Geo News PT2 51: the ultimate mass murder

Written by Hamish Campbell

Hamish Campbell

Oruatemanu, also known as Arrow Rocks, at the eastern end of Whangaroa Bay, in Northland, has recently become a focus of international scientific attention. This unprepos­sessing, uninhabited islet, surrounded by a broad rock platform and surmounted by two vegetated crests that resemble inclined arrow heads, may be of passing interest to boaties, but to geologists it is pure gold. For the rocks on Oruatemanu preserve a record of a pivotal moment in Earth’s history known as the Permian—Triassic boundary.

Something profoundly disturbing and destructive afflicted the Earth 251 million years ago, bringing the Permian period to a sudden close. The exact nature and cause of this event remains a mystery, but the result was almost total annihilation of life on Earth. It was the ultimate mass murder, and has been described as the greatest mass extinction event of all time, with more than 90 per cent of all life forms de­stroyed. But what sort of event could he so cataclysmic yet leave no trace? This is the question that continues to puzzle geologists.

It could be said that geology came of age as a science in the mid-1800s when it delivered the concept of extinction. The notion that life forms were created only to later be extinguished shook the foundations of Western understanding of the natural world, an understanding that was firmly rooted in biblical revelation. It is ironic to think that the first university professorship in geology was raised in 1818 for the Reverend William Buckland at Oxford University, that bastion of the Church of England, expressly to investigate and document evidence of Noah’s great flood.

Much water has flowed under the bridge since then, and extinction is now accepted as a regular occurrence. We hear about species threatened by extinction almost daily, and less commonly comes news that a once flourishing species of animal or plant can no longer be found, and is therefore deemed extinct.

Extinction is the ultimate fate of all species. Paleontologists have suggested that the average duration of a species is about four million years. Some endure for much longer, others depart after a rela­tively short appearance on life’s stage. But something very dramatic has to happen to annihilate large numbers of species simultaneously: something catastrophic and of global proportions.

The geological record tells us that there have been a number of mass extinctions through time. Indeed, the geological time scale is based on a series of such extinctions, with individual divisions named on the basis of apparently sudden disappearances and appear­ances of certain fossils. Only now, however, is the scientific world really coming to grips with the causes of each extinction event. It has become a matter of some urgency as the magnitude of human impact on the global environment hits home.

Perhaps the best known and understood mass extinction is the Cretaceous-Tertiary event 65 million years ago (CT65) that wiped out the dinosaurs, but this event eliminated only about 50 per cent of all life forms. The Permian-Triassic event was much worse.

It seems odd to think of time having boundaries. We don’t refer to the BC-AD boundary, the June-July boundary or even the Friday Saturday boundary. The use of “boundary” in the context of geological time is largely historical, in that the term originally related to recognisable formations of layered sedimentary rock which were characterised by distinctive fossils. One formation lay on top of another, and each had a physical boundary. Only later was it realised that the rocks proxied as a record of time itself, and the bounda­ries between formations provided more definite markers than some ill-defined point within a formation.

The Permian was named in 1841 by Sir Roderick Murchison (for whom Murchison, in the South Island, is named) after the Perm Basin, just west of the Urals in Russia. The Triassic was named by Friedrich August von Alberti in 1834 to encompass three distinc­tive rock formations exposed in Germany (Bunter, Muschelkalk and Keuper). Oruatemanu’s signifi­cance lies in a distinctive sequence of thinly layered coloured sedimentary rocks found on the island. The original sediments that gave rise to these layers accumu­lated very slowly in deep oceanic waters between 260 and 230 million years ago. In other words, they are of Permian and Triassic age and they span the Permian—Triassic boundary.

The rocks are rich in microscopic fossil plankton. In fact, the original deep-sea sediments that make up these rocks would have accumulated as oozes composed almost exclusively of the tiny, exquisitely sculptured and ornamented siliceous skeletons of planktonic animals called radiolarians.

Amidst these deposits (referred to as cherts) are smaller amounts of clay-rich rocks, or argillites, and volcanic ash beds, or tuffs. Resting on basalt lava flows that erupted on the sea floor during Permian time, this remarkable pile of sediments is less than 150 metres thick, yet represents a 30-million­year record of oceanic history. Ructions in the Earth have since tipped the pile on its side, so geologists now have the convenience of walking across time rather than having to climb it.

Exposed sequences of marine sedimentary rock that span the Permian-Triassic boundary are extremely rare, especially in the Southern Hemisphere. This occurrence in Northland is exceptional in several respects and may reveal important clues as to what happened.

To this end, radiolarian experts are systematically exploring the memory banks of Earth history locked away on Oruatemanu. It is painstaking work, requiring centimetre-by-centimetre sampling, and it involves researchers from eight Japanese universities, the University of Auckland and the Institute of Geological and Nuclear Sciences. The project is led by Yoshiaki Aita, of Utsunomiya University, and is supported by Japanese research money.

There are other se­quences in the South Island that span the Permian-Triassic boundary but Oruatemanu is exceptional because of the deep marine nature of the sediments (laid down in ocean depths greater than 3 km) and because of the wealth of fossils in the rocks. Depth is significant because in shallow water a cataclysmic disturbance would have disrupted the geological record. In contrast, the deep sea is the most stable environment on Earth.

The study will reveal how the most basic of life’s processes, ocean productiv­ity, changed across the PT boundary, and will also divulge any unusual minerals or alterations to radioactive materials that might be associated with an asteroid impact or similar phenom­enon.

The PT puzzle may soon be solved.