Getting to the bottom of 200,000 years of history

A team of researchers from the ARC Centre of Excellence for Australian Biodiversity and Heritage, (CABAH) led by Professor Michael Bird from JCU, used a custom-made raft to float a rig to the middle of Girraween Lagoon, near Darwin.  Over five days they used a hydraulic jack hammer to drive a series of plastic tubes into the lagoon bed.

By hammering the one and two-metre wide tubes to a depth of almost 20 metres, they retrieved a collection of sediment cores that contain environmental evidence from before the last Ice Age.

Girraween Lagoon is a significant Aboriginal site and gained a place in modern Australian popular culture as a location for filming the iconic water bottle scene in Crocodile Dundee.

“It’s like a tape recorder looking back into the past - the further you go into the sediment, the further you go back in time,” said Prof. Bird, a geologist and leader of the CABAH Landscapes theme.

“We can see the interval when humans were first in the area. So, in due course, we will be able to learn more about the environment they were inhabiting.”

The cores, frozen and cut into half-metre lengths, represent a “brilliant record” of the past and will be closely examined over the next two years, Prof. Bird said. In the laboratory, they can be sliced again and analysed to look for chemical evidence of environmental changes such as vegetation, fires and rainfall.

The microscopic clues will be used to piece together the answers to fundamental questions about the past, such as the impact of human arrival, megafauna extinction and whether the climate during the last interglacial period 130,000 years ago was similar to today.

The field work took place following extensive discussions with the Traditional Owners, The Larrakia Nation. During a recent field trip, a group of young Larrakia Rangers spent time with the CABAH team, learning about the research.

Girraween is an Aboriginal word, meaning ‘place of flowers’. Palaeoecologist Dr Cassandra Rowe from JCU has been monitoring vegetation in and around the lagoon and is excited at the prospect of examining pollen grains from the sediment cores.

Working with slices of about two cubic centimetres, Dr Rowe is able to isolate pollen grains and examine them under a microscope.

“The pollen has preserved really nicely,” Dr Rowe said. “I have around 130 different pollen types, representing different plants. The detail that gives us is quite extraordinary.”

“Pollen needs a permanently moist position to preserve well. To have this site like Girraween that’s preserved everything in great detail, providing information on vegetation structure and composition, is really exciting,” she added.

The team, including Michael Brand, Costijn Zwart, Chris Wurster and Xennephone Hadeen, bring together diverse expertise and will also use the cores to learn about past use of fire and signs of climate change.

Link to graphic here.

Link to YouTube video here.