Wednesday, January 20, 2016

Sea Turtles and Atomic Bombs

Fig.1. Hawksbill sea turtle, Eretmochelys imbricata
(Credit: Clark Anderson/Aquaimages, 2006)

At 5:30 a.m. on July 16, 1945, the first nuclear weapon was detonated under code name “Trinity.” Although disagreement still remains as to the start or nature of the Anthropocene, many scientists agree that nuclear weapons testing caused a widespread anthropogenic signal in the geologic record. In a couple short decades, these weapons tests doubled the amount of the radioactive isotope carbon-14 in our atmosphere. The isotope in the air entered plants during photosynthesis and animal tissues as animals ate the plants, leaving behind a radioactive trace in the natural world.
A recent study published in the Proceedings of the Royal Society B examined atomic bomb fallout to estimate the life and death of wild sea turtles. Kyle Van Houtan, an ecologist with the National Oceanic and Atmospheric Administration (NOAA) and Duke University, worked with fellow researchers to analyze hard tissue from the shells of 36 hawksbill sea turtles collected since the 1950s.

The huge amount of carbon-14 in the atmosphere due to nuclear weapons testing has been decreasing due to radioactive decay. As these levels drop down to a baseline, scientists can calculate how old an animal or plant was based on the amount carbon-14 in their tissues.

Van Houtan et al. examined carbon-14 levels in the sea turtle shells and found that Hawaiian hawksbills, in particular, deposited eight growth lines annually. This accurate time scale - compared with background carbon-14 levels in Hawaiian corals - allowed the researchers to determine specific growth rates and reproductive maturity of sea turtle populations in the wild.

Fig.2. The “Baker” explosion during Operation Crossroads, a U.S. military nuclear
 weapon test at Bikini Atoll, Micronesia in 1946 (Credit: U.S. Dept. of Defense, 1946)

This technique improves upon past methods that measure growth in these animals, including using length as a proxy for maturity. In addition to offering a reliable estimate for lifespan, it could also reveal why Hawaiian hawksbill populations have declined despite concerted conservation efforts. The study showed that female Hawaiian hawksbills reach reproductive maturity at 29 years - much later than other sea turtle populations around the world. This may explain why the numbers of Hawaiian hawksbills are not rebounding as quickly as expected.

Such news is worrying because it affects the ecosystem as a whole. As sea turtles suffer from hunting or trafficking, declining numbers disrupt the reef dynamic and affect the food supply as a whole. The hawksbill sea turtle (Eretmochelys imbricata) is of particular interest due to its critically endangered status

Nevertheless, this study is a result of university scientists, law enforcement, and museum staff working together. The shells used came from turtles that died naturally or were harvested for their decorative shells as part of wildlife trade. The animals ultimately wound up in collections and allowed scientists to understand sea turtle populations.

Between atomic bomb fallout and links to wildlife trade, this study is a small part of a larger conversation about humans and their environment. The Trinity nuclear test in 1945 marked a profound shift in how human activity affected the planet. These turtle collections - among other natural history collections - bear witness to the scale of this impact, revealing the depth of change that has occurred since the onset of the Atomic Age.

Hawksbill Sea Turtle. [n.d.]. National Geographic. Retrieved from:

Van Houtan, K. S., Andrews, A. H., Jones, T. T., Murakawa, S. K. K., & Hagemann, M. E. (2016, January 6). Time in tortoiseshell: a bomb radiocarbon-validated chronology in sea turtle scutes. Proceedings of the Royal Society B, vol. 283 (1822). doi: 10.1098/rspb.2015.2220

Walters, C. N., et al. (2016, January 8). The Anthropocene is functionally and stratigraphically distinct from the Holocene. Science, vol. 351 (6269). doi. 10.1126/science.aad2622

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