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Plant Opals Release Secrets

Tiny pieces of opal that develop naturally in living plants are offering scientists a new tool to investigate New Zealand's past. "Opal phytoliths" are grains of opal that form mainly in the stems and leaves of grasses, trees and other plant life, and accumulate in the upper layers of soil when plants decay. Different types of plants tend to have different types of phytoliths, so analysis of phytoliths found in soils and sediments in particular areas can give important clues to their history, explains Victoria University chemist Dr Cyril Childs.

Phytoliths are typically about 20 microns (two thousandths of a centimetre) in length; many are colourless but others range up to dark brown. Childs and fellow scientists Professor Renzo Kondo (from Obihiro University, Japan) and Ian Atkinson (of Landcare Research) have written a book on New Zealand opal phytoliths.

The researchers characterised the nature of phytoliths from a wide range of New Zealand grasses, ferns and trees, and classified them by shape and size. Then they used this information in interpreting the distribution of phytolith types they found in soil and sediment samples from a wide range of sites.

In present-day New Zealand soils, grass phytoliths are generally more abundant in the South Island than in the North, while the reverse applies to tree phytoliths. Tree phytoliths are particularly prominent in high-rainfall areas. Grass phytoliths are related to drier areas, areas that were too wet for forest, and young soils from volcanic ash where forest had no time to develop. As forest covered most of New Zealand for so long, and humans arrived only recently, forest-type phytoliths are widely found in areas that have changed from forest to grasslands since human settlement began.

The scientists looked in particular at soil from Te Ngae Road in Rotorua, digging down to take a section representing the last 20,000 years. Analysis of phytoliths showed that the full-Glacial to late-Glacial period (20,000 to 11,250 years ago) was cold and dry, as indicated by phytoliths from grasses. After that, the period of soil formation represented by the overlying Waiohau Ash (deposited by a volcanic eruption about 11,250 years ago) was wetter and warmer than previously, as evidenced by the dominant presence of phytoliths from both trees and grasses.

From 8860 to 930 years ago, phytoliths from trees -- probably mainly from beeches, rewarewa, tawa and rata -- are dominant. In modern soils (from 650 years ago to the present) phytoliths from both grasses and trees are present. The rise in proportion of grass phytoliths at the start of this time may be due to Polynesian settlement with a resulting decrease in forest cover and increase in grassland.

The group looked back as far as 500,000 years through a section of buried soils from Rangitatau East Road between Wanganui and Taranaki, although weathering of the older phytoliths made it harder to identify their original shapes. Even so, much useful information was gathered, including indications that about 20,000 years ago snow tussock and short tussock were common there.

Weathering was also a problem with most of the phytoliths found in the Antarctic marine sediments. These samples were from cores brought up from below the sea surface by the CIROS-1 and CIROS-2 programmes. A further complication was the presence of other types of silica particles, especially those produced by sponges and diatoms. More detailed studies in the future may yield useful information.

Analysis of phytoliths from preserved moa faeces found in Takahe Valley, Fiordland, gave some evidence of the birds' diet, mainly tussock grasses. For the future, Childs sees exciting possibilities for further research using the opal phytoliths.

"Opal phytolith analyses have been applied in many types of studies overseas -- archaeology, paleoethnobotany, ecology, geology, geography, forensic investigations, and in studies of animal diets," he says. "There are many opportunities here in New Zealand for similar applications; the field is wide open."