Shark teeth have long fascinated humans. From your average beach-comber stumbling across one embedded in the sand, to a paleontologist trying to determine their evolutionary significance to scientists studying their role in marine food webs… everybody loves a good shark tooth. But for the first time, researchers are looking at them from a different angle — one that could help reconstruct the history of ocean oxygen levels.
The uranium isotope composition (δ238U) of seawater is a powerful tool for studying past marine anoxia, or oxygen-depleted conditions, which have been linked to mass extinctions and climate shifts. Typically, carbonates like limestone are used to track these isotope signatures, but they can be altered after deposition, leading scientists to explore alternative archives. Shark teeth, with their hard enameloid made of crystalline fluorapatite, might offer a more stable record. Since uranium readily binds to phosphate, fossilized shark teeth could, in theory, preserve the original seawater uranium isotope signal.
To test this idea, Haoyu Li of the California Institute of Technology and a team of researchers analyzed uranium isotopes in 39 fossilized shark teeth from diverse locations, including Banks Island in the Arctic, the Gulf of Mexico, and Peru’s Pisco Basin. These teeth spanned a wide age range, from the modern era back to the Cretaceous period. The results revealed that modern shark teeth contain negligible uranium—less than one part per billion—while fossil teeth show significantly higher concentrations, reaching several hundred parts per million. This suggests that uranium is not incorporated into teeth while the shark is alive but instead enters the structure after the shark passes, during burial and fossilization.
The isotope data showed a wide range of δ238U values, from -0.72 to +0.57 ‰, and δ234U values from -162.1 to +969.7 ‰. Li and the team say this variation suggests two key findings: first, that diagenetic overprinting is common, meaning that the uranium signal in many fossilized teeth does not fully reflect original seawater values. Second, that the uranium isotope ratios are influenced by local depositional environments, meaning shark teeth from different regions may record different chemical signatures based on the conditions in which the animal was buried. Despite these complications, the uranium isotope variations in shark teeth are comparable to those found in marine carbonates, suggesting that some samples — those with minimal diagenetic alteration — could still provide useful insights into ancient ocean conditions!
Li’s work highlights both the potential and the challenges of using shark teeth as an archive for reconstructing past ocean oxygen levels. Their mineral composition makes them more resistant to alteration than carbonates, but they are clearly not immune to post-depositional changes. The team suggest that any future research done will need to refine the methods used for identifying teeth that have undergone minimal uranium exchange, and for distinguishing primary seawater signals from diagenetic influences. But if successful, shark teeth could become a valuable tool for studying marine anoxia, helping scientists better understand past climate events and predict future ocean changes in response to global warming.
