When you swim through the azure waters of the Great Barrier Reef, there is one entity on the seafloor that’s almost impossible to miss: the giant clam. Its huge, fluted shell frames a soft mantle shimmering with electric blue, green and gold.

As the world’s largest living bivalve, Tridacna gigas can reach 1.2 metres, weigh more than 200 kilograms and live for over a century. Across their long lives, these clams record their daily life in their shells, creating an autobiographical shell diary.

In our new research, we read one of these shell diaries to reconstruct month-by-month climate conditions at Jiigurru (also known as the Lizard Island Group), in the northern Great Barrier Reef.

The shell’s story reveals that, before widespread industrial-era warming, Jiigurru’s summers were cooler than today, while winters were surprisingly similar.

How can a shell keep a diary?

As a giant clam grows, the cycle of day and night produces microscopic daily lines in its shell, while the changing seasons produce broader annual bands. Like tree rings, these layers turn the shell into a diary.

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.

Geochemistry is the language in which this climate diary is written. By reading that language, climate researchers can........

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