May 12, 2003
Seafloor sediments hold clues to
By Kasey White
Scientists from UCSC and other institutions around the world arrived
in Rio de Janeiro last week after spending two months at sea on the
research ship JOIDES Resolution near an ancient
chain off Africa, known as the Walvis Ridge.
Scientists aboard the JOIDES Resolution display a sediment
core showing evidence of global warming 55 million years ago.
From left to right: Christina Riesselman (USA), Kacey Lohman (USA),
professor of Earth sciences James Zachos (cochief scientist),
Appy Suijs (Netherlands), and Stephen Schellenberg (USA).
Photo: Courtesy of Joint Oceanographic Institutions.
There they studied evidence of a massive release of
methane that caused
extreme global warming 55 million years ago.
This extraordinary episode of global warming, often referred to as
the Paleocene-Eocene Thermal Maximum, is unique in Earth's history in
terms of magnitude and rate of warming, as well as in the manner in
which it began.
Scientists think that roughly 2,000 gigatons (2 trillion
tons) of methane,
a potent greenhouse gas, was released to the ocean and atmosphere 55
million years ago, triggering a runaway process of global
The scientists on Ocean Drilling Program (ODP) Leg 208 set
out to test
this hypothesis about the origin of the Paleocene-Eocene
Geochemists speculate that methane escaped from submarine
ice crystals that trap methane and are distributed in
sediments on the
outer edges of continental margins worldwide. For reasons that remain
unknown, the clathrates suddenly began to decompose on a
increasing the amount of methane in the atmosphere and
ocean. This decomposition
process appears to have lasted for a period of 40,000
warming the planet by more than 5 degrees Celsius.
Sediments far below the seafloor hold evidence of the
and its consequences.
"The rapid release of methane 55 million years ago should have
left a distinct signature in the form of calcite-poor or
the distribution and thickness of which would be controlled
by the total
mass of methane released," said James Zachos, cochief scientist
of ODP Leg 208 and professor of Earth sciences at UCSC.
This is because the rapid release of such a large mass of
its subsequent oxidation to carbon dioxide would have significantly
altered ocean chemistry. The added carbon dioxide would
the overall acidity or corrosiveness of seawater. This, in
increase the dissolution of calcite shells of
microplankton, which are
the dominant component of seafloor sediments, leaving
behind only nonsoluble
clays. The dissolution of calcite would initiate in the deepest parts
of the ocean and rapidly spread upwards as additional carbon dioxide
entered the ocean. The overall extent and duration of
ultimately be controlled by the total mass of methane released.
"By establishing the vertical extent of carbonate dissolution,
as well as the duration of dissolution, it should be
possible to determine
the total amount of carbon dioxide that was added to the ocean during
the event," Zachos said.
Retrieving the sediments proved to be a formidable
challenge. To accurately
reconstruct past climate, technicians needed to retrieve
without deformation of layers and other structures.
"Imagine the difficulties of retrieving a layer of
1 meter thick with the consistency of mud from 200 to 300
the seafloor at a water depth of 4,800 meters, without disturbing the
centimeter-scale layering," said Dick Kroon, cochief scientist
from Vrije Universiteit Amsterdam.
Technicians used a device known as the Advanced Piston Corer (APC)
to obtain the cores. The extreme stress placed on the
quickly began to take its toll on the APC. In the very
first hole, the
steel core barrel literally snapped in half from the
applied while attempting to extract it from the sediment. The lower
half of the barrel could not be retrieved, and the hole had
to be abandoned.
On several other occasions, the steel rods that secure the
came back to the surface twisted or bent.
"At this point, we were a little concerned about
whether we could
achieve our objectives," Zachos said.
Determined, the drilling crew made the necessary repairs, adjusted
their drilling strategy, and pushed on. Their efforts ultimately paid
off; the thin boundary layer was recovered at five sites in
between 2,500 and 4,800 meters. Remarkably, at each site
the layer was
fully intact and in perfect condition.
Initial examination of the sediments provided immediate insight into
the scale of calcite dissolution during this event. Micah Nicolo, a
graduate student at Rice University, remarked, "When the cores
were opened in the ship's lab, we were stunned by the
change in colors
of the sediment, from bright white carbonate to deep red clays."
Each core, regardless of depth, yielded a sequence of carbonate-rich
sediment dissected by a distinct, dark clay layer, varying
from 100 to 50 centimeters. The base of each clay layer, regardless
of depth, contained essentially no calcite--indicating dissolution of
calcite sediment throughout the ocean.
Ellen Thomas of Wesleyan University studied additional
impacts of the
"The extent of dissolution may explain why so many
organisms that precipitate calcite shells became extinct at
The scale of carbonate dissolution recorded in these cores
It is suggestive of a much larger flux of methane, possibly
estimates. It may also point toward an additional source of
"It far exceeds what has been estimated by models
assuming a release
of 2,000 gigatons of methane," Kroon said.
The initial results also suggest that the deposition of
on the deeper reaches of the seafloor did not resume for at
years, and that the total recovery time to a "normal state"
took as long as 100,000 years. This result suggests that
from these extreme events takes considerable time.
The cores recovered on this leg may also provide insight
into the ultimate
cause of the thermal maximum. Toward the end of the Paleocene epoch,
the planet was slowly warming due to rising levels of carbon dioxide
emitted from volcanoes.
"Several of us suspect that the melting of clathrates and rapid
release of methane was initiated by gradual warming that pushed the
climate system across a physical threshold," Zachos said.
Once started, the release of methane and resultant warming
release of more methane, a positive feedback effect. This phenomenon
is a concern for future global warming, Zachos said.
Studies of the sediment cores from Walvis Ridge and others recently
recovered in the Atlantic and Pacific Oceans will allow scientists to
test these and other ideas about the origin of the Paleocene-Eocene
ODP is an international partnership of scientists and
organized to study the evolution and structure of the
Earth. It is funded
principally by the U.S. National Science Foundation, with substantial
contributions from its international partners. The Joint
Institutions (JOI) manages the program. Texas A&M University is
responsible for science operations, and Lamont-Doherty
of Columbia University is responsible for logging services.
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