Thursday, 12 October 2017

The zeal in Zealandia: revealing a lost continent

From an outsider's standpoint, geology appears to be a highly conservative science. As I have mentioned on numerous occasions, it seems astonishing that it took over four decades for Alfred Wegener's continental drift hypothesis to be formalised - via the paradigm-shifting discovery of sea floor spreading - into the theory of plate tectonics. I suppose that like evolution by natural selection, the mechanism, once stated, seems blindingly obvious in hindsight.

Regardless, the geological establishment appears to have been stubbornly opposed to the ideas of an outsider (Wegener was a meteorologist) who was unable to provide proof of an exact mechanism. This was despite the fact that the primary alternative, hypothetical submerged (but extremely convenient) land bridges, appear even more far-fetched.

Over the past few decades geophysical data has been accumulating that should generate rewrites of texts from the most basic level upwards. Namely, that the islands making up New Zealand are merely the tip of the iceberg, accounting for just six per cent of a mostly submerged 'lost' continent. Once part of the Southern Hemisphere's Gondwana, in 1995 the newly discovered continent was given the name Zealandia. Approximately five million square kilometres in size, it broke away from the Australasian region of Gondwana around 70-80 million years ago.

After a decade or two of fairly lacklustre reporting, 2017 seems to be the year in which Zealandia is taking-off in the public domain. First, the Geological Society of America published a paper in February. stating that Zealandia should be officially declared as a continent. Then in July the drill ship Joides Resolution began the two month long Expedition 371, a research trip under the International Ocean Discovery Programme (IODP). Scientists from twelve countries undertook deep sea drilling, gaining data on plate tectonics, palaeontology and climate history as well as research directly relevant to understanding the geology of the newest continent.

It is surprising then to learn that geologists first mooted the idea as early as the 1960s but that apart from some marine core samples collected in 1971, no-one undertook the necessary ocean-based research until very recently. Earth resources satellites have helped somewhat, but nothing could replace the evidence that emerged with deep drilling of the seabed. Therefore I wonder what has sparked the sudden interest in an idea that has been around for so long?

One possibility is the large amount of data that the international geological community required to prove the theory beyond doubt, coupled with the fact that this sort of research has little in the way of an obvious immediate practical benefit. It is extremely expensive to undertake deep sea drilling and few vessels are equipped for the purpose. Joides Resolution itself will be forty years old next year, having undergone several years' of refit to keep it going. Those areas of sea bed with potential oil or gas deposits may gain high-fidelity surveying, but compared to fossil fuels, fossil biota and sea bed strata research are very much at the whim of international project funding. In the case of the IODP, governments are cutting budgets on what are deemed non-essential projects, so it remains to be seen whether the intended follow-up trips will occur.

It would be disappointing if there was no further research as despite the acceptance of Zealandia, there is still a great deal of disagreement about what is known as the Oligocene Drowning. I first came across the notion of an eighth continent in the excellent 2007 book In Search of Ancient New Zealand, written by geologist / palaeontologist Hamish Campbell and natural history writer Gerard Hutching. The reason that over ninety per cent of Zealandia is underwater is due to the lack of thickness of its continental land mass - only 20-30km - making it far less buoyant than other continents.

But has this submerged percentage varied during the past eighty million years? There are some very divided opinions about this, with palaeontologists, geneticists and other disciplines taking sides with different camps of geologists. These can be roughly summarised as Moa's Ark versus the Oligocene Drowning, or to be more precise, what percentage, if any, of New Zealand's unique plants and animals are locally-derived Gondwanan survivors and how many have arrived by sea or air within the past twenty or so million years?

The arguments are many and varied, with each side claiming that the other has misinterpreted limited or inaccurate data. If Zealandia has at any time been entirely submerged, then presumably next to none of the current fauna and flora can have remained in situ since the continent broke away from Gondwana. The evidence for and against includes geology, macro- and micro-fossils, and genetic comparisons, but nothing as yet provides enough certainty for a water-tight case in either direction. In Search of Ancient New Zealand examines evidence that all Zealandia was under water around twenty-three million years ago, during the event known as the Oligocene Drowning. However, Hamish Campbell's subsequent 2014 book (co-written with Nick Mortimer) Zealandia: Our continent revealed discusses the finding of land-eroded sediments during this epoch, implying not all the continent was submerged.

It's easy to see why experts might be reticent to alter their initial stance, since in addition to the conservative nature of geology there are other non-science factors such as patriotism at stake. New Zealand's unusual biota is a key element of its national identity, so for New Zealand scientists it's pretty much a case of damage it at your own peril! In 2003 I visited the predator-free Karori Wildlife Reserve in Wellington. Six years later it was rebranded as Zealandia, deliberately referencing the eighth continent and with more than a hint of support for Moa's Ark, i.e. an unbroken chain of home-grown oddities such as the reptile tuatara and insect weta. With the nation's reliance on tourism and the use of the '100% Pure New Zealand' slogan, a lot rests on the idea of unique and long-isolated wildlife. If the flightless kakapo parrot for example turns out not to be very Kiwi after all, then who knows how the country's reputation might suffer.

What isn't well known, even within New Zealand, is that some of the best known animals and plants are very recent arrivals. In addition to the numerous species deliberately or accidentally introduced by settlers in the past two hundred years, birds such as the silvereye / waxeye (Zosterops lateralis) and Welcome swallow (Hirundo neoxena) are self-introduced, as is the monarch butterfly.

The volcanic island of Rangitoto in Auckland's Hauraki Gulf is only about six centuries old and yet - without any human intervention - has gained the largest pohutukawa forest in the world, presumably all thanks to seeds spread on the wind and by birds. Therefore it cannot be confirmed with any certainty just how long the ancestors of the current flora and fauna have survived in the locality. A number of New Zealand scientists are probably worried that some of the nation's best-loved species may have arrived relatively recently from across the Tasman; a fossil discovered in 2013 suggests that the flightless kiwi is a fairly close cousin of the Australian emu and so is descended from a bird that flew to New Zealand before settling into an ecological niche that didn't require flight.

Other paleontological evidence supports the Moa's Ark hypothesis: since 2001 work on a lake bed at St Bathans, Central Otago has produced a wide range of 16 million year-old fossils, including three bones from a mouse-sized land mammal. The diversity of the assemblage indicates that unless there was some uniquely rapid colonisation and subsequent speciation, there must have been above-water regions throughout the Oligocene. In addition, whereas the pro-underwater faction have concentrated on vertebrates, research into smaller critters such as giant land snails (which are unable to survive in salt water conditions) supports the opposite proposition.

So all in all, there is as yet no definitive proof one way or the other. What's interesting about this particular set of hypotheses is the way in which an array of disciplines are coming together to provide a more accurate picture of New Zealand's past. By working together, they also seem to be reducing the inertia that has led geology to overlook new ideas for far too long; Zealandia, your time has come!