A single giant clam shell from the Great Barrier Reef contains a month-by-month record of the region's climate from 200 years ago, researchers have found. The 58-centimeter shell was collected from Jiigurru, in the Lizard Island Group of the northern reef.

Scientists from the University of Melbourne and James Cook University used radiocarbon dating to show the clam lived between 1785 and 1827, near the end of the Little Ice Age. The study was published in the journal Global and Planetary Change.

Like tree rings, the clam's shell holds microscopic daily growth lines and broader annual bands. By measuring oxygen and carbon isotopes in the layers, the team reconstructed sea surface temperatures and rainfall patterns of that era.

The reconstruction showed temperatures 0.6 to 0.9 degrees Celsius cooler on average than measurements from 1995 to 2015. Summers were 0.9 to 1.2 degrees cooler, while winter temperatures were almost the same as today. The record also shows El Nino-like and La Nina-like cycles.

The find adds a rare month-by-month marine record to the Southern Hemisphere climate story, where historical data is thin. The researchers say similar shells from other Indo-Pacific reefs could reveal more about past climate shifts.