Sand Grains Reveal How Humans Change Australia

Piles of sandy sediment dumped by floods along our rivers over thousands of years are proving to be continental history books more orderly and revealing than many accounts penned by human authors.

CSIRO scientists Dr Jon Olley and Dr Gary Caitcheon and Professor Bob Wasson from the Australian National University are pioneering a revolutionary way to date single grains of sand. This is enabling researchers, for the first time, to accurately reconstruct major events in Australia’s river valleys and flood plains over millenia.

Optically stimulated luminescence, or OSL, has the power to explain how European settlement has transformed the landscape, how aboriginal management changed Australia - and how the continent’s rivers functioned when it was grazed by giant marsupials before humans arrived.

It can also help us achieve a dramatic improvement in the way we look after our country, the researchers say.

The layers of sediment are like books, each layer a different page in the story of a river and its catchment. Tiny grains of quartz buried in them provide the dates for each event.

Together they tell a story about how humans modified the landscape, burning and felling the bush, eroding the soil, changing the river flood patterns, altering the chemical composition of water and sediments.

“Humans are like glaciers in their impact on a landscape,” Dr Olley explains. “We remove cover, increase erosion, change things profoundly. The traces of this activity can be seen in the layers of sediment that line our river systems.” But the tiny grains also reveal much more. They could become life savers - warning Australians planning to build their homes on flood plains how often the mega-floods occur which might sweep them away. In most parts of Australia, the flood record goes back barely a century - but we know from research that now and again there are immense floods, many times greater than those we have seen so far. How frequently they occur, and which places are most at risk is a matter of life and death.

“In the sides of river gorges there are often little shelves which still hold the sediment of ancient floods. By dating them we may get an idea how long it has been since the river last ran so high, and how frequently such large floods occur. If planners are seriously interested, they can plan our communities to minimise the hazard of these catastrophic events.”

The team is currently dating a site on the Murrumbidgee which they believe records major flood events for the last 2000 years and shows a clear change in the river from the time of European settlement.

The grains of quartz sand act like tiny clocks. When they encounter sunlight, it releases energy stored in the crystal lattice, setting the clock to zero. But once the grain is buried away from light, it begins to accumulate radioactive energy from the surrounding soil, and from this the time since burial can be calculated.

Dr Olley’s team take core samples containing quartz grains from the middle of a sediment layer, and subject them to 14 different processes and measurements. Ultimately they expose the sand grain to blue-green light, and the answering flash as the crystal releases its stored energy reveals the time elapsed since it was buried.

“Australia’s quartz grains are particularly suitable because they shine really brightly. They are very sensitive to environmental radiation, because they have been dosed, buried and exposed many, many times. This creates sites in the crystal lattice to trap the energy.”

But the technique also applies to other semi-arid countries, and Dr Olley is working with scientists to measure the impact of human activity on the African landscape of Tanzania.

“In Africa we’re looking at 1000 years of really poor landscape management. In Australia we’ve had only a hundred years or so, with some recent attempts to lift our game. If we can understand what happens in each process, it will help us determine which landscape management and restoration methods are most effective - so it has important practical consequences for how we look after Australia,” he says.

More information:

Dr Jon Olley, CSIRO Land & Water, john.olley@clw.csiro.au