Hole U1335C. Ongoing projects included the organization of the
storerooms and inventory updates for the shipboard laboratories. Six pallets of
cores were moved to the refrigerated container on the bridge deck in
preparation for offload in Honolulu.
A program to
inspect for safety and tag all plugged in appliances was begun by the
Electronics Specialists. A fire and boat drill was held on April 20 for the
entire ship's complement. Staff members were shown how to lower the lifeboats.