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> Instead, the team decided to measure levels of radioactive carbon-14 in fibres in the centre of the shark’s eye lens. Such measurements reflect levels of radi

> Instead, the team decided to measure levels of radioactive carbon-14 in fibres in the centre of the shark’s eye lens. Such measurements reflect levels of radiocarbon in the ocean when the lens was first formed. Measurements of 28 female Greenland sharks, made during surveys in 2010–13, suggested that the largest of them (at 5.02 metres long) must have been between 272 and 512 years old at the time. >levels of radiocarbon in the ocean when the lens was first formed But wouldn't that tell you the age of the rocks/minerals that were used to make the lens? Or is the lens living tissue that dies after formation?

deleted 10y ago
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CamperBob2 10y ago
It's a pretty cool experiment. C14 is formed continuously in the upper atmosphere and enters the carbon cycle via CO2 absorption by plants. Since the ratio of C12 to C14 in the environment is known to be reasonably stable over time, the C12:C14 ratio in a given biological sample can be used to infer how long it's been since the organism stopped participating in the carbon cycle (i.e., died.) The basic idea is simple enough, but the dating process is complicated by various factors that have to be calibrated out. The environmental C14:C12 ratio got a lot lower when we started burning fossil fuels in the 1800s. Later, setting off a bunch of massive nukes in the 1950s had the opposite effect. The lens of the eye is living tissue, so it can be carbon-dated. In this study, the researchers actually took advantage of the nuclear contamination problem. They noted that only the smallest, most immature specimens exhibited the unnaturally high C14:C12 ratio characteristic of items from the period before the test ban treaties took effect. Bigger sharks had clearly absorbed most of the carbon in their lenses over a much longer period of time[1]. Since they have a good idea of the rate at which the sharks grow, they could infer that the larger specimens must have been growing for hundreds of years before the nuclear tests. [1] Things get a bit hand-wavy here; the article suggests that the shark lens stops absorbing carbon after it's formed. Meanwhile, Wikipedia says that the human lens continues to grow.

deleted 10y ago

[deleted]

CamperBob2 10y ago

It's a pretty cool experiment. C14 is formed continuously in the upper atmosphere and enters the carbon cycle via CO2 absorption by plants. Since the ratio of C12 to C14 in the environment is known to be reasonably stable over time, the C12:C14 ratio in a given biological sample can be used to infer how long it's been since the organism stopped participating in the carbon cycle (i.e., died.) The basic idea is simple enough, but the dating process is complicated by various factors that have to be calibrated out. The environmental C14:C12 ratio got a lot lower when we started burning fossil fuels in the 1800s. Later, setting off a bunch of massive nukes in the 1950s had the opposite effect. The lens of the eye is living tissue, so it can be carbon-dated. In this study, the researchers actually took advantage of the nuclear contamination problem. They noted that only the smallest, most immature specimens exhibited the unnaturally high C14:C12 ratio characteristic of items from the period before the test ban treaties took effect. Bigger sharks had clearly absorbed most of the carbon in their lenses over a much longer period of time[1]. Since they have a good idea of the rate at which the sharks grow, they could infer that the larger specimens must have been growing for hundreds of years before the nuclear tests. [1] Things get a bit hand-wavy here; the article suggests that the shark lens stops absorbing carbon after it's formed. Meanwhile, Wikipedia says that the human lens continues to grow.