Sperm and killer whale teeth add one growth layer a year until midlife, bomb radiocarbon shows

The reference dates in this experiment come from the atmosphere. Atmospheric nuclear weapons testing from the mid-20th century released large amounts of radiocarbon, and the fallout entered ecosystems on land and in the sea ^[5]. Ordinary radiocarbon dating relies on the isotope's decay. This team used something else: the sharp rise in radiocarbon while testing ran, and its decline after testing stopped ^[6]. Each tooth layer was measured on SUERC's accelerator mass spectrometer ^[4], and its radiocarbon placed it before, during or after the tests ^[6]. If the layers are annual, their count should agree with that curve ^[2].

According to phys.org's account of the paper in Frontiers in Marine Science ^[7], the layers formed yearly during early to midlife. The account describes this as some of the first direct evidence that the layers are annual ^[3]. In the animals' senior years, the layers appear to become less reliable as an age marker ^[3].

The denominator is two sperm whales and three killer whales, all dead on Scottish shores in past decades ^[7][20]. I think five animals are enough to show the method works on old specimens. They are too few to say at what age the drift begins, or whether it begins at the same age in each species. The summary does not report how many layers were sampled per tooth or how closely the counts tracked the curve.

Moby shows how the method bounds an age from below. He was at least 43 when he died in 1997, because the innermost layers of his teeth formed before fallout reached the marine environment ^[10]. That puts his birth in 1954 or earlier ^[19]. His skull is on display in the National Museum of Scotland ^[13]. The study's second sperm whale was younger than Moby when he stranded ^[11].

Stable isotopes of carbon and nitrogen in the same layers trace changes in diet and where an animal was feeding ^[9]. Read with the radiocarbon, they gave the team clues to where each whale called home, what it ate and how long it lived ^[18]. The killer whale ranked among the oldest known males of his type ^[12] is the kind of animal whose late-life layers the study found least dependable as an age marker ^[3].

The work was led by SUERC: Centre for the Isotope Sciences, with the universities of Glasgow, Edinburgh and St Andrews and National Museums Scotland ^[8]. Its lead author, Kieran Tierney of SUERC ^[17], first saw Moby as a child. "My parents took me to see Moby when he arrived in the Firth of Forth in 1997, when I was 6 years old," he said ^[15]. "To be able to hold one of Moby's teeth 30 years later and do work that helps us understand a bit of the life he had is something I'm really proud of," he said ^[16].