IT WAS the Earth’s biggest ever fart. More than a trillion tonnes of methane
burst suddenly from the ocean 55 million years ago, sending temperatures soaring
and killing thousands of species in the ocean depths. But it also triggered an
evolutionary bonanza among land mammals, including early primates. Without it we
might not be here today.
The first evidence for this prehistoric mega-fart was unearthed in 1991, but
for years it has left researchers baffled. Why did it happen? Why so suddenly,
and why such a widespread effect? Now at last scientists are beginning to
understand what lay behind this extraordinary outburst, and are realising that
the Earth is capable of creating dramatic climate changes all on its own,
without human help.
The first whiff of trouble came from a hole drilled 2100 metres into the Maud
Rise, a submarine ridge beneath the Weddell Sea off Antarctica. Examining the
ancient sediment removed from the hole, geologists James Kennett from the
University of California at Santa Barbara and Lowell Stott from the University
of Southern California, Los Angeles, found evidence of a sudden mass extinction
of organisms living on the seafloor at the end of the Palaeocene era. Between a
half and two-thirds of all bottom-dwelling animals disappeared within perhaps a
thousand years.
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Kennett and Stott soon discovered that researchers had detected similar
deep-sea extinctions from the same time in marine sediments in both the
Caribbean and Europe. This was clearly a global event, one of the largest
extinctions of the past 90 million years. And yet, curiously, it did not kill
either marine organisms at the surface or plants and animals on land.
What happened? Since no surface species died, it could not have been an
asteroid strike of the kind that saw off the dinosaurs 10 million years before.
Looking closely at the chemistry of fossils in the drilled core, though, Kennett
and Stott uncovered their first clue. The precise ratios of different isotopes
of carbon and oxygen found in fossil remains give a picture of the environmental
conditions at the time the organisms lived. It quickly became clear from the
core that something dramatic had disturbed the chemistry of the planet’s
biosphere 55 million years ago, at what geologists call the Palaeocene/Eocene
(P/E) boundary.
First, there had been a big decrease in the proportion of oxygen-18 in the
fossils. This isotope is very sensitive to temperature change, so Kennett and
Stott concluded that ocean temperatures had soared at the same time as the
extinction. The deepest water, where the extinctions occurred, warmed from 11 to
15 °C within about a thousand years.
There was something even more intriguing in the isotopic record from this
time in both the Maud Rise core and others around the world. Organic matter in
rocks across the P/E boundary reveals a sudden isotopic shift from carbon-13 to
carbon-12. “The isotope shift occurred in rocks from the deep ocean, the shallow
ocean and on land,” says Jerry Dickens of James Cook University in Townsville,
Australia. “Whatever happened at the P/E boundary changed the isotopic
composition of all the material in the global carbon cycle.”
One big burst
That’s a prodigious amount of carbon: some 40 trillion tonnes or 200 times
the current store of carbon in the world’s forests. What’s more, the change was
extraordinarily fast by geological standards—taking less than 10 000
years. There’s only one way to release such a huge amount of carbon-12 so
quickly—fossil fuels. Geological deposits of coal, oil or methane are
greatly enriched with carbon-12. So the search was on to find a fossil fuel
source that could have released its carbon in a massive burst, 55 million years
before the Industrial Revolution.
A subterranean source such as carbon dioxide from volcanic eruptions or
hydrothermal vents was a possibility. But Dickens has ruled this out. He
calculated that if CO2 in the Earth’s mantle at that time contained a
proportion of carbon-12 similar to today’s levels, the annual volcanic eruption
rate would have to be boosted to some hundred times the average rate over the
past billion years. “Such high rates are improbable…unprecedented in the
geological record,” he says.
So the mystery source of carbon was certainly not CO2, but it could
have been methane. Initially, wetlands were suggested as the source for this
methane. In 1992 Jim Zachos and Lisa Cirbus Sloan, both then at the University
of Michigan at Ann Arbor, estimated that by the end of the Palaeocene there were
three times as many methane-generating swamps and wetlands as today.
But Dickens argues that even this large methane source was much too small to
explain the flush of carbon-12 through the biosphere at the P/E boundary. The
real source, he suggests, can only have been a lesser known but even larger
reservoir very rich in carbon-12: methane hydrates. Confined by high pressure
and low temperature, these are stable, crystalline lattices of water molecules
encasing methane gas. Release the pressure or raise the temperature, however,
and they will shatter, quickly releasing large amounts of methane.
Massive reservoirs
Methane hydrates exist in large quantities in permafrost and marine
sediments. In the latter, the gas is generated by bacteria in the mud. Seismic
surveys have revealed large reservoirs of methane hydrates in the top few
hundred metres of sediments on the floors of all the oceans, usually just beyond
the edge of continental shelves. Many of these structures trap even larger
stores of gaseous methane that has bubbled up from beneath.
Methane “leaks” from hydrate lattices have been blamed for everything from
oil-well blowouts to the mysterious disappearance of ships and planes in the
Bermuda Triangle. But could they have released methane into the environment at
the P/E boundary in sufficient quantities to trigger mass extinctions in the
ocean depths and upset global climate at the surface? Dickens thinks so. He
estimates that between 1 and 10 trillion tonnes of methane are tied up today in
or beneath hydrates. And much of this could, in theory, be released quickly.
“We know that the stability of hydrates is sensitive to changes in
temperature,” says Dickens. He calculates that the warming of the deep ocean
revealed in the fossil records would have heated the sediments enough to release
perhaps a trillion tonnes of methane, enough to cause the observed global shift
in carbon isotopes. He accepts that this scenario is “largely untested”. Indeed,
he has devoted most of a paper to be published in Geology to pointing
out the uncertainties. But, he says, “right now, most everybody studying the P/E
boundary seems to accept that the release of methane hydrates is the only
plausible explanation.”
