June 13, 2005
New findings show a slow recovery from extreme
global warming episode 55 million years ago
By Tim Stephens
Most of the excess carbon dioxide pouring into the atmosphere
from the burning of fossil fuels will ultimately be absorbed
by the oceans, but it will take about 100,000 years.
Professor of Earth sciences James Zachos inspects a sediment
core just recovered from the seafloor on the Ocean Drilling
Program (ODP) ship JOIDES Resolution.
Photo: Takashi Hasegawa
A red clay layer in sediment cores
drilled from the seafloor was deposited 55 million years
ago, when a massive release of carbon dioxide caused global
warming and ocean acidification. The clay layer was created
by the dissolution of the calcium carbonates that give the
surrounding layers their pale color. This section of split
core was recovered from 130 meters below the seafloor at
a water depth of 4.8 kilometers in the Angola Basin, South
Photo: Isabella Raffi
That is how long it took for ocean chemistry to recover from
a massive input of carbon dioxide 55 million years ago, according
to a study published this week in the journal Science.
James Zachos, professor of Earth sciences at UCSC, led an international
team of scientists that analyzed marine sediments deposited
during a period of extreme global warming known as the Paleocene-Eocene
Thermal Maximum (PETM). Sediment cores drilled from the ocean
floor revealed an abrupt change in ocean chemistry at the start
of the PETM 55 million years ago, followed by a long, slow recovery.
"Most people have not thought about the long-term fate
of all that carbon and what's involved in removing it from the
system. There is a long timescale for the recovery, tens of
thousands of years before atmospheric carbon dioxide will start
to come back down to preindustrial levels," Zachos said.
Earlier studies using computers to run numerical models of
Earth's carbon cycle have calculated similarly long timescales
for absorption of the carbon dioxide currently being released
into the atmosphere from fossil fuels, he said.
"Our findings are consistent with what the models have
been showing for years. What we found validates those geochemical
models," Zachos said.
The oceans have a tremendous capacity to absorb carbon dioxide
from the atmosphere. Results from a large international research
effort published last year indicated that the oceans have already
absorbed nearly half of the carbon dioxide produced by humans
in the past 200 years--about 120 billion metric tons of carbon.
When carbon dioxide dissolves in water it makes the water more
acidic. Ocean acidification starts at the surface and spreads
to the deep sea as surface waters mix with deeper layers. The
sediment cores studied by Zachos and his coworkers showed the
effects of a rapid acidification of the ocean during the PETM.
The acidification was more severe than they had expected, suggesting
that the amount of carbon dioxide that entered the atmosphere
and triggered global warming during the PETM was much greater
than previously thought.
The leading explanation for the PETM is a massive release of
methane from frozen deposits found in the deep ocean near continental
margins. The methane reacted with oxygen to produce huge amounts
of carbon dioxide. Both methane and carbon dioxide are potent
greenhouse gases and caused temperatures to soar during the
PETM. Average global temperatures increased by about 9 degrees
Fahrenheit (5 degrees Celsius), and the fossil record shows
dramatic changes during this time in plant and animal life,
both on land and in the oceans.
Previous estimates for the amount of greenhouse gas released
into the atmosphere during the PETM were around 2,000 billion
tons of carbon. Zachos said at least twice that much would be
required to produce the changes observed in this new study.
"This is similar to the estimated flux from fossil fuel
combustion over the next three centuries," he said. "If
we combust all known fossil fuel reserves, that's about 4,500
billion tons of carbon. And now we know that the recovery time
for a comparable release of carbon in the past was about 100,000
The study's conclusions hinge on the effects of ocean acidification
on the chemistry of calcium carbonate, the mineral from which
certain kinds of phytoplankton (microscopic algae) and other
marine organisms build their shells. When these organisms die,
their shells rain down onto the seafloor.
Marine sediments are typically rich in calcium carbonate from
these shells, but increased acidity causes it to dissolve. The
dissolution of calcium carbonate enables the ocean to store
large amounts of carbon dioxide in the form of bicarbonate ions.
"The calcium carbonate sitting on the seafloor increases
the ocean's buffering capacity, so that it can eventually neutralize
most of the changes in acidity caused by the carbon dioxide
accumulating in the atmosphere," Zachos said.
