Published on the 11th June 2015 by ANSTO Staff
ANSTO and the Chinese Academy of Sciences are uncovering centuries of climate history locked inside the shells of enormous molluscs.
In September 2014, the Royal Australian Navy frigate HMAS Sydney sailed down the east coast of Australia from Darwin to Sydney. The voyage was routine, but in its hull was an unusual cargo: the shells of five giant clams, destined for research institutes and museums in Sydney and Canberra.
Weighing more than 100 kg each, two of the massive shells were bound for ANSTO’s research laboratories at Lucas Heights. The organisation’s scientists hope that the shells will contain evidence of the history of temperatures in the Coral Sea over the past 150 years.
By analysing this evidence, they hope to gain a better understanding of the monsoon winds that strongly affect the climate in the North and East of Australia.
The arrival of the shells was like “Christmas come early,” says Professor Henk Heijnis, a Senior Principal Research Scientist at ANSTO’s Institute for Environmental Research. He couldn’t wait to get his hands on them.
The shells will play an important part in a collaboration between ANSTO and scientists at the Chinese Academy of Sciences in Xi’an, who have pioneered the use of giant clam shells to piece together the history of monsoons in the South China Sea.
The prestigious Academy is home to the “crème de la crème of Chinese science”, says Heijnis. The aim is for ANSTO and the Academy to pool their expertise to thoroughly understand the trends and patterns of the region’s monsoons. The work involves analysing sediments in extinct volcano craters and stalagmites, in addition to the clam shells, to unravel tropical climate history.
The project was the brainchild former Head of the Institute for Environmental research at ANSTO, Professor John Dodson, and Professor Zhisheng An, of the Chinese Academy of Sciences in Xi’an.
“The skills and equipment at ANSTO are complementary in many ways to the capabilities of the Chinese researchers,” said Heijnis. “So we are sharing expertise and infrastructure.”
Shells reveal past sea temperatures
Giant clams live for about 150 years. Each year, a new layer is added to their shells, like the rings on a tree. By measuring the ratio of the elements calcium and strontium in each layer, scientists can calculate the sea surface temperature at the time it was laid down.
Ocean temperatures are important for those who live on land, too. By affecting winds and ocean currents, sea surface temperatures determine the levels of rainfall over the land, contributing to phenomena such as “the wet”, when monsoon winds bring rain to Australia’s north.
The Chinese scientists had already used both fossil and current clam shells to provide a picture of the climate in their region. One fossil shell they found was 2500 years old, providing temperature data from more than two millennia ago.
“They’re building up the jigsaw puzzle nicely,” says Heijnis. “You can start to see the bigger picture of climate in that region now.”
Confiscated clams put to use
The story of how Heijnis came to get hold of some Australian clam shells of his own is a tale of ingenuity and collaboration. Turning to the internet, he discovered Shane Penny, a scientist at the Northern Territory’s Department of Primary Industry and Fisheries in Darwin who was studying giant clams for his PhD.
Heijnis enquired if Penny knew where he could come by some of the shells. To his surprise, the fisheries scientist emailed him a photo of 194 clam shells the Australian Fisheries Management Authority had confiscated from fishing boats that had been illegally harvesting giant clams in Australian waters.
“It’s unfortunate that nearly 200 giant clams were taken, but it has provided one of those unique once-in-a-lifetime opportunities,” says Penny.
By a lucky chance, some space was available on HMAS Sydney, which was returning from exercises off Darwin, so in no time two shells were on their way to ANSTO.
Analysis relies on ANSTO facilities
ANSTO has all the facilities needed to analyse the shells, says Heijnis. First, his team will use a high-precision optical and X-ray scanner to determine the number and position of the annually-added layers, formally called laminations. Then tiny samples will be drilled out from within each layer and analysed, using a type of imaging known as inductively coupled plasma – atomic emission spectroscopy, to measure the calcium-to-strontium ratios.
Finally the particle accelerators at ANSTO will be used to pinpoint when the laminations were laid down. “ANSTO is the home of a lot of landmark national infrastructure, and because of its quality we actually get quite a lot of interactions with people in other parts of the world,” said Heijnis.
The information that comes from examining the shells will extend the sea temperature records for the Coral Sea back, perhaps to 150 years ago. In another part of the climate study, the ANSTO scientists and their Chinese collaborators are looking at stalagmites from caves near the town of Katherine in the Northern Territory. Stalagmites go back further in time than the clam shells, and can provide valuable information on rainfall patterns.
“The more it rains, the more calcium carbonate gets dissolved from the rock and the bigger the stalagmite grows,” Heijnis explained. “The variation in the rainfall is also linked with sea-surface temperatures so there is a direct connection to the clam research.”
In most circumstances, scientists use ANSTO’s carbon-dating facilities to calculate the age of clams, lake sediments and some recent stalagmites. For older stalagmites where all the carbon is part of very ancient carbonate rocks, the ratio between uranium and thorium reveals their age.
The final approach that ANSTO is using on the project is to examine the muddy depths of lakes and rivers. Just as clam shells add a new lamination, each year layers of sediment settle onto lake and river-beds, providing scientists with another kind of timeline to study the history of monsoon rains.
“We’ve got some cores from the Kakadu area and I’m just sampling them for radio-carbon dating to see what time frame they cover,” Heijnis said. “If we’re lucky it could be millennia.”
Last year, he and Chinese scientists sampled sediments from lakes in craters of extinct volcanoes on Hainan Island in the South China Sea. The data is expected to reveal rainfall over tens of thousands of years.
“Because the craters have high walls they are like a very narrow bucket, which is the best way to record rainfall,” he said. “It’s a natural rain gauge.”
An was delighted with the success of the project so far and looked forward to young Australians and young Chinese scientists collaborating into the future, looking at the variability in the tropical system which affects both our countries.