Rock key to mystery of prehistory

Scientists are developing a 25-million year snapshot, reports TONY GEE.

A 25 million-year snapshot of the Earth's history is gradually giving up its secrets to an international team of scientists in the Far North.

The long-term study might also shed light on the cataclysmic events that caused mass extinctions several hundred million years ago.

The study centres on a small rocky outcrop known as The Arrows, about 500m offshore from Marble Bay, south of Whangaroa, on Northland's east coast.

New Zealand and Japanese scientists consider the Arrows to be of extreme significance because rock on the islet represents around 25 million years of geological history captured in ancient, deep marine sedimentary deposits dating back 270 million years.

The deposits in The Arrows formation span the Triassic and Permian periods and the site is one of only three places in the world where this is known to occur. The others are in Thailand and Turkey.

The Arrows - known to Maori as Oruatemanu - and some rock formations a few hundred metres away on shore at Marble Bay are the oldest known rocks in the North Island.

They are several hundred metres wide, about 100m long, rise vertically and have some vegetation growing on them.

Around 100m of rock are available for geological study.

The Arrows were recorded on maps early last century, but their significance was not realised until a visiting Japanese exchange-programme student wrote a report on the islet in 1986.

Japanese interest in the report eventually led to active geological study and a three-year project which now involves the Institute of Geological and Nuclear Sciences in Lower Hutt, Auckland University and seven Japanese universities.

A palaeontologist and Te Papa geologist, Dr Hamish Campbell, a member of the institute, leads the New Zealand component of the project, which is now halfway through.

The Japanese are especially interested in the deposits of fossilised plankton (known collectively as radiolarians) layered within ancient marine sediments on The Arrows.

Samples are collected and taken to Japan for laboratory analysis at the seven universities involved in the project.

The parent rock is dissolved in acid, allowing the fossilised plankton to be identified under a scanning microscope.

"It's extraordinary," said Dr Campbell.

"There are tiny, exquisite skeletons, some only one-tenth of a millimetre in size."

Fish teeth have also been identified.

"All this work is being paid for by the Japanese taxpayer and in the process, we're learning a lot about New Zealand."

Dr Campbell said the sediments containing the fossilised plankton would have accumulated in water more than 2km deep, between 245 and 270 million years ago.

"That 25 million-year timespan represents a geological record of deep marine deposition," he said.

"There is entire rock produced by plankton."

Although such ancient deposits occurred elsewhere, it was rare for geologists to see examples so well exposed as they were on The Arrows.

"We're breaking new ground.

"This is new science. We're learning about the evolution of plankton from these rocks."

The period represented in the sequences of deep marine sedimentary rock on The Arrows is also significant because it spans a time in the Earth's history when mass extinction of life occurred because of an unknown catastrophic event.

A popular theory blames the impact of a comet, but this is hard to prove because ice makes up a large part of comets and leaves no trace.

Scientific analysis of the Arrows may advance a theory about what changed the Earth so dramatically several hundred million years ago.

* The oldest known rocks in New Zealand, dating back around 510 million years, are in the Cobb Valley area of northwest Nelson.