Sea turtle shells are glassine, speckled and swirled. What's more, they are also biological clocks indicating ages, reproductive maturity and growth rates of turtle populations living in the wild, according to a new study by NOAA researchers and Duke University.

Those researchers conducted radiocarbon dating of atomic bomb fallout that shows up in the sea turtle shells. With this new technique, we may learn more about the lack of recovery of some populations of endangered sea turtles worldwide. Their findings were recently published in the journal Findings of the Royal Society B. 

More specifically, the team looked at hard tissue from 36 deceased hawksbill sea turtle shells gathered since the 1950s. The turtles involved either died from natural causes or were harvested for their shells as part of the tortoiseshell trade. In order to obtain the specimens, the scientists worked with museum archives, law enforcement and federal agencies, according to a release. 

Here's how it worked: The researchers compared the bomb-testing radiocarbon built up in each shell to the radiocarbon (also from bomb-testing) present in Hawaii's corals. From this, they learned each turtle's approximate age. Because carbon-14 was more present in the Earth's biosphere from the mid-1950s to around 1970 resulting from Cold War-era nuclear tests, but have dropped since then, scientists can tell an organism's age based on the level of carbon-14 it contains.

They also did a comparison of the collected specimens' radiocarbon information with those of other hawksbills, wild and captive, with known growth rates. Before this, less precise methods were used to learn sea turtles' age, growth and maturity. These included looking at turtle length and conducting analysis of unfinished growth layers in bone tissue that is hollow, according to a statement.

Two other learnings were also involved: It turns out that the turtles were omnivores at least until a period in the 1980s, but are now mostly herbivores."Such a dramatic decline in their food supply could delay growth and maturity, and may reflect ecosystem changes that are quite ominous in the long term for hawksbill populations in Hawaii," said Kyle Van Houtan, fisheries research ecologist at NOAA's Pacific Islands Fisheries Science Center and a professor at Duke.

The hawksbills in the Hawaii population also seem to begin breeding at 29 years, much later than other hawksbill populations worldwide--this may explain why this population of turtles hasn't rebounded, noted Van Houtan in the release.

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