This map shows the continental shelf punctuated by deep submarine canyons and ravines that lead down several kilometres into the depths of the Queensland Trough. The fossil reef, at around 50 m below sea level, can be seen as pink bumps along the top of the shelf's edge.
Credit: Robin Beaman/JCU/Southern Surveyor
I'm sitting in front of a time machine. We're deep in the bowels of a survey ship, 50 km off the coast of Cairns. It's mid-afternoon and the room hums with the quiet whirr of a bank of busy computers and the distant clanking of the engine room.
"The machine is just giving us an update of what the present situation is," says Robin Beaman, my tour guide to the Palaeolithic past of this part of Australia. "It's not something you can leave and forget, it's a very complicated machine and it needs watching and tweaking occasionally."
He taps away at the computer, calibrating it. There's a ping, and an array of data and scribbly lines dance across the screen. It doesn't mean much to me. But, somewhere beneath the ship, pulses of sound energy have been unleashed and they're bringing back a grainy picture of the past.
Beaman takes me to another screen that renders the stream of data into something I can understand. He shows me an aerial view of an ancient river bed. A waterway meanders between limestone hills.
"In fact, this may even have been a waterfall at some stage," he muses, concentrating hard and pointing at one part of the image. "It almost has waterfall-like features, with the rivulets coming through here ... [and] on either side of the river channel is the drowned reef."
It's not a time machine in the strictest sense, but this multi-beam or 'swath' echosounder is, for the first time, giving us a vivid picture of an ancient coastline which met its demise long ago. It's a high-tech sonar mapping device that simultaneously sends out 135 individual pulses of sound.
This carpet of sound bounces off the seafloor, as far as two kilometres beneath us in some places, and brings back the first detailed, three-dimensional (3-D) topographical maps of the ocean bottom around Australia's Great Barrier Reef.
Twenty thousand years ago, with much of our planet's water locked up in Ice Age glaciers, sea levels were much lower than they are today. So even though we're 50 km out from the modern coast of Queensland, we're floating above an ancient shoreline from a time when this part of Australia looked very different.
"During the last Ice Age, the Great Barrier Reef as we know it was high and dry. It was a marshy mangrove plain and Aborigines would have been roaming across the landscape hunting and fishing," says Beaman, a marine geophysicist with the Cairns campus of James Cook University (JCU). "That's not to say these coral reefs didn't exist; but they existed as limestone hills covered with eucalypt forest."
On the seaward side of this ancient shoreline, though, is the scientist's real quarry. It's an ancient reef that runs in deeper waters, all along the edge of the continental shelf.
Beyond that, the seafloor drops away to the deep abyss; but this narrow strip, an average of 40 to 70 m deep and a few hundred metres wide, was once teeming with a plethora of hard and soft corals and brightly coloured reef fish.
Then, over hundreds of years, sea levels rose so rapidly that the reef was unable to grow in height fast enough to keep up. It 'drowned', and today languishes in deeper waters, a more thinly populated shadow of the ecosystem it once was.
