The History of Atmospheric Nuclear Weapons Tests & Determination of the Life Span of Whale Sharks.
The paper to which I will very briefly refer is this one: Annual Bands in Vertebrae Validated by Bomb Radiocarbon Assays Provide Estimates of Age and Growth of Whale Sharks (Joyce Jia Lin Ong et al., Frontiers in Marine Science, Vol 7 Page 188 (2020))
This paper is open sourced and is available to the public at the link, so I will not excerpt it extensively but only offer a brief explanation of some background on what about historical nuclear testing involves determination of the age of whale sharks.
As many people know, carbon-14, the radioactive isotope of carbon occurs widely as a result of the high energy radiation flux flowing from the sun at a reasonably constant rate. Interaction of these high energy particles with the overwhelmingly dominant isotope of nitrogen, nitrogen-14, results in a nuclear reaction in which N-14 is transformed into C-14. The radioactive half-life of carbon 14 is roughly 5700 years, relatively short on a geological time scale. Thus were it not being created by natural processes, it would not exist on Earth.
A system in which a radionuclide that is decaying according to the (exponential) nuclear decay law while simultaneously forming at a constant rate - this is the situation in nuclear reactors - will reach a state where it will asymptotically approach a constant value where it is impossible for the quantity to increase beyond a point defined by what we call "The Bateman Equilibrium."
This situation, established over millennia, allows for the famous "radiocarbon dating" technique, since living things exchange radioactive carbon with the environment but when they die, this exchange stops and the concentration falls as the C-14 decays back to N-14.
However, beginning in the 1950's, the long established Bateman equilibrium was disturbed with the dubious practice of open air nuclear weapons testing, particularly with the development of thermonuclear weapons which utilized nuclear fusion triggered by fission driven implosions at extraordinary high temperatures. In these thermonuclear tests, the lithium in lithium deuteride is converted into tritium (early thermonuclear weapons used tritium directly) and fused in the following nuclear reactions: 6Li(n,α )3H and 2H(3H,n)4He. In the first, Lithium-6 absorbs a neutron to release tritium, and in the second, deuterium (hydrogen-2) fuses with radioactive tritium (hydrogen-3) to yield helium and a neutron. The neutron is an order of magnitude higher in energy than typical fission neutrons, the latter being produced in an energy distribution of between 1 to 2 MeV (million electron Volts), the former at a well defined energy of about 14.1 MeV. In air, these 14.1 MeV neutrons have a probability (nuclear reaction cross section) to produce carbon-14 when a nitrogen-14 atom is struck by a neutron and ejects a proton: 14N(n,p)14C.
Since open air nuclear testing stopped nearly completely in the 1960's, albeit after the awful Soviet "Tsar Bomba" test that briefly had a power output equivalent of nearly 2% of the sun's output, this spike put a time marker in all living systems that basically set the clock around 1963, when the final concentration of 14C in the environment was measured.
Each year 0.0122% of a sample of 14C decays back to 14N. Thus by measuring what percentage of C14 has decayed, one can know when the organism was alive. In the 57 years since 1963, 0.691% has decayed.
Thus with precise instrumentation to measure the amount of carbon 14 in a sample one can calculate exactly how long ago a living thing has formed, very precisely in fact since 1963 since the 14C ratio was measured then rather than assumed, as in pre-1963 carbon 14 ratios based on an assumed Bateman equilibrium depending on a consistent nuclear particle flux from the sun.
It has been speculated that the bones of Whale Sharks are rather like tree rings, that surfaces lay down in them in a regular time related pattern. However, proof was lacking. Since bones incorporate some calcium carbonate as well as calcium phosphate however, sensitive testing of the bones should show a distribution of carbon-14 to carbon 12 and 13 ratios that are age dependent, and tied closely to the spike that occurred in the decade before 1963, from the US "Mike" test, the first thermonuclear explosion in 1953 to the last test in 1963.
The authors found that it did, and that thus whale shark bones are like tree rings. From this it is possible to determine when a particular whale shark was born and when it died from its bones.
A brief excerpt from the paper, as a preface to opening the full paper, which is, again, open to the public:
It is critical that age estimates provided by otoliths and vertebrae are accurate, since uncertainty or underestimates surrounding these ages can lead to stock collapses of exploited species (e.g., orange roughy Hoplostethus atlanticus; Smith et al., 1995), or compromise the effectiveness of recovery programs for species that are threatened or endangered. For this reason, many studies have sought to validate the timing of the production of growth bands (Campana, 2001). A common approach is to tag individuals with a chemical marker such as oxytetracycline (OTC) that is laid down within an otolith or vertebrae. Individuals are released and when recaptured at some time in the future, the tag acts as a time stamp that allows the rate of deposition of subsequent growth bands to be determined. For large fishes and sharks that are relatively long-lived and difficult to tag and recapture, validation of annual banding patterns can also be obtained through an analysis of bomb radiocarbon in vertebrae. Above-ground testing of thermonuclear weapons in the 1950s and 60s increased the ratio of carbon 14 isotopes in the atmosphere that were then mixed into the ocean, passed up food webs and incorporated into marine organisms. As a result, the timing of the deposition of bands can be validated by comparing carbon isotope values within vertebrae, with an isotope baseline chronology of known age (Campana, 2001; Campana et al., 2002; Goldman et al., 2012).
The whale shark, Rhincodon typus, is a huge (up to 18 m length; Mcclain et al., 2015), highly migratory, filter-feeding shark found in all tropical and warm temperate seas (Compagno, 2001; Chen et al., 2002; Stevens, 2007). It forms aggregations in productive coastal areas and is a highly valued target for marine eco-tourism (e.g., Huveneers et al., 2017). However, the whale shark has recently been classified as Endangered (IUCN Red List; Pierce and Norman, 2016) and there is now an urgent need for reliable and accurate information on age and growth of the species in order to develop effective conservation and management strategies. At present, there is relatively little demographic data available, especially for large or mature individuals. Using X-radiography, Wintner (2000) analyzed the growth bands in whole vertebrae of juveniles that had stranded on the coast of South Africa to develop an initial growth curve for the species...
I am proud that this work was conducted in my State's State University system, Rutgers, the State University of New Jersey. (One of my sons is a Rutgers graduate.)
Have the best weekend you can under the circumstances.
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