Geo News

New Zealand awash

Natural historians in New Zealand have long been preoccupied with the antiquity and origins of our native land-dwelling plants and animals, especially our more distinctive icons, such as moa, kiwi and tuatara. New biochemical technolo­gies are providing fresh insights into the genetic lineage of our flora and fauna, and hence their ancestry. But this informa­tion doesn't necessarily tell us anything about where the ancestral forms lived, or where the genetic changes took place. It is assumed that it was in New Zealand, but was it? This is a good question, for several reasons. First, the fossil evidence of our national iconic animals and birds is very restricted in terms of time: there are no known moa, kiwi or tuatara fossils older than two million years. Second, in terms of geological history, there may not have been any land at all within the New Zealand region about 25 million years ago. If the New Zealand landmass has emerged from the sea within the last 30 million years, then all of our native plants and animals have arrived and evolved since then. Such a scenario is almost unthinkable to contemplate. Conventional wisdom tells us that our native flora and fauna are descended from the "living cargo" that originally came with the New Zealand continent, Zealandia, when it split from Gondwana. But some geologists are now question­ing that view. What is not in question is that New Zealand was originally part of the Gondwana supercontinent, and that it rifted away about 85 million years ago. It did so as a result of major thermal effects within the Earth's mantle that broke the crust asunder. Much of the granite in the South Island and Stewart Island relates to this thermal event. The chunk of crust that rifted away did not have the present-day configuration of the New Zealand landmass. It was much larger—about half the size of Australia—and is referred to as the "New Zealand continent" or Zealandia. It is amusing to recall that the early European explorers in our part of the world came here to search (in vain) for the great southern continent they called Terra Australis. If only they had known that it exists as Zealandia—largely sub­merged! An approximation of its size and shape, can be seen in bathymetric maps of the greater New Zealand region. What is impressive is not just the extent of Zealandia, but the fact that most of it now lies 1000 metres below sea level. Geological exploration of today's submarine part of Zealandia tells us that it was indeed dry land when it rifted away from Gondwana. The evidence is unequivocal: well beneath the sea floor there are sedimentary sequences with extensive coal deposits that could only have formed on land in low-lying forest and swamp environments between 70 and 50 million years ago. So what happened to make it sink? First, the thermally buoyant Zealandia cooled down. Sinking associated with cooling is perhaps more familiar with air masses in the atmosphere, but it is well authenticated by geologists in rocks as well. Second, Zealandia was either unusually thin to begin with or has become so. It is only about 25 km thick, not thick enough to be emergent as land. The thicker a block of crust, the higher it rides. Crust that underlies most of the oceans is only about 7 km thick, and the really high places on Earth are sup­ported by 70-80 km of crust, such as beneath the Altiplano, in South America, and the Tibetan Plateau, in Asia. Within the context of Zealandia, it is the present New Zealand landmass that is anomalous. It could be said that the only reason it exists above water at all is because of the vigorous nature of plate collision between the Pacific Plate and Australian Plate that commenced less than 30 million years ago. This collision has literally pushed New Zealand up, creating all our mountain ranges in the process, and the forces responsible are still in action; still holding New Zealand's head above water. Terrestrial geological evidence also supports a largely submerged Zealandia, in the form of the wide spread occurrence of Oligocene limestone. Limestone is a white rock consisting of the skeletal remains of marine organisms ranging from plankton and algae to whales. It is largely devoid of sediments eroded from the land.Oligocene limestone, deposited between 34 and 24 million years ago, is best preserved in those parts of New Zealand that are well removed from the active plate boundary collision zone, such as in Northland and the Chatham Islands, but is best known in the King Country near Te Kuiti (Waitomo Caves), and inland north Otago and Canterbury. Pockets of it are preserved deep within the Southern Alps, near Queenstown, and in the nether regions of the Rakaia, Rangitata and Waimakariri River valleys of Canterbury, such as in the Castle Hill Basin. The distribution of this limestone is fascinating. Imagine a sheet of white rock mantling the whole of New Zealand, and which has now been eroded away so that only fragments remain. Extend the sheet again and imagine Zealandia totally submerged. A biological wipe-out. Just a few days of submergence, let alone hundreds of years (or thousands, or a million), would he sufficient to wipe the New Zealand slate clean and destroy all terrestrial life. With subsequent re­emergence of land, life would become re-estab­lished, replenished primarily from the nearest land, notably Australia. By virtue of distance, birds would be among the first animals to arrive. Plants, insects, lizards, amphibians and mammals would have to take their chances. The idea of a clean slate in the Oligocene appeals to many geologists and paleontologists in that it may explain why there isn't a much greater diversity of animals in New Zealand today. When Zealandia split from Gondwana, dinosaurs turtles and flying reptiles were among the cargo, and surely the full spectrum of other Cretaceous land animals was present as well—mammals, lizards, snakes, amphibians—although we have as yet no fossil evidence of these. We know what put paid to the dinosaurs (effects of a major meteorite impact), but we need another mechanism to rid Zealandia, and hence New Zealand, of all other animals. Total submergence of the land would do the trick. New Caledonia can be thought of as a distal northern extension of Zealandia. Could it have been totally submerged in the Oligocene, too? The only known fossil-bearing sedimentary rock of Oligocene age in New Caledonia is, indeed, limestone. Like New Zealand, New Caledonia has a legacy of continental Gondwanan origins com­plete with Late-Cretaceous coal measures. Furthermore, the only native mammals in New Caledonia are bats, and it, too, has flightless birds and no snakes. The similari­ties are striking! Most geologists believe that the New Zealand land area in the Late Oligocene was greatly reduced, and some have suggested that it was only 18 per cent of the present area—a reduction that would have led to an evolutionary crisis in the terrestrial biota. Yet it remains possible that there may have been a period when there was no emergent land at all. The implications are considerable, so far as the origin of our New Zealand terrestrial species and ecosystems are con­cerned, and therefore the possibility warrants further investigation.

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