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The friction of the tides slows down the Earth's rotation: this is known as tidal braking.
The effect, though small, is measurable by the high-precision clocks used by astronomers, and so can be established directly as well as on theoretical grounds: at present, the effect amounts to a day getting longer by 2.3 milliseconds over the course of a century (see here for more details).
It is possible to use these growth patterns to date recent shells (and so the sediments that contain them) in a manner analogous to dendrochronology.
However, there is a more interesting way of using this data, which we shall discuss in the remainder of this article.
Some corals lay down distinct bands of skeletal calcium carbonate on a daily basis and also display seasonal patterns, so that they keep count both of days and of years.If a change of one day per year corresponds to the passage of 10 million years, then this limits the precision with which we can resolve the age of a shell or coral.What is more, the change in day length is not as predictable as the decay of radioactive isotopes.So by calculating how tidal braking has changed the number of days in a year or a lunar month, we can put a date on the organisms: for example, a coral showing 375 daily growth bands per year must have grown around 100 million years ago.
The number of days per year or per lunar month changes so slowly over time that we cannot expect sclerochronology to be as precise as radiometric methods such as U-Pb.In the same way, mussels deposit their growth bands every low tide, but also show variations according to the phase of the moon, so that they keep count both of low tides and of lunar months.