JiigurruA shell’s nearly 10,900 daily growth lines span about 30 years.

Summers there ran cooler than today’s from about 1785 to 1827, while winters were similar.

Oxygen isotopes in 784 drilled samples give sea temperature, carbon isotopes give rainfall, and daily growth lines supply the dates.

The Global and Planetary Change study rests on a single 58-centimetre shell from the Jiigurru lagoon.

How each outlet framed it
Phys.org
explains how daily growth lines and annual bands in giant clam shells record seawater geochemistry changes month-by-month over centuries

Sources: Phys.org