Drowning in questions

By Rebecca Priestley In Science

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A map of undersea New Zealand shows the current land mass is the emergent part of a much larger continent. It extends past Chatham Island to the east, past Campbell Island to the south, northwest to New Caledonia, and in 1995 was named Zealandia.

In 2001, Hamish Campbell and another geologist raised the idea that there might have been a time during which the entire Zealandia continent was underwater. Although no firm evidence of a total submergence has ever been found, as Campbell rightly points out in his new e-book, The Zealandia Drowning Debate, “science consists of ideas that have been disproved and ideas that have yet to be disproved”. The Zealandia drowning hypothesis, therefore, is an idea that has yet to be disproved.

A map of undersea New Zealand shows a much larger land mass that stretches northwest to New Caledonia.

In this short e-book, a clear and engaging presentation of a scientific debate that suits the electronic format, Campbell explains the drowning hypothesis and looks at the “major sticking points from both geological and biological viewpoints”.

Drawing on his own fieldwork in the Chatham Islands – a discussion peppered with “biogenic ooze”, “foraminifera” and “planktonic blooms” – Campbell explains the deposition of sediments and the differences between sediments derived from marine and terrestrial sources. Between 25 and 22 million years ago, geologists say, there is little evidence of terrestrial sediment in any part of Zealandia – it all appears to have a marine origin. What’s more, there is evidence that most of New Zealand’s present land surface – “and probably all of it”, says Campbell – is about 23 million years old. That’s part of the geological argument that the entire continent could have been submerged during the Oligocene period.

Many biologists have a problem with this idea because it would mean that all New Zealand terrestrial plants and animals, including iconic species such as tuatara, moa, kauri and kiwi, have arrived from elsewhere – most likely Australia – and became established and evolved only within the past 23 million years. This clashes with the established view that many of these species evolved from creatures that were on the New Zealand land mass when it split from Gondwanaland 80 million years ago.

What would help solve the debate? “More fossil-lake bonanzas,” says Campbell, like the old lake sites being explored near St Bathans and Middlemarch in Otago. In the meantime, he says, “neither side in the drowning debate can be absolutely sure”. Although Zealandia was substantially underwater about 23 million years ago, there might have been islands that remained above water as a refuge for continuous habitation of plants and animals.

Some of New Zealand’s seaweeds. Photo/Robyn Sivewright.

Drowning or no drowning, New Zealand’s remoteness and isolation has led to the evolution of many endemic organisms – species unique to New Zealand – including seaweeds. New Zealand Seaweeds is a guide to common species found around New Zealand’s main islands, written by Niwa scientist and seaweed expert Wendy Nelson. The book is worth opening for the gorgeous pictures alone – full-colour photographs alongside illustrations by Nancy Adams – but the scientific prose is also a pleasure to read.

Seaweeds, writes Nelson, can take the form of “crusts, tubes, bladders, cushions, blades, feathery filaments”, in texture they can be “cartilaginous, silky, rubbery, crisp, papery or slimy”, and they can produce “mucilages and slimes” to protect the plant from desiccation and to enhance the dispersal of gametes and spores.

This lovely book includes an introductory essay then chapters on green, brown and red seaweeds, covering more than 200 species from microscopic algae to giant kelps.

The Zealandia Drowning Debate: Did New Zealand Sink Beneath the Waves?, by Hamish Campbell (BWB Text, e-book, $4.99). New Zealand Seaweeds: An Illustrated Guide, by Wendy Nelson (Te Papa Press, $79.99).

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