Rocks of all ages tell story

A few hundred metres off the coast of Northland stand tiny islets that may tell the story of the biggest catastrophe in the history of life on Earth.

Arrow Rocks, just off Tauranga Bay about 40km north of the Bay of Islands, are so small that you can clamber round the main rock in about 10 minutes.

The other rock is even smaller, but more treacherous, rising from the waves like a sharp arrow head - or, as the Maori saw it, a bird's beak. They named the islet Oruatemanu, "two birds" or perhaps "the bird's home".

You can see at once why these rocks have drawn geologists from several Japanese universities this week for their eighth field trip in as many years. In the eroded cliff faces and caves, layer upon layer of multicoloured rocks have been twisted into rollercoaster patterns by years of folding and deformation.

These are not just any old rocks. This is one of a handful of places on the globe where you can see rocks that were laid down just before, during and immediately after something awful that happened 251 million years ago.

Almost instantaneously, geologically speaking, 90 per cent of the living species that existed at that time were wiped out - a far worse disaster even than the meteorite that hit Mexico's Yucatan Peninsula 65 million years ago and destroyed about half the species of that time, including the dinosaurs.

At the end of what geologists call the Permian era, named after the Russian city of Perm where 250 million-year-old rocks were first found, the dinosaurs had not yet evolved.

Only about 120 million years before, the first amphibians had ventured out of the protection of the sea on to a raw land.

By Permian times a whole zoo of land animals had evolved - creatures called synapsids, or mammal-like reptiles including huge plant-eaters the size of rhinos, and sabre-toothed meat-eaters that jumped on the backs of the plant-eaters and ripped their skins with their teeth.

The land was covered in mosses and ferns, with a few early trees around the margins of lakes and rivers. Spiders, beetles and a wide variety of insects had evolved in the undergrowth. The sea teemed with tiny plankton, snails, seafloor plants and fish.

Then suddenly, 251 million years ago, the fossil record preserved at places such as Arrow Rocks shows that most of these life forms disappeared. Plants died and were replaced by fungus. Of 74 species of amphibians and reptiles, only two survived.

Auckland University geologist Bernhard Sporli, who is working with the Japanese on Arrow Rocks, says it was a terrible time.

"You had general wildfires, dust that went into the atmosphere. The effects were not measured in months but in years. It could have been dark for a whole year."

With no sunlight, photosynthesis stopped and plants died. Eventually the animals that fed on them followed.

But the causes are a mystery.

"We just don't know," says Dr Chris Hollis of the Institute of Geological and Nuclear Sciences, who was at Tauranga Bay this week.

Arrow Rocks may help to find the answer because they are a rare place that was deep under the sea 251 million years ago, and has been lifted up through massive folding to reveal the clear layers marking the remains of soil, plants and animals that were washed down rivers into the sea.

The Japanese-New Zealand team has not yet found the "smoking gun" - the precise layer where life died, which may be a layer of charcoal.

But they have found a sharp change in fossil plankton between the layers above and below the dead layer, confirming that life changed dramatically in New Zealand as well as in Russia and other parts of the world.

Scientific detectives have fingered two prime suspects for the cause of the crisis, and they may both be guilty.

First, a meteorite. It is only 13 years since a young Canadian graduate student found the 195km-wide crater on the Yucatan Peninsula left by a meteorite 65 million years ago, but that impact is now widely accepted as the blow that finally killed the dinosaurs. It has been natural for scientists to look for a similar cause of the much bigger end-Permian event.

A team led by Assistant Professor Luann Becker at the University of California has found a series of clues.

In 2001, she found helium and argon gases trapped inside complex carbon molecules in sediments laid down at the end of the Permian in China and Japan. Analysis showed that the gases were chemically the same as helium and argon from meteorites.

Last year she reported finding meteorite fragments in another end-Permian outcrop in Antarctica. In July this year she said she had found the meteorite crater itself, with telltale glassy rocks, in an underwater structure called the Bedout High, originally identified by oil drillers off the northwest coast of Australia.

But each of her findings has been roundly attacked by other scientists. American, British and Australian researchers argued, for example, that the Bedout High is probably a volcanic structure rather than a meteorite crater. In response, Becker acknowledged that her evidence was not "definitive", but merely raised a hypothesis for further investigation.

Te Papa geologist Hamish Campbell, also at Arrow Rocks this week, suggests that the damage may have been caused by a comet, a lump of carbon dioxide, ice and dust that could cause a lot of damage without leaving any chemical "signature" in the rocks.

But British geologist Mike Benton, who published a book last year on the Permian extinction (When Life Nearly Died, (Thames & Hudson, London, available in Auckland Public Library), believes there is no need to postulate any extraterrestrial object, because the extinction can be explained by much more certain events originating in the Earth itself.

His chief suspect is an enormous volcanic event in Siberia which lasted for about 600,000 years at the right time, 251 million years ago.

This was not the kind of isolated volcano we know today, where molten lava bursts out of the Earth's mantle and typically builds up a mountain around it such as Ruapehu.

Instead, the Siberian event was a "flood" eruption, where basalt lava poured out of long cracks in the ground. It spread over 3.9 million sq km, about one-and-a-third times the size of Australia.

Benton believes that the carbon dioxide and other "greenhouse gases" released by the eruption eventually warmed the Earth enough to melt the vast stores of frozen methane on the sea floor around the continents. This created a "runaway greenhouse" effect that raised world temperatures by 6C and killed off many species that could not cope with either the heat, the higher levels of carbon dioxide or "acid rain".

This is still only a theory. Benton notes that there have been bigger flood eruptions at other times that did not kill 90 per cent of all species.

"Maybe it was simply a coincidence of factors," he writes.

"The end-Permian was the only time when the continents were fully assembled into a single supercontinent [see accompanying story] and when there was a large-scale flood basalt eruption episode.

"During the later large-scale basalt eruptions, the continents had drifted apart and maybe life had diversified sufficiently in the different continents and oceans to be able to resist a range of severe climatic changes ...

"One thing is clear, however. The biggest mass extinction of all time did happen 251 million years ago, and even if we cannot yet fully explain why, it is important to look at the consequences of cutting life down to 10 per cent or less of its normal diversity. There are lessons to be learnt."