The bomb pulse

Answers concerning the life-history traits of sea turtles sometimes come from the most unlikely sources. Most recently, the nuclear tests during the mid-twentieth century proved key to determining the age and growth rates of sea turtles in Hawaii. To understand the improbable link between Hawaiian sea turtles and nuclear weapons, we have to go back in time by over half a century.

During the 1940s and early 1960s nuclear tests were being carried out by various nations across the globe. As a direct result of this, the concentration of carbon-14 (14C) in the atmosphere nearly doubled within a decade. In 1963 over 100 nations signed a treaty agreeing to ban nuclear weapon tests in the atmosphere, in outer space and under water. Since then the atmospheric concentration of 14C has been decreasing at a steady rate due to natural exchanges with the biosphere. As atmospheric 14C is assimilated in the biosphere it can be found in all plants and in the animals that eat them. Interestingly, the 14C concentrations inside an organism mirror those present in the atmosphere and because the temporal change in the concentrations of 14C is well-documented, scientists can accurately determine the age of an organism based on its 14C content. This technique, known as bomb-radiocarbon dating (or bomb-pulse dating), is similar to the more widely-known radiocarbon dating used to date fossils.

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The levels of carbon-14 in the atmosphere have been relatively stable over long time periods, with the exception of a large addition of carbon-14 in 1955–1963 as a result of nuclear bomb tests. The boxed region in a is shown in more detail in b. (Source: Nature)

A team of researchers from NOAA and Duke University recently applied bomb-radiocarbon dating to the hard tissue of 36 hawksbill turtle shells collected since the 1950s. This allowed them to approximate growth rate and reproductive maturity of these turtles and gave them new insights into this hawksbill population. Their results, published in Proceedings of the Royal Society B, show that this sea turtle population starts breeding at an average age of 29 years (range from 23 to 36 years). This is much later than other populations of this species, and may be a reason why this population, one of the smallest in the world, is not rebounding. In addition, the research reveals that these turtles’ diet has changed over time: they were omnivores until the 1980s but are now mostly herbivores. This indicates a dramatic change in the turtles’ food supply, which could be a sign of long-term ecosystem changes occurring in Hawaii.

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Researchers have generated new growth curves for the Hawaiian hawksbill sea turtles by studying the interior structure of the turtles’ posterior marginal scutes and using bomb-radiocarbon dating.  (Source: Proceedings of the Royal Society B)

Bomb-radiocarbon dating appears to be more accurate in assessing the development of sea turtles than previously-used methods. It also has many applications in other research fields, including investigating Nazi war crimes and accurately determining wine vintages. However, as the atmospheric 14C issued from the bomb pulse disappears, so does our ability to use it to accurately age organisms. It is estimated that the bomb pulse will die out within the next two decades. Until then, there is no doubt that the bomb pulse will continue to be of unexpected scientific significance.

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