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Invisible volcanic glass shards found in Tasmanian wetland sediment may have originated from the supereruption of New Zealand’s Taupō Volcano 26,000 years ago.
It’s said the world is so vast and the fragments it’s made from are so tiny, that it’s impossible to grasp how truly big and smallthey are. Imagine then, the scientific task of reconciling the two should a massive chunk of the Earth’s surface happen to be blown violently and distantly asunder across space and time. Undeterred, this has been Jenni Hopkins’ grail for the past six years. Her extraordinary challenge has been to identify the origins of a sprinkling of volcanic glass, so microscopic as to be invisible to the naked eye, discovered during an unrelated field study in the wilds of northern Tasmania.
The silica-rich shards were found 2.5m below the soil surface, buried in peaty sediment extracted at Yellow Marsh in the remote upper catchment of the Medway River, better known for trout fishing than geological significance.
Radiocarbon dating of plant spores above the cryptotephra and an exhaustive regime of chemical analysis at Hopkins’ Victoria University laboratory in Wellington, New Zealand, led her to a startling discovery.
The shards are almost certainly remnants of the Kawakawa/Oruaniu event, the world’s most recent so-called supereruption and thought to be one of the largest volcanic events in human history.
That it happened almost 26,000 years ago and 2500km from where the core samples were taken, intensified the degree of difficulty Dr Hopkins had to overcome big time.
“The methods were very challenging and time-consuming,” she says, as though understatement is a prerequisite to explaining such endeavour.
“The glass shards were very small and not common in the sediments.
“Cryptotephra are notoriously difficult to extract and analyse which is why rhyolitic volcanic ash found in Australia has not necessarily been linked to known eruption sources before.”
Shards of volcanic glass from the eruption that formed Lake Taupō have been found in Tasmania. Photo / Getty Images
The months-long blast itself was enormous.
Almost 1200 cubic kilometres of rock, magma and other material were spewed across a landmass which today roughly measures 1600km by 450km. In some places, ignimbrite or volcanic rock, was deposited 200m deep.
All of the North Island and part of the South, both of which were considerably larger at the time, were blanketed in 10cm of ash.
Particles from the eruption survive throughout New Zealand and can be found terrestrially 900km to the east within the Chatham Islands.
Previous modelling suggests they would also have drifted to eastern Australia.
However, no evidence of this had been found until Dr Peter McIntosh, Earth Sciences and Cultural Heritage Manager with Tasmania’s Forest Practices Authority, began probing the long-term impact of Aboriginal land management at Yellow Marsh in 2017.
Testing on a 2.5m deep soil sample retrieved by one of his research partners, University of Queensland student Judith Vink, revealed a range of pollen samples across the transition between the peak of the last Ice Age and the Holocene epoch which began 11,700 years ago.
Also embedded within a 2cm slither of sediment at the base of the core were a number of clear volcanic shards.
McIntosh says most people, scientists included, would have dismissed the oddly-shaped fragments but Vink, a PhD candidate, realised they may be important.
“When Judith described the unusual particles she spotted under the microscope I knew they had to be volcanic ash,” McIntosh recalls.
“The Oruanui supereruption ... was the most likely candidate because it was the largest globally in the last 70,000 years.”
To collect enough of the ‘invisible’ glass, they drilled a second core using hand and gauge augers to a depth of 3m before the arduous testing process began.
“I had a suspicion it was going to be a really great find, so was more than happy to dedicate the time to getting it done,” Dr Hopkins says of the laboratory marathon.
No supereruptions are thought to have occurred in Australia over the past 3 million years, although there have been active volcanoes including South Australia’s Mt Gambier and Mt Schank.
That aside, rhyolitic shards, possibly from an eruption in Papua New Guinea, have also been found in northeast Queensland but a lack of comparison data has rendered their origin inconclusive.
To nail down the genesis of the Yellow Marsh deposits, Hopkins developed her own technique, known as density separation.
Once individual shards were identified and counted on microscope slides, they were geochemically analysed using an electron microprobe.
“We really were pushing the boundaries of analysis on this type of sample,” says the English-born volcanologist.
“Extracting and concentrating these tiny shards and getting them analysed is exceptionally challenging.”
If the analysis is correct, the research group believes the discovery at Yellow Marsh would allow Last Glacial period deposits in the southwest Pacific, including those in Australia, New Zealand and Antarctica, where they have also been found, to be irrefutably linked.
“In turn, this allows the varying environments during this time period (between 20,000 and 26,000 years ago) to be compared,” Dr Hopkins says.
