Tracking of marine oxygen loss through fossilized shark teeth

The shark’s teeth have long been fascinated by people. From your average beach average that stumbles into an embedded in the sand, to a paleontologist who tries to determine their evolutionary importance for scientists studying their role in marine networks … They all love a good shark tooth. But for the first time, researchers are watching them from another angle – one that can help rebuild the history of ocean oxygen levels.

The composition of the uranium isotope (Δ238U) of seawater is a powerful tool to study past marine anoxia, or poor oxygen conditions, which have been related to mass disappearances and climate shifts. Typically, carbonates as limestone are used to follow these isotopia signatures, but they can be changed after deposition, leading scientists to explore alternative archives. The shark teeth, with their strong enamel made of crystalline fluorapatitis, can offer a more stable record. Since uranium is easily connected to phosphate, the teeth of fossilized sharks can, in theory, preserve the original signal of the sea uranium isotope.

To test this idea, Haoyu Li of the Institute of Technology in California and a team of researchers analyzed uranium isotopes in 39 sharks fossilized by various locations, including the island of Banks in the Arctic, the Gulf of Mexico and the Pisco Basin of Peru. These teeth traveled a wide range of age, from the modern era again to the Cretan period. The results found that modern shark teeth contain negligible uranium – less than a portion per billion – while fossil teeth show significantly higher concentrations, reaching several hundred parts per million. This suggests that the uranium is not included in the teeth while the shark is alive, but rather enters the structure after the shark passes, during burial and fossilization.

The isotope data showed a wide range of values ​​δ238U, from -0.72 to +0.57 ‰, and values ​​δ234u from -162.1 to +969.7. Li and the team say this change suggests two main findings: first, that the diagenic overload is common, means that the uranium signal in many fossilized teeth does not fully reflect the original seawater values. Second, that uranium isotope reports are affected by local deposit environments, which means that shark teeth from different regions can record various chemical signatures based on the conditions under which animals were buried. Despite these complications, the changes of uranium isotope in the shark teeth are comparable to those found in marine carbonates, suggesting that some samples – those with minimal diagenic change – can still provide useful knowledge in ancient ocean conditions!

Li’s work underlines both the potential and the challenges of using shark teeth as an archive for rebuilding past ocean oxygen levels. Their mineral composition makes them more resistant to change than carbonates, but they are not clearly immune to post -deposition changes. The team suggests that any future research done will need to refine the methods used to identify the teeth that have undergone minimal uranium exchange, and for distinguishing primary sea signals from diagenic impacts. But if they are successful, shark teeth can become a valuable tool to study marine anoxia, helping scientists better understand past climate events and predict future ocean changes in response to global warming.