Sediments deposited at the start of the PETM show an abrupt
transition from carbonate-rich ooze to a dark-red clay layer
in which the carbonate shells are completely gone. Above the
clay layer, the carbonates gradually begin to reappear.
This transition at the Paleocene-Eocene boundary was already
well known from previous studies of sediment cores by Zachos
and others. The new Science paper, however, presents
the first results from a series of sediment cores covering the
PETM over a broad range of depths in the ocean. The cores were
recovered in 2003 from Walvis Ridge in the southeastern Atlantic
This series of sediment cores enabled the researchers to trace
changes in ocean chemistry over time at different depths in
the ocean. This is important because the chemical equilibrium
between solid calcium carbonate (calcite) and dissolved calcium
and carbonate ions changes with depth. The dissolution of calcite
increases not only with acidity, but also at the colder temperatures
and higher pressures found in the deep ocean.
At a certain depth--currently 4 kilometers (2.4 miles) in the
southern Atlantic--the calcite shells of dead plankton drifting
down from the surface waters begin to dissolve. The point at
which the dissolution rate exceeds the supply rate of calcite
from above is called the carbonate compensation depth (CCD).
The distinctive layers of clay that mark the PETM in sediment
cores indicate that those sites were below the CCD at the time
those sediments accumulated.
In the series of sediment cores from different depths on Walvis
Ridge, Zachos and his coworkers observed a rapid shoaling (rising
toward the surface) of the CCD due to the acidification of ocean
"The CCD shoaled quickly from below the deepest site to
above the shallowest site, producing a clay layer with no carbonate.
And then the carbonate starts to reappear, first at the shallowest
site, then deeper, eventually reaching the deepest site,"
Zachos said. "The time lag before the carbonates start
to reappear is about 40 to 50 thousand years, and then it's
another 40 thousand years before you see the normal carbonate-rich
The dissolution of calcium carbonate provides only temporary
storage of carbon dioxide. When the dissolved ions recombine
to form calcite again, carbon dioxide is released. The long-term
storage of carbon dioxide is accomplished through chemical weathering
of silicate rocks, such as granite and basalt, on the land.
As weathering removes carbon dioxide, however, the same buffering
process that slowed the accumulation of carbon dioxide in the
atmosphere starts to operate in reverse, gradually releasing
stored carbon from the ocean back into the atmosphere.
"The ocean's role is to act like a temporary store for
the carbon until these chemical weathering processes can remove
it from the system. This is the theory of ocean carbonate chemistry
that we were taught in graduate school, and here is a case study
where you can actually see it happen," Zachos said.
These changes in ocean chemistry during the PETM coincided
with a sharp reduction in marine biodiversity. For example,
many species of bottom-dwelling phytoplankton that form calcite
shells went extinct, possibly as a direct result of ocean acidification,
Within the past year, scientists have begun to detect similar
changes in ocean chemistry in response to the rise in atmospheric
carbon dioxide from fossil fuel consumption and other human
activities. Researchers have also begun to worry about the potential
ecological effects of ocean acidification. Whatever the effects
may be of current increases in atmospheric carbon dioxide, we
will probably have to live with them for a long time.
"Even after humans stop burning fossil fuels, the impacts
will be long-lasting," Zachos said.
In addition to Zachos, the authors of the Science paper
are Ursula Röhl of the University of Bremen, Germany; Stephen
Schellenberg of San Diego State University; Appy Sluijs of Utrecht
University, The Netherlands; David Hodell of the University
of Florida; Daniel Kelly of the University of Wisconsin, Madison;
Ellen Thomas of Wesleyan University and Yale University; Micah
Nicolo of Rice University; Isabella Raffi of G. d'Annunzio University,
Italy; Lucas Lourens of Utrecht University; Heather McCarren,
a graduate student in Earth sciences at UCSC; and Dick Kroon
of Vrije University, The Netherlands.
This study was conducted as part of a five-year interdisciplinary
project funded by the National Science Foundation to investigate
the consequences of greenhouse warming for biocomplexity and
biogeochemical cycles. In a related study led by Lourens and
published this week in the journal Nature, the researchers
reported on a similar, less extreme global warming event that
occurred 2 million years after the PETM.
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