It also helps to explain other events that occurred at the same time. Though
many species in the deep ocean became extinct, ones that could tolerate low
oxygen levels flourished. And many shells made from calcium carbonate dissolved.
These two changes suggest, respectively, a decrease in the availability of
oxygen and an increase in dissolved CO2, making the water more acid.
Methane would have been rapidly oxidised to CO2in the water, removing
oxygen in the process.
If methane did emerge from the depths 55 million years ago to transform our
planet, what released it? Most researchers believe that something caused the
water in the deep ocean to warm up, destabilising the methane hydrates and
triggering the release of their methane. The likeliest cause of this deep
warming is a sudden change in ocean circulation patterns.
The ocean circulation system is driven from a small number of places at the
surface where water becomes unusually saline. This super-saline water is denser
than the surrounding water and sinks, often right to the ocean floor. Through
most of the oceans’ history, this process of “deep-water formation” seems to
have occurred in polar regions such as the North Atlantic and around Antarctica,
as it does today. Here, where seawater freezes to form ice caps, the liquid
water that remains becomes increasingly salty. Once it has sunk, the water takes
a thousand-year circuit through the ocean depths before surfacing again,
travelling along what oceanographers call the “conveyor belt”.
It seems likely, says Dickens, that this system broke down 55 million years
ago, allowing warmer water from middle or tropical latitudes to penetrate the
ocean depths. He points out that even without the methane, the Earth had been
warming relatively rapidly over the previous several million years. At some
point, he says, “something snapped”.
Zachos and Dickens argue that the warming that had already been occurring
took the whole ocean-climate system over a threshold. Stott and Kennett think
they have an idea how this may have happened. They suggest that global warming
had gradually reduced the freezing of polar waters to the point where sinking
ceased. Deep-water formation would then have occurred largely at warmer
latitudes, probably mainly in the Tethys Seaway—a vastly expanded version
of today’s Mediterranean Sea. High evaporation rates there would have made
surface waters increasingly saline, sending warm water to the depths, where the
hydrates were waiting.
Tim Bralower of the University of North Carolina in Chapel Hill, on the other
hand, believes the outburst of methane occured when the climate system received
an extra kick, rather than simply passing a threshold. Last year he and his
colleagues showed that a series of thick layers formed when volcanic ash rained
down on the ancient Caribbean at the same time as the global shift in carbon
isotopes. Such a dramatic increase in volcanic activity would have shrouded the
upper atmosphere in a sulphate haze, cooling tropical ocean waters and making them sink
(This Week, 8 November 1997, p 20).
They would still have been warm
enough to give a thermal kick to the cold waters of the ocean deeps.
Whatever the details, one lesson of the story seems to be that natural
systems on the Earth’s surface are able to cause major and sudden changes to the
environment, without the intervention of asteroids, sunspots or any other
outside force. Another is that even a slow process of global warming can trigger
abrupt feedbacks that accelerate the process by unleashing reservoirs of
greenhouse gases such as methane hydrates.
It’s unlikely that the same sort of event could happen again today: modern
seawater is much colder than it was at the P/E boundary, and the ocean-climate
system is much further from the threshold. Even so, scientists believe that the
upheaval that occurred 55 million years ago can help us tackle our current
climate worries by helping to improve the models used to predict the effect of
our own fossil fuel inputs. What’s more, without the P/E “heat wave”, humans
might not be around today. While the warming 55 million years ago gradually
faded as the planet reabsorbed the excess carbon and the deep oceans settled
down, the evolution of the planet’s mammals had already taken a dramatic and
irreversible turn.
“At the same time as the great warming, there was a major evolution and
dispersal of new kinds of mammals,” says Chris Beard, vertebrate palaeontologist
of the Carnegie Museum of Natural History in Pittsburgh. Among those on the
geographical and evolutionary move were all kinds of ungulates—including
the ancestors of horses, zebras, rhinos and cattle—and primates, notably
the predecessors of today’s lemurs and tarsiers.
On the move
“Until that moment we have no record of primates or hoofed ungulates,” says
Jerry Hooker of the Natural History Museum in London. “They suddenly appeared in
northern Asia, Europe and North America. Obviously they came from somewhere. We
guess that they were evolving and dispersing out of tropical Asia, where our
fossil record is still very poor.”
“Climate change must have been behind a lot of what happened,” says Hooker.
Beard argues that soaring temperatures caused vegetation zones to move north,
with herbivores following, and at the same time caused warming across the far
northern regions of Asia, North America and Europe. Many mammals that had
previously been blocked from the far north by the cold temperatures took their
chance and migrated across the bridge where the Bering Straits are today, with
some moving on to Western Europe. At that time, says Beard, the North Atlantic
hadn’t opened up, and a land link remained via Greenland.
For Beard, the climatic convulsions at the P/E boundary heralded “the dawn of
the age of mammals”. And among the new primates evolving in the balmy conditions
were the omomyids, which emerged from Asia and spread into North America and
Europe at the start of the Eocene. They are widely regarded as the ancestors of
simians, who in turn spawned the stock that produced humans (New
Scientist, Science, 28 May 1994, p 18).
An asteroid blasting into the Earth 65 million years ago may have seen off
the then planetary overlords, the dinosaurs. But it seems that it took another
global convulsion 10 million years later, fermented in the depths of the oceans,
to launch the new masters on their way.
