The stories of sea turtles can be told from their shells. Borrowing tools from archaeology, researchers have been able to reconstruct a timeline of where they’ve been, what they’ve eaten and how the environment around them has been changing.
Sea turtles are the oceans’ wanderers. They live for up to a hundred years, and meander hundreds of thousands of miles. The reptiles grow continuously through life, and as they do, their shell grows with them. New tissue is laid down in layers, with the oldest on the outside and the newest on the inside. As the tissue forms, it incorporates chemicals from the environment, so each layer becomes a record of conditions from that time.
By studying the chemical fingerprints inside each layer, scientists have been able to learn about the turtles’ lives and the state of the water around them. “It’s a bit like sea turtle forensics,” says Bethan Linscott from the University of Miami. “The shells are effectively recording environmental stress in the ocean.”
The problem was that they couldn’t tell how much time each layer represented. Now research presented in Marine Biology, changes that.
Linscott and her colleagues analysed shell samples from 24 stranded sea turtles – loggerheads (Caretta caretta) and green turtles (Chelonia mydas) – collected along the Florida coast between 2019 and 2022. They took tiny biopsies from the hard part of the shell and sliced the samples into sections, each one twentieth of a millimetre thick.
Each layer was radiocarbon dated and compared with the mid-20th-century 'bomb pulse', a spike from nuclear weapons testing that acts as a calibration point in the marine environment. Borrowing a statistical method used in archaeology to date sediment samples, they were then able to calculate that, on average, each layer represents a time span of seven to nine months.
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The timelines of different turtles could then be compared, revealing periods when all the animals grew more slowly. These patches coincided with major environmental disturbances in the waters around Florida, including harmful algal blooms known as ‘red tides,’ and disruptive accumulations of Sargassum seaweed.
Being able to match changes in sea turtle behavior, such as foraging patterns and dietary differences, with environmental change is important, because it gives researchers a way to assess how the animals are impacted by climate change and other external factors.
“Our findings can help scientists better understand how marine ecosystems are changing and how species respond to those changes,” says Linscott.
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