Researchers have extracted a month-by-month climate record spanning 30 years from a 58-centimetre giant clam shell collected at Jiigurru, revealing that northern Great Barrier Reef summers were significantly cooler over two centuries ago than they are today.
Jiigurru Giant Clam Preserves a 30-Year Shell Diary
Beneath the azure surface of the northern Great Barrier Reef, researchers found an unexpected climate archive inside the fluted shell of a giant clam. The specimen, belonging to the species Tridacna gigas, grew in the lagoon at Jiigurru, also known as the Lizard Island Group. Known as the world’s largest living bivalve, these creatures can reach 1.2 metres in length, surpass 200 kilograms in weight, and live for more than a century.
As the bivalve grows, the cycle of day and night deposits microscopic growth lines inside the shell, while seasonal shifts produce broader annual bands. By placing the shell under a microscope, researchers counted nearly 10,900 daily growth lines, determining that the mollusk lived for approximately 30 years.
The growth lines provide the dates, just as we date entries in our own diaries. The surrounding seawater writes the content by shaping the chemistry of each new layer.
Bohao Dong Research Assistant, ARC Centre of Excellence for Indigenous and Environmental Histories and Futures, The University of Melbourne, Amy Prendergast Associate Professor in The School of Geography, Earth and Atmospheric Sciences and Chief Investigator for the ARC Centre of Excellence for Indigenous and Environmental Histories and Futures, The University of Melbourne, and Sean Ulm Director, ARC Centre of Excellence for Indigenous and Environmental Histories and Futures, James Cook University
Geochemical Sampling Reveals Pre-Industrial Temperatures
To read the environmental history encoded in the shell, researchers drilled 784 tiny holes into the shell layers to extract powder samples. In each sample, they measured the ratios of oxygen and carbon isotopes. Oxygen isotopes provided estimates for past sea surface temperatures, while carbon isotopes offered clues regarding local rainfall patterns.
Radiocarbon dating placed the clam’s lifespan between approximately 1785 and 1827, positioning its existence near the end of the Little Ice Age, a pre-industrial cooling period. When the team compared their isotope-derived temperatures against modern sea surface measurements recorded in the same lagoon from 1995 to 2015, distinct seasonal differences emerged.
Annual average temperatures 225 years ago ran 0.6 to 0.9°C cooler than modern baselines. Summer readings showed an even sharper drop, registering 0.9 to 1.2°C cooler than today, whereas winter temperatures remained remarkably similar to contemporary seasonal averages.
Shifting El Niño and La Niña Cycles in the Early 1800s
Beyond seasonal warmth, the carbon isotope data traced a distinct rhythm alternating between wetter and drier intervals. The temperature records also fluctuated in repeating two- to three-year cycles that mirror modern El Niño-like and La Niña-like variability.
The shell data shows that warmer, drier El Niño-like episodes occurred more frequently toward the close of the 1700s. By contrast, cooler and wetter La Niña-like conditions grew more frequent during the early 1800s, encompassing the strongest variability events captured within the entire 30-year record.
Collaborative Research and the Wider Southern Hemisphere Record
Jiigurru holds deep cultural significance for Dingaal and Ngurrumungu Aboriginal communities, with local archaeological excavations at shell midden sites demonstrating at least 6,500 years of Aboriginal occupation.
The findings, published in the journal Global and Planetary Change, help address a notable geographic gap in climate data.
Other regional archives align with the Jiigurru shell data. Corals retrieved from other sectors of the Great Barrier Reef and north-western Australia point toward cooler historical seas, while Antarctic ice cores and glacier archives indicate concurrent shifts in southern atmospheric and oceanic circulation.