Terminology of geological time: Establishment of a community standard

Abstract

It has been recommended that geological time be described in a single set of terms and according to metric or SI (Systeme International d'Unites) standards, to ensure worldwide unification of measurement. While any effort to improve communication in sci- entific research and writing is to be encouraged, we are also concerned that fundamental differences between date and duration, in the way that our profession expresses geological time, would be lost in such an oversimplified terminology. In addition, no precise value for 'year' in the SI base unit of second has been accepted by the international bodies. Under any circumstances, however, it remains the fact that geologi- cal dates - as points in time - are not relevant to the SI. Known dates may define durations, just as known durations may define dates, or dates may simply be punctual references that support historical narratives, but dates are not quantities. Furthermore, dates, as datum points, belong to a specific type of guiding information that is in constant use not only by the disciplines that explore the unwritten past, but in the physical sciences and engineering as well. Accordingly, we recommend a new standardization of the distinction between geohistorical date, in years before present expressed in 'annus', symbol 'a', with the multiples 'ka', 'Ma', and 'Ga' for thousands, millions and billions of years ago, ac- cording to a convention that has been very widely adopted during the last 30 years, and geohistorical duration ,e xpressed in 'year', symbol 'yr', with multiples 'kyr', 'Myr' and 'Gyr', respectively, as the most appropriate among the various formats in the current literature. Agree- ment on these two sets of terms throughout the wide community that deals with paleochronology would remove a false impression of impro- visation and uncertainty as to appropriate terminology, and would lead to more effective communication in areas where a simplified but needlessly SI-conisistent terminology would be less, not more useful.