KEY POINTS:
New Zealand is the emerging tip of Zealandia, a drowned continent which completely submerged to a depth of 1-3km about 23 million years ago. It was an inexorable sinking - an inching descent over aeons to a submarine state, bringing creeping extinction to all land life.
That's the startling new finding of In Search of Ancient New Zealand - a largely geological story interrogated from the memory banks of rocks. Our landscape, once thought to be shaped by epochs of weathering, has in fact been cut by the sea.
The theory adds a new chapter to the story of our birth some 83 million years ago, when Zealandia was rent asunder from the eastern margin of the super-continent Gondwanaland. It's a chapter disrupting the previous "Moa's Ark" theory that Zealandia drifted away from its mothership carrying a cargo of flora and fauna that continued to evolve in isolation.
In Search of Ancient New Zealand documents a drowning. "As it rifted away, Zealandia was stretched and thinned. The inevitable consequent loss of buoyancy resulted in slow sinking. Zealandia was to behave like a slowly sinking ship for 60 million years."
The book's lead author, Dr Hamish Campbell, arrives at this view not because evidence for sinking is strong but rather because evidence for the presence of continuous land, and hence continuous terrestrial life, isn't there. "The scientific evidence is so weak we are obliged to consider the possibility of complete drowning," says Campbell, explaining thesis by scientific stalemate. Total submersion can't be proved, but neither can continuous land.
The book gets the jump on a scientific paper penned by several geologists and biologists including Campbell, due to be published in Geological Magazine in December. What rescued New Zealand from beneath the ocean was vigorous pushing activity at the boundary of the Pacific and Australian tectonic plates. The colliding plates forced New Zealand up from the depths, not unlike the demigod Maui of Maori myth who pulled up a huge fish from the sea that became the North Island of Aotearoa.
"We have entertained the possibility New Zealand arose from a state of total immersion, total inundation by the sea," says the book. "An obvious implication of this is that all New Zealand's native plants and animals have somehow arrived and evolved within the past 23 million years."
Which may at last answer such questions as: Why, except for a few bat species and perhaps a rodent, do we have no native land mammals? Why, given that we were once part of the same land mass, do we have no marsupials like Australia? And why, given some 83 million years of evolution, is there not more biodiversity here?
Campbell says the idea won't seem radical to many New Zealand geologists because it's one many have thought about. The so-called "Oligocene drowning" of about 25 million years ago when New Zealand's land mass was thought to be at its smallest is well documented. Campbell and others began thinking about total submersion following a multi-disciplinary research project in the Chatham Islands begun in 2004 and finished this year. The key geological finding was that the islands must have been under the sea until about 2 million years ago. The biological work also confirmed the island's plants and animals have been separated from their mainland ( New Zealand) relatives for not more than 2-3 million years.
Armed with new understanding of how life arrives and evolves on a new land, the scientists turned their attention to New Zealand. A significant piece in the sinking puzzle was found by University of Otago associate professor Chuck Landis who turned the traditional view on its head. In Search of ... explains: "This will come as a shock to many New Zealanders who have been taught that extensive flat surfaces in New Zealand's landscape are a product of the slow wearing down of land by erosion to form a peneplain ... For generations we have all been tricked ... The conspicuous planar surfaces in our landscape are all marine-cut."
Campbell points to a photograph in the book of the Hawkdun Range in central Otago. "I just love the majesty - here we've got mountains that have been pushed up and, lo and behold, there's the remnant of an erosion surface that could only be cut by the sea on top." Although most of us wouldn't know it, Campbell says there are "zillions" of old sea floors in our landscape that we're walking around on.
Further geological evidence for our underwater existence comes from the lack of sediment. "If there is no land there is no sediment," says Campbell noting that land sediment - debris, gravel, sand, silt and mud - can only be generated from pre-existing rock by erosion. Its absence in our rock layers suggests an absence of land.
What is present around New Zealand and dating to 23 million years ago is limestone - pure biogenic sediments comprising the skeletal remains of marine organisms.
"Limestones represent distance from land and absence of erosion of rock and basically open water. The geological record speaks of the sinking of Zealandia since the moment we broke away from Gondwanaland."
The fossil record also backs up the drowning theory with little evidence to indicate antiquity, but plenty to show life, including plants, insects, some bats and even fresh water crocodiles that were here 16-19 million years ago. There's also one very interesting fossil of a mouse-sized primitive mammal that traces back to Gondwanan lineage. If the theory gains credence, biologists will have plenty of work ahead figuring out how terrestrial life arrived after the land emerged from the sea. And if it was flown, blown or rafted here, where did it come from?
There are, however, a few glitches in the submergent picture. How, for example, to explain the kauri, the moa, and the tuatara which did have ancestors in pre-drowning times? Campbell reverts to the stalemate argument pointing out the the fossil record for all of the above is weak, and there may be other explanations for their return post-drowning. But he also doesn't exclude the possibility of small islands - perhaps in South Otago - where coal fields may predate the sinking. Mostly however, the geological evidence for islands isn't there either.
The net result is that Campbell and his colleagues will be redrawing geologist and palaeontologist Charles' Fleming's 1960s maps - the first attempt to depict the changing shape of the New Zealand land mass over the past 58 million years. The new maps will show no land in the North Island and next to none in the South Island during the Oligocene and early Miocene period of 23-24 million years ago.
Campbell's enthusiasm for geology and palaeontology is irrepressible, both in print and when he talks.
Asked which photos in the book he particularly likes Campbell picks a superbly detailed fossil grayling - a New Zealand native freshwater fish that became extinct in the 1920s. "That's absolutely luscious. I love the artistic quality. We palaeontologists are constantly on the look out for shapes in rock and obviously the more shapely the more interesting."
Campbell, who works at the Institute of Geological and Nuclear Sciences in Wellington and at Te Papa, got into geology because he wanted to find out what his father, John Campbell, a geologist to whom the book is dedicated, did.
"My father was surprisingly non-communicative and he didn't drive, so we didn't go on field trips. By the time I got to the seventh form at Otago Boys' in Dunedin I still didn't have a clue what Dad did. We used to ask and he would say: 'I'm a university teacher' - it tells you nothing."
Campbell took geology at Otago University to find out what his father got up to. From his first lecture - ironically on submarine geology - he fell in love with the topic and went on to complete a doctorate at Cambridge.
Campbell says the book, written in conjunction with former Forest and Bird magazine editor Gerard Hutching, is aimed at the educated general public who want to know more about the sunken eighth continent of Zealandia. But it's a knowledge that may make some brains hurt - as they try to decipher how buoyant continental crust floats like cream on milk and moves (and sinks) as a function of flow within the denser oceanic crust.
Campbell thinks the public will also enjoy the history of New Zealand's geological community - "images of ordinary people who have made a difference by coming up with an interesting idea or perspective." He hopes too the book will help in the two questions he's commonly asked. "How old are things in our landscape and how do we know that they are that old?"
As In Search of ... shows, there are several answers. On the one hand New Zealand is a relatively young 23 million years fresh out of the sea. But that doesn't mean we don't have old rocks - our oldest dating from 505 million years ago. Dating them is done either by deducing a relative age from fossils, or by measuring the radioactive decay of rock elements such as uranium or potassium. But that's just basic rock reading. If you know how to wield a mass spectrometer you can interrogate the rock for other mysteries and find in our ubiquitous greywacke, for example, zircons of 3 billion years ago. New Zealand is indeed as old as the hills.
"There's a huge adventure-discovery side to geology and that's what most geologist really do enjoy about it," says Campbell.
"You never know what you're going to find and every time you go out you've almost certainly gained new knowledge."
