by Georgia Barrington-Smith, 31st October 2024
The Environmental History of the Great Barrier Reef, as told by a Giant Clam Shell
Although archaeology has made waves on land, we know comparatively little about the history beneath our shores.
A few centuries ago, the Earth experienced a Little Ice Age (LIA) with harsh winters and cool summers. Whilst scientists have been able to reconstruct what that environment was like in the Northern Hemisphere, the conditions in the Southern Hemisphere have remained relatively unknown.
That’s where giant clams come in! By investigating giant clams (Tridacnidae spp.), Bohao Dong, University of Melbourne Ph.D. student and AINSE PGRA scholar, was able to uncover information about the southern hemisphere’s environmental history stretching back centuries.
Digging for clues in the ocean floor
Giant clams have a rapid growth rate, and preserve ultra-high resolution paleo-environmental information in bands within their shells. Just as tree trunk rings that tell us the age of a tree, giant clam shells provide monthly, or even daily, insights into environmental conditions throughout the clams’ lifetime.
With environmental records preserved in their shells, the giant clam can be thought of as a historian lying in shallow seawater, writing an environmental diary over the course of its life. The dates of the diary entries can be read through radiocarbon dating, while the contents of the diary include details such as the temperature of the surrounding seawater, salinity, dissolved inorganic carbon, and the total amount of daylight experienced.
Uncovering the secrets of the past
The clam used for Bohao’s research was collected from Lizard Island (Jigguru), Great Barrier Reef, Australia. Its enormous shell was 58 cm long, 30 cm wide and 20 cm high.
Using carbon-14 dating techniques at ANSTO’s Centre for Accelerator Science (CAS), Bohao and his research collaborators were able to determine that this particular clam lived approximately 1800-1830 CE, before the industrial revolution.
The team undertook isotopic composition analysis on 10,534 shell increments that were laid down by the clam each day of its life, revealing that the clam lived during a time when summer sea surface temperatures were 1.2℃ cooler than they are today. This finding showed it was likely that the same cold events observed during the Little Ice Age in the Northern Hemisphere also occurred in the Southern Hemisphere.
By investigating trace element (Strontium/Calcium) ratios using inductively coupled plasma–atomic emission spectroscopy (ICP-AES) analysis at CAS, ANSTO, the team revealed the clam shell had received less incoming sunlight over its lifetime than it would have received in today’s climate. After matching the timeline of the clam’s life with markers of volcanic activity in the South Pacific, scientists pieced together that the low incoming sunlight was likely because of volcano eruption events in the 19th Century.
This research is currently ongoing, with the team hoping to uncover more information about the energy supply to the local food web, El Niño-Southern Oscillation (ENSO) activities, and human-environment interactions during the LIA period in the Great Barrier Reef and greater South Pacific.
AINSE are proud to spotlight Bohao’s research as he continues to use nuclear science techniques to uncover the mysteries of the ocean’s past, in order to better protect our marine environments into the future.
Keep up to date with our regular research spotlights, including a highlight of Alex Bastick’s research into how pipelines impact the marine food web!