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http://www.bbc.co.uk/science/horizon/2002/dayearthdied.shtml
The Day The Earth Nearly Died – programme summary
250 million years ago, long before dinosaurs roamed the Earth, the land =
and oceans teemed with life. This was the Permian, a golden era of biodiver=
sity that was about to come to a crashing end. Within just a few thousand y=
ears, 95% of the lifeforms on the planet would be wiped out, in the biggest=
mass extinction Earth has ever known. What natural disaster could kill on =
such a massive scale? It is only in recent years that evidence has begun to=
emerge from rocks in Antarctica, Siberia and Greenland.=20
uot;
=
The demise of the dinosaurs, 65 million years ago (at the so-called K/T =
boundary), was as nothing compared to the Permian mass extinction. The K/T =
event killed off 60% of life on Earth; the Permian event 95%. Geological da=
ta to explain the destruction have been hard to find, simply because the ro=
cks are so old and therefore subject to all kinds of erosion processes. It =
seems plausible that some kind of catastrophic environmental change must ha=
ve made life untenable across vast swathes of the planet.=20
een"
The world's biggest volcanoes
In the early 1990s, the hunt for evidence headed for a region of Siberia=
known as the Traps. Today it's a sub-Arctic wilderness but 250 million=
years ago, over 200,000km=B2 of it was a blazing torrent of lava. The Sibe=
rian Traps were experiencing a 'flood basalt eruption', the biggest=
volcanic effect on Earth. Instead of isolated volcanoes spewing out lava, =
the crust split and curtains of lava were released. And the Siberian flood =
eruption lasted for millions of years. Could volcanic activity over such a =
long time alter the climate enough to kill off 95% of life on Earth?=20
Vincent Courtillon used a much smaller flood basalt eruption, in Iceland=
in 1783, as the basis for some calculations. Writing in the 18th century, =
Benjamin Franklin (then American Ambassador in Paris) described 1784 as a y=
ear without a summer. Ash from the eruption blacked out the sky and crops f=
ailed across Europe. Courtillon extrapolated the climatic impact of the Sib=
erian Trap eruption from the records of the Icelandic event. He deduced tha=
t a 'nuclear winter' lasting decades would be followed by rapid glo=
bal warming due to the increased level of greenhouse gases in the post-erup=
tion atmosphere.=20
e same place"
Vincent believes the disruption of cooling followed by warming could cau=
se the Permian extinction but other geologists disagree. Peter Ward returne=
d to the Siberian Trap data to estimate the amount of carbon dioxide – and =
global warming – that could result. His worst case scenario is a temperatur=
e rise of 5=B0C, enough to kill off many species but not the 95% wipeout th=
at ended the Permian.=20
If the Siberian eruptions were not deadly enough, what other effects mig=
ht be at work? To try to answer that, Michael Rampino set out to establish =
an even more fundamental piece of data: how long did the extinction take? H=
e studied rock sedimentation rates in the Alps and concluded that the Permi=
an killer had stalked the planet for just 8,000-10,000 years, far less than=
had been thought. His mind turned to ways of causing such catastrophic des=
truction in – on geological timescales – the blink of an eye. He wanted to =
explore the possibility of a meteorite strike.=20
The hunt for meteor evidence
Meteor strikes that wipe out life may sound like sci-fi but it's gen=
erally accepted that an impact sparked the K/T extinction and the end of th=
e dinosaurs. That meteorite was 10km wide and left a crater in what is now =
the Gulf of Mexico. The dust raised by such an impact could make global tem=
peratures plummet overnight. How big would any Permian meteorite have to be=
? Rampino suggests one just 50% bigger could cause sufficient environmental=
change. There is one huge flaw in this argument: where is the crater?=20
Adrian Jones models the effects of impact on the Earth's geological =
crust. He has a hunch that meteorite crater hunters are looking for the wro=
ng thing. After an impact, the crust rebounds to form a large shallow crate=
r. If the meteorite if truly massive though, an extra process occurs. The c=
ombined heat of the impact and rebound is enough to melt the crust. Lava fl=
oods through and the crater disappears beneath new crust. If he's right=
, the Permian meteorite crater can't be found because it doesn't ex=
ist.=20
n its wake"
All of which serves to help proponents of the meteorite impact theory. I=
ts detractors, though, point out that meteors leave several trails in their=
wake – fragments of minerals that have come from space. Greg Rettaleck mou=
nted an expedition in the mid-1990s looking at Permian rock beds in the Ant=
arctic. Some of the quartz grains looked like they had been fractured by a =
very energetic process – a meteorite?=20
Although this was evidence for a strike of some sort, there were unanswe=
red questions as well. The K/T meteorite left a trail of iridium – characte=
ristic of space materials – around the world. Yet there is no evidence the =
Permian strike did the same.=20
to end"
Paul Wignall is a British geologist who doubts a meteorite caused the ma=
ss extinction 250 million years ago. In the late 1990s he had a hunch of a =
way to prove his beliefs, a good idea of where to look for new evidence: Gr=
eenland. Permian rocks are hard to find because they are usually just thin =
layers, yet his trip yielded rock beds metres thick. This was more than jus=
t new evidence; it was the best he could have hoped to find.=20
Carbon copious
The Greenland rock told a very different story to that Michael Rampino h=
ad found in the Alps. Instead of a rapid event of under 10,000 years, the e=
xtinction beds Wignall examined lasted 80,000 years and showed three distin=
ctive phases in the plant and animal fossils they contained. The extinction=
appeared to kill land and marine life selectively at different times. Such=
a long process contradicted the catastrophic meteorite theory but Wignall =
couldn't explain what=20
had come close to killing all life on Earth. His best clue was the c=
arbon isotope balance in the rock, which showed an increase in carbon-12 ov=
er time. The standard explanation – rotting vegetation – could not have cau=
sed such a marked effect. Wignall was curious what this could mean.=20
An answer came from geologist Gerry Dickens, who knew just how to get la=
rge amounts of carbon-12 rapidly, thanks to his work with offshore drilling=
companies in the USA. He had spent time helping them try to tap reserves o=
f frozen methane hydrate from the seabed of the Gulf of Mexico. He knew met=
hane hydrate is found around many of the world's coasts. Dickens wonder=
ed how large a rise in sea temperature was necessary to cause the solid che=
mical to gasify and ascend to the atmosphere. Experiments suggested a rise =
of 5=B0C would be sufficient. And he was amazed to see how much gas came fr=
om pieces of solid methane hydrate that were placed in water.=20
When Paul Wignall learned of Dickens' findings, he used his carbon-1=
2 data to estimate how much methane hydrate would have to be released to af=
fect the isotope balance. Methane is one of the most potent greenhouse gase=
s and he deduced that unlocking frozen methane hydrate would have caused a =
temperature rise of 4-5=B0C over time. Not enough to kill off 95% of life o=
n Earth but he realised this was a compounded effect. A rise of about 5=B0C=
must already have occurred to prompt the frozen methane to melt. The combi=
ned temperature rise of 10=B0C is generally accepted as a figure able to ca=
use truly mass extinction.=20
So it seems likely there were two Permian killers. The Siberian T=
raps did erupt, contributing first to a nuclear winter cooling effect (caus=
ed by dust) and and then to global warming (due to greenhouse gases). Over =
40,000 years, some land animals gradually died out while life in the seas l=
ived relatively calmly on, as the water temperature gently rose. Then the s=
eas gave up their frozen methane. In just 5,000 years, there was massive lo=
ss of species from the world's oceans. In a third and final phase of th=
e extinction, the Permian killer returned to stalk the land for another 35,=
000 years. By the end of that process, 95% of the Earth's species were =
extinct.=20