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This
story is expanded from the print version.
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Activities by the Ocean Drilling Program, or ODP, during 2001 focused on topics ranging from Cenozoic sea-level history to hot spot motions to characterizing the Mesozoic and Paleogene ocean. On two cruises, the research vessel JOIDES Resolution installed sea-floor observatories Following are some of the highlights, not including results that depend on detailed, shore-based studies in progress.
Sea-level snapshots
Sea-level history was the theme of ODP Leg 194, during which a series of eight
sites were drilled through Oligocene-Holocene, mixed-carbonate and siliciclastic
sediments of the Marion Plateau along northeast Australia. Mapping the
seismic sequence stratigraphy supplied a geometric framework for finding the
best drill sites to sampleboth sea-level highstand and sea-level lowstand sequences.
Lithostratigraphic and biostratigraphic data obtained from the Marion Plateau's
two Neogene carbonate platforms reveal that platform architecture was controlled
by a series of complexly related factors including sea-level change, bottom-current
action, and biological assemblages. Surprisingly, Leg 194 results show that
the oldest platform phase of the southern carbonate platform developed in a
topographic depression, rather than on a topographic high. Furthermore, the
steep-sided geometry of both Marion Plateau carbonate platforms is typical of
tropical to subtropical environments, whereas Leg 194 cores document a cool
subtropical faunal assemblage in these same platforms.
Filling the global seismic network
A major goal of Legs 195 and 196 was to install sea-floor observatories. The
first part of Leg 195 was devoted to coring South Chamorro Seamount, a serpentine
mud volcano on the forearc of the Mariana subduction system; and to installing
a long-term observatory that will collect data on the geochemistry of the sub-sea
floor interstitial fluids. The second part was devoted to coring and casing
a hole in the Philippine Sea abyssal sea floor, and installing a broadband seismometer
570 meters below the sea floor under 5.7 kilometers of water. By filling a large
gap in the global seismic station grid, the observatory will enable improved
resolution of global tomographic studies. Moreover, the observatory will permit
more precise study of the seismic structure of the crust and upper mantle of
the Philippine plate, as well as better resolution of earthquake locations and
mechanisms in the northwest Pacific subduction zone.
Seeing more in the drill hole
Leg 196 was the second of a two-leg program in the Nankai Trough. The sediments
were cored extensively on the first, Leg 190. Leg 196 focused on logging while
drilling, or LWD, and on installing long-term hydrogeological observatories
(ACORKs) at two sites near the toe of the Nankai prism: Site 808 at the deformation
front and Site 1173, a reference site about 12 kilometers seaward.
LWD is a method for measuring physical parameters of the rocks close to the
bit as the drill penetrates. Using this strategy, researchers on the ship can
gather important information about in situ conditions and stratigraphic
intervals that may not be fully sampled by coring. The ACORK, a major new development
over the original CORK device, is designed to allow portions of the borehole
to be isolated for separate, long-term fluid sampling and pressure measurements.
The ACORK was successfully emplaced in Hole 1173B, and then configured to monitor
the conditions in basement and in the overlying Lower Shikoku Basin formation.
The ACORK in Hole 808I was intended to penetrate just to the décollement,
with an emphasis on determining the conditions at the frontal thrust, a fractured
zone about 160 meters below the thrust, and the décollement. As a result
of extreme deterioration of drilling conditions and failure of the underreamer,
a device that enlarges a drill hole, drillers had to stop 36 meters short of
the goal. But the ACORK remains a viable installation.
Hot spot on the move
The bend in the Hawaiian-Emperor seamount chain is often cited as the best example
of a record of a change in the direction of plate motion as the plate passes
over a fixed hot spot. Alternatively, the bend might document movement of the
Hawaiian hot spot relative to the Pacific lithosphere. Basement sites were drilled
in the Emperor seamount trend during Leg 197 to test the latter hypothesis.
The objective was to obtain cores from lava flows suitable for paleomagnetic
paleolatitude and radiometric age determinations. Researchers obtained cores
from Detroit Seamount dating to between 76 million and 71 million years ago;
Nintoku Seamount from about 56 million years ago; and Koko Seamount from about
48 million years ago.
Paleolatitudes of these seamounts, suggested by preliminary shipboard analysis,
clearly differ from the latitude of Hawaii, and are consistent with results
from earlier drilling at Suiko Seamount and Detroit Seamount. The shipboard
analysis of paleolatitude vs. age for the Emperor seamounts must be supported
by shore-based paleomagnetic studies and radiometric age determinations. However,
the available data suggest that the Emperor seamounts record the rapid southward
motion of the Hawaiian hotspot in the mantle, requiring a major change in how
we view this seamount chain as a record of mantle convection and plate motions.
Climate boundaries
The mid-Cretaceous, between 125 million and 85 million years ago, and early
Paleogene, between 60 million and 45 million years ago, were characterized by
some of the most equable and warm climates of the Phanerozoic. Leg 198, on the
Shatsky Rise in the northwest Pacific, was designed to pinpoint the causes and
consequences of these intervals of global warmth. Eight sites were drilled on
a broad-depth transect, recovering a virtually complete section from the Jurassic/Cretaceous
boundary to the Holocene. Four of the sites show a remarkable record across
the Cretaceous/Tertiary boundary. The lithostratigraphy of the boundary successions
is similar in all of the holes: the uppermost Maastrichtian whitish nanofossil
ooze contrasts strongly with the pale-orange, clay-rich lowermost Danian foraminiferal
ooze. The contact is mixed by bioturbation, but otherwise the boundary interval
is undisturbed and stratigraphically complete. Leg 198 also cored the first
igneous rocks from Shatsky Rise, predominantly diabase sills.
Linking to biostratigraphy
Researchers on Leg 199 targeted the paleoceanographic record of the Paleogene
by coring a transect across the equatorial region of the central Pacific west
of Hawaii. Their goal is to improve our knowledge of the Paleogene biostratigraphy
and to link that record to paleomagnetic chronostratigraphy and to seismic stratigraphy.
Excellent, continuous sedimentary records were obtained at eight sites. Leg
199 results place new constraints on the late Paleocene and early Eocene equatorial
position and permit significant extension and refinement of the results of earlier
studies.
Leaving Honolulu just before Christmas of 2001, JOIDES Resolution embarked
on Leg 200 and the last full year of drilling operations for ODP. Brief descriptions
of the science proposed for the final Legs of the program can be found at the
Ocean Drilling Program's JOIDES Office
site and Ocean Drilling Program Science
Operator site.ODP is set to conclude seagoing operations in September 2003.
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