Wikibooks: Historical Geology/Sclerochronology

• Format: Book
• Language: English
• Subjects: Arctica islandica
• Online Access: https://en.wikibooks.org/wiki/Historical_Geology/Sclerochronology

In this article we shall discuss the basis of sclerochronology a method of dating shells and [[corals]] by analysis of their growth patterns. =Growth patterns in shells and corals= When shelly organisms grow many types lay down bands of new growth in a way that regularly reflects the passage of time for example laying down one growth band per day ( ) or one growth band every low tide as mussels do. 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. 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. The photograph above right shows the clam Arctica islandica a popular species with sclerochronologists growth bands are visible in the shell. 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. =Tidal braking= 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 for more details). This may not sound like much but it adds up over the course of 100 million years that would add up to a change in the length of a day of 38 minutes. This means that in the past days must have been shorter. As the length of a year is constant this means that in the past there must have been more days per year using the present rate of slowing as a basis there would have been about ten more days per year a 100 million years ago. We should note that in fact scientists do not